Forest

Service

United States Department of Agriculture

FHTET-2017-01

September 2017

Forest Health Technology

Enterprise Team

Biology and Biological Control of

Common Gorse and Scotch Broom
Cover Photo Credits

Top: Common gorse (Forest and Kim Starr, Starr Environmental, bugwood.org).

Left of common gorse, top to bottom: Agonopterix umbellana (Janet Graham); 

Exapion ulicis (Janet Graham); Sericothrips staphylinus (Fritzi Grevstad, Oregon 

State University); Tetranychus lintearius (Eric Coombs, Oregon Department of 

Agriculture, bugwood.org).

Bottom: Scotch broom (Eric Coombs, Oregon Department of Agriculture, Eric Coombs, Oregon Department of Agriculture, 

bugwood.org).

Right of Scotch broom, top to bottom: top to bottom:Bruchidius villosus (Jennifer Andreas, 

Washington State University Extension); Exapion fuscirostre (Laura Parsons, 

University of Idaho, bugwood.org); Leucoptera spartifoliella (Eric Coombs, 

Oregon Department of Agriculture, bugwood.org).

How to cite this publication:

Andreas, J.E., R.L. Winston, E.M. Coombs, T.W. Miller, M.J. Pitcairn, 

C.B. Randall, S. Turner, and W. Williams. 2017. Biology and Biological Control 

of Scotch Broom and Gorse. USDA Forest Service, Forest Health Technology 

Enterprise Team, Morgantown, West Virginia. FHTET-2017-01.

References to pesticides appear in this publication. Publication 

of these statements does not constitute endorsement or 

recommendation of them by the U.S. Department of Agriculture, 

nor does it imply that uses discussed have been registered. Use of 

most pesticides is regulated by state and federal laws. Applicable 

regulations must be obtained from the appropriate regulatory agency 

prior to their use.

CAUTION: Pesticides can be injurious to humans, domestic animals, desirable 

plants, and fish and other wildlife if they are not handled and applied properly. Use all 

pesticides selectively and carefully. Follow recommended practices given on the label 

for use and disposal of pesticides and pesticide containers.

CAUTION:

PESTICIDES

The use of trade, firm, or corporation names in this publication is for the information 

and convenience of the reader. Such use does not constitute an official endorsement 

or approval by the U.S. Department of Agriculture or the Forest Service of any product 

or service to the exclusion of others that may be suitable.

The Forest Health Technology Enterprise Team (FHTET) was created in 

1995 by the Deputy Chief for State and Private Forestry, USDA Forest 

Service, to develop and deliver technologies to protect and improve the 

health of American forests. This book was published by FHTET as part of the 

technology transfer series. This publication is available online at:

http://bugwoodcloud.org/resource/pdf/commongorse.pdf
Federal Recycling Program

Printed on recycled paper

In accordance with Federal civil rights law and U.S. Department of Agriculture (USDA) civil 

rights regulations and policies, the USDA, its Agencies, offices, and employees, and institutions 

participating in or administering USDA programs are prohibited from discriminating based on 

race, color, national origin, religion, sex, gender identity (including gender expression), sexual 

orientation, disability, age, marital status, family/parental status, income derived from a public 

assistance program, political beliefs, or reprisal or retaliation for prior civil rights activity, in any 

program or activity conducted or funded by USDA (not all bases apply to all programs). Remedies 

and complaint filing deadlines vary by program or incident. 

Persons with disabilities who require alternative means of communication for program information 

(e.g., Braille, large print, audiotape, American Sign Language, etc.) should contact the responsible 

Agency or USDA’s TARGET Center at (202) 720-2600 (voice and TTY) or contact USDA through 

the Federal Relay Service at (800) 877-8339. Additionally, program information may be made 

available in languages other than English. 

To file a program discrimination complaint, complete the USDA Program Discrimination Complaint 

Form, AD-3027, found online at http://www.ascr.usda.gov/complaint_filing_cust.html and at any 

USDA office or write a letter addressed to USDA and provide in the letter all of the information 

requested in the form. To request a copy of the complaint form, call (866) 632-9992. Submit 

your completed form or letter to USDA by: (1) mail: U.S. Department of Agriculture, Office of the 

Assistant Secretary for Civil Rights, 1400 Independence Avenue, SW, Washington, D.C. 20250-

9410; (2) fax: (202) 690-7442; or (3) email: program.intake@usda.gov . 

Biology and Biological Control

of Common Gorse and Scotch Broom

Jennifer E. Andreas, Rachel L. Winston, Eric M. Coombs, 

Timothy W. Miller, Michael J. Pitcairn, Carol Bell Randall, 

Susan Turner, and Wyatt Williams

For additional copies of this publication, contact:

Richard Reardon

U.S. Forest Service

Forest Health Technology Enterprise Team

180 Canfield Street

Morgantown, WV 26505

304-285-1566

rreardon@fs.fed.us

This publication is available online at 

http://bugwoodcloud.org/resource/pdf/commongorse.pdf

Jennifer E. Andreas

Integrated Weed Control Program

Washington State University Extension

2606 W. Pioneer 

Puyallup WA 98371

253-445-4657

jandreas@wsu.edu
Jennifer E. Andreas, Invasive Plant Biocontrol Specialist, Integrated Weed 

Control Project, Washington State University Extension, Puyallup, WA; 

jandreas@wsu.edu; www.invasives.wsu.edu

Rachel L. Winston, Environmental Consultant, MIA Consulting LLC, 

Sandpoint, ID; rachel@getmia.net

Eric M. Coombs, Biological Control Entomologist (retired), 

Oregon Department of Agriculture Plant Division, Salem, OR; 

eric.coombs@comcast.net

Timothy W. Miller, Extension Weed Scientist, Washington State University 

Extension, Mount Vernon, WA; twmiller@wsu.edu

Michael J. Pitcairn, Senior Environmental Scientist, Pest Detection/ 

Emergency Projects Branch, California Department of Food & Agriculture, 

Sacramento, CA; mike.pitcairn@cdfa.ca.gov

Carol Bell Randall, Entomologist, USDA Forest Service, Forest Health 

Protection, Coeur d’Alene, ID; crandall@fs.fed.us

Susan Turner, Invasive Plant Biocontrol Specialist, Ministry of Forests, 

Lands and Natural Resource Operations, Kamloops, British Columbia; 

Susan.Turner@gov.bc.ca

Wyatt Williams, Invasive Species Specialist, Oregon Department 

of Forestry Private Forests Division, Salem, OR; 

wyatt.williams@oregon.gov; http://tinyurl.com/ODF-ForestHealth

We are very grateful to Fritzi Grevstad (Oregon State University), 

Richard Hill (Landcare Research), Todd Neel (USDA FS), and Paul Pratt 

(USDA ARS WRRC) for help with specific sections of this manual, and 

to Janis Matsunaga (Hawaii Department of Agriculture) and Pat Conant 

(Hawaii Department of Agriculture, retired) for help gathering information. 

We would like to thank all of the county weed coordinators and land 

managers that we have worked with through the years for encouraging us 

to develop our series of “Biology and Biological Control ” manuals. Some 

of the material in this manual was adapted from past manuals in the 

series; we wish to acknowledge the authors of the original material. The 

layout of this manual was designed by Wendy W. Harding. We appreciate 

all of the photographers who granted permission for the use of photos. 

We also extend our gratitude to Richard Reardon (Forest Service-Forest 

Health Technology Enterprise Team or FHTET) for producing this guide.

Authors

Acknowledgments
Contents Chapter 1: Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Responding to the Threat of Gorse and Scotch Broom . . . . . . . . . . . 3

The Invasion Curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Management of Gorse and Scotch Broom Infestations . . . . . . . . . . . 6

Classical Biological Control of Weeds . . . . . . . . . . . . . . . . . . . . . . . . . 7

Code of Best Practices for Classical Biological Control of Weeds . . . 9

Biological Control of Gorse and Scotch Broom . . . . . . . . . . . . . . . . . 10

Is Biological Control of Gorse and Scotch Broom Right for You? . . . 11

About This Manual . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Chapter 2: Getting to Know Gorse and Broom . . . . . . . . . . . . . . . . . 14

Taxonomy and Related Species . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Gorse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Biology and Ecology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Habitat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Commonly Confused Species . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Scotch Broom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Biology and Ecology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Habitat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Commonly Confused Species . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Broom and Gorse Comparison Tables . . . . . . . . . . . . . . . . . . . . . 31-32

Chapter 3: Biology of Gorse and Scotch Broom 

 Biological Control Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Insects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Beetles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Butterflies and Moths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Thrips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

True Bugs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Mites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Fungi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Gorse and Scotch Broom Biological Control Agents (General) . . . . 37
Gorse Biological Control Agents (North America and Hawaii) . . . . . 37

Exapion ulicis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Gorse seed weevil

Tetranychus lintearius . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Gorse spider mite

Gorse Biological Control Agents (Hawaii Only) . . . . . . . . . . . . . . . . . 42

Agonopterix umbellana . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Gorse soft shoot moth

Apion sp. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

Gorse seed weevil

Pempelia genistella . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Gorse colonial hard shoot moth

Sericothrips staphylinus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Gorse thrips

Stenopteriapion scutellare . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Gorse stem-mining weevil

Uromyces pisi f. sp. europaei . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Gorse rust

Scotch Broom Biological Control Agents . . . . . . . . . . . . . . . . . . . . . 48

Bruchidius villosus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Broom seed beetle

Exapion fuscirostre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Scotch broom seed weevil

Leucoptera spartifoliella . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

Scotch broom twig miner

Unapproved Natural Enemies of Gorse and Scotch Broom . . . . . . . 60

Aceria genistae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

Agonopterix nervosa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

Arytainilla spartiophila . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

Comparison Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65-69

Chapter 4: Elements of a Gorse and Scotch Broom 

 Biological Control Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

Before You Begin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

Determining the Scope of the Problem . . . . . . . . . . . . . . . . . . . . . 71

Defining Goals and Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

Understanding Gorse and Scotch Broom 

 Management Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

Developing, Implementing, and Managing 

 a Gorse and Scotch Broom Biological Control Program . . . . . . . . 73

Selecting Biological Control Agent Release Sites . . . . . . . . . . . . 73

Establish goals for your release site . . . . . . . . . . . . . . . . . . . . 73

Determine site characteristics . . . . . . . . . . . . . . . . . . . . . . . . . 73

Note land use and disturbance factors . . . . . . . . . . . . . . . . . . 74

Survey for presence of biological control agents . . . . . . . . . . . 75

Record ownership and access . . . . . . . . . . . . . . . . . . . . . . . . . 75

Contents 

(continued)
Choosing the Appropriate Biological Control Agents 

 for Release . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

Biocontrol agent efficacy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

Biocontrol agent availability . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

Release site characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

Obtaining and Releasing Gorse and Scotch Broom 

 Biological Control Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

Factors to consider when looking for sources 

 of biological control agents . . . . . . . . . . . . . . . . . . . . . . . . . . 80

Collecting Gorse and Scotch Broom 

 Biological Control Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

Collection methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

Racket and beat sheet . . . . . . . . . . . . . . . . . . . . . . . . . . 82

Sweep netting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

Aspirating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

Sorting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

Transferring infested plant material . . . . . . . . . . . . . . . . . 86

Light traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

Methods by species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

Gorse soft shoot moth (Agonopterix umbellana) . . . . . . . 87

Broom seed beetle (Bruchidius villosus) . . . . . . . . . . . . . 88

Scotch broom seed weevil (Exapion fuscirostre) . . . . . . 88

Gorse seed weevil (Exapion ulicis) . . . . . . . . . . . . . . . . . 88

Scotch broom twig miner (Leucoptera spartifoliella) . . . . 89

Gorse thrips (Sericothrips staphylinus) . . . . . . . . . . . . . . 89

Gorse spider mite (Tetranychus lintearius) . . . . . . . . . . . 89

Release Containers for Gorse and Scotch Broom 

 Biological Control Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

Transporting Gorse and Scotch Broom 

 Biological Control Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

Keep the containers cool at all times . . . . . . . . . . . . . . . . . 90

Transporting short distances . . . . . . . . . . . . . . . . . . . . . . . . 90

Shipping long distances . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

Other factors to consider . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

Avoiding common packaging mistakes . . . . . . . . . . . . . . . . 93

Purchasing Gorse and Scotch Broom 

 Biological Control Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

Releasing Gorse and Scotch Broom 

 Biological Control Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

Establish permanent location marker . . . . . . . . . . . . . . . . . 94

Record geographical coordinates 

 at release point using GPS . . . . . . . . . . . . . . . . . . . . . . . 94

Prepare map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

Complete relevant paperwork at site . . . . . . . . . . . . . . . . . 95

Set up photo point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

Release as many biocontrol agents as possible . . . . . . . . . 95

Contents 

(continued)
Regulations for the Transfer of Gorse and Scotch Broom 

 Biological Control Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

Documenting, Monitoring, and Evaluating 

 a Biological Control Program . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

The Need for Documentation . . . . . . . . . . . . . . . . . . . . . . . . . . 99

Information Databases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

Biological Control of Weeds: A World Catalog

 of Agents and their Target Weeds (database) . . . . . . . . . 100

EDDMapS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

Monitoring Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

Assessing biological control agent populations . . . . . . . . . 101

Assessing the status of gorse and Scotch broom 

 and co-occurring plants . . . . . . . . . . . . . . . . . . . . . . . . . 101

Qualitative (descriptive) vegetation monitoring . . . . . . 104

Quantitative vegetation monitoring. . . . . . . . . . . . . . . . . 104

Assessing impacts on nontarget plants . . . . . . . . . . . . . . . 105

Chapter 5: An Integrated Gorse and Scotch Broom 

 Management Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

Components of Integrated Weed Management Programs . . . . . . . 108

Education and Outreach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

Inventory and Mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

Prevention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

EDRR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

Weed Control Activities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

Biological Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

Physical Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

Pulling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

Cutting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

Mowing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

Cultural Practices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

Burning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

Grazing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

Seeding competitive species . . . . . . . . . . . . . . . . . . . . . . . 117

Chemical Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

Contents 

(continued)
Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

Selected References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

Chapter 1: Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

Chapter 2: Getting to Know Gorse and Scotch Broom . . . . . . . . . . 132

Chapter 3: Biology of Gorse and Scotch Broom 

 Biological Control Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

Chapter 4: Elements of a Gorse and Scotch Broom 

 Biological Control Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137

Chapter 5: An Integrated Gorse and Scotch Broom 

 Management Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138

Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140

Appendix I: Troubleshooting Guide: When Things Go Wrong . . . . 140

Appendix II: Biological Control Agent Release Form . . . . . . . . . . . 141

Appendix III: Gorse and Broom Biological Control Agent 

 Monitoring Form . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

Appendix IV: Gorse and Scotch Broom 

 Qualitative Monitoring Form . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

Appendix V: Scotch Broom Seedpod 

 Quantitative Monitoring Form . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

Contents 

(continued)



Chapter 1. Introduction

Overview Common gorse (Ulex europaeus L., Figure 1-1a) and Scotch broom (Cytisus 

scoparius [L.] Link, Figure 1-1b) are related woody shrubs native to Europe. 

Both species were introduced to North America in the 1800s as ornamentals and 

later widely used in erosion control and as natural livestock fences. Both species 

escaped cultivation and are now invasive throughout western and eastern North 

America. Common gorse (hereafter referred to simply as gorse, its most common 

name) and Scotch broom continue to be available for purchase in the United 

States, though the sale of these two species is now illegal in states where they are 

classified as regulated plants or noxious weeds.

Although several exotic broom species have been intentionally or accidentally 

introduced to North America, Scotch broom is by far the most common and 

problematic for land managers. Gorse and Scotch broom have been the primary 

targets of biological control efforts in the United States, and are the focus of 

this manual. Additional information for understanding and differentiating other 

related broom species is given in Chapter 2.

Figure 1-1. a. Gorse; b. Scotch broom. (a. Forest and Kim Starr, Starr Environmental, bugwood.org; b. Eric Coombs, Oregon 

Department of Agriculture, bugwood.org)

a b
2 Chapter 1: Introduction

Gorse is present in eight states in eastern North America as well as four states 

(including Hawaii) and one province in western North America (Figure 1-2a). 

Scotch broom is present in 19 states and two provinces in eastern North America 

and eight states (including Hawaii) and one province in western North America 

(Figure 1-2b). Though both gorse and Scotch broom are sparsely distributed on 

hillsides and along roadsides in the East, the worst infestations occur in western 

North America. Neither species is established in central states or provinces of 

North America.

Throughout their native and introduced ranges, gorse and Scotch broom are 

found at open sites, including hillsides, pastures, roadsides, river banks, dry river 

beds, chaparral, grasslands, degraded coastal dunes, forest edges and clear cuts, 

and fallow fields. They are most commonly found in cool, temperate regions in 

coarse, dry to semi-moist soils with low fertility. Severe winter temperatures, 

extensive summer drought, and heavy shading limit the distribution of both 

species.

Gorse and Scotch broom compete aggressively for light, water, and nutrients, 

and are a major concern for displacing native and/or more desirable species in 

natural areas, grasslands, and commercial forests. Because of their nitrogenfixing ability, both species have an advantage over competing vegetation in poor 

soils. Cattle avoid grazing both species; older growth is unpalatable, and toxic 

compounds in seeds have resulted in livestock death. Wildlife, goats, and sheep 

will browse young growth and flowers of both gorse and Scotch broom; however, 

both gorse and broom frequently form dense, impenetrable thickets that block 

animal access to water and more desirable forage. Gorse and Scotch broom are 

extreme fire hazards due to the high oil content of foliage and seeds and the large 

amount of dead growth/litter in plant centers and beneath their canopies. Both 

species are long-lived (15-30 years) and their seeds can remain viable in the soil 

for 30 years or more, exacerbating their negative impacts. 

Figure 1-2. North American and Hawaiian distribution of: a. gorse; b. Scotch broom. Some 

states and provinces are more heavily infested than others. (USDA PLANTS Database, 

EDDMapS)

a b
Chapter 1: Introduction 3

Responding to the 

Threat of Gorse 

and Scotch Broom

Gorse and Scotch broom are invasive species not native to North America whose 

introduction causes or is likely to cause economic or environmental harm. In 

Washington State, Scotch broom could cause an estimated $59 million in direct 

losses to range, timber, and hunting lands. If it were to spread into 35 percent of 

productive land, primarily in western Washington, it may cost $142.8 million 

in business activity, including the loss of 660 jobs and over $36 million in lost 

wages. In Oregon alone, Scotch broom currently causes a nearly $39 million loss 

in economic activity. This loss would increase to nearly $180 million if Scotch 

broom successfully invades all suitable/susceptible habitat in Oregon. Likewise, 

gorse currently causes $441,000 of damage in Oregon; however, it only infests a 

fraction of its suitable habitat. Should all susceptible acres be infested, the loss of 

economic activity from gorse in Oregon would exceed $205 million.

A general management response to the threat of invasive species is based on 

four key elements or intermediate outcomes: prevention and preparedness, 

eradication, containment, and asset-based protection. In order to ensure a timely 

and appropriate management response, land managers must continually monitor, 

evaluate, and report new gorse and Scotch broom infestations and evaluate how 

gorse and broom responded to each control effort. Research and development 

informed by the observations and needs of land managers will play a critical role 

in the eventual success or failure of gorse and broom prevention and management 

activities in their invaded range. 

Prevention and Preparedness

Preventing high-risk invasive species from establishing is the most cost-effective 

approach to managing the threat they pose. Considerable resources and planning 

are required to maintain prevention of a large number of species. Preparedness 

encompasses all the activities and resources necessary to successfully manage 

new invasions.

Eradication

Eradication is generally only possible in the early stages of establishment when 

distribution and abundance of the invasive species are low. This approach can be 

almost as cost-effective as prevention. 

Containment

Where an invasive species cannot be eradicated, there can be substantial net 

benefit gained from preventing its further spread. Containment involves measures 

to eradicate outlying (satellite) infestations and prevent spread beyond the 

boundaries of core infestations (those that are too large and well established to 

eradicate). Obtaining a high degree of community support is a prerequisite for 

any long-term containment program.

Asset-Based Protection

An asset-based approach to managing an invasive species is appropriate once 

it has become so widespread that it would be inefficient to control the species 

everywhere it occurs, and containment would provide a low return on investment. 
4 Chapter 1: Introduction

The asset-based approach manages the invasive species only where reducing 

their adverse effects provides the greatest benefits by achieving protection and 

restoration outcomes for specific highly valued assets.

Monitoring, Evaluation, and Reporting

For science-based programs, such as invasive species management, monitoring, 

evaluation, and reporting are elements of adaptive management, whereby 

programs are continually reviewed and analyzed to ensure that their approaches 

are consistent with and supportive of any changes in environmental response, 

community expectation, or scientific knowledge.

Research and Development

The knowledge that comes from research and development is critical to 

implement evidence-based management approaches. In many cases, substantial 

advances in invasive species management will require development of new 

techniques and acquisition of greater and new knowledge. The investment in 

research needs to be sufficient to ensure future management is not seriously 

constrained by insufficient research and development support.

The invasion curve (Figure 1-3) shows that eradication of an invasive species 

such as gorse or Scotch broom becomes less likely and control costs increase 

as an invasive species spreads over time. Prevention is the most cost-effective 

solution, followed by eradication. If a species is not detected and removed early, 

intense and long-term control efforts will be unavoidable.

While gorse and Scotch broom infest large acreages in some regions, there are 

entire states and provinces where both weeds are absent or are present at very 

low population levels. The diversity of gorse and broom populations, from absent 

to widespread and abundant, throughout their potential range requires land 

managers to coordinate their management response to gorse and broom across 

larger landscapes to prevent current infestations from spreading into uninfested 

areas. 

Identifying where gorse and Scotch broom are on the invasion curve in a 

particular area is the first step to taking management action. Inventorying and 

mapping current gorse and broom populations, coupled with research efforts to 

predict where gorse and broom are most likely to inhabit (Figure 1-4a,b), enables 

land managers to concentrate resources in areas where gorse and broom are likely 

to invade, and then to treat individual plants and small populations before it is too 

late to remove them. 

Biological control is one of many control methods available to land managers, 

but biological control is not appropriate for areas on the left side of the invasion 

curve (species absent [prevention] – small number of localized populations 

[eradication]). Biological control as a control method is best suited to gorse and 

broom populations in the later phases of the invasion curve (rapid increase in 

distribution and abundance [containment] – widespread and abundant throughout 

potential range [asset based protection]). 

The Invasion 

Curve
Chapter 1: Introduction 5

Figure 1-3. Generalized invasion curve showing actions appropriate to each stage. (© State of Victoria, Department of 

Economic Development, Jobs, Transport and Resources. Reproduced with permission.)

Figure 1-4. Current distribution (black dots) and habitat suitability index (red high, green low) for: a. gorse; b. Scotch broom. 

(The Research Group, LLC 2014)

a b
6 Chapter 1: Introduction

Management 

of Gorse and 

Scotch Broom 

Infestations

Successful management of gorse and Scotch broom is an intensive process which 

requires land managers to continuously inventory, map, and assess the extent 

and severity of infestations. Land managers must also understand the benefits 

and shortcomings of each weed control method, alone and in combination, 

when applied to gorse and Scotch broom. Pulling or digging seedlings and 

young, individual gorse and broom plants is feasible; pulling large plants may 

exacerbate the problem as damaged roots and stems generate new shoots, and the 

disturbance of large pulling efforts provides a perfect environment for gorse and 

broom seedling recruitment. Mowing is similarly only feasible on young gorse 

and broom plants, and it may also exacerbate the problem by triggering re-growth 

and spreading seeds. Cutting large plants flush to the ground and chemically 

treating the stump has proven effective, though this method is time intensive 

and expensive for large infestations. Burning often facilitates gorse and broom 

re-sprouting and seedling establishment, and it can be dangerous due to the high 

flammable oil content of these species. Grazing gorse and broom can be effective 

on young growth, but larger plants are typically unpalatable or toxic, and grazing 

may have severe negative, long-term consequences for other components of 

the plant community. Chemical control (using herbicides) provides control of 

small infestations where monitoring and re-treatment can occur as necessary; 

however, it can be impractical, prohibitively expensive, and damaging to desired 

vegetation when treating very large infestations. Due to the inherent difficulties 

in managing gorse and broom throughout their invaded range, a biological 

control program was initiated in the 1920s. Many of the biological control agents 

that have since been approved for use against common gorse and Scotch broom 

in North America and/or Hawaii are already widespread. However, there are 

some infestations where biocontrol agents are not currently present or where 

the populations present might benefit from augmentative releases. This manual 

discusses the biological control of gorse and Scotch broom in North America and 

Hawaii, within the larger context of an integrated management strategy.

The most effective weed management strategies are based on regular inventory 

and monitoring of target weed populations, application of one or multiple weed 

control methods, evaluation of treatment efficacy, additional inventory and 

mapping, and adjustment of control methods as needed to meet management 

objectives in response to changing weed populations through time. 

Integrated Pest Management (IPM) incorporates additional activities that enable 

land managers to address the threat of gorse and broom invasions in infested 

as well as uninfested areas across a landscape. Integrated pest management 

activities include education and outreach, inventory and mapping, prevention 

methods, and control methods (physical control [pulling or mowing], cultural 

control [revegetation, grazing, or fire], chemical control [herbicides], and 

biological control). IPM relies on the development of realistic pest management 

objectives, accurate pest identification and mapping, appropriate prevention 

and control methods, and post-treatment monitoring to ensure current pestmanagement activities are meeting gorse and broom management goals.
Chapter 1: Introduction 7

Land managers choose control methods, either alone or in combination, that 

enable them to achieve their gorse and broom management goals or objectives 

in the most cost-effective manner. No single control method will enable 

managers to meet their gorse and broom management goals in all environments 

or instances. Control method(s) employed will depend on the size and location 

of the infested area and specific management goals (e.g., eradication vs. weed 

density reduction). Small patches of gorse and broom may be eliminated through 

a persistent physical or chemical control program, but large infestations will often 

require the use of additional control methods. A combination of control methods 

consistently applied, evaluated, and adjusted through time is usually necessary to 

attain and maintain weed management goals for gorse and Scotch broom.

Most invasive plants (weeds) in the United States are not native to North 

America; they arrived with immigrants, through commerce, or by accident from 

different parts of the world. These non-native plants are generally introduced 

without their natural enemies, the complex of organisms that feed on or attack 

the plant in its native range. A lack of natural enemies is thought to be one reason 

plant species become invasive weeds when introduced to areas outside of their 

native range.

Biological control of weeds (also called “biocontrol” of weeds) is the deliberate 

use of living organisms to limit the abundance of a target weed. In this manual, 

biological control refers to “classical biological control,” which reunites hostspecific natural enemies from the weed’s native range with the target weed in its 

introduced range. Natural enemies used in classical biological control of weeds 

include different organisms, such as insects, mites, nematodes, and pathogens. In 

North America, most weed biological control agents are plant-feeding insects, of 

which beetles, flies, and moths are among the most commonly used. 

Biological control agents may attack a weed’s flowers, seeds, roots, foliage, 

and/or stems. Effective biological control agents seldom kill weeds outright, but 

work with other stressors such as moisture or nutrient shortages to reduce vigor 

and reproductive capability, or facilitate secondary infection from pathogens—all 

of which compromise the weed’s ability to compete with other plant species. 

Once established, root- and crown-feeding biocontrol agents are usually more 

effective against perennial plants that primarily spread by root buds. Flowerand seed-feeding biocontrol agents are typically more effective on annual or 

biennial plants that spread only by seed. Regardless of the plant part attacked by 

biocontrol agents, the aim is always to reduce populations and vigor of the target 

weed. 

Although weed biological control is an effective and important weed 

management tool, it does not work in all cases and should not be expected to 

eradicate the target weed. Even in the most successful cases, biocontrol often 

requires multiple years before impacts become noticeable. When classical 

Classical 

Biological Control 

of Weeds
8 Chapter 1: Introduction

biological control alone does not result in an acceptable level of weed control, 

other weed control methods (e.g. physical, cultural, or chemical control) may be 

incorporated to achieve desired results. For a more in-depth description of weed 

control methods in the context of gorse and broom management, please refer to 

Chapter 5.

There are advantages and disadvantages to biological control of weeds as a 

management tool. These are listed in Table 1-1. 

To be approved for release in North America, weed biocontrol agents must be 

host specific, meaning they must feed and develop only on the target weed, or in 

limited cases, on a few closely related plant species. They must never feed on any 

crop or protected plant species; attack on ornamental plants may be minimally 

tolerated and is evaluated on a case-by-case basis. Rigorous testing is required to 

confirm that biocontrol agents are host specific and effective. Potential biocontrol 

agents often undergo five or more years of testing to ensure that rigid host 

specificity requirements are met, and results are vetted at a number of stages in 

the approval process. 

The United States Department of Agriculture’s Animal and Plant Health 

Inspection Service – Plant Protection and Quarantine (USDA-APHIS-PPQ) is 

the federal regulatory agency responsible for providing testing guidelines and 

authorizing the importation of biocontrol agents into the USA. The Canadian 

Food Inspection Agency (CFIA) serves the same regulatory role in Canada. 

Federal laws and regulations are in place to identify and avoid potential risks 

to native and economically valuable plants and animals that could result 

from exotic organisms introduced to manage weeds. The Technical Advisory 

Group (TAG) for Biological Control Agents of Weeds is an expert committee 

with representatives from USA federal regulatory, resource management, and 

Advantages Disadvantages

Target specificity Will not work on every weed in every setting 

Continuous action Permanent; cannot be undone

Long-term cost-effective; can 

provide sustained control at 

the landscape scale

Funding and testing candidate biocontrol agents is 

expensive; measurable impact may take years or 

even decades to materialize

Integrates well with other 

control methods

Approved biocontrol agents are not available for all 

exotic weeds

Generally environmentally 

benign

Like all weed control methods, nontarget effects are 

possible, but pre-release testing reduces the risks

Self-dispersing, even into 

rough or difficult to access 

terrain

Unpredictable level of control; does not eliminate 

weed

Table 1-1. Advantages/disadvantages of classical biological control as a 

weed management tool
Chapter 1: Introduction 9

environmental protection agencies, and regulatory counterparts from Canada 

and Mexico. TAG members review all petitions to import new biocontrol agents 

into the USA, and make recommendations to USDA-APHIS-PPQ regarding the 

safety and potential impact of prospective biocontrol agents. Weed biocontrol 

researchers work closely with USDA-APHIS-PPQ and TAG to accurately assess 

the environmental safety of potential weed biocontrol agents. In addition, some 

states in the USA have their own approval process to permit field release of weed 

biocontrol agents. In Canada, the Biological Control Review Committee (BCRC) 

draws upon the expertise and perspectives of Canadian-based researchers (e.g., 

entomologists, botanists, ecologists, weed biological control scientists) from 

academic, government, and private sectors for scientific review of petitions 

submitted to the CFIA. The BCRC reviews submissions for compliance with the 

North American Plant Protection Organization’s (NAPPO) Regional Standards 

for Phytosanitary Measures (RSMP) No. 7. The BCRC also reviews submissions 

to APHIS. The BCRC conclusions factor into the final TAG recommendation 

to APHIS on whether to support the release of the proposed biocontrol agent 

in the USA. When release of a biocontrol agent is proposed for both the USA 

and Canada, APHIS and the CFIA attempt to coordinate decisions based on the 

assessed safety of each country’s plant resources.

Biological control practitioners have adopted the International Code of Best 

Practices for Biological Control of Weeds. The Code was developed in 1999 by 

delegates and participants in the Tenth International Symposium for Biological 

Control of Weeds to both improve the efficacy of, and reduce potential negative 

impacts from, weed biological control. In following the Code, practitioners 

Code of 

Best Practices 

for Classical 

Biological Control 

of Weeds

International Code of Best Practices 

for Classical Biological Control of Weeds1

1. Ensure that the target weed’s potential impact justifies release of 

 non-endemic biocontrol agents

2. Obtain multi-agency approval for target

3. Select biocontrol agents with potential to control target

4. Release safe and approved biocontrol agents

5. Ensure that only the intended biocontrol agent is released

6. Use appropriate protocols for release and documentation

7. Monitor impact on the target

8. Stop releases of ineffective biocontrol agents or when control is achieved

9. Monitor impacts on potential nontargets

10. Encourage assessment of changes in plant and animal communities

11. Monitor interaction among biocontrol agents

12. Communicate results to public

1

Ratified July 9, 1999, by the delegates to the X International Symposium on Biological Control

 of Weeds, Bozeman, MT
10 Chapter 1: Introduction

reduce the potential for causing environmental damage through the use of weed 

biological control by voluntarily restricting biocontrol activities to those most 

likely to result in success and least likely to cause harm.

There are several resources that provide additional information about general 

weed biocontrol practices and specific weed-biocontrol systems, which can be 

found in the Chapter 3 references under Andreas et al. 2017, Coombs et al. 2004, 

and Winston et al. 2014b.

Although many species “hitchhiked” on gorse and Scotch broom plants when 

they were introduced from Europe, four of these species are now credited 

with the earliest incidences of biological control of gorse and broom in North 

America. The gorse tip moth Agonopterix nervosa, the broom gall mite Aceria 

genistae, the broom psyllid Arytainilla spartiophila, and the broom seed beetle 

Bruchidius villosus were first recorded between 1915 and 2005 on gorse and/or 

Scotch broom growing in Canada and/or the USA. Following the initiation of the 

gorse and broom biocontrol program in the 1920s, 11 additional biocontrol agents 

were released in the USA after being officially screened for safety, suitability, 

host specificity, and efficacy. Six of these species were released only in Hawaii 

and only on gorse.

The first tested and approved biocontrol agent, the gorse seed weevil Exapion 

ulicis (Figure 1-5), was released in Hawaii in 1926 and in the continental United 

States in 1953. From 1960-1994 three additional biological control agents were 

approved for release in the continental USA, including the Scotch broom seed 

weevil Exapion fuscirostre, the broom twig miner Leucoptera spartifoliella, and 

the gorse spider mite Tetranychus lintearius. The Scotch broom seed beetle 

B. villosus, accidentally introduced into the eastern USA in 1918, was tested for 

host specificity and approved for redistribution in the continental USA in 1998. 

From 1958-2000, six additional species were tested, approved, and released only 

in Hawaii, including the gorse soft shoot moth Agonopterix umbellana (formerly 

Biological Control 

of Gorse and 

Scotch Broom

Figure 1-5. Adult Exapion 

ulicis, the gorse seed 

weevil. (Janet Graham)
Chapter 1: Introduction 11

A. ulicetella), the gorse weevil Apion sp., the gorse colonial hard shoot moth 

Pempelia genistella, the gorse thrips Sericothrips staphylinus, the gorse stemgalling weevil Stenopterapion scutellare, and the gorse rust Uromyces pisi

f. sp. europaei. 

The four Scotch broom and gorse biological control agents currently established 

in Canada arrived adventively from Europe or naturally spread from the 

continental USA. 

When biological control is successful, biocontrol agents increase in abundance 

until they suppress (or contribute to the suppression of) the target weed. As local 

target weed populations are reduced, their biological control agent populations 

also decline due to starvation and/or dispersal to other target weed infestations. 

In many biocontrol systems, there are fluctuations over time with the target weed 

becoming more abundant, followed by increases of its biocontrol agent, until the 

target weed/biocontrol agent populations stabilize at a much lower abundance. 

As stated in Table 1-1, biological control is not effective in every weed system 

or at every infestation. Furthermore, many of the biocontrol agents currently 

approved for use against gorse and Scotch broom are already widespread. We 

recommend that you develop an integrated weed management program in which 

biological control is one of several control methods considered. Here are some 

questions you should ask before you begin a biological control program:

Is my management goal to eradicate the weed or reduce its abundance? 

Biological control does not eradicate target weeds, so it is not a good fit with 

an eradication goal; however, depending on the target weed, which biological 

control agent is used, and land use, biological control can be effective at reducing 

the abundance of a target weed to an acceptable level.

How soon do I need results: this season, one to two seasons, or within five to 

ten years?

Biological control requires time and patience to work. Generally, it can take 

one to three years after release to confirm that biological control agents are 

established at a site, and even longer for biocontrol agents to cause significant 

impacts to populations of the target weed. For some weed infestations, 5-30 years 

may be needed for biological control to reach its weed management potential.

What resources can I devote to my weed problem? 

If you have only a small gorse and broom problem (< 1 acre or 0.4 ha), 

weed control methods such as pulling and/or herbicides, followed by regular 

monitoring for re-growth and re-treatment when necessary, may be most 

effective. These intensive control methods may allow you to achieve rapid 

control and prevent the weeds from spreading and infesting additional areas, 

especially when infestations occur in high-priority treatment areas such as travel 

corridors where the weeds are more likely to readily disperse. Where broom or 

gorse are well established over a large area (>1 acre or 0.4 ha), and resources are 

limited, biological control may be the most economical weed control option.

Is Biological 

Control of Gorse 

and Scotch Broom 

Right for You?
12 Chapter 1: Introduction

Is the weed the problem, or a symptom of the problem? 

Invasive plant infestations often occur where desirable plant communities have 

been or continue to be disturbed. Without restoration of a desirable, resilient plant 

community, and especially if disturbance continues, biological control is unlikely 

to solve your weed problems.

The ideal biological control program:

1. Is based upon an understanding of the target weed, its habitat, land use and 

condition, and management objectives

2. Is part of a broader integrated weed management program

3. Has considered all weed control methods and determined that biological 

control is a suitable option based on available resources and weed 

management objectives

4. Has realistic weed management goals and timelines

5. Includes resources to ensure adequate monitoring of the target weed, the 

vegetation community, and populations of biological control agents.

This manual provides information on the biology and ecology of gorse and 

Scotch broom and each of their biological control agents, with emphasis on North 

America. It also presents guidelines to establish and manage approved biological 

control agents as part of an integrated gorse and broom management program.

Chapter 1: Introduction provides introductory information on gorse and Scotch 

broom (including their distribution, habitat, and economic impact) and classical 

biological control.

Chapter 2: Getting to Know Gorse and Scotch Broom provides detailed 

descriptions of the taxonomy, growth characteristics and features, invaded 

habitats, and occurrence of gorse and Scotch broom in North America. It also 

describes how to differentiate gorse and Scotch broom from related and lookalike species.

Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents 

describes biological control agents of gorse and Scotch broom, including details 

on each biocontrol agent’s native range, original source of releases in North 

America, parts of gorse and broom plants attacked, life cycle, description, host 

specificity, known nontarget effects, habitat preferences, and availability. This 

chapter is particularly useful for identifying biological control agents in the field.

Chapter 4: Elements of a Gorse and Scotch Broom Biological Control 

Program includes detailed information and guidelines on how to plan, 

implement, monitor, and evaluate an effective gorse and Scotch broom biological 

control program. Included are guidelines and methods for:

• Selecting and preparing biological control agent release sites

• Collecting, handling, transporting, shipping, and releasing biological 

control agents

• Monitoring biological control agents and vegetation

About This Manual
Chapter 1: Introduction 13

Chapter 5: An Integrated Gorse and Scotch Broom Management Program 

discusses the role of biological control in the context of an integrated gorse and 

Scotch broom management program.

The Glossary defines technical terms frequently used by those involved in gorse 

and Scotch broom biological control and found throughout this manual.

References lists selected publications and resources utilized to compile this 

manual.

Appendices:

I. Troubleshooting Guide: When Things Go Wrong

II. Biological Control Agent Release Form

III. Gorse and Broom Biological Control Agent Monitoring Form

IV. Gorse and Scotch Broom Qualitative Monitoring Form

V. Scotch Broom Seedpod Quantitative Monitoring Form
14

Chapter 2: Getting to Know Gorse and Scotch Broom

Taxonomy and 

Related Species

Common gorse, hereafter referred to simply as gorse, and Scotch broom belong 

to the pea family (Fabaceae or Leguminosae). Members of the pea family 

produce unique five-petal flowers that resemble sailboats. A single large petal 

comprises the banner, two petals form the wings, and two fused petals form the 

keel (Figure 2-1).

Gorse is in the genus Ulex. There are approximately 20 species in this genus, all 

of which are thorny evergreen shrubs with green stems and yellow flowers in the 

form of a 2-lipped sailboat. No species of Ulex are native to North America, and 

other than common gorse, no other Ulex species occur in North America. 

Scotch broom is in the genus Cytisus, which is represented by approximately 

65 species worldwide. No species in this genus are native to North America, 

though four species and two cultivated hybrids are currently established in North 

America. Of these, only Scotch 

broom and Portuguese broom 

(Cytisus striatus) are considered 

invasive and problematic in the USA. 

Three closely related genera contain 

additional broom species considered 

invasive and problematic in the 

USA, including Genista, Spartium, 

and Retama. Scotch broom is by far 

the most problematic broom species 

present in North America and the 

primary target of broom biological 

control efforts and this manual. All 

of the invasive brooms are thornless 

shrubs with green stems and flowers 

in the form of a 2-lipped sailboat. 

The related invasive brooms are 

described in greater detail later in this 

chapter on page 31. Figure 2-1. Pea flower diagram: 

a. single-petal banner; b. one of two singlepetal wings; c. two-petal keel. (Tony Wills)

a

b

c
Chapter 2: Getting to Know Gorse and Scotch Broom 15

Gorse Scientific Name

Ulex europaeus L.

Other Names

Common gorse, furze, whin, European gorse

Classification

Kingdom Plantae Plants

 Subkingdom Tracheobionta Vascular plants

 Superdivision Spermatophyta Seed plants

 Division Magnoliophyta Flowering plants

 Class Magnoliopsida Dicotyledons

 Subclass Rosidae

 Order Fabales

 Family Fabaceae (Leguminosae) Pea family

 Genus Ulex Gorse

 Species Ulex europaeus L. Gorse

History

Gorse, a native of Western Europe, was intentionally introduced to North 

America in the 1800s as an ornamental and as a hedge plant to contain livestock. 

It was recorded escaping cultivation by 1900. Gorse was introduced to Hawaii in 

the late 1800s as a hedge plant and was considered invasive there by 1925.

Description

At a Glance 

Gorse (Figure 2-2) is an evergreen shrub typically growing 3-13 feet (1-4 m) 

tall from a woody, multi-branched root system. Stems are hairy when young and 

less so as the plant ages. Leaves are alternate and three-parted when the plant 

is young and are reduced to scales or thick spines as the plant ages. Flowers are 

Figure 2-2. Gorse plant. 

(Wendy DesCamp, 

Washington State Noxious 

Weed Control Board)
16 Chapter 2: Getting to Know Gorse and Scotch Broom

yellow and occur either singly in leaf axils or in numerous clusters on the ends of 

older branches. Flowers are characteristic of the pea family with petals forming 

a banner and keel (similar to a boat). Seedpods are hairy, turning black with age. 

They grow to 0.8 inches (2 cm) long.

Roots

Gorse develops a large taproot up to 2 feet (60 cm) long (Figure 2-3a) with 

multiple branching lateral roots occurring in the top 4 inches (10 cm) of soil. 

Gorse stems growing low along the ground sprout adventitious aerial roots 

(Figure 2-3b). All roots have numerous nodules that contain nitrogen-fixing 

bacteria, allowing gorse to colonize nutrient poor soils and outcompete other 

plant species.

Stems

Plants may grow prostrate or erect. Prostrate plants typically occur in exposed, 

windy locations. Erect plants grow 3-13 feet (1-4 m) tall and are often as wide as 

they are tall. When growing in locations with dense vegetation, gorse produces a 

single main stem. At more open sites, gorse produces multiple densely branched 

stems. Stems of young plants are soft, gray-green, and hairy. As the plants age, 

stems remain green but become woody, angled, and terminate in a spine 1.5-2.5 

inches (3.8-6.3 cm) long. Mature stems appear leafless and covered with spines 

(Figure 2-3c). Both stems and spines photosynthesize.

Leaves

Leaves are small, alternate, and three-parted when the plant is young, and are 

reduced to scales or stiff spines as the plant ages. Spines and leaves have a waxy 

coating. Mature plants are densely covered in sharp spines; spines are 1.8-2.6 

inches (4.6-6.6 cm) long and end in a yellow point (Figure 2-3c,d). Plants are 

evergreen; the green scales and spines are present on stems year-round.

Flowers

Flowers occur either singly in leaf axils or in numerous clusters on the ends 

of older branches. Flowers are yellow, 0.5-1 inch (1.3-2.5 cm) long, and 

characteristic of the pea family with petals forming a banner and keel (similar to 

Figure 2-3. Gorse: a. underground root system; b. adventitious aerial roots; c. stems; d. spines. (a.,c.,d. Nancy Ness, Grays 

Harbor Noxious Weed Control Board; b. ©Phil Bendle, Friends of Te Henui, T.E.R:R.A.I.N)

a b c d
Chapter 2: Getting to Know Gorse and Scotch Broom 17

a boat, Figure 2-4a). Plants begin flowering from 18 months to 3 years of age. In 

North America, flowering occurs in early spring with a smaller secondary bloom 

in late fall at some locations. 

Fruits and Seeds

Seedpods (legumes) are hairy and green, turning black with age (Figure 2-4b). 

They grow 0.5-0.8 inches (1.3-2 cm) long and contain 1-6 seeds. The oval 

seeds are 0.1-0.15 inches (3-4 mm) across, hard, shiny, and dark brown or black 

(Figure 2-4c). A mature plant can produce up to 18,000 seeds annually.

See Figure 2-5 on the next page for a line drawing of key gorse diagnostic traits.

Figure 2-4. Gorse: a. flowers; b. seedpods; c. seeds in pod. (a. Jennifer Andreas, 

Washington State University Extension; b. Forest and Kim Starr, Starr Environmental, 

bugwood.org; c. Steven Conaway, Penn State University, bugwood.org)

a b

c
18 Chapter 2: Getting to Know Gorse and Scotch Broom

Figure 2-5. Illustration of gorse key traits. (Britton, N.L., and A. Brown. 1913. An illustrated flora of the northern United States, 

Canada and the British Possessions. 3 vols. Charles Scribner’s Sons, New York. Vol. 2: 349., USDA PLANTS database) 
Chapter 2: Getting to Know Gorse and Scotch Broom 19

Biology and Ecology

Gorse spreads by seed only, but it can also regenerate from the root crown after 

the stem is damaged (Figure 2-6). Mature seedpods split open rapidly in dry 

weather, ejecting seeds up to several feet (a few meters), though most fall within 

3.2 feet (1 m) of the plant. Seeds are transported by insects, birds, humans, other 

animals, waterways, and vehicles/equipment. Due to their thick seed coats, seeds 

can remain viable in the soil for up to 30 years. 

The highest rates of germination occur in moist soils and in open, disturbed 

soils with limited competing vegetation. Most seeds germinate in spring or 

early summer; germination rates are highest after seed scarification. Seedlings 

are sensitive to shading from other plants and survive better in areas with little 

competition for light. Juvenile plants have small, three-parted leaves. These are 

reduced to scales and spines as the plant ages. Mature plants photosynthesize 

with their spines and green stems. Plants begin flowering at 18 months to three 

years of age and continue to grow for 25-30 years. In North America, flowering 

occurs in early spring with a smaller secondary bloom in late fall at some 

locations. When seedpods mature, they dry out and burst open with an audible 

popping sound. This action, known as dehiscing, helps scatter seeds short 

distances.

Habitat

Soil disturbance is an important contributor to gorse seedling establishment. 

Gorse can often be found creating dense infestations on hillsides, pastures, 

Figure 2-6. Gorse sprouting from cut stem. (Jennifer Andreas, Washington State University 

Extension)
20 Chapter 2: Getting to Know Gorse and Scotch Broom

roadsides, river banks, dry river beds, chaparral, grasslands, degraded coastal 

dunes, forest edges, and fallow fields (Figure 2-7a-f). A variety of habitat types 

and plant communities can be invaded by gorse following heavy grazing, 

cultivation, logging, and burning. Gorse does best in cool, temperate regions. 

Severe winter temperatures, extensive summer drought, and heavy shading limit 

its distribution. Gorse performs best in coarse, well-drained, dry to semi-moist 

soils with low fertility and in areas without significant competing vegetation. 

Distribution

As of 2017, gorse has been declared noxious in four states and one Canadian 

province (Figure 2-8a). Though gorse is considered established in 12 states 

(including Hawaii) and one Canadian province (Figure 2-8b,c), it is most 

abundant and problematic in western North America and Hawaii (Figure 2-8c).

Comments

Mature gorse plants contain approximately 2 to 4 percent flammable oils. This, 

in combination with the large amount of dry branches and spines throughout 

the plant centers and beneath their canopies, can create an extreme fire hazard 

year round. Once ignited, gorse can burn rapidly and with high intensity. In 

1936, a wildfire fueled primarily by gorse swept through the coastal community 

of Bandon, Oregon, killing thirteen people and destroying much of the town. 

Established gorse plants are rarely killed by fire, so post-fire gorse populations 

can regenerate rapidly by both seed recruitment and by re-sprouting from basal 

stems.

Figure 2-7. Gorse infestations: a. along a river and encroaching forested mountainsides; b. on an urban sidewalk; c. in a golf 

course; d. in pastures, roadsides and cleared areas in a rural town; e. in coastal scrubland; f. in an abandoned field. (a., c.-f. 

Wyatt Williams, Oregon Department of Forestry; b. King County Noxious Weed Control Board)

a b c

d e f
Chapter 2: Getting to Know Gorse and Scotch Broom 21

Figure 2-8. Gorse: a. noxious weed and/or regulated species listings; b. establishment by states and provinces; 

c. establishment by counties and districts. (EDDMapS, USDA PLANTS Database, Washington State Noxious Weed Control 

Board, The Research Group LLC 2014, British Columbia IAPP [accessed 30 November 2016], Clements et al. 2001)

Gorse is still available for purchase in the USA, though the sale of this species 

is now illegal in states where it is classified as a regulated plant or noxious weed 

(Figure 2-8a). 

a b

c
22 Chapter 2: Getting to Know Gorse and Scotch Broom

Commonly Confused Species

Numerous species present in North America have yellow, pea-like flowers 

similar to gorse; however, most potential look-alikes are not shrubs and do not 

have sharp spines in places of leaves. Camelthorn (Alhagi maurorum) is a spiny 

shrub with pea-like flowers similar to gorse (Figure 2-9a,b). Camelthorn can be 

differentiated by its pink or maroon flowers, by growing only 2-3 feet 

(0.6-0.9 m) tall from a rhizomatous root system, and by its elliptic leaves 

that remain persistent on mature stems. Brooms are the species most closely 

resembling gorse. Brooms can be readily differentiated by their lack of spines 

and by having a less dense, open appearance. Species of broom resembling gorse 

are listed in Table 2-1 on page 31, along with key characteristics in Table 2-2 

(page 32) that can be used for accurate identification. 

Figure 2-9. Camelthorn (Alhagi maurorum), a potential look-alike for gorse; a. plant; 

b. close-up of features. (a.,b. Franklin County Noxious Weed Control Board) 

a b
Chapter 2: Getting to Know Gorse and Scotch Broom 23

Scotch Broom Scientific Name

Cytisus scoparius (L.) Link

Other Names

Broom, broomtops, common broom, English broom, European broom, Irish 

broom, Scot’s broom, Cytisus scoparius (L.) Link subsp. scoparius, Cytisus

scoparius subsp. andreanus (Puiss.) Dippel, Sarothamnus scoparius (L.) Wimm. 

ex W.D.J. Koch

Classification

Kingdom Plantae Plants

 Subkingdom Tracheobionta Vascular plants

 Superdivision Spermatophyta Seed plants

 Division Magnoliophyta Flowering plants

 Class Magnoliopsida Dicotyledons

 Subclass Rosidae

 Order Fabales

 Family Fabaceae (Leguminosae) Pea family

 Genus Cytisus Broom

 Species Cytisus scoparius (L.) Link Scotch broom

History

Scotch broom is considered native throughout Europe and also the Canary 

Islands. It was intentionally introduced to North America in the 1800s as an 

ornamental, fodder for domestic sheep, and erosion control. It was reported 

invasive by 1860. Scotch broom was introduced to Hawaii as an ornamental, 

possibly as early as the 1800s, though it was reportedly first collected growing 

on Hawaii Island in 1909.

Description

At a Glance 

Scotch broom (Figure 2-10) is a shrub typically growing 3-10 feet (1-3 m) tall 

from a forked taproot. Stems are hairy when young and less so as the plant ages. 

Stems are 5-angled or star-shaped in cross section. Leaves are alternate and 

three-parted, and are deciduous early in the season and in times of stress. Flowers 

are yellow, appear singly or in clusters of two, and are characteristic of the 

pea family with petals forming a banner and keel (similar to a boat). Seedpods 

can grow up to 3 inches (7.5 cm) long; they are flattened and have hair on the 

margins, turning brown at maturity.
24 Chapter 2: Getting to Know Gorse and Scotch Broom

Figure 2-10. Scotch broom 

plants in flower. (Jennifer 

Andreas, Washington 

State University Extension)

Roots

Scotch broom develops a large, forked taproot over 2 feet (60 cm) long (Figure 

2-11a) with multiple branching lateral roots growing shallowly just beneath 

the soil surface. All roots have numerous nodules that contain nitrogen-fixing 

bacteria, allowing broom to colonize nutrient poor soils and outcompete other 

plant species.

Stems

Scotch broom plants may grow prostrate or erect. Prostrate plants typically 

occur in exposed, windy locations. Erect plants grow 3-10 feet (1-3 m) tall 

and are often as wide as they are tall. When growing in locations with dense 

vegetation or shade, Scotch broom produces a single main stem. At more open 

sites, Scotch broom produces multiple densely branched stems. Stems of young 

plants are hairy. As the plants age, stems become woody, hairless, and 5-angled 

or star-shaped in cross section (Figure 2-11b). All stems are green and used in 

photosynthesis. Leaves are deciduous early in the season, leaving stems bare and 

green (Figure 2-11c). 

Leaves

Leaves are small, alternate, three-parted (separated into three leaflets), and appear 

in early spring. Each leaflet is elliptical, 0.3-0.7 inches (5-20 mm) long, and 0.06-

0.3 inches (1.5-8 mm) wide (Figure 2-11d). The bottom sides of leaflets are often 

fuzzy with short hairs. Leaves are deciduous early in the growing season and in 

times of stress. When leaves fall from the plant, the remaining bare green plant 

stems are the primary source of photosynthesis. 

Flowers

Flowers occur either singly or in clusters of two in leaf axils or in numerous 

clusters on the ends of older branches. Flowers are 0.6-1 inch (1.5-2.5 cm) 

long, and characteristic of the pea family with petals forming a banner and keel 

(similar to a boat, Figure 2-12a and 2-14). Most flowers are yellow, though they 

can vary from off-white and creamy yellow to orange, red, and a combination of 

these colors (Figure 2-12a-f). Plants begin flowering from 18 months to 3 years 

of age. Flowering usually occurs in early spring, though sporadic flowering can 

occur throughout the year. 
Chapter 2: Getting to Know Gorse and Scotch Broom 25

a b c d

Figure 2-11. Scotch broom: a. root; b. stems; c. upper stems becoming bare as leaves fall; d. leaves. (a. Nancy Ness, Grays 

Harbor Noxious Weed Control Board; b. Robert Vidéki, Doronicum Kft., bugwood.org; c. Steve Dewey, Utah State University, 

bugwood.org; d. Jennifer Andreas, Washington State University Extension)

Figure 2-12. Scotch broom flower color variation: a. yellow (most typical); b. cream and orange; c. yellow and red; d. red with 

pink; e., f. landscape images with variable flower colors. (a.-c. Jennifer Andreas, Washington State University Extension; 

d.-f. Eric Coombs, Oregon Department of Agriculture, bugwood.org)

a b c d

e f
26 Chapter 2: Getting to Know Gorse and Scotch Broom

Fruits and Seeds

Seedpods (legumes) are green, turning dark brown with age (Figure 2-13a). 

They are flattened with hairs along the margins (Figure 2-13b). Seedpods grow 

1-2.8 inches (2.5-7 cm) long and 0.3-0.5 inches (8-13 mm) wide and contain 

3-12 seeds each. The oval seeds are 0.1-0.15 inches (3-4 mm) long, hard, smooth, 

and brown (Figure 2-13c). A mature plant can produce up to 15,000 seeds 

annually.

See Figure 2-14 for a line drawing of key Scotch broom diagnostic traits.

Figure 2-13. Scotch broom: a. green and brown seedpods (immature and mature); 

b. immature seedpod with hairs along the margins; c. seeds in a mature pod. 

(a.-c. Jennifer Andreas, Washington State University Extension)

a b

c
Chapter 2: Getting to Know Gorse and Scotch Broom 27

Figure 2-14. Illustration of Scotch broom key traits. (Encyclopædia Britannica Eleventh Edition, Vol. 4: 650)
28 Chapter 2: Getting to Know Gorse and Scotch Broom

Biology and Ecology

Similar to gorse, Scotch broom spreads by seed only, but it can also regenerate 

from the root crown if the stem is damaged. Mature seeds pods split open rapidly 

in dry weather, ejecting seeds up to several feet (a few meters), though most fall 

within 3.2 feet (1 m) of the plant. Seeds are transported by insects, birds, humans, 

other animals, waterways, and vehicles/equipment. Due to their thick seed coats, 

seeds can remain viable in the soil for 30 years; some sources claim seeds remain 

viable for up to 80 years under proper storage conditions. 

The highest rates of germination occur in moist soils and in open, disturbed 

soils with limited competing vegetation. Most seeds germinate in spring or 

early summer; germination rates are highest after seed scarification. Seedlings 

are sensitive to shading from other plants and survive better in areas with little 

competition for light. Scotch broom leaves are deciduous early in the growing 

season and after stress, leaving the bare green plant stems as the sole source for 

photosynthesis. Plants begin flowering at 18 months to three years of age and 

continue to grow for up to 30 years, though most plants only live for 15 years. 

Flowering usually occurs in early spring though an occasional plant may bloom 

throughout the year. When seedpods mature, they dry out and burst open with an 

audible popping sound. This action, known as dehiscing, helps scatter seeds short 

distances.

Habitat

Scotch broom seedling establishment is facilitated by soil disturbance. The weed 

can often be found creating dense infestations on timber clear cuts, hillsides, 

pastures, roadsides, river banks, dry river beds, chaparral, grasslands, degraded 

coastal dunes, forest edges, and fallow fields (Figure 2-15a-f). A variety of 

habitat types and plant communities can be invaded by Scotch broom, especially 

following logging, flooding, and burning. The weed does best in cool, temperate 

regions, but is able to survive Mediterranean climates if summer droughts are 

not extensive. Severe winter temperatures, extensive summer drought, and 

heavy shading limit its distribution. Scotch broom performs best in sandy, welldrained, dry to semi-moist soils with low fertility and in areas without significant 

competing vegetation. 

Distribution

Scotch broom has been declared noxious and/or regulated in nine states 

(including one state where it has not yet been recorded, Figure 2-16a). As of 

2017, it is considered established in 27 states (including Hawaii) and three 

Canadian provinces (Figure 2-16b,c). 
Chapter 2: Getting to Know Gorse and Scotch Broom 29

Figure 2-15. Scotch broom infestations: a. in a logging clear cut; b. surrounding a reservoir; c. on a roadside; d. on an open, 

disturbed hillside; e. in a pasture; f. on an eroded slope. (a.-f. Jennifer Andreas, Washington State University Extension)

a b c

d e f

Figure 2-16. Scotch broom: a. noxious and/or regulated species listings; b. establishment by states and provinces; c. 

establishment by counties and districts. (EDDMapS, USDA PLANTS Database, Washington State Noxious Weed Control 

Board, The Research Group LLC 2014, British Columbia IAPP [accessed 30 November 2016], Peterson and Prasad 1998)

a b

c
30 Chapter 2: Getting to Know Gorse and Scotch Broom

Comments

While mature Scotch broom plants have a lower oil content than gorse, they 

are still flammable and considered a fire hazard. As plants age, the ratio of dry 

wood to moist green material increases. Consequently, dense and mature stands 

of Scotch broom could be highly flammable. Furthermore, Scotch broom’s fire 

hazard potential is increased by its frequent occurrence on steep slopes. Scotch 

broom seed germination is stimulated by fire, which may lead to rapid post-fire 

recolonization. 

Although still sold and planted for its beauty as an ornamental, Scotch broom’s 

negative environmental impact has caused its sale to become restricted or, in 

some places, illegal. Where distribution of the species is restricted, many sterile 

or less aggressive varieties are still sold, including Burkwood’s broom and 

moonlight broom (Figure 2-17). It is important to note these are the same species 

as Scotch broom (Cytisus scoparius); extreme caution should be applied when 

considering these varieties in an ornamental setting.

Commonly Confused Species

Numerous species present in North America have yellow, pea-like flowers similar 

to Scotch broom, including the native goldenbanners (Thermopsis spp., Figure 

2-18a) and exotic birdsfoot trefoil (Lotus corniculatus, Figure 2-18b), which all 

may be mistaken for young Scotch broom. Goldenbanners, birdsfoot trefoil, and 

most other potential look-alikes are not shrubs so can be easily differentiated. 

Gorse resembles Scotch broom with its similar shrub habit, yellow, pea-like 

flowers, legume fruit, and green stems. Gorse differs in that mature plants are 

covered with sharp spines rather than leaves. Other exotic broom species most 

closely resemble Scotch broom. Table 2-1 contains photographs and Table 2-2 

lists key characteristics useful for differentiating these species from Scotch 

broom and from each other. 

Figure 2-17. Moonlight broom, a 

commercially available ornamental variety 

of Cytisus scoparius. (Jennifer Andreas, 

Washington State University Extension)

Figure 2-18. Potential look-alike species for young Scotch broom: a. mountain 

goldenbanner (Thermopsis montana); b. birdsfoot trefoil (Lotus corniculatus). 

(a. Peganum; b. Ohio State University Weed Lab, bugwood.org) 
Chapter 2: Getting to Know Gorse and Scotch Broom 31

Table 2-1. Other exotic broom species present in North America similar in appearance to Scotch broom 

and gorse

Bridal veil broom: a. Jean-Paul Peltier, b. Javier Martin, c. Fouad Msanda; French broom: a. Philipp Weigell, b. Calibas, c. Xemenendura; 

Portuguese broom: a., b. ©2011 Vernon Smith, c. ©2011 Zoya Akulova; Spanish broom: a., b. Jennifer Andreas, Washington State University 

Extension, c. Eugene Zelenko; Scotch broom: a.-c. Jennifer Andreas, Washington State University Extension. 

a. Plant b. Flowers c. Pods

Bridal veil broom 

Retama monosperma

French broom 

Genista monspessulana

Portuguese broom 

Cytisus striatus

Spanish broom 

Spartium junceum

Gorse 

Ulex europaeus

Scotch broom 

Cytisus scoparius
32 Chapter 2: Getting to Know Gorse and Scotch Broom

Table 2-2. Key characteristics for differentiation of other exotic broom species present in North America 

similar to Scotch broom and gorse

Plant Trait

Species

Preferred 

Habitat Height Leaves Stems Flowers Pods

Bridal veil broom

Retama 

 monosperma

Disturbed, 

open areas; 

rocky, 

infertile 

soil; dry 

conditions

3-10 ft 

(1-3 m)

Single; <1⁄3 in 

(3⁄4 cm); linear; hairy 

above and below; 

sparse; quickly 

deciduous

All: slender, 

green, drooping

White; 1⁄2 in 

(11⁄4 cm); clusters 

2-20 along stems; 

early summer

Circular; 

1⁄3-1⁄2 in 

diameter 

(1 cm); 

inflated; 

smooth

French broom

Genista 

 monspessulana

Disturbed, 

open; high 

pH soil; 

mesic 

habitat

3-10 ft 

(1-3 m)

3-parted; leaflets 

1⁄3-1⁄2 in (1-11⁄4 cm); 

hairy; numerous; 

on plant year-round

Young: slender, 

green, ridged; 

Mature: brown, 

round

Yellow; ≤1⁄2 in 

(11⁄4 cm); clusters 

4-10 at branch 

ends; 

spring/summer

Linear; 1⁄2-1 in 

(11⁄4-21⁄2 cm); 

dense hair

Portuguese broom

Cytisus striatus

Disturbed, 

open; mesic 

habitat

3-10 ft 

(1-3 m)

Single to 3-parted; 

leaflets 1⁄3-1⁄2 in 

(1-11⁄4 cm); smooth 

above, hairy below; 

sparse; deciduous 

early

Young: slender, 

green; 

Mature: brown, 

woody

Pale yellow; ≤1 in 

(21⁄2 cm); clusters 

1-2 in leaf axils; 

spring/summer

Linear; 

1⁄2-11⁄2 in 

(11⁄4-4 cm); 

inflated; 

dense hair

Spanish broom

Spartium junceum

Disturbed, 

open 

areas; dry 

conditions

5-15 ft 

(11⁄2-

41⁄2 m)

Single; 1⁄2-1 in 

(11⁄4-21⁄2 cm); oval, 

smooth-margined; 

deciduous early

Young: slender, 

green, round, 

rush-like; 

Mature: woody, 

round

Yellow; ≤1 in 

(21⁄2 cm); clusters 

of several at 

current-year branch 

ends; summer into 

fall

Linear; 

2-4 in 

(5-10 cm); 

slightly 

flattened; 

dense hair

Gorse 

Ulex europaeus

Disturbed, 

open; 

well-drained 

soil; mesic 

habitat

3-13 ft 

(1-4 m)

Young: 3-parted, 

small; 

Mature: leaves 

reduced to spines 

13⁄4-21⁄2 in 

(41⁄2-61⁄2 cm), 

ending in yellow 

point, on plant 

year-round

Young: soft, 

green, hairy; 

Mature: woody, 

green, hairless, 

terminate in 

spine ≤1 in 

(21⁄2 cm)

Yellow; 1⁄2-1 in 

(11⁄4-21⁄2 cm); singly 

in leaf axils or large 

clusters at stem 

ends of mature 

plants; early spring

Linear; 

1⁄2-3⁄4 in 

(11⁄4-2 cm); 

inflated; 

dense hair

Scotch broom

Cytisus scoparius

Disturbed, 

open; 

well-drained 

soil; mesic 

habitat

3-10 ft 

(1-3 m)

3-parted; 1⁄3-3⁄4 in 

(≤1⁄3 cm); smooth 

above, hairy below; 

deciduous early

Young: slender, 

green, hairy; 

Mature: woody, 

hairless, 5-

angled crosssection

Yellow; 1⁄2-1 in 

(11⁄4-21⁄2 cm); 

clusters 1-2 in 

leaf axils or large 

clusters at stem 

ends of mature 

plants; early spring

Linear; 

1-23⁄4 in 

(21⁄2-7 cm); 

flattened; 

hair only 

along 

margins
33

Chapter 3: Biology of Gorse and Scotch Broom Biological 

 Control Agents

Classical biocontrol agents may be found in a number of taxonomic groups. 

The majority of approved biocontrol agents are invertebrates in the kingdom 

Animalia and the phylum Arthropoda. More specifically, most biocontrol 

agents are insects (class Insecta) in the orders Coleoptera (beetles), Lepidoptera 

(butterflies and moths), and Diptera (true flies). In addition to insects, there are 

also mites (arthropods in the class Arachnida), nematodes (kingdom Animalia 

and phylum Nematoda), and fungi (kingdom Fungi) biocontrol agents. The gorse 

and Scotch broom biocontrol agents currently approved for use in North America 

include four species of insects (three beetles and a moth) and one mite species. 

Six additional biocontrol agents that were approved for use in Hawaii include 

five insects (two beetles, two moths, and a thrips) and one species of rust fungus. 

Three accidentally introduced species currently not approved for redistribution 

but commonly found on gorse or Scotch broom in the continental USA include 

two insects (a moth and a psyllid) and a mite. The taxonomic groups of all 

approved and unapproved gorse and broom biocontrol agents are described in 

greater detail in the following sections.

Insects are the largest and most diverse class of animals. Basic knowledge of 

insect anatomy and life cycle will help in understanding insects, and recognizing 

them in the field.

Most insects used in weed biocontrol have complete metamorphosis, which 

means they exhibit a life cycle with four distinct stages: egg, larva, pupa, and 

adult (Figure 3-1). All insects have an exoskeleton (a hard external skeleton) 

and a segmented body divided into three regions (head, thorax, and abdomen, 

Figure 3-2a,b). Adult insects have three pairs of segmented legs attached to the 

thorax, and a head with one pair each of compound eyes and antennae. 

Because insects have an external skeleton, they must shed their skeleton in order 

to grow. This process of shedding the exoskeleton is called molting. Larval 

stages between molts are called “instars.” Larvae of insects with complete 

metamorphosis generally complete three to five instars before they molt into 

pupae. During the pupal stage, insects change from larvae to adults. Insects do 

not feed or molt during the pupal stage. Adult insects emerge from the pupal 

stage and do not grow or molt.

Insects
34 Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents

Figure 3-1. Line drawings of a beetle life cycle showing complete metamorphosis.

(University of Idaho)

Figure 3-2. Line drawings of insect anatomy: a. beetle; b. moth. (a., b. adapted from

Biological Control of Weeds in the West, Rees et al. 1996)

Thorax

Abdomen

Head Thorax

Abdomen

Head

a b
Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents 35

Beetles (Order Coleoptera)

Adult beetles are hard-bodied insects with tough exoskeletons. Adult beetles 

possess two pairs of wings. The two front wings, called elytra, are thickened and 

meet in a straight line down the abdomen, forming a hard, shell-like, protective 

covering (Figure 3-2a). The two hind wings are membranous and used for flight; 

these are larger and are folded under the elytra when not in use. Beetle larvae 

are grub- or worm-like with three small pairs of legs, allowing some to be quite 

mobile. Many are pale white with a brown or black head capsule, though some 

may be quite colorful and change markedly in appearance as they grow. Beetles, 

like those used for gorse and broom biocontrol, have chewing mouthparts.

Butterflies and Moths (Order Lepidoptera)

Adult Lepidoptera have two pairs of membranous wings that are covered with 

powder-like scales. Adult butterflies/moths have prominent antennae and coiled 

mouthparts that are adapted to siphoning sap and nectar from plant flowers. They 

can be bright- or dull-colored, and males and females of the same species do not 

always have the same coloration. Many adult butterflies/moths feed very little, if 

at all. Lepidoptera larvae (known as caterpillars) have a toughened head capsule, 

chewing mouthparts, and a soft body; they are mobile and active feeders. The 

pupal stage can be naked or enclosed in a cocoon, depending on the species. 

Thrips (Order Thysanoptera)

Thysanoptera undergo incomplete metamorphosis with only three distinct stages: 

egg, nymph, and adult. There is no true pupal stage for this order of insects. 

Adult thrips can be wingless or have two pairs of stalk-like wings with long hair 

fringing the margins. There are two actively feeding nymphal instars for all thrips 

and 2-3 inactive (non-feeding) instars. Nymphs somewhat resemble adults, but 

they lack wings and functional reproductive organs. Adult and active nymphal 

stages of thrips feed by piercing the plant with their straw-like mouthparts and 

sucking out the cell contents. 

True Bugs, Including Psyllids (Hemiptera)

True “bugs” are in the order Hemiptera and undergo incomplete metamorphosis 

with only three distinct life stages: egg, nymph, and adult. There is no true pupal 

stage for this order of insects. Adult Hemiptera possess two pairs of wings. The 

hind wings are membranous; the front wings are generally hardened at their base 

and membranous at their tips, but the broom psyllid has entirely membranous 

front wings. Psyllid nymphs molt multiple times, and each subsequent instar 

more closely resembles adults. Psyllid nymphs and adults feed by piercing the 

plant with their straw-like mouthparts and sucking out the cell contents.
36 Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents

Like insects, mites are in the phylum Arthropoda and have an exoskeleton; 

however, they belong to a different class, Arachnida, whose adult members are 

typically characterized by having 8 legs (compared to the 6 legs of adult insects). 

In some mite species, the first immature stage is called larva; mites in this stage 

have only 6 legs. The second immature stage is called nymph and has 8 legs. 

Nymphs are usually very similar in appearance to adults (Figure 3-3). Some 

mite species do not have a larval stage, and some mite families have only 4 legs. 

Larvae, nymphs, and adults all feed by piercing and sucking cell contents.

Fungi belong to their own kingdom (Fungi). The fungus described in this manual 

is a rust, which is in the phylum Basidiomycota. Rust fungi are obligate parasites; 

they require a living host to complete their life cycle. Rusts typically attack 

leaves and stems of the host plant. Rust infections usually appear as numerous 

rusty, orange, yellow, or even white colored spots (pustules) that rupture 

the leaf surface and release spores that resemble colored powder (typically 

yellow, orange, or brown). Most rust infections are local spots but some may 

spread internally through the plant. Rusts spread from plant to plant mostly 

by windblown spores, although insects, rain, and animals may aid in the rust 

transmission and infection process. 

The life cycle of rust fungi can be very complicated. Rust fungi can produce 

up to five distinctive spore types which have different functions from infesting 

a new host plant, re-infecting the same host plant, and producing pustules on 

infected plant leaves and stems.

Mites

Fungi

Figure 3-3. Mite life cycle. (Rachel Winston, MIA Consulting)
Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents 37

Gorse and 

Scotch Broom 

Biological Control 

Agents

Biocontrol agents used on North American gorse and broom attack the stems, 

seeds, and foliage. Each biocontrol agent is described in detail in the following 

sections, separated by approved species attacking gorse in North America 

and Hawaii, approved species attacking Scotch broom in North America, and 

species accidentally introduced to North America but not currently approved for 

redistribution. All gorse and broom biocontrol agents already established in North 

America and/or Hawaii are summarized and compared in Tables 3-1 through 3-5 

near the end of this chapter.

Gorse Biological 

Control Agents 

(North America 

and Hawaii)

Exapion ulicis (Forster)

Gorse seed weevil

Synonym: Apion ulicis (Forster)

Order Coleoptera

Family Brentidae

Native Distribution Western Europe

Target Weed Gorse

North America Original Source USA: England

First Release USA: 1953 (CA)

Nontarget Effects None reported

Hawaii Original Source England, France

First Release 1926

Nontarget Effects None reported

Description

Eggs are round, small, and translucent yellow. Larvae are cream colored with 

brown head capsules, C-shaped, and can reach 3 mm in length (Figure 3-4a). 

Pupae are typically 3 mm long and cream colored, becoming dark gray with age. 

Adults are gray with very long, slightly curved snouts and brownish gray legs 

(Figure 3-4b). Faint stripes are sometimes apparent on their elytra, and they are 

typically 2-3 mm long.

Life Cycle

In North America, overwintering adults emerge during late winter-early spring 

depending on location. Adults feed on gorse flowers and foliage (Figure 3-4c), 

and deposit eggs into young seedpods. Larvae begin hatching in early to late 

spring and feed on developing seeds. Larvae develop through three instars. 

Pupation occurs in seedpods, and new adults emerge in late summer. Adults do 

not chew their way out of the seedpod, instead relying on the plant’s dehiscing 

mechanism to escape. Seedpods dehisce, or dry out, and burst open at maturity 

in order to spread their seeds. Adults feed on spines and stems of gorse and then 

overwinter among gorse foliage. There is one generation per year (Figure 3-5).
38 Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents

Figure 3-4. Exapion ulicis: a. larvae and damage within a gorse seedpod; b. adult; c. adults on gorse flower.

(a. George Markin, USDA Forest Service, bugwood.org; b. Janet Graham; c. Eric Coombs, Oregon Department of Agriculture,

bugwood.org)

a b c

Figure 3-5. Schematic life cycles of Exapion ulicis and gorse in North America. Bars indicate the approximate length of

activity for each life stage; dates will vary depending on local conditions. The life cycle of E. ulicis differs in Hawaii where

both the biocontrol agent and gorse are subjected to different climatic conditions. Black bars represent the beetle’s inactive

overwintering period.

Habitat Preference

Similar to its host plant, the gorse seed weevil does best in open, sunny sites and 

at locations with dense gorse infestations. Its distribution is limited in regions 

with cold winters, shade, only scattered host plants, and in salt spray zones along 

coastlines.

Damage

Adult feeding leads to the destruction of stem tissue but without significantly 

harming the attacked plant. Larval seed feeding (Figure 3-4a) may reduce seed 

output. While this does not kill existing gorse plants, it can help reduce the rate of 

spread of gorse populations.
Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents 39

Current Status and Availability

This beetle is widespread and abundant on gorse in the western USA 

(Figure 3-6), though its overall impact is limited. From 30 to 95 percent of 

seedpods are attacked, but this does not reduce established stand density. At 

best, it may slow the rate of spread of gorse; however, it is ineffective on seed 

maturing in autumn/winter. This species is currently not known to be present 

in Canada.

In Hawaii, E. ulicis is established on Maui and Hawaii Island. Aggressive 

chemical/burning control programs have destroyed gorse at some locations, 

bringing about a collapse of the weevil populations, followed by a slow 

biocontrol agent recovery. Attack rates have varied by year on both islands. 

Annual attack rates of up to 95 percent of seedpods have had only limited 

impact on the invasiveness of gorse, likely due to the long-lived seed bank and 

subsequent plant recruitment. 

Comments

In Hawaii, feeding damage by E. ulicis may increase the susceptibility of gorse 

plants to the pathogenic fungus Colletotrichum sp.

Figure 3-6. Current

establishment of Exapion 

ulicis on gorse in North

America and Hawaii.

(Winston et al. 2014a)
40 Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents

Tetranychus lintearius Dufour

Gorse spider mite

Order Acari

Family Tetranychidae

Native Distribution Europe

Target Weed Gorse

North America Original Source USA: England, Portugal, 

Spain via New Zealand

First Release USA: 1994 (CA, OR)

Nontarget Effects None reported

Hawaii Original Source England, Portugal, Spain via 

New Zealand via USA (OR)

First Release 1995

Nontarget Effects None reported

Description

Eggs are tiny, round, and largely transparent. First instar (larval) mites are light in 

color and have six legs. Second to fourth instar (nymphal) mites have eight legs, 

are brown, and resemble small adults. Adults also have eight legs, are brick red in 

color, and are up to 0.5 mm long (Figure 3-7).

Life Cycle

Adults form a colony with large amounts of webbing on the terminal branches 

of gorse. Females lay eggs year-round on infested shoots. Hatching mites 

complete four immature stages, with larvae and nymphs feeding on plant tissue. 

Adults feed on stems and spines and live up to four weeks. There are up to six 

generations per year, with all stages capable of overwintering (Figure 3-8).

Figure 3-7. Tetranychus lintearius adults. (a. Rich Lee, San Juan County Noxious Weed

Control Board; b. Eric Coombs, Oregon Department of Agriculture)

a b
Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents 41

Figure 3-8. Schematic life cycles of gorse and Tetranychus lintearius. There are multiple generations of T. lintearius annually

(up to 6); all stages are capable of actively overwintering. The life cycle of gorse differs in Hawaii where it is subjected to

different climatic conditions.

Habitat Preference

The mite does best in warm, open gorse patches and away from the ocean. 

Damp, ocean-side infestations or heavily shaded forest patches are seldom 

attacked. Tetranychus lintearius can be somewhat cold hardy, but severe winter 

temperatures limit populations. 

Damage

Large populations of this mite produce extensive amounts of webbing over 

mite colonies on gorse terminal branches (Figure 3-9a,b). Larval, nymphal, and 

adult feeding stunts branch growth and reduces flowering (Figure 3-9c), thus 

contributing to a reduction in the spread of gorse. Heavily infested plants are 

killed by the extensive feeding.

Current Status and Availability

This biocontrol agent was initially widely distributed on gorse throughout the 

western USA, even leading to an 80 percent reduction in gorse flowering in 

Oregon. It was most effective in open patches in inland areas. Populations have 

since decreased significantly due to heavy predation by beetles and predatory 

Figure 3-9. Tetranychus lintearius webbing: a. with mass of adults; b. over large infested gorse patch; c. covering gorse stems

with T. lintearius feeding damage. (a. Rich Lee, San Juan County Noxious Weed Control Board; b. Eric Coombs, Oregon

Department of Agriculture; c. Steven Conaway, Penn State University, bugwood.org)

a b c
42 Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents

mites. Weak webbing indicates predators are likely present. Tetranychus 

lintearius is now considered an ineffective biocontrol agent in the continental 

USA. This species is currently not known to be present in Canada.

In Hawaii, Tetranychus lintearius is established on Maui and Hawaii Island. It 

initially provided partial to substantial control of gorse until the year 2000, when 

predacious mites may have first appeared. Populations of T. lintearius are now 

limited on both islands where it is established (Figure 3-10).

Figure 3-10. Current

establishment of

Tetranychus lintearius on

gorse in North America

and Hawaii (Winston et al.

2014a) 

Gorse Biological 

Control Agents 

(Hawaii Only)

Six gorse biological control agents were approved and released only in Hawaii. 

Each of these is described in the following section. Because these species are not 

currently approved for release in the continental USA or Canada, less detail is 

given for their descriptions, life cycles, and current status. 

Agonopterix umbellana (Fabricius)

Gorse soft shoot moth

Order: Lepidoptera

Family: Oecophoridae

Synonym: Agonopterix ulicetella (Stainton)

Description and Life Cycle

Adults are light brown with dark brown or black longitudinal lines on the 

front wings that fade as the adult ages. Adults are typically 12 mm long with 

a wingspan of 21 mm, and they have long, dark antennae (Figure 3-11a). 

Overwintering adults emerge during late winter/early spring and lay eggs in 

gorse leaf and spine axils. Eggs are bright yellow, barrel-shaped, and 1 mm long. 
Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents 43

Larvae hatch in late spring and spin silken tubes on gorse buds, feeding on new 

shoots and spines. Attack by multiple larvae can defoliate an entire shoot and 

kill the developing tip (Figure 3-11b). There are five larval instars. First to fourth 

instars are dark brown with dark spots on their sides. Fifth instars are olive green 

with dark side spots, and can be up to 20 mm long (Figure 3-11c). Pupation 

occurs in the silken feeding tubes. Adults emerge in late summer and overwinter 

in gorse foliage. There is one generation per year.

History and Current Status

Agonopterix umbellana was collected from England and released in Hawaii 

from 1988. A second population of warmer-adapted individuals collected from 

Portugal was released from 1991, in an attempt to increase establishment at lower 

elevation sites in Hawaii. Both releases resulted in successful establishment, and 

subsequent records do not differentiate between the two populations. 

This species is currently established on two islands (Figure 3-12). It was 

initially widespread on Hawaii Island, but an aggressive chemical/burning 

control program in 2001/2002 destroyed the gorse, resulting in a collapse of 

the A. umbellana population. Agonopterix umbellana recovered and was again 

abundant by 2010. On Maui, this biocontrol agent is well established only at 

high elevations (> 3,280 feet or 1,000 m), where gorse is also most problematic. 

Overall in Hawaii, while larval feeding can destroy a high percentage of gorse 

shoot tips and sometimes leads to dieback, plants frequently compensate by 

initiating growth of new shoots later in the season when A. umbellana is no 

longer active. Impact is therefore limited. Parasitism may also contribute to low 

impact at some sites.

Figure 3-11. Agonopterix umbellana: a. adult (body length top line, wingspan length bottom line); b. silken feeding tubes and

damage to gorse; c. larva. (a.,b. Fritzi Grevstad, Oregon State University; c. Eric Coombs, Oregon Department of Agriculture,

bugwood.org)

a b c
44 Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents

Figure 3-12. Current

establishment of

Agonopterix umbellana

on gorse in Hawaii

(Winston et al. 2014a)

Apion sp.

Gorse seed weevil

Order: Coleoptera

Family: Brentidae

The exact identification of this species is unknown, but it is possibly Apion 

uliciperda Pandelle.

Description and Life Cycle

Not much is known about this species beyond it being closely related and 

morphologically similar to Exapion ulicis (already described in the previous 

section). The description and life cycle of E. ulicis are, therefore, repeated here. 

Adults of the unknown Apion sp. have been described as being slightly larger and 

a deeper gray compared to E. ulicis. 

Adult E. ulicis are gray with very long, slightly curved snouts and brownish gray 

legs. Faint stripes are sometimes apparent on their elytra, and they are typically 

2-3 mm long. In the continental United States, overwintering adult E. ulicis

emerge during early spring, feed on gorse flowers and foliage, and deposit eggs 

into young gorse seedpods. Eggs are round, small, and translucent yellow. Larvae 

hatch in late spring and early summer and feed on developing seeds. Larvae are 

cream colored, C-shaped, can reach 3 mm in length, and develop through three 

instars. Pupae are typically 3 mm long and become dark gray with age. Pupation 

occurs in seedpods, with adults emerging in late summer. Adults feed on spines 

and stems of gorse and then overwinter among gorse foliage. There is one 

generation per year.
Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents 45

History and Current Status

After several of the first releases of E. ulicis failed to result in establishment in 

Hawaii, a species closely related to E. ulicis encountered in Spain and Portugal 

was released on Maui in 1958. This release did not result in successful 

establishment, and future efforts with this species were abandoned once it 

was determined that E. ulicis had finally successfully established. 

Pempelia genistella (Duponchel)

Gorse colonial hard shoot moth

Order: Lepidoptera

Family: Pyralidae

Description and Life Cycle

Larvae overwinter within a silken feeding web. They become active in spring 

and feed on gorse spines, leaves, buds, shoots, and flowers beneath their silk 

web. This feeding causes damaged foliage and stems to turn brown and die. 

Larvae have green and brown stripes and can be up to 25 mm long. Pupae are 

dark reddish-brown. Pupation occurs within the silk web in early summer. Adults 

emerge in summer and lay eggs at the base of mature spines on growing gorse 

shoots. Adults are light brown with black, brown, and white markings on their 

wings; males have a small tuft at the base of antennae. Adults are 10-15 mm long 

with a wingspan of 26-29 mm. Larvae emerge in late summer to early fall and 

congregate to spin a coarse creamy-gray silken web with many tunnels, often 

at the base of current gorse growth. There are typically 2-9 larvae per web, and 

overwintering occurs within webs. There is one generation per year.

History and Current Status

Individuals collected from Portugal were released on Hawaii Island in 1996. 

This moth initially established and was recovered in small amounts. Gorse 

at the release sites was subsequently exterminated by fire and herbicides, 

and P. genistella populations do not appear to have survived.

Sericothrips staphylinus Haliday

Gorse thrips

Order: Thysanoptera

Family: Thripidae

Description and Life Cycle

Adults are tiny (~1 mm long), black, and have white wing pads (Figure 3-13a,b). 

They are typically wingless, though some winged individuals do occur. Winged 

forms are more abundant when population densities are high. Adults lay eggs 

in slits within young stems of gorse. Eggs are pale yellow, cylindrical, and 

~0.3 mm long. There are two actively feeding nymphal instars and two inactive 

(non-feeding) instars. Nymphs are creamy-yellow and look increasingly similar 
46 Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents

to adults as they molt between instars (Figure 3-13a). The gorse thrips completes 

multiple generations during the warmer months of the year. At cold, highelevation sites, adults overwinter among gorse foliage. Elsewhere in Hawaii, 

adults are active throughout winter. The entire life cycle of a single generation 

is approximately 6-8 weeks. Adults and feeding nymphs pierce gorse stems and 

suck out the contents of mesophyll cells (Figure 3-13b). This results in a mottled, 

blotchy appearance of attacked tissue (Figure 3-13c). At high numbers, the gorse 

thrips can reduce gorse growth and flowering, and kill seedlings.

History and Current Status

Sericothrips staphylinus was collected from England and Portugal and released 

in Hawaii in 1991. A second release was made with individuals from France in 

1992. Both release events resulted in successful establishment, and subsequent 

records do not differentiate between the different source populations. 

This species is currently established only on Hawaii Island (Figure 3-14). After 

becoming widespread, populations decreased, possibly due to predation. Even at 

the highest observed densities, feeding discoloration was only occasionally found 

on mature gorse plants, but plant death was not observed. Impact is therefore 

limited. 

This species is currently under review for possible release in the continental 

USA. Additional host range testing was completed on 63 species, and a 

petition for field release was submitted in 2012 to the TAG and has since been 

recommended for release. USDA-APHIS-PPQ approval is currently pending.

Figure 3-13. Sericothrips staphylinus: a. adult (foreground) and nymph (background, see arrow); b. adults on gorse foliage;

c. feeding damage to gorse tissue in left pot. (a.-c. Fritzi Grevstad, Oregon State University)

a b c
Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents 47

Figure 3-14. Current

establishment of

Sericothrips staphylinus on

gorse in Hawaii. (Winston

et al. 2014a)

Stenopterapion scutellare (Kirby)

Gorse stem-mining weevil

Order: Coleoptera

Family: Brentidae

Synonym: Apion scutellare Kirby, Perapion scutellare (Kirby)

Description and Life Cycle

Adults are dark gray with very long, slightly curved snouts and grayish-black 

legs. Faint stripes are sometimes apparent on their elytra, and they are typically 

4-5 mm long. Adults emerge in spring and deposit eggs into growing shoot tips 

of gorse. Eggs are round, small, and translucent yellow. The shoot continues to 

grow, but within a month of the oviposition, a 1 cm gall forms in which the larva 

develops by feeding on galled tissue. Galling does not kill attacked shoots, but it 

halts or significantly reduces their growth. Larvae are cream colored, C-shaped, 

can reach 5 mm in length, and develop through three instars. Pupae are typically 

4 mm long and become dark gray with age. Larvae overwinter in galls. Pupation 

occurs in galls by late winter/early spring, and new adults emerge in spring. 

There is one generation per year.

History and Current Status

Several releases of S. scutellare were made in Hawaii from 1961 to 1991 

utilizing individuals collected from Portugal, Spain, and France. All attempts 

failed for unknown reasons, and this biocontrol agent is not believed to have 

established.
48 Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents

Uromyces pisi f. sp. europaei M. Wilson & D.M. Hend.

Gorse rust

Class: Pucciniomycetes

Order: Pucciniales

Description and Life Cycle

Rust fungi produce up to five spore stages throughout the growing season. In the 

spring, overwintering spores germinate and infest the stem and spine surfaces 

of gorse, forming masses of reddish-brown and powdery pustules. Pustules 

spread rapidly from plant to plant; they are easily dispersed by both wind and 

rain. Multiple cycles may be produced throughout the year. Infected plants can 

experience stunted growth and reduced seed production.

History and Current Status

Uromyces pisi f. sp. europaei collected from England was released on Hawaii 

Island in 2000. A single pustule was observed at the release site two years 

following release; however, all subsequent surveys have failed to yield this 

biocontrol agent. It is believed U. pisi f. sp. europaei did not establish.

Scotch Broom 

Biological Control 

Agents

Bruchidius villosus (Fabricius)

Broom seed beetle 

Order Coleoptera

Family Chrysomelidae

Native Distribution Europe

Target Weed Scotch broom, 

French broom

North America Original Source USA: Accidental 

introduction 

CAN: Accidental 

introduction

First Documentation USA: 1918 (MA) 

CAN: 2001 (BC)

First Redistribution USA: 1998 (OR) 

CAN: 2006 (BC)

Nontarget Effects None reported

Description

Eggs are tiny, white, and oval-shaped (Figure 3-15a,b). Larvae are an off-white 

color with brown head capsules and can reach up to 2 mm in length. Pupae are 

gray or brown and up to 2 mm long. Adults can also be up to 2 mm long. They 

have gray-black bodies, antennae, and legs. Both their elytra and snouts are short 

(Figure 3-15d).
Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents 49

a b c d

Figure 3-15. Bruchidius villosus: a. three eggs on a Scotch broom seedpod; b. larval tunnel extending from an egg; c. larva

feeding completely within the left seed; d. adult. (a.-d. Jennifer Andreas, Washington State University Extension)

Life Cycle

Overwintering adults emerge in spring when broom begins to flower. They 

congregate on flowers to feed on pollen, which helps stimulate ovary maturation. 

Eggs are laid on the seedpod. Hatching larvae burrow into the seedpod wall, 

sometimes forming visible tunnels (Figure 3-15b), before entering into and 

feeding on developing seeds. Larvae develop through four instars completely 

within seeds (Figure 3-15c, left seed); generally there is one larva per seed. 

Pupation occurs within the seed coat. New adults emerge in late summer, leaving 

behind round emergence holes in seeds (Figure 3-16). Adults do not chew their 

way out of the seedpod, instead relying on the plant’s dehiscing mechanism 

to escape. Seedpods dehisce, or dry out, and burst open at maturity in order to 

spread their seeds. Adults overwinter away from the host plant. There is one 

generation per year (Figure 3-17).

Figure 3-16. Bruchidius 

villosus adult and feeding

damage to a Scotch broom

seed. (Jennifer Andreas,

Washington State

University Extension)
50 Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents

Figure 3-17. Schematic life cycle of Bruchidius villosus and Scotch broom in North America. Bars indicate the approximate

length of activity for each of the beetle’s life stages; dates will vary depending on local conditions. Black bars represent the

inactive overwintering period.

Habitat Preference

The broom seed beetle does best in meadows or on hillsides with southern 

exposure. It may perform poorly in heavily shaded, cold, high elevation, and/or 

damp sites.

Damage

Larval feeding on developing seeds (Figures 3-15c, 3-16) reduces viable seed 

production. One beetle typically kills one seed. While this does not kill existing 

broom plants, it can help reduce the rate of spread of broom populations and 

may have long-term impacts by reducing seed recruitment as established plants 

senesce. 

Current Status and Availability

Bruchidius villosus was unintentionally introduced to North America. First 

reported on Scotch broom in Massachusetts in 1918, it later spread naturally 

along the east coast of the USA. Individuals from these unintentionally 

introduced populations were tested for host specificity following USDA-APHIS 

TAG protocols and approved for redistribution in the USA. Beginning in 1998, 

the broom seed beetle was deliberately transferred from North Carolina to 

Scotch broom growing in Oregon and Washington. By 2001, the beetle spread 

naturally to French broom (Genista monspessulana) growing in Oregon and to 

Scotch broom growing in British Columbia, Canada. From 2006-2008 it was 

redistributed within British Columbia on Scotch broom. In 2003, the beetle was 

recorded as present on Scotch Broom in Nova Scotia.

This beetle is now widespread on Scotch Broom in the northwestern USA 

(Figure 3-18a) where its abundance is variable but increasing. In 2014, a study 

evaluating the attack rate of B. villosus and Exapion fuscirostre on Scotch broom 

seeds was conducted in California, Oregon, Washington, and British Columbia. 

Across 30 sites in Washington, the average B. villosus attack rate was 44.2% but 
Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents 51

ranged from 0 to 87.4%. At 32 sites in Oregon, an average of 40.8% seeds were 

destroyed with a range of 1.8 to 83.5%. At three sites in California, the average 

B. villosus attack rate was 0.6% with a range of 0 to 1%. Subsequent surveys in 

California in 2015 and 2016 found the abundance of B. villosus to have steadily 

increased; in 2016, at 16 sites where it was recovered, an average of 13% of 

seeds were destroyed (range 1-32%). Studies are continuing as it is still unclear 

if densities and attack rates are sufficiently high to decrease plant recruitment. 

Parasitism is typically low but may limit biocontrol agent populations in some 

regions, and B. villosus seems to be less affected than E. fuscirostre. This species 

appears to outcompete E. fuscirostre at sites where they both occur. In 2014, 

B. villosus was the dominant species at 27/28 Washington sites and 22/32 Oregon 

sites. It was not intentionally released in California but has self-dispersed at least 

100 miles south from the Oregon border. It appears to be a recent invasion, and 

populations of B. villosus are likely to continue increasing and spreading further 

south. Bruchidius villosus is also widespread on French broom in southwestern 

Oregon (Figure 3-18b), though its impact on this weed has not been formally 

evaluated.

In Canada, B. villosus has established at sites in both the coastal/lower mainland 

and southeastern interior areas of British Columbia, although its abundance and 

impact on Scotch broom are still unknown. In 2014, an average of 59% of seeds 

were destroyed across 10 sites with a range from 0.7 to 98%. This species is more 

active than the other adventive Scotch broom beetle, E. fuscirostre. In 2014, 

B. villosus was the dominant species at 90% of the study sites; E. fuscirostre was 

entirely absent from 60% of the sites. 

a b

Figure 3-18. Current North American establishment of Bruchidius villosus: a. on Scotch broom; b. on French broom. (Winston

et al. 2014a)
52 Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents

The previously mentioned 2014 study evaluated the combined impact of 

B. villosus and E. fuscirostre on Scotch broom seed development. Although 

B. villosus is more abundant than E. fuscirostre at many sites, the additive affect 

between the two species increases the amount of overall seed destruction. The 

average attack rate on Scotch broom seeds across 10 sites in British Columbia 

was 69.1 percent but ranged at individual sites from 0.7 to 98 percent seed 

destruction. The highest attack rates were at sites in the southwestern mainland; 

the lowest seed destruction occurred at sites on Vancouver Island and in the 

interior (Figure 3-19a). At 30 sites in Washington, an average of 56.4 percent 

of seeds were destroyed with a range of 0 to 92.5 percent. The highest level 

of attack was in the Puget lowlands, and the lowest rate of attack was on the 

Olympic Peninsula (Figure 3-19a). Across 32 sites in Oregon, the average attack 

rate was 67.3 percent and ranged from 6.1 to 91.4 percent. The highest attack 

rates were found in the Willamette Valley with slightly lower rates at higher 

elevations; the lowest seed destruction occurred along the Oregon coast 

(Figure 3-19b). At the three sites in California, the average attack rate was 

37.4 percent and ranged from 33.4 to 41.9 percent. Seed destruction was fairly 

consistent across the sampled regions; however, with so few sites it is unclear 

whether there is greater variation in biocontrol agent populations at other Scotch 

broom infestations (Figure 3-19c). Further monitoring of seed destruction is 

necessary to evaluate fluctuations in insect populations and associated attack 

rates over time.

Comments

Though B. villosus was first an accidental introduction in the USA, it is approved 

for redistribution within the USA and was intentionally redistributed in British 

Columbia, Canada from 2006-2008. 

The weevil Exapion fuscirostre also attacks seedpods of Scotch broom (see next 

section). Late instar larvae of B. villosus can be differentiated from E. fuscirostre

in that B. villosus larvae feed completely within broom seeds (Figure 3-20a), 

to the extent their presence can be difficult to detect unless seeds are dissected. 

Exapion fuscirostre larvae cause external feeding damage to seeds which is 

obvious when the pod is first opened (Figure 3-20a). Adult B. villosus are black 

and have much shorter snouts and elytra than adult E. fuscirostre (Figure 3-20b).

While there are no reports of nontarget effects in the United States, B. villosus

was found to attack Lupinus arboreus in a common garden experiment in France. 

This is currently being assessed in California and Washington.
Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents 53

a b

c

Figure 3-19. Attack rates of Scotch broom seeds by Bruchidius villosus and Exapion fuscirostre in 2014 at sites in: a. British

Columbia and Washington; b. Oregon; c. California. At each site, red represents the proportion of seeds damaged by the

biocontrol agents and blue represents the proportion of intact, undamaged seeds. (Maps prepared by Perry Beale, Washington

State Department of Agriculture)

Figure 3-20. Comparison of Bruchidius villosus (right) and Exapion fuscirostre (left): a. larvae attacking Scotch broom seeds;

b. adults. (a. Thomas Shahan, Oregon Department of Agriculture; b. Jennifer Andreas, Washington State University Extension)

a b
54 Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents

Exapion fuscirostre (Fabricius)

Scotch broom seed weevil

Synonym: Apion fuscirostre Fabricius

Order Coleoptera

Family Brentidae

Native Distribution Europe

Target Weed Scotch broom

North America Original Source USA: Italy 

CAN: Accidental 

introduction via USA

First Release USA: 1964 (CA)

First Documentation CAN: 2006 (BC)

First Redistribution CAN: 2007 (BC)

Nontarget Effects None reported

Description

Eggs are small, white to yellowish, and round. Larvae are an off-white color with 

brown head capsules (Figure 3-21a). They can be up to 2.5 mm in length while 

adults can be up to 3 mm. Pupae are cream colored and up to 3 mm long. Adults 

have brown bodies with two long, silver or tan bands that run down either side of 

their bodies (one on each side). Their snouts are long and curved, and they have 

light brown legs (Figure 3-21b).

Life Cycle

Overwintering adults emerge in early spring when Scotch broom begins to flower 

and feed on stems and flowers. Females must feed on Scotch broom flowers in 

order to produce eggs. Eggs are laid inside the seedpod, with hatching larvae 

feeding on developing seeds. Larvae feed half in and half out of attacked seeds, 

developing through three instars and pupating within the seedpod. New adults 

emerge in late summer. Adults do not chew their way out of the seedpod, instead 

Figure 3-21. Exapion fuscirostre: a. larva and damage to a seed; b. adult. (a. Thomas

Shahan, Oregon Department of Agriculture; b. Laura Parsons, University of Idaho,

bugwood.org)

a b
Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents 55

relying on the plant’s dehiscing mechanism to escape. Seedpods dehisce, or dry 

out, and burst open at maturity in order to spread their seeds. Adults overwinter 

in soil litter. There is one generation per year, though generations sometimes 

overlap as adults are frequently active year-round (Figure 3-22).

Habitat Preference

The Scotch broom seed weevil does best in meadows or on hillsides with 

southern exposure. It performs poorly in heavily shaded, cold, high elevation, 

and/or damp sites (e.g. in direct contact with ocean spray).

Damage

Adult feeding (Figure 3-23a) causes terminal shoot dieback, but does not kill 

the plant. Larval feeding on developing seeds (Figures 3-21a, 3-23b) reduces 

viable seed production. While this does not kill existing broom plants, it may 

help reduce the rate of spread of Scotch broom populations; however, the overall 

efficacy of this biocontrol agent is questionable due to high seed production and 

the longevity of viable seeds in the seed bank.

Figure 3-22. Schematic life cycle of Exapion fuscirostre and Scotch broom in North America. Bars indicate the approximate

length of activity for each of the beetle’s life stages; dates will vary depending on local conditions. Black bars represent the

inactive overwintering period.

Figure 3-23. Exapion fuscirostre: a. adult feeding damage, bottom stem; b. emerged adult and larval feeding damage. (a. Eric

Coombs, Oregon Department of Agriculture, bugwood.org; b. Jennifer Andreas, Washington State University Extension)

a b
56 Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents

Current Status and Availability

Though E. fuscirostre is moderately abundant on Scotch broom in the western 

USA (Figure 3-24), its impact is generally low. At most sites its observed seed 

reduction rates are likely insufficient to impart significant control of Scotch 

broom populations alone, but it may contribute to a slowed rate of spread. In 

2014, a study evaluating the attack rate of E. fuscirostre and Bruchidius villosus

on Scotch broom seeds was conducted in California, Oregon, Washington and 

British Columbia. Across 30 sites in Washington, the average E. fuscirostre attack 

rate was 12.2 percent but ranged from 0 to 50.1 percent at individual sites. At 

32 sites in Oregon, an average of 26.5 percent seeds were destroyed with a range 

of 0 to 83.5 percent, and at three sites in California, the average E. fuscirostre

attack rate was 36.8 percent with a range of 32.4 to 41.9 percent. 

Parasitism is typically low but may limit populations in some regions. Exapion 

fuscirostre seems to be more affected by parasitism than B. villosus, which may 

contribute to B. villosus’s ability to outcompete E. fuscirostre at sites where 

they both occur. In 2014, E. fuscirostre was the secondary species at most sites 

in Washington and Oregon. It was the dominant species at only 1/28 sites in 

Washington and 10/32 sites in Oregon but was the dominant species at all three 

California sites.

Exapion fuscirostre spread naturally from the USA to British Columbia, Canada 

by 2006. It was intentionally redistributed within British Columbia from 

2007-2008. As of 2014, its abundance and impact in Canada appear to be less 

than the other adventive Scotch broom beetle, B. villosus. Across 10 sites in 

southern British Columbia, the average attack rate was 10.2 percent with a range 

from 0 to 51.9 percent. It was the dominant species at only one of the 10 sites, 

contributed to up to 26 percent of seed destruction at three sites, and was absent 

from the six remaining sites.

Figure 3-24. Current

establishment of Exapion 

fuscirostre on Scotch

broom in North America.

(Winston et al. 2014a)
Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents 57

The previously mentioned 2014 study evaluated the combined impact of 

B. villosus and E. fuscirostre on Scotch broom seed development. Although 

B. villosus is more abundant than E. fuscirostre at many sites, the additive affect 

between the two species increases the amount of overall seed destruction. The 

average attack rate on Scotch broom seeds across 10 sites in British Columbia 

was 69 percent but ranged at individual sites from 0.7 to 98 percent seed 

destruction. The highest attack rates were at sites in the southwestern mainland; 

less seed destruction occurred at sites on Vancouver Island and in the interior 

(Figure 3-19a). At 30 sites in Washington, an average of 56.4 percent of seeds 

were destroyed with a range of 0 to 92.5 percent. The highest level of attack 

was in the Puget lowlands and the lowest rate of attack was on the Olympic 

Peninsula (Figure 3-19a). In Oregon, across 32 sites, the average attack rate was 

67.3 percent and ranged from 6.1 to 91.4 percent. The highest attack rates were 

found in the Willamette Valley with slightly lower rates at higher elevations; the 

lowest seed destruction along the Oregon coast (Figure 3-19b). At the three sites 

in California, the average attack rate was 37.4 percent and ranged from 33.4 to 

41.9 percent. Seed destruction was fairly consistent across the sampled regions; 

however, with so few sites it is unclear whether there is greater variation in 

biocontrol agent populations at other Scotch broom infestations (Figure 3-19c). 

Further monitoring of seed destruction is necessary to evaluate fluctuations in 

insect populations and associated attack rates over time.

Comments

The beetle B. villosus also attacks seedpods of Scotch broom (see previous 

section). Late instar larvae of E. fuscirostre can be differentiated from B. villosus

once seedpods are opened in that E. fuscirostre larvae feed half in and half out 

of attacked seeds (Figure 3-20a). Bruchidius villosus larvae feed completely 

enclosed within seeds (Figure 3-20a), so individual seeds must be dissected to 

confirm the species is present. Adult E. fuscirostre have much longer snouts and 

elytra than adult B. villosus (Figure 3-20b).

Leucoptera spartifoliella (Hübner)

Scotch broom twig miner

Order Lepidoptera

Family Lyonetiidae

Native Distribution Europe

Target Weed Scotch broom

North America Original Source USA: France, but also found 

already present in USA

First Release USA: 1960 (CA)

Nontarget Effects None reported
58 Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents

Description

Eggs are tiny, oval, and white. Larvae are green-brown, translucent, appear 

somewhat flattened, and can reach 3-4 mm in length. Pupae are contained within 

white, silky cocoons 4-5 mm long (Figure 3-25a). Adults are small (3-5 mm long) 

and are seldom seen. They are white with white antennae, feathered wing tips, 

and have pale gold markings (Figure 3-25b).

Life Cycle

Adults lay eggs in late summer on young Scotch broom stems when broom has 

finished flowering. Larvae hatch in late summer and early autumn and tunnel 

into young shoots to feed (Figure 3-25c). Larvae develop through six instars over 

several months and overwinter in the stems of Scotch broom. Larvae emerge in 

early spring and spin cocoons on broom stems (Figure 3-25a) or the undersides 

of broom leaves, where they pupate. New adults emerge in late spring and early 

summer when broom flowers. There is one generation per year (Figure 3-26).

a b c

Figure 3-25. Leucoptera spartifoliella: a. pupa in a cocoon; b. adult; c. damage to a Scotch broom stem. (a.,b. Eric Coombs,

Oregon Department of Agriculture, bugwood.org; c. © Charlie Streets)

Figure 3-26. Schematic life cycle of Leucoptera spartifoliella and Scotch broom in North America. Bars indicate the

approximate length of activity for each of the moth’s life stages; dates will vary depending on local conditions.
Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents 59

Habitat Preference

The Scotch broom twig miner moth does best at low-elevation infestations with 

moderate temperature and ample moisture. 

Damage

Larval mining causes dieback of Scotch broom stems; however, plants often resprout new stems below the sites of damage. 

Current Status and Availability

Though this biocontrol agent was intentionally introduced to the USA in 1960, 

it was found to have already been present in California, Oregon, and Washington. 

It was likely imported on ornamental plants prior to 1940. Both the intentional 

and adventive populations have since intermixed and are no longer differentiated. 

Leucoptera spartifoliella is now widespread on Scotch broom in California and 

Oregon, but is present at only limited sites in Washington (Figure 3-27). High 

moth numbers can deform Scotch broom plants and cause stem dieback, but plant 

density is not affected. Because attacked plants often re-grow below the sites of 

damage, the overall impact of this biocontrol agent is negligible. Populations 

are also heavily parasitized and do not fare well in hot, dry sites. This species is 

currently not known to be present in Canada.

Figure 3-27. Current

establishment of

Leucoptera spartifoliella

on Scotch broom in North

America. (Winston et al.

2014a)
60 Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents

Three accidentally introduced species that are established on broom in North 

America are covered in this manual; one of these is also established on common 

gorse. Several other unapproved natural enemies are established on common 

gorse and Scotch broom in North America, including Aceria davidmansoni, 

Dictyonota fuligosa, Gargara genistae, Melanotrichus concolor, M. virescens, 

and Selenophoma juncea. These species are not included herein because they 

play a more minor role in regulating gorse and/or broom populations or they 

have also been found attacking additional desirable species. It is illegal to 

intentionally move any unapproved natural enemies to new areas in the 

USA. Care should be taken when transferring approved biocontrol agents to 

ensure unapproved species are not also included in transferred material.

Aceria genistae (Nalepa)

Order: Acari

Family: Eriophyidae

Host: Scotch broom

Description and Life Cycle

All stages are tiny and best viewed with a microscope. Larvae and nymphs 

are white to orange and 0.10-0.12 mm long (Figure 3-28a). Adults are white 

to orange (typically orange) and have a worm-like appearance (Figure 3-28b). 

They have two pairs of developed legs near their heads and can be 0.16-0.225 

mm long. All stages feed on stem bud tissue by extracting sap from plant cells. 

This induces the development of galls 5-30 mm in diameter, which serve as 

protective housing to hundreds of mites. Galls are the best indication of mite 

presence (Figure 3-28c,d). As galls grow, they become increasingly hairy until 

they senesce, at which time mites migrate to new buds to form new galls. Galls 

may develop faster and have greater impact at hot, dry sites. There can be 

several generations per year. Mite numbers appear to be greatly reduced during 

overwintering. All stages are capable of overwintering within new buds. 

Unapproved 

Natural Enemies 

of Gorse and 

Scotch Broom

Figure 3-28. Aceria genistae: a. larva/nymph (see arrows) between gall hairs; b. magnified adult; c. galls; d. extensive damage

to a Scotch broom plant. (a.,d. Eric Coombs, Oregon Department of Agriculture, bugwood.org; b. Paul Pratt, USDA ARS

WRRC; c. Jennifer Andreas, Washington State University Extension)

a b c d
Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents 61

History and Current Status

Aceria genistae was first recorded in the USA by 2005 as an accidental 

introduction on Scotch broom in Oregon and Washington (Figure 3-29). The 

mite is currently abundant in Washington where its overall impact is moderate, 

as it reduces Scotch broom flowering and plant biomass and, in some cases, 

may cause stem and plant mortality (Figure 3-28d). In Oregon, the mite is 

widespread but only abundant locally with a slight overall impact. In British 

Columbia, Canada, sightings of Aceria genistae were first reported in 2007, and 

identification was later confirmed in 2010; it has had only minor impact to date. 

The mite was first recorded in California in 2014. Though its distribution in 

California is still limited, the mite has significant impact at well-established sites, 

reducing plant growth and reproduction and sometimes causing plant death.

Aceria genistae underwent host specificity testing in Washington. It fed heavily 

on Lupinus densiflorus, an endangered species in Canada, during no-choice 

greenhouse tests but has not been found on naturally-occurring L. densiflorus

populations. This species is currently not approved for redistribution in the 

United States.

A mite originally identified as A. genistae was recorded on French broom and 

gorse in California in the 1990s. This mite has since been identified as a different 

species, Aceria davidmansoni.

Figure 3-29. Current

establishment of Aceria 

genistae on Scotch broom

in North America. (Winston

et al. 2014a)
62 Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents

Agonopterix nervosa (Haworth)

Order: Lepidoptera

Family: Oecophoridae

Synonyms: Depressaria nervosa Haw., Depressaria costosa Haw.

Hosts: Gorse, Scotch broom, Portuguese broom

Description and Life Cycle

Adults are 10-15 mm long with variable coloring. Typical adults have white 

or yellowish wings with small gray to brown mottling, sometimes appearing 

as stripes on wing veins (Figure 3-30a). Their wingspan is 16-22 mm. 

Overwintering adults emerge during early spring to lay eggs on stems and 

leaf axils of gorse, Scotch broom, and Portuguese broom. Eggs are yellowish, 

cylindrical, and 1 mm long. Larvae hatch in late spring and spin tubes of plant 

material on shoot tips of their host plant. Larvae feed on young leaves, shoot tips, 

and flower buds, which stunts stem growth and reduces seed production. Larvae 

vary in color from yellowish-gray to brown and can be up to 15 mm long (Figure 

3-30b). There are five larval instars. The brown pupae (Figure 3-30c) are 10-15 

mm long. Pupation occurs within the feeding tubes. New adults emerge in late 

summer and overwinter in their host plant foliage. There is one generation per 

year.

History and Current Status

This species was accidentally introduced to Canada. It was recorded on gorse 

in British Columbia by 1915 and has since been recorded on Scotch broom in 

British Columbia as well. Its overall abundance and impact on gorse and Scotch 

broom in Canada are unknown. 

Agonopterix nervosa likely spread from Canada to the USA in the 1920s. It 

attacks both Scotch broom and gorse in California, Oregon, and Washington 

(Figure 3-31a), though it is more effective on gorse. It may stunt shoots (Figure 

3-31b) and reduce seed production (Figure 3-31c), but overall its impact on both 

a b c

Figure 3-30. Agonopterix nervosa: a. adult (body length top line, wingspan length bottom line) b. larva and Scotch broom

stems; c. pupa. (a.,b. Eric Coombs, Oregon Department of Agriculture; c. Jennifer Andreas, Washington State University

Extension)
Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents 63

weed species is limited. Populations are heavily parasitized in the USA. This 

moth also attacks Portuguese broom (Cytisus striatus) in Oregon. This species is 

not approved for redistribution in the United States.

Figure 3-31. Agonopterix 

nervosa: a. distribution

on gorse and Scotch

broom in North America;

b. damage to a gorse

shoot tip and spines;

c. damage to a Scotch

broom flower. (a. Winston

et al. 2014a; b. Jennifer

Andreas, Washington

State University Extension;

c. Eric Coombs, Oregon

Department of Agriculture)

a

b c

Arytainilla spartiophila (Förster)

Order: Hemiptera

Family: Psyllidae

Host: Scotch broom

Description and Life Cycle

Overwintering eggs are embedded in Scotch broom stems beneath a waxy cap. 

Tiny, orangey-brown nymphs (<2 mm long) hatch in early spring and gather 

near new leaf buds to feed. Nymphs feed primarily along the stem, rarely on the 

leaves themselves. Nymphs grow through five instars before developing into pale 

brown aphid-like adults (2-3 mm long) with clear wings (Figure 3-32). Adults 

feed on new growth of Scotch broom, lay eggs, and die by early summer. There 

is one generation per year.
64 Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents

History and Current Status

This accidentally introduced species was first recorded on Scotch broom in 

Washington, USA in 1935. It is now widespread in California, Oregon, and 

Washington (Figure 3-33) where it is the most common and abundant of Scotch 

broom natural enemies. It has also been reported on Scotch broom in Virginia, 

though its abundance and impact there are unknown. High densities can reduce 

Scotch broom growth and may weaken plants stressed from competition, making 

them vulnerable to pathogens; however, the overall impact of this psyllid is likely 

limited. In New Zealand, Candidatus Liberibacter europaeus, a new and possibly 

pathogenic bacteria thought to have been introduced along with Arytainilla 

spartiophila, appears to be damaging Scotch broom plants. It has not been found 

in the USA; consequently, the psyllid does not have the same impact in North 

America. This species is not approved for redistribution in the USA; it is 

currently not known to be present in Canada. Note: The honeydew produced 

by this species interferes with late-season collections of approved Scotch broom 

biocontrol agents.

Figure 3-33. Current

establishment of Arytainilla 

spartiophila on Scotch

broom in North America

(Winston et al 2014a)

Figure 3-32. Arytainilla 

spartiophila adult.

(Landcare Research Ltd.,

New Zealand)
Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents 65 Table 3-1. Traits of biological control agents established on gorse and Scotch broom in the continental USA and Canada. Red bolded species are not approved for redistribution in the USA. (continued on next page) Biological Control Agent Species Attacked Generations/ Year Overwintering Stage, Location Adult Egg Larva Pupa Aceria genistae Unapproved Scotch broom Multiple All stages in communal galls on host plant Microscopic Nymphs: all stages white to orange and microscopic (0.10-0.12 mm long). Early stages with 6 legs; later stages with 8 legs. Agonopterix nervosa Unapproved Gorse, Scotch broom, Portuguese broom One Adults among foliage of host plant Cylindrical; yellowish;

1 mm long; laid onstems, leaf axils ofhost

Vary from yellow-grayto brown; 15 mm long;5 instars

Within cocoonsof plant materialspun by larva

Arytainilla  spartiophilaUnapproved

Scotch broom One Eggs embedded

in Scotch broomstems

Tiny; Not visible asembedded in broomstem beneath a waxycap

Nymphs: all stages tiny; orangey-brown;<2mm long; 5 stages

Bruchidius  villosus Broom seed

beetle

Scotch broom,French broom

One Adults in soil litter

away from thehost plant

Tiny; oval-shaped;white; laid on thebroom seedpod

Cream colored withbrown head capsule;2 mm long; 4 instars;inside broom seedswithin pods

Gray or brown;2 mm long; insidebroom seedswithin pods

Exapion  fuscirostre Scotch broom

seed weevil

Scotch broom One Adults in soil litter Small; round; white

to yellowish; laidin Scotch broomseedpods

Cream colored withbrown head capsules;3 mm long; 3 instars;exposed in broomseedpods

Cream colored;3 mm long;exposed in broomseedpods

Exapion ulicis Gorse seed

weevil

Gorse One Adults among

gorse foliage

Small; round;translucent yellow;laid in gorseseedpods

Cream colored withbrown head capsules;3 mm long; 3 instars;exposed in gorseseedpods

Cream becomingdark gray withage; 3 mm long;exposed in gorseseedpods

Leucoptera  spartifoliella Scotch broom

twig miner

Scotch broom One Larvae in broom

stems

Tiny; oval-shaped;white; laid on youngbroom stems afterflowering ceases

Green-brown,translucent; a bitflattened; 3-4 mm long

Within white, silkycocoons spunby larva; 4-5 mmlong

abcdefg
66 Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents Table 3-1 (continued). Traits of biological control agents established on gorse and Scotch broom in the continental USA and Canada. Red bolded species are not approved for redistribution in the USA. a. Aceria genistae, numerous galls housing hundreds of mites each; b. Agonopterix nervosa; c. Arytainilla spartiophila; d. Bruchidius villosus; e. Exapion fuscirostre; f. Exapion ulicis; g. Leucoptera spartifoliella; h. Tetranychus lintearius. (a.,b.,g.,h. Eric Coombs, Oregon Department of Agriculture, bugwood.org; c. Landcare Research Ltd., New Zealand; d. Jennifer Andreas, Washington State University Extension; e. Laura Parsons, University of Idaho, bugwood.org; f. Janet Graham) Biological Control Agent Species Attacked Generations/ Year Overwintering Stage, Location Adult Egg Larva Pupa Tetranychus lintearius Gorse spider mite Gorse Up to 6 All stages inside communal webbing Tiny; round; largely transparent Nymphs: first stage light in color with 6 legs; second to fourth stage brown with 8 legs

h

Table 3-2. Traits of biological control agents established on gorse in Hawaiia. Agonopterix umbellana; b. Exapion ulicis; c. Sericothrips staphylinus; d. Tetranychus lintearius. (a.,b. Janet Graham; c. Fritzi Grevstad, Oregon State University; d. Eric Coombs,Oregon Department of Agriculture, bugwood.org)

Biological 

Control Agent

Species 

Attacked

Generations/ 

Year

Overwintering Stage, 

Location Adult Egg Larva Pupa

Agonopterix  umbellana Gorse soft shoot

moth

Gorse One Adults among gorse

foliage

Barrel-shaped; brightyellow; 1 mm long; laidin axils of gorse leavesand spines

Dark brown to olivegreen with dark sidespots; 20 mm long; 5instars

Within silkencocoons spun bylarva

Exapion ulicis Gorse seed

weevil

Gorse One Adults among gorse

foliage; some adultsactively feed throughwinter

Small; round;translucent yellow; laidin gorse seedpods

Cream colored withbrown head capsules;3 mm long; 3 instars;exposed in gorseseedpods

Cream becomingdark gray withage; 3 mm long;exposed in gorseseedpods

Sericothrips  staphylinus Gorse thrips

Gorse Multiple At cold sites, adults

among gorse foliage; atwarmer sites adults/nymphs active throughwinter

0.3mm long;cylindrical; paleyellow; laid in slits ingorse stems

Nymphs: all stages creamy-yellow; lookincreasingly similar to adults as they moltbetween stages; 4 stages

Tetranychus  lintearius Gorse spider

mite

Gorse Up to 6 All stages inside

communal webbing

Tiny; round; largelytransparent

Nymphs: first stage light in color with 6legs; second to fourth stage brown with 8legs

abcd
Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents 67 Month Gorse Life Stage Exapion ulicis Tetranychus lintearius Agonopterix nervosa Unapproved January Seeds germinate; older plants continue to grow overwinter Adults overwinter in soil litter All stages overwinter; mites continue life cycle and feeding Adults overwinter in foliage of host plant February Flowering peaks for individuals ≥18 months old; seedpods begin to develop; previous-year seeds germinate March Adults emerge and feed on gorse flowers and foliage; eggs are laid in young gorse seedpods; hatching larvae feed on developing seeds April Seedpods begin to mature; some split at maturity; seed dispersal commences; juvenile plants have small, 3-parted leaves; leaves reduced to spines as plants age Egg-laying continues; hatching larvae feed on developing seeds Larvae, nymphs, and adults feed on stems and spines of gorse within webbing covering large colonies; life cycle repeated for up to six generations per year Overwintered adults become active; eggs are laid on stems and leaf axils of hosts May Larvae continue feeding; pupation begins in seedpods; new adults emerge in seedpods Hatching larvae spin feeding tubes, feed on young leaves, shoot tips, flower buds June Seed dispersal continues as seedpods split with maturity Pupation continues; new adults continue emerging in seedpods July New adults escape pods when pods split naturally Pupation occurs in feeding tubes August New adults emerge September Smaller secondary bloom at some locations; seed dispersal continues Adults overwinter in soil litter Adults overwinter in foliage of host plant October All stages overwinter; mites continue life cycle and feeding November Seeds germinate; older plants continue to photosynthesize overwinter with spines/green stems December Table 3-3. Comparison of North American-established biocontrol agent activity, according to gorse growth stages. Plant and biocontrol agent stages will vary by climate and location. The red bolded species is not approved for redistribution in the USA; it is included in this table to aid in proper identification to ensure it is not inadvertently collected and redistributed along with approved biocontrol agents.
68 Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents Month Scotch Broom Life Stage Bruchidius villosus Exapion fuscirostre Leucoptera spartifoliella Aceria genistae Unapproved Agonopterix nervosa Unapproved Arytainilla spartiophila Unapproved January Seeds germinate; older plants grow leaves Adults overwinter in soil litter away from host plant Adults overwinter in soil litter Overwintering larvae feed in stems of Scotch broom All stages overwinter; mites continue life cycle and feeding, though populations and activity are reduced during colder months; found primarily in buds Adults overwinter in foliage of host plant Eggs overwinter embedded in Scotch broom stems beneath a waxy cap February March Seeds germinate; older plants begin losing leaves; flowering begins Adults continue to overwinter; few adults begin emerging Adults continue to overwinter; few adults begin emerging Nymphs hatch, feed on Scotch broom stems through five stages April Seeds continue to germinate; older plants photosynthesize with green stems; flowering peaks for individuals ≥18 months old; seedpods begin to develop Adults continue emerging, feed on pollen, begin mating; eggs are laid on developing seedpods Adults continue emerging, feeding on Scotch broom terminal stems and flowers; mating begins; eggs are laid in seedpods Larval feeding continues; pupation begins in cocoons on broom stems All stages feed on shoot buds and stem tips, inducing the formation of hairy galls. Eggs are laid within the galls. Hatching mites feed within galls. The entire process is repeated through several generations throughout the growing season. Galls begin to senesce and dry out later in the season; mites begin to migrate to buds Overwintered adults become active; eggs are laid on stems and leaf axils of hosts May Larvae begin hatching and feed within seeds Adults continue laying eggs; larvae hatch and feed within seeds Hatching larvae spin feeding tubes, feed on shoot tips and within flowers June Larval feeding continues; pupation begins Larval feeding continues; pupation begins Pupation continues; adults emerge Adults feed on Scotch broom stems, lay eggs in Scotch broom stems, die July Seedpods begin to split at maturity; seed dispersal commences Pupation continues; new adults begin emerging, remain in seedpods until pod splits naturally Pupation continues; new adults begin emerging, remain in seedpods until pod splits naturally Eggs are laid on Scotch broom stems; hatching larvae tunnel into young shoots Pupation occurs in feeding tubes Eggs overwinter embedded in Scotch broom stems beneath a waxy cap August New adults emerge September Seed dispersal continues Adults overwinter in soil litter away from host plant Adults overwinter in soil litter Larvae feed in stems of Scotch broom during fall and winter Adults overwinter in foliage of host plant October Seeds germinate; older plants continue to photosynthesize overwinter with green stems; some plants grow leaves All stages overwinter; mites continue life cycle and feeding, though populations and activity are reduced during colder months November December Table 3-4. Comparison of North American-established biocontrol agent activity, according to Scotch broom growth stages. Plant and biocontrol agent stages will vary by climate and location. Red bolded species are not approved for redistribution in the USA; they are included in this table to aid in proper identification to ensure they are not inadvertently collected and redistributed along with approved biocontrol agents.
Chapter 3: Biology of Gorse and Scotch Broom Biological Control Agents 69 Table 3-5. Comparison of Hawaii-established biocontrol agent activity, according to gorse growth stages. Plant and biocontrol agent stages will vary by climate, elevation, and location. Month Gorse Life Stage Agonopterix umbellana Exapion ulicis Sericothrips staphylinus Tetranychus lintearius January Flowering peaks for individuals ≥18 months old; seedpods begin to develop at high-elevation sites; previous-year seeds germinate Adults are inactive in foliage of gorse Some adults are inactive overwinter, others feed on gorse flowers and foliage; eggs are laid in young gorse seedpods; hatching larvae feed on developing seeds At some cold, high- elevation sites, adults are inactive in gorse foliage; at warmer sites, adults and nymphs are active throughout winter Larvae, nymphs, and adults feed on stems and spines of gorse underneath webbing covering large colonies; life cycle repeated for up to six generations per year February March Eggs are laid in gorse spine and leaf axils Larvae feed on developing seeds; pupation begins at some locations Multiple generations completed in warmer months of the year; adults and nymphs feed on gorse green cells; eggs are laid in slits in gorse stems April Seedpods begin to split at maturity; seed dispersal commences; juvenile plants have small, 3-parted leaves; leaves reduced to spines as plants age Larvae spin silken feeding tubes on gorse buds; larvae feed on new shoots and spines New adults emerge in seedpods; adults escape when pods split naturally May Seed dispersal continues as seedpods split with maturity June Pupation occurs in feeding tubes July Plants continue growing/ producing spines; seed dispersal continues New adults emerge Adults feed on gorse foliage August September Flowering begins at some low-elevation sites for individuals ≥18 months old; seed dispersal continues Adults become inactive in foliage of gorse October November Flowering begins at high elevation sites for individuals ≥18 months old; seedpods begin to develop at low-elevation sites; previous-year seeds germinate Some adults become inactive overwinter, others feed on gorse flowers and foliage; eggs are laid in young gorse seedpods December At some cold, high- elevation sites, adults are inactive in gorse foliage; at warmer sites, adults and nymphs are active throughout winter
70

Chapter 4: Elements of a Gorse and Scotch Broom 

 Biological Control Program

Before You Begin Biological control is one of many weed control methods available to land 

managers, but biological control is not appropriate for areas where gorse or 

Scotch broom are not present or where a small number of localized populations 

occur. Biological control as a control method is best suited to gorse or broom 

populations in the later phases of the invasion curve, where populations are 

experiencing a rapid increase in distribution and abundance, or where gorse and 

broom are widespread and abundant throughout their potential range (asset based 

protection, Figure 1-3 repeated here in Figure 4-1).

Figure 4-1. Generalized invasion curve showing actions appropriate to each stage. (© State of Victoria, Department of 

Economic Development, Jobs, Transport and Resources. Reproduced with permission.)
Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program 71

The results of using biological control to treat gorse and Scotch broom may vary 

greatly from site to site for a variety of reasons. Land managers should develop 

treatment programs that complement management activities and objectives 

unique to the area. This is accomplished by first understanding the scope of the 

gorse and Scotch broom problem, defining overall goals for the gorse and broom 

management program, and understanding the control methods available for 

accomplishing the goals. 

Determining the Scope of the Problem

The first step should be to develop a distribution map of gorse and Scotch broom 

at a scale that will allow you to address the problem in a manner consistent 

with your overall land-management objectives and available weed management 

resources. The most appropriate scale may encompass a large landscape with 

a variety of site characteristics and land uses managed by many different land 

owners/managers—all of whom contribute to mapping efforts (Figure 4-2a). In 

large management areas with significant gorse and Scotch broom infestations and 

limited resources, aerial mapping of large patches of gorse and broom may be 

sufficient to identify priority areas for additional survey and weed management 

activities. In other management areas with small, discrete gorse and broom 

infestations, or where an infestation’s characteristics affect your ability to meet 

management objectives, your weed management strategy might have to include 

more extensive mapping and analysis of the scope of the infestations (e.g., size, 

density, cover, or location in relation to roads and waterways over time) 

(Figure 4-2b).

In many cases, it may prove useful to check for existing gorse and broom 

distribution data before collecting your own. Several different agencies and 

organizations maintain weed distribution databases, including state agricultural 

Figure 4-2. Scotch broom data for: a. counties with Scotch broom in the state of Washington; b. infestations of differing 

densities at Howe Farm in Kitsap County, Washington. (a. Washington State Department of Agriculture; b. Kitsap County 

Parks)

a b
72 Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program

departments, provincial ministries (e.g. British Columbia IAPP Application), 

invasive plant/species councils, USDA PLANTS database, EDDMapS, and 

many others. EDDMapS can be particularly useful for land mangers interested 

in creating gorse and Scotch broom distribution maps for their area. By 

visiting www.eddmaps.org and creating a free account, users can view existing 

distribution maps for gorse and Scotch broom or other weeds at the state, county, 

or point level. By selecting the GIS view option, users can view gorse and broom 

data on various backgrounds and zoom into different scales, add hand drawn 

labels, boundaries, points and other shapes to the map, perform measurements 

such as perimeter estimates or distance between points, add new gorse and broom 

data from user shapefiles, edit the management status of various infestations, and 

print finished maps (see page 100 for more information on EDDMapS).

Defining Goals and Objectives

Goals broadly define the “what” or desired outcome of management; objectives 

define the “how” or specific activities through which desired outcomes can be 

achieved. To be effective, objectives must be SMART: specific, measurable, 

achievable, realistic, and timely. Defining your weed management goals and 

objectives is the crucial first step in developing a successful biological control 

program. By defining what you want to achieve, you will be able to determine if, 

when, and where you should use biological control.

As precisely as possible, you must define what will constitute a successful gorse 

and broom management program. For example, the objective of “...a noticeable 

reduction in gorse and Scotch broom density over the next ten years...” might 

be achievable, but it uses a subjective measurement of success that is open to 

observer bias. Alternatively, the objective of “...a 50 percent reduction in gorse 

and Scotch broom stems over the next three years...” is objectively measurable 

(and therefore SMART). If your goal is to reduce the abundance of gorse and 

broom, then biological control might be an appropriate weed management tool; 

however, by itself biological control will not completely and permanently remove 

gorse and broom from the landscape. If your goal is to eradicate gorse and 

broom, then you should plan to employ other weed control techniques instead of, 

or in addition to, biological control (see Chapter 5 for more details).

Understanding Gorse and Scotch Broom Management Options

Once you determine the scope of your gorse and Scotch broom infestations and 

define your overall program goals, review the weed control methods available 

(biological control, physical treatments, cultural practices, and herbicides), 

and determine the conditions (when, where, if, etc.) under which it might be 

appropriate to use each method or combination of methods (see Chapter 5). 

Consult commercial, agency, or university biological control experts, cooperative 

weed management area partners, or county weed coordinator/supervisors to 

learn about other gorse and Scotch broom management activities (herbicide use, 

mowing, etc.) underway or planned for your area, and the level and persistence of 

control that might be achieved by each.
Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program 73

Identify the resources that will be available for weed management activities, 

and determine if they will be consistently available until you meet your weed 

management program objectives. If resources are not currently available, or will 

not be available consistently, identify what will happen at the treatment site if 

planned management activities are not implemented. This information will help 

you determine the best management activities to use as you initiate and continue 

your integrated gorse and broom management program. 

With a map of gorse and broom infestations in your management area, an 

understanding of your land management goals, well defined weed management 

objectives, and a list of the weed control methods available with the level of 

control you can realistically expect from each, you can identify sites where 

biological control would be a good fit, alone or in combination with other control 

methods.

When biological control is deemed suitable for treating your gorse and broom 

infestations, there are several important factors to consider. These include 

selecting appropriate release sites, obtaining and releasing biocontrol agents, 

and monitoring the success of the program. Familiarity with all aspects of a 

biocontrol program before beginning will greatly facilitate its implementation 

and increase its chances of success. These items are discussed in their own 

sections below. If problems are encountered following the initiation of a 

biological control program, refer to the troubleshooting guide in Appendix I 

for potential solutions.

Selecting Biological Control Agent Release Sites

Establish goals for your release site

You must consider your overall management goals for a given site when you 

evaluate its suitability for the release of biological control agents. Suitability 

factors will differ depending on whether the release is to be:

1. a general release, where biological control agents are simply released for 

gorse and broom management,

2. a field insectary (nursery) release, used primarily to mass produce 

biological control agents for redistribution to other sites, or 

3. a research release, used to investigate biological control agent biology 

and/or the biocontrol agent’s impact on the target weed and nontarget plant 

community.

A site chosen to serve one of the roles listed above may also serve additional 

functions over time (e.g., biological control agents might eventually be collected 

for redistribution from a research or general release).

Determine site characteristics 

For practical purposes, no gorse and broom infestation is too large for biocontrol 

releases; however, it might not be large enough (Figure 4-3a). Very small, 

isolated patches of gorse and broom may not be adequate for biological control 

Developing, 

Implementing, and 

Managing a Gorse 

and Scotch Broom 

Biological Control 

Program
74 Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program

agent populations to build up and persist and are often better treated with other 

weed control methods, such as physical control or herbicides. An area with 

at least 1 acre (0.40 hectares) of gorse or Scotch broom is the minimum size 

to better ensure a successful biological control agent release site, but larger 

infestations are more desirable (Figure 4-3b), especially if the land manager 

hopes to someday use the release site as a field insectary. However, smaller 

infestations may be acceptable release sites in some cases, such as critical 

habitat zones where disturbance from physical control would be detrimental or 

sites where herbicides are prohibited. If the Scotch broom or gorse populations 

are extensive within a region but the individual population is below an acre, 

biocontrol agents can be released to establish populations and encourage spread 

throughout the region. In addition, control of Scotch broom and gorse may be 

considered a low priority in some regions and is often overlooked for intensive 

management. In these cases, land managers may wish to use biocontrol as a way 

to reduce further weed spread. Nevertheless, biocontrol agents disperse more 

easily in contiguous gorse and broom infestations than in infestations with only 

a few scattered plants and distant patches. If your biological control program 

goals involve evaluating the program’s efficacy, establish permanent monitoring 

sites before you release any biocontrol agents. The monitoring sites will require 

regular inspections, so consider the site’s ease of accessibility, terrain, and slope.

Note land use and disturbance factors 

Release sites should experience little to no regular disturbance. Abandoned 

fields/pastures, vacant lots, and natural areas are good choices for biological 

control agent releases. Sites where insecticides are used should not be utilized for 

biocontrol agent releases. Such sites include those near wetlands that are subject 

to mosquito abatement or near agricultural fields or orchards where pesticide 

applications occur regularly. Roadside infestations along dirt or gravel roads 

with heavy traffic should also be avoided; extensive dust makes gorse and broom 

Figure 4-3. Gorse infestations: a. too small for biological control; b. appropriate for biological control. (a. Jennifer Andreas, 

Washington State University Extension; b. Nancy Ness, Grays Harbor Noxious Weed Control Board)

a b
Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program 75

plants less attractive to biocontrol agents. Do not use sites where significant land 

use changes will take place, such as intensive reforestation, road construction, 

cultivation, building construction, and mineral or petroleum extraction. If supply 

of biocontrol agents is limited, prioritize release sites that are not regularly 

mowed, pulled, burned, or treated with herbicides.

Survey for presence of biological control agents

Always examine your prospective release sites to determine if gorse and Scotch 

broom biological control agents are already present. Many of the biological 

control agents currently approved for use in North America and/or Hawaii are 

already widespread. If a biocontrol agent you are planning to release is already 

established at a site, you may want to consider making the release at another 

site where the biocontrol agent is not yet present. If observed biocontrol agent 

populations are low at a site, you can release additional biocontrol agents at that 

site to augment the existing population.

Record ownership and access

If you release biological control agents on private land, it is a good idea to select 

sites on land likely to have long-standing, stable ownership and management. 

Stable ownership will help you establish long-term agreements with a landowner, 

permitting access to the sites to sample or harvest biological control agents 

and collect insect and vegetation data for the duration of the project. This is 

particularly important if you are establishing a field insectary site, because five 

years or more of access may be required to complete insect harvesting or data 

collection. General releases of biological control agents to control gorse and 

Scotch broom populations require less-frequent and short-term access; you may 

need to visit such a site only once or twice after initial release. When releasing 

biocontrol agents on private land, it may be a good idea to obtain the following:

• written permission from the landowner allowing use of the area as a release 

site,

• written agreement with the landowner allowing access to the site for 

monitoring and collection for a period of at least six years (three years for 

establishment and buildup and three years for collection),

• permission to put a permanent marker at the site, and

• written agreement with the landowner that land management practices at 

the release site will not interfere with biological control agent activity

The above list can also be helpful for releases made on public land where the 

goal is to establish an insectary. In particular, an agreement should be reached 

that land management practices will not interfere with biological control agent 

activity (e.g. chemically spraying or physically destroying the weed infestation). 

It is often useful to visit the landowner or land manager at the release site 

annually to ensure they are reminded of the biological control endeavors and 

agreement. Always re-check with the landowner prior to inspecting release sites; 

in some cases the ownership may have changed.
76 Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program

You may wish to restrict access to release locations, especially research sites and 

insectaries, and allow only authorized project partners to visit the sites and collect 

biocontrol agents. The simplest approach is to select locations that are not visible 

to or accessible by the general public. To be practical, most if not all of your 

sites will be readily accessible, so in order to restrict access you should formalize 

arrangements with the landowner or manager. This will require you to post notrespassing signs, install locks on gates, etc. (Figure 4-4).

Another consideration is physical access to a release site. You will need to drive 

to or near the release locations, so determine if travel on access roads might 

be interrupted by periodic flooding or inclement weather. You might have to 

accommodate occasional road closures by private landowners and public land 

managers for other reasons, such as wildlife protection.

Choosing the Appropriate Biological Control Agents

for Release

You should consider several factors when considering which biological control 

agent to release at a site, including biocontrol agent efficacy, availability, and site 

preferences (Tables 4-1 and 4-2).

Figure 4-4. “No 

disturbance” sign. (Alan 

Martinson, Latah County 

Weed Control, and Paul 

Brusven, Nez Perce 

BioControl Center)
Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program 77 Table 4-1. Summary of general characteristics and site preferences of gorse and Scotch broom biological control agents established in North America. Species in red are not approved for redistribution in the USA; they are included in this table to aid in proper identification to ensure they are not inadvertently collected and redistributed along with approved biocontrol agents. Biocontrol Agent Characteristics Site Characteristics Species Plant Species and Part Attacked Efficacy Availability Favorable Conditions Unfavorable Conditions Bruchidius villosus Broom seed beetle Seeds of Scotch broom, French broom Reduces viable seed production; unclear if densities and attack rates are high enough to decrease plant populations Widespread and high abundance in OR,WA and BC; low abundance in CA Open areas, meadows and hillsides with southern exposure Heavy shade, cold, damp, possibly high elevations Exapion fuscirostre Scotch broom seed weevil Seeds of Scotch broom Reduces viable seed production, but often insufficient to control Scotch broom populations Widespread but moderately abundant in CA, ID, OR, WA; low abundance in BC, it is less effective than B. villosus Open areas, meadows and hillsides with southern exposure Heavy shade, cold, damp, high elevations Exapion ulicis Gorse seed weevil Seeds of gorse Reduces viable seed production, but insufficient to control gorse populations Widespread and abundant in CA, OR, WA; not known to be present in Canada Open, sunny, dense gorse infestations Cold winters, shade, scattered gorse plants, salt spray zones Leucoptera spartifoliella Scotch broom twig miner Stems of Scotch broom Mining causes dieback of stems; however plants often re-sprout below sites of damage so overall impact is negligible; parasitized Widespread in CA, OR but limited in WA; not known to be present in Canada Low elevations, moist, moderate temperatures Dry, high elevations, cold winters Tetranychus lintearius Gorse spider mite Stems, spines, leaves of gorse Stunts branch growth, reduces flowering; initially very effective by killing heavily infested plants; heavy predation now makes it ineffective Limited abundance in CA, OR, WA; not known to be present in Canada Warm, open, away from the ocean Damp, ocean-side, shade Aceria genistae Stems and stem buds of Scotch broom Mite galls reduce seed production, plant biomass and may cause stem or plant mortality; medium impact overall in WA, less abundant in CA, OR and BC where impact is limited Widespread in WA; established but abundant only locally in OR, CA, and BC; not approved for redistribution in USA Hot, dry, mild winter temperatures; spreads quickly through windy corridors Extreme winter temperatures, excessive precipitation Agonopterix nervosa Shoot tips, flowers of gorse, Scotch broom, Portuguese broom Stunts stem growth and reduces seed production; most effective on gorse, but overall impact to any attacked species is limited Widespread in OR and WA, uncommon in CA; highly parasitized; abundance in BC unknown; not approved for redistribution in USA Sunny, below 2,600 feet (800m) in elevation High elevation, shade or excessive overcast conditions Arytainilla spartiophila Stems and new growth of Scotch broom At high densities reduces growth and weakens plants, making them more susceptible to pathogens; overall impact is likely limited Widespread in CA, OR and WA; not approved for redistribution in USA; not known to be present in Canada Adapted to variety of conditions throughout northwestern North America No specific unfavorable conditions determined to date
78 Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program

Biocontrol Agent Characteristics Site Characteristics

Species

Plant Species 

and Part Attacked Efficacy Availability

Favorable 

Conditions

Unfavorable 

Conditions

Agonopterix 

 umbellana 

Gorse soft shoot 

 moth

Shoots and spines 

of Scotch broom

High populations 

defoliate stems, destroy 

shoot tips, and can 

lead to plant dieback; 

however most attacked 

plants recover after the 

moths are no longer 

active

Widespread on 

Hawaii Island; 

abundant on 

Maui only at high 

elevations (> 3,280 

feet or 1,000 m)

High 

elevations, cool 

temperatures

Low elevations, 

hot

Exapion ulicis 

Gorse seed weevil

Gorse Reduces viable 

seed production, but 

insufficient to control 

gorse populations

Variable 

abundance from 

year to year on 

Maui and Hawaii 

Island

Open, sunny, 

dense gorse 

infestations

Cold winters, 

shade, 

scattered gorse 

plants, salt 

spray zones

Sericothrips 

 staphylinus 

Gorse thrips

Stems of gorse At high densities, 

reduces gorse growth 

and flowering; however, 

only limited impact has 

been observed in the 

field

Established 

only on Hawaii 

Island and with 

low abundance, 

possibly due to 

predation

No specific 

favorable 

conditions 

determined to 

date

No specific 

unfavorable 

conditions 

determined to 

date

Tetranychus 

 lintearius 

Gorse spider mite

Stems, spines, 

leaves of gorse

Stunts branch growth, 

reduces flowering; 

initially very effective by 

killing heavily infested 

plants; heavy predation 

now makes it ineffective

Limited abundance 

on Maui and 

Hawaii Island, 

likely due to 

predation

Warm, open, 

away from the 

ocean

Damp, oceanside, shade

Table 4-2. Summary of general characteristics and site preferences of gorse biological control agents 

established in Hawaii

Biocontrol agent efficacy

Efficacy refers to the ability of the biological control agent to directly or 

indirectly reduce the population of the target weed below acceptable damage 

thresholds or cause weed mortality resulting in control. It is preferable to 

release only the most effective biocontrol agents rather than releasing all 

biocontrol agents that might be available for a target weed. Consult with local 

weed biological control experts, neighboring land managers, and landowners 

to identify the biocontrol agent(s) that appear(s) most effective given local site 

characteristics and management scenarios.

Biocontrol agent availability 

Five approved biological control agents are currently established on gorse or 

Scotch broom in the continental United States, though their availability varies 

greatly between species and sites. The seed weevil Exapion ulicis is the most 
Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program 79

widespread biocontrol agent on gorse and is readily available for collection in 

California, Oregon, and Washington. The spider mite Tetranychus lintearius

is also established on gorse in California, Oregon, and Washington, but its 

abundance is limited by predation. Neither species is overly effective in 

controlling gorse. 

Of the Scotch broom biological control agents approved for redistribution, the 

seed beetle Bruchidius villosus is the most widespread. Though abundant in 

Oregon and Washington, it remains to be seen if densities and attack rates are 

sufficiently high to decrease Scotch broom populations. The twig-mining moth 

Leucoptera spartifoliella is widespread in California and Oregon but limited in 

Washington, while the seed weevil Exapion fuscirostre is moderately abundant 

in California, Idaho, Oregon, and Washington. The overall impact of both 

L. spartifoliella and E. fuscirostre on Scotch broom is minimal. 

Several unintentionally introduced species are established on gorse and/or 

broom in the United States. Refer to “Unapproved Natural Enemies of Gorse 

and Broom” (pages 60-64) for more information. None are approved for 

redistribution in the United States; three are mentioned herein to prevent 

their inadvertent collection and redistribution as they are commonly 

encountered when working with the approved biocontrol agents. The moth 

Agonopterix nervosa attacks both gorse and Scotch broom, though its impact 

is limited on both species. Populations of A. nervosa are widespread in Oregon 

and Washington and limited in California. The psyllid Arytainilla spartiophila is 

widespread on Scotch broom; however, it has limited impact on Scotch broom 

populations. The mite Aceria genistae is also widespread on Scotch broom in 

portions of northwestern North America where its impact varies from slight to 

heavy. 

Four of the species established in the continental United States are also 

established in Canada; A. genistae, B. villosus, and E. fuscirostre are established 

on Scotch broom while A. nervosa is established on both gorse and Scotch 

broom. None were intentionally introduced to Canada, though two of these 

species (B. villosus and E. fuscirostre) were intentionally redistributed for 

a short time within British Columbia. Bruchidius villosus is widespread in 

British Columbia, though it remains to be seen if densities and attack rates are 

sufficiently high to decrease Scotch broom populations. The impact of the other 

three established species on gorse and Scotch broom in Canada is either limited 

or unknown.

Since 1926, eight biological control agents have been released on gorse in 

Hawaii. The two weevils Apion sp. and Stenopterapion scutellare, the shoot 

moth Pempelia genistella, and the rust Uromyces pisi f. sp. europaei all failed 

to establish. The moth Agonopterix umbellana and the weevil E. ulicis are both 

widespread and abundant on Hawaii Island and Maui at different times and 

locations, though both have limited impact on gorse populations. The thrips 

Sericothrips staphylinus and the mite T. lintearius are both limited in Hawaii, 

likely due to predation.
80 Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program

Federal and state departments or commercial biological control suppliers may be 

able to assist you in acquiring biocontrol agents not yet available but permitted 

for use in your area (see Obtaining and Releasing Gorse and Broom Biological 

Control Agents, below). In the United States, state departments of agriculture, 

county weed managers, extension educators, or federal and university weed 

biological control specialists should be able to recommend in-state collection 

sites where appropriate. Remember that in the United States, interstate transport 

of biological control agents requires a USDA-APHIS-PPQ 526 Permit (see 

Regulations for the Transfer of Gorse and Scotch Broom Biological Control 

Agents, page 99). Get your permits early to avoid delays.

Release site characteristics 

General physical site and biological preferences for each biocontrol agent have 

been developed from anecdotal observations and experimental data. These are 

listed in Tables 4-1 and 4-2 to help land managers ensure that biocontrol agents 

are released in sites with suitable conditions. 

Obtaining and Releasing Gorse 

and Scotch Broom Biological Control Agents

You can obtain gorse and Scotch broom biological control agents by collecting 

or rearing them yourself, having someone collect them for you, or by purchasing 

them from a commercial supplier. This section provides information on 

collecting and purchasing gorse and broom biocontrol agents, with emphasis 

on Bruchidius villosus (continental United States). Exapion fuscirostre and 

E. ulicis are both already moderately abundant in Northwestern North America, 

but both have limited impact on gorse or Scotch broom so are not the highest 

priority for redistribution. Tetranychus lintearius and Leucoptera spartifoliella

are heavily preyed upon or parasitized in the continental United States so are not 

highly recommended for further redistribution. Aceria genistae, Agonopterix 

nervosa, and Arytainilla spartiophila are not approved for redistribution in 

the United States, but they are widely distributed at many sites.

All four gorse biological control agents established in Hawaii are not highly 

effective against their target weed and are likely most effective when combined 

with complementary control methods. Agonopterix umbellana and Exapion 

ulicis are both at least moderately abundant on Hawaii Island and Maui, but 

both have limited impact on gorse populations so are not the highest priority for 

redistribution. Sericothrips staphylinus and Tetranychus lintearius populations 

are limited due to predation, so are not highly recommended for further 

redistribution.

Factors to consider when looking 

for sources of biological control agents

You do not need to take a lottery approach and release all approved biological 

control agents at a site in the hopes that one of them will work. Some biological 

control agents will not be available even if you want them, and some are already 

widespread and/or have been shown to have little or no effectiveness in certain 
Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program 81

areas. The best strategy is to release the best biocontrol agent. Ask the county, 

state, or federal biological control experts in your area for recommendations of 

biocontrol agents for your particular project.

If available, biological control agents from local sources are best. Using local 

sources increases the likelihood that biocontrol agents are adapted to the climate 

and site conditions present and are available at appropriate times for release 

at your target infestation. Using locally sourced biological control agents also 

reduces the possibility of accidentally introducing biocontrol agent pathogens or 

natural enemies to your area. Local sources may include neighboring properties 

or other locations in adjacent counties/districts. Remember that in the United 

States, interstate transport of biological control agents requires a USDA-APHISPPQ 526 Permit (see Regulations for the Transfer of Gorse and Scotch Broom 

Biological Control Agents, page 99). Get your permits early to avoid delays.

Some USA states, counties, and universities have field collection days at 

productive insectary sites (Figure 4-5). On these days, land managers and 

landowners are invited to collect or receive locally collected gorse and Scotch 

broom biological control agents for quick release at other sites. These sessions 

are an easy and often inexpensive way for you to acquire biological control 

agents. They are good educational opportunities as well, because you may see 

first-hand any impacts the various biocontrol agents might be having on gorse 

and broom plant communities. 

Figure 4-5. Scotch broom field day. (Jennifer Andreas, Washington State University 

Extension)
82 Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program

Typically, field days are conducted at several sites in a state and on several 

dates. Although designed for intrastate collection and redistribution, out-of-state 

participants may be welcome to participate (remember that USDA-APHISPPQ 526 Permits are required for interstate movement and release of biological 

control agents). Contact county weed supervisors, university weed or biological 

control specialists, or federal weed managers for information about field days in 

your region.

Collecting Gorse and Scotch Broom Biological Control Agents

Planning and timing of collection is critical. For all species, it is usually 

most efficient to scout the potential collection site well in advance to ensure 

your desired species is present at suitable densities. The species of biological 

control agent and weather characteristics at your collection and release site 

will determine the best time in the season to collect. Ensure that all necessary 

collection supplies are on hand. Also, accurate identification of the biological 

control agents is essential. General guidelines for collecting gorse and broom 

biological control agents are listed in the following sections and in Tables 4-3 

and 4-4. 

For all species, collect only on a day with good weather. Do not collect in the 

rain; arthropods will hide and become difficult to find in rainy weather, excess 

moisture causes adverse effects, and biocontrol agents may drown in wet 

collection containers. 

Collection methods

Racket and beat sheet: The most common method for collecting gorse 

and Scotch broom biocontrol agents is to use a tool such as a racket to tap 

the biocontrol agents off of their host plant foliage and onto a beat sheet 

(Figure 4-6a), tray (Figure 4-6b), or sweep net (Figure 4-6c) placed strategically 

beneath the branch being tapped. Biocontrol agents thus tapped off the foliage 

can then be gathered directly using an aspirator or sorted later using a sorting 

tray and aspirator (see below). Avoid disturbing the gorse or broom before 

tapping because this will often cause beetles to fly away. While this method is 

most commonly used for Bruchidius villosus, Exapion fuscirostre, and E. ulicis, 

it can also be useful for collecting Sericothrips staphylinus (established only 

in Hawaii). Bruchidius villosus is quick to fly when warm, which can make 

collections challenging. It is best to collect in cool temperatures (e.g., early 

morning), and keep the beat sheet in the shade while aspirating.

Sweep netting: A sweep net consists of a conical canvas or muslin bag held open 

on one end by a sturdy wire hoop 10-15 inches (25-38 cm) in diameter attached 

to a handle 3 feet (0.9 m) long (Figure 4-6c). They can be purchased from 

entomological, forestry, and biological supply companies, or you can construct 

them yourself. Sweeps are made by swinging the net through the plant canopy 

and collecting insects off the foliage. It is best to use no more than 25 sweeps 

(10 sweeps for delicate biocontrol agents such as moths and thrips) before 

removing the biocontrol agents from the net. Removing material at regular 

intervals reduces the potential harm that could result from knocking biocontrol 
Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program 83

Biocontrol 

Agent

Biocontrol 

Agent Stage

Target 

Plant Plant Stage Timing Method Notes

Bruchidius 

 villosus 

Broom seed 

 beetle

Adult Scotch 

broom

Flowering; 

seedpods 

forming

April to May

Use a racket to tap 

adults from flowers and 

stems onto a beat sheet 

or tray, then aspirate, or 

tap adults into a sweep 

net and sort beetles in 

cages

Already widespread, but 

should be redistributed 

to Scotch broom 

infestations where not 

currently established; 

most effective biocontrol 

agent

Exapion 

 fuscirostre 

Scotch broom 

 seed weevil

Adult Scotch 

broom

Flowering; 

seedpods 

forming

April to May

Use a racket to tap 

adults from flowers and 

stems onto a beat sheet 

or tray, then aspirate, or 

tap adults into a sweep 

net and sort beetles in 

cages

Already moderately 

abundant and (often) 

limited impact makes 

it a lower priority for 

redistribution compared 

to B. villosus

Exapion ulicis 

Gorse seed 

 weevil

Adult Gorse

Flowering; 

seedpods 

forming

March to 

mid-April

Use a racket to tap 

adults from flowers and 

stems onto a beat sheet 

or tray, then aspirate, or 

tap adults into a sweep 

net and sort beetles in 

cages

Already widespread and 

limited impact makes 

it a low priority for 

redistribution

Leucoptera 

 spartifoliella 

Scotch broom 

 twig miner

Pupa Scotch 

broom Flowering April to June

Hand collect pupae from 

broom foliage and rear 

in cages

Low impact, already 

widespread, and heavy 

parasitism make it a 

very low priority for 

Adult redistribution Scotch 

broom

Flowering to 

seedpods 

maturing

June to 

August

Use light traps to attract 

adults

Tetranychus 

 lintearius 

Gorse spider 

 mite

All stages Gorse

Throughout 

growing 

season

March to 

October; 

most 

abundant 

August to 

September

Clip infested stems and 

transfer to uninfested 

sites

Heavy predation makes 

this a low priority for 

redistribution

Table 4-3. Recommended timetable and methods for collecting approved gorse and Scotch broom 

biological control agents in North America. Methods are listed in the order of ease of collection. 

Plant and biocontrol agent stages will vary by climate and location.
84 Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program

Biocontrol 

Agent

Biocontrol 

Agent Stage

Target 

Plant

Plant Stage Timing Method Notes

Agonopterix 

 umbellana 

Gorse soft 

 shoot moth

Larva Gorse Seedpods 

maturing; 

seeds 

dispersing

April to May Clip stems infested with 

larvae and transfer to 

uninfested sites

Already moderately 

widespread and limited 

impact makes it a low 

priority for redistribution

Exapion ulicis 

Gorse seed 

 weevil

Adult Gorse Flowering; 

seedpods 

forming

November to 

February

Use a racket to tap 

adults from flowers 

and stems onto a beat 

sheet or tray, then 

aspirate, or collect 

using a sweep net

Already widespread 

and limited impact 

makes it a low priority 

for redistribution

Sericothrips 

 staphylinus 

Gorse thrips

Adults and 

Nymphs

Gorse All stages Most 

abundant 

March to 

November

Use a racket to tap 

adults and nymphs 

from stems and flowers 

onto a white beat sheet 

or tray, then aspirate, or 

clip infested stems and 

transfer to uninfested 

sites

If low observed impact 

is due to predation, 

then low priority for 

redistribution; otherwise 

could be redistributed 

to Maui and other sites 

on Hawaii Island 

Tetranychus 

 lintearius 

Gorse spider 

 mite

All stages Gorse All stages Throughout 

the year

Clip infested stems and 

transfer to uninfested 

sites

Heavy predation makes 

this a low priority for 

redistribution

Table 4-4. Recommended timetable and methods for collecting established gorse biological control 

agents in Hawaii. Methods are listed in the order of ease of collection. Plant and biocontrol agent stages 

will vary by location.

Figure 4-6. Gorse and Scotch broom collection methods: a. racket and beat sheet; b. racket and tray; c. sweep net. 

(a.,b. Eric Coombs, Oregon Department of Agriculture, bugwood.org; c. Laura Parsons, University of Idaho, bugwood.org)

a b c
Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program 85

agents around with debris, and reduces the opportunity for predator insects and 

spiders swept up with the biocontrol agents from incapacitating or devouring the 

biocontrol agents. 

Because Scotch broom and gorse plants are stiff and large, it can be difficult 

and inefficient to sweep the plants for adults. Rather, plants can be tapped (as 

mentioned in the section above) to collect adults in the sweep net and sorted later 

in a controlled environment. A large amount of flowers, other insects, and spiders 

are also collected when tapping material into a sweep net. This material can be 

transferred into other bags (cloth or plastic) and kept in a cool environment until 

it can be sorted through to retrieve the biocontrol agents. Using sieves to sort 

through the largest material can be helpful. Cages are recommended for sorting 

Bruchidius villosus since they are quick to fly when warm and can become 

unmanageable. The presence of spiders creating webs and the honeydew from 

the broom psyllid can damage or kill biocontrol agents, so it is important to sort 

through the material quickly. This method is good for large scale collections but 

may be more labor intensive. This method can be useful for B. villosus, 

Exapion fuscirostre, E. ulicis, and Sericothrips staphylinus.

Aspirating: An aspirator is a device used to suck biocontrol agents from a 

surface into a collection vial. Aspirators can be used to collect insects out of 

a sweep net or cage or off a sorting tray or sheet (see below). A variety of 

aspirators can be purchased from entomological, forestry, and biological supply 

companies, or you can construct one yourself. Simple aspirators are powered 

by mouth suction, manually by using an aspirating bulb, or mechanically using 

a modified hand vacuum. Mouth-powered aspirators contain rubber tubing for 

inhaling (Figure 4-7a) and an insect tube for collecting insects (Figure 4-7b) into 

a storage vial. Inline filters (e.g. HEPA filters, Figure 4-7c) are commercially 

available to prevent unintentional inhalation or swallowing of particles or debris 

during mouth aspiration. At the very least, mouth aspirators should be equipped 

with fine-mesh screening on the vial end of the tubing held in the mouth 

(Figure 4-7d) so that insects and small particles are not inhaled. 

Figure 4-7. Aspirator 

components: a. suction 

tube; b. insect tube; 

c. fine particle filter; 

d. larger particle screen. 

(Jennifer Andreas, 

Washington State 

a University Extension)

b c

d
86 Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program

Sorting: The racket/beat sheet and sweep net collection methods are not 

selective, so other insects and spiders are usually collected along with the 

biocontrol agents. Sorting separates the biocontrol agents from unwanted 

organisms and debris (such as unapproved insects/mites, predators, or weed 

seeds) collected along with the biocontrol agents. The easiest way to sort is to 

empty the contents of the net or tray into a plastic tub and aspirate the biocontrol 

agents out of the debris. If the collected material is first chilled in a cooler, the 

biocontrol agents will move more slowly and will be easier to catch and sort. 

Bruchidius villosus and Exapion fuscirostre adults both move very quickly so 

cages are often the most efficient environments for sorting (Figure 4-8a,b). 

Use an aspirator (described above) to sort the biocontrol agents. To speed up 

the sorting process, count out a set number of biocontrol agents 2-5 times (for 

example, 200 or 500 adult Bruchidius villosus beetles) into separate collecting 

vials. Tap the bottom of vial to knock down all the beetles, then mark the fill 

level on the vial. Use the average fill level based on those 2-5 collection vials to 

collect approximate release-size densities of biocontrol agents without needing 

to count out each individual insect. Remember that adults of the various gorse 

and broom biocontrol species are different in size, so different release density fill 

levels should be used for each species.

Transferring infested plant material: This method is applicable for the mite 

Tetranychus lintearius, the moth Agonopterix umbellana (established only in 

Hawaii), and the thrips Sericothrips staphylinus (established only in Hawaii). 

Gorse stems infested either with the mite, thrips, or larvae of A. umbellana can 

be clipped (Figure 4-9a), stored in a breathable but sealable container (described 

further on page 90), and moved to new sites where the mite, moth, or thrips are 

not yet present. Care should be taken not to spread gorse seeds to new sites as 

this may introduce new genetic material. Care should also be taken to avoid 

spreading other plant or insect species to new sites as this may inadvertently 

create future problems.

Figure 4-8. Sorting gorse and Scotch broom biocontrol agents: a. in a cage with a mouth-suction aspirator; b. in a cage with a 

mechanical aspirator. (a. Jennifer Andreas, Washington State University Extension; b. Sharlene E. Sing, USDA FS RMRS)

a b
Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program 87

Light traps: Light traps are used to collect nocturnal biocontrol agents (typically 

moths) that are otherwise difficult to collect during the day. This method is not 

often used for collecting Leucoptera spartifoliella, but can be used for adults. 

Construct a wire or wooden framework to support a battery-operated lantern and 

beneath it a large funnel (with a wide enough opening for large insects) that rests 

inside a wide-mouth jar with Scotch broom plant material in the bottom 

(Figure 4-9b). Place it in a sheltered place near a Scotch broom infestation. Start 

the light at dusk, and empty it in the morning. Alternatively, prop up a white 

sheet to serve as a reflecting surface, and place a lantern in front of it on a stool. 

Hand-collect the moths attracted to the sheet as they land on the surface. Many 

similar-looking moths may be attracted with this method, so it is important all 

moths are properly identified as L. spartifoliella before being transferred.

Methods by species

Gorse soft shoot moth (Agonopterix umbellana, established only in Hawaii):

Gorse stems infested with A. umbellana can be gathered during early spring when 

larvae are active on gorse stems, shoots, and spines. Depending on location, the 

collecting period is generally from March to May. Stems should be contained in a 

sealable but breathable container (described further in the next section) and then 

transferred to new gorse sites where they should be placed in direct contact with 

uninfested gorse stems (taking care not to spread gorse seeds to new sites as this 

may introduce new genetic material). Once at the new sites, the moth larvae will 

relocate to living, uninfested stems.

a b

Figure 4-9. Gorse and Scotch broom collection methods: a. clipping and collecting gorse 

stems infested with the gorse spider mite, Tetranychus lintearius; b. light trap for collecting 

adult moths. (a. Eric Coombs, Oregon Department of Agriculture, bugwood.org; 

b. Jerry Payne, USDA ARS, bugwood.org)
88 Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program

Broom seed beetle (Bruchidius villosus): Collect adult beetles by tapping stems 

over beat sheets or trays with tools such as rackets to dislodge the insects, or by 

sweeping the ends of broom stems and branches. Collection material should then 

be sorted as soon as possible and the adult B. villosus aspirated. Collect the seed 

beetles during cool temperatures, usually early in the morning. The seed beetle 

is quick to fly when warm and will be challenging to aspirate. A cage is the most 

efficient sorting environment, and first chilling the collected material may make 

the insects move more slowly. Do not collect during and immediately after rain 

events. Water sitting on the flowers will soak the beat sheet or sweep net and 

make sorting the seed beetle difficult. Adults can also drown if too much water 

is present. The optimal time to collect is in early spring when Scotch broom 

flowers; depending on location, the collecting period is generally from April to 

May. This beetle is already widespread throughout much of northwestern North 

America, but it should be redistributed to Scotch broom sites where it is not 

already established. 

Scotch broom seed weevil (Exapion fuscirostre): Collect adult beetles by 

tapping stems over beat sheets or trays with tools such as rackets to dislodge 

the insects, or by sweeping the ends of broom stems and branches. Collection 

material should then be sorted as soon as possible and the adult E. fuscirostre

aspirated. Collect the seed weevils during cool temperatures, usually early in 

the morning. While E. fuscirostre is not as quick to fly as B. villosus, it can still 

move quickly when warm and will be challenging to aspirate. A cage is the 

most efficient sorting environment, and first chilling the collected material may 

make the insects move more slowly. Do not collect during and immediately after 

rain events. Water sitting on the flowers will soak the beat sheet or sweep net 

and make sorting the seed weevil difficult. Adults can also drown if too much 

water is present. The optimal time to collect is in early spring when Scotch 

broom flowers; depending on location, the collecting period is generally from 

April to May. This weevil is already moderately widespread throughout much 

of northwestern North America but has limited overall impact to Scotch broom 

populations. Consequently, it is a low priority for redistribution. 

Gorse seed weevil (Exapion ulicis): Collect adult beetles by tapping stems 

over beat sheets or trays with tools such as rackets to dislodge the insects, or 

by sweeping the ends of gorse stems and branches. Collection material should 

then be sorted and the adult E. ulicis aspirated. Collect the seed weevils during 

cool temperatures, usually early in the morning. The optimal time to collect is in 

early spring when gorse flowers; depending on location, the collecting period is 

generally from February to March. This beetle is already widespread throughout 

much of the northwestern United States but has limited overall impact to gorse 

populations. Consequently, it is a low priority for redistribution. 
Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program 89

Scotch broom twig miner (Leucoptera spartifoliella): Because larvae are 

heavily parasitized, it is best to collect this biocontrol agent in either the pupal 

or adult stage. Pupae can be hand-collected in late spring (mid-April to mid-June 

depending on location) and reared out in cages or in breathable, clear containers. 

Any parasitoids that emerge should be separated and destroyed. Emerging adults 

can then be safely transferred to new Scotch broom patches. Alternatively, light 

traps can be used to trap the nocturnal adults. The optimal time to collect is in 

summer when Scotch broom is flowering to seedpod maturation; depending on 

location, the collecting period is generally from June to August. This species is 

already widely distributed throughout the northwestern United States with limited 

impact, so it is a low priority for redistribution. 

Gorse thrips (Sericothrips staphylinus, established only in Hawaii): Collect 

all stages by tapping stems over beat sheets or trays with tools such as rackets 

to dislodge the insects, or by sweeping the ends of gorse stems and branches. 

Breathing on stems may increase collection success as CO2 will cause the thrips 

to drop from places where they may have been hiding. Collection material should 

then be sorted and the S. staphylinus aspirated. Alternatively, clip gorse stems 

infested with S. staphylinus, place them in a sealable but breathable container 

(described further in the next section), and then transfer them to new gorse 

sites where they should be placed in direct contact with uninfested gorse stems 

(taking care not to spread gorse seeds to new sites as this may introduce new 

genetic material). The downfall with the stem collection method is not knowing 

the number of thrips being transferred to the new site. Collect the thrips during 

the heat of the day. The optimal time to collect is during the warmest months 

of the growing season when all stages of the thrips are most active; depending 

on location, the collecting period is generally from March to November. This 

biocontrol agent was initially widespread in Hawaii, but populations have since 

decreased, and very little impact to gorse populations has been observed. If 

the population reduction is due to predation, this species is a low priority for 

redistribution. Otherwise, the biocontrol agent should be redistributed throughout 

Maui and Hawaii Island. 

Gorse spider mite (Tetranychus lintearius): Gorse stems infested with 

T. lintearius can be gathered throughout the growing season. Depending on 

location, the collecting period is generally from March to November. Tetranychus 

lintearius is heavily impacted by predatory mites and beetles, and care should be 

taken to ensure predators are not transferred along with T. lintearius. Weak 

T. lintearius webbing indicates predators are likely present. Collected stems 

should be contained in a sealable but breathable container (described further in 

the next section) and then transferred to new gorse sites where they should be 

placed in direct contact with uninfested gorse stems (taking care not to spread 

gorse seeds to new sites as this may introduce new genetic material). Once at the 

new sites, the mites will relocate to living, uninfested stems.
90 Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program

Release Containers for Gorse and Scotch Broom 

Biological Control Agents

The manner in which biological control agents are handled during transportation 

to the release site will affect whether they will survive and multiply at the new 

site. To reduce mortality or injury, it is best to redistribute the biocontrol agents 

the same day they are collected.

Following collection, biocontrol agents should be transferred to release 

containers intended to protect them (and to prevent insects and mites from 

escaping en route). When large sections of infected stems are transferred between 

sites to redistribute Tetranychus lintearius or Agonopterix umbellana (established 

only in Hawaii), the stems should be stored in large, breathable bags made of 

paper or gauze. Paper and gauze bags provide sufficient ventilation while plastic 

bags may cause moist plant material to rot or drown the biocontrol agents. 

When only smaller infected plant segments are used in the transfer of the beetles 

Bruchidius villosus, Exapion fuscirostre, and E. ulicis, the moth Leucoptera 

spartifoliella, or the thrips Sericothrips staphylinus, release containers should be 

rigid enough to resist crushing but also ventilated to provide adequate airflow 

and reduce condensation. Un-waxed paperboard cartons are ideal; they are 

rigid, permeable to air and water vapor, and are available in many sizes. As an 

alternative, you can use release containers made of either light-colored lined or 

waxed paper (e.g. ice cream cartons are particularly suitable; see Figure 4-10a)

or plastic, providing they are ventilated; simply poke numerous holes in the 

container or its lid with an ordinary push pin or thumb tack, and cover the holes 

with a fine mesh screen. Be sure the holes are not large enough to allow the 

biocontrol agents to escape. Untreated paper bags (lunch bags) work well for 

transporting biocontrol agents short distances; however, they are fragile and offer 

little physical protection for the material within, must be sealed tightly to prevent 

biocontrol agents from escaping, and some biocontrol agents are capable of 

chewing through them. Do not use glass or metal release containers; they are 

breakable and make it difficult to regulate temperature, airflow, and humidity.

Fill release containers half full with loosely crumpled paper towels or tissue 

paper to provide a substrate for biocontrol agents to rest on and hide in, and to 

help regulate humidity. Include a small amount of Scotch broom or gorse sprigs, 

depending on the biocontrol agent’s preferred host. Sprigs should be free of 

seeds, flowers, dirt, spiders, and other insects and should not be placed in water 

in the release container. Seal the release container lids with masking or label tape 

or with tightly fitting rubber bands. If you are using paper bags, fold over the tops 

several times and staple them shut. Be sure to label each container with (at least) 

the biological control agent(s) name, the number of biological control agents in 

the container, the collection date and site, and the name of the person(s) who did 

the collecting (Figure 4-10b).
Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program 91

Figure 4-10. Cardboard release containers: a. for transporting gorse and Scotch broom biocontrol agents; b. properly labelled. 

(a. Martin Moses, University of Idaho, bugwood.org; b. Jennifer Andreas, Washington State University Extension)

Transporting Gorse and Scotch Broom 

Biological Control Agents

Keep the containers cool at all times 

Once you collect and package the biocontrol agents, maintain them at 

temperatures between 50 and 65 °F (10-18 °C). If possible, place the release 

containers in large coolers equipped with frozen ice packs. Do not use ice cubes 

unless they are contained in a separate, closed, leak-proof container. Wrap the 

ice packs in crumpled newspaper or bubble wrap to prevent direct contact with 

release containers and to absorb any condensation that forms. Place extra packing 

material in coolers to prevent ice packs from shifting and damaging biocontrol 

agent containers. As an alternative to coolers with ice packs, electric car-charged 

coolers may be utilized, provided the cycle is set to cool and not warm. Always 

keep coolers out of direct sun, and only open them when you are ready to release 

the biocontrol agents. If you cannot release them immediately, place them in 

a refrigerator for short-term storage (no lower than 40 °F [4.4 °C]) until you 

transport or ship them (which should occur as soon as possible and preferably not 

longer than 48 hours).

Transporting short distances 

If you can transport your biocontrol agents to their release sites within 3 hours 

after collection, and release them the same day or early the next, you need not 

take any measures other than those already described.

a b
92 Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program

Shipping long distances

If you will be shipping your biocontrol agents to their final destination, use a 

bonded carrier service with guaranteed overnight delivery (e.g., USPS, FedEx, 

UPS, or DHL) and send the recipient the tracking number for the package. 

In such cases, the release containers should be placed in insulated shipping 

containers with one or more ice packs. Some specially designed foam shippers 

have pre-cut slots to hold small biocontrol agent containers and ice packs 

(Figure 4-11). This construction allows cool air to circulate but prevents direct 

contact between the ice and the release containers. Laboratory and medical 

suppliers sell foam “bioshippers” that are used to transport medical specimens 

or frozen foods. If neither foam product is available, you can use a heavy-duty 

plastic cooler which also may be better suited to large gorse or broom stems 

infected with Tetranychus lintearius or Agonopterix umbellana (established only 

in Hawaii). Please note that for safety reasons, dry ice cannot be used for 

transporting biocontrol agents.

Careful packaging is very important regardless of the shipping container you use. 

Ice packs need to be wrapped in crumpled newspaper, wrapping paper, or bubble 

wrap, and should be firmly taped to the inside walls of the shipping container to 

prevent them from bumping against and possibly crushing the release containers 

during shipping. Empty spaces in the shipping container should be loosely filled 

with crumbled or shredded paper, bubble wrap, packing “peanuts,” or other soft, 

insulating material. Use enough insulation to prevent release containers and 

ice packs from shifting during shipment, but not so much that air movement is 

restricted. Enclose all paperwork accompanying the biocontrol agents (including 

copies of permits and release forms) before sealing the shipping container. For 

additional security and protection, you may place the sealed shipping containers 

or coolers inside cardboard boxes.

Figure 4-11. Commercially 

made shipping container. 

(University of Idaho, 

bugwood.org)
Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program 93

Avoiding Common Packaging Mistakes

Crushing: Secure all material included in the shipping container so that blue ice, 

bundles of plant material, etc., do not become loose and move around in transit 

thereby crushing, tearing, or popping open release containers and killing or 

scattering the biocontrol agents inside.

Escape: Close release containers lids securely with rubber bands or easily 

removable/resealable tape (e.g., masking tape) to prevent biocontrol agents 

from escaping into the shipping container.

Excess heat: Do not expose release containers to direct sunlight or temperatures 

above 65 °F (18 °C). Avoid shipping delays that can expose biocontrol agents to 

high temperatures.

Excess moisture: Remove spilled or excess water in release and shipping 

containers. Do not ship weed sprigs with any type of water source (e.g., floral 

foam or tubes) inside release containers. Add crumpled paper towels to release 

containers to absorb incidental moisture or condensation.

Lack of ventilation: Provide adequate ventilation; use air-permeable release 

containers or make air holes in plastic containers with push pins or other small 

diameter tools, covering the holes with a fine mesh screen to prevent the 

escape of mobile biocontrol agents. 

Starvation: Provide sufficient food, and do not store release containers with 

biological control agents more than 48 hours. 

Stress: Provide root-, flower- and seed-free sprigs of the target weed (free also of 

other weed species’ seeds, flowers, dirt, spiders, or other insects) and crumpled 

paper towels where biocontrol agents can shelter; avoid over-crowding.

Other factors to consider

• Make your overnight shipping arrangements well before you collect 

your biological control agents, and make sure the carrier you select can 

guarantee overnight delivery.

• Plan collection and packaging schedules so that overnight shipments can 

be made early in the week. Avoid late-week shipments that may result in 

delivery on Friday through Sunday, potentially delaying release of the 

biocontrol agents for several days.

• Clearly label the contents of containers and specify that they contain 

perishable material.

• Check with a prospective courier to make sure that they can accept this 

type of cargo and will not treat the packages in ways that could harm 

the biological control agents. If the courier cannot guarantee that such 

treatments will not occur, choose a different carrier.

• Contact personnel at the receiving end, tell them what you are shipping and 

when it is due to arrive, provide a tracking number, verify that someone 

will be there to accept the shipment, and instruct them to open the package 

and place the release containers directly into a refrigerator until the 

biocontrol agents can be released (as soon after receipt as possible).
94 Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program

Purchasing Gorse and Scotch Broom 

Biological Control Agents

While gorse and Scotch broom biocontrol agents are currently not available 

commercially, suppliers may provide these species in the future. At that time, 

county weed managers, extension educators, or university weed or biological 

control specialists may be able to recommend one or more suppliers. Make 

sure that a prospective supplier is reputable, can provide healthy individuals 

of the species you want (parasite- and pathogen-free), and can deliver them to 

your area at a time appropriate for field release (you will want to know where 

and when the biocontrol agents were collected). Avoid purchasing biocontrol 

agents from a supplier who collects biocontrol agents from an environment 

significantly different from your planned release location. Interstate shipments of 

gorse and broom biological control agents by commercial suppliers also require 

a USDA PPQ 526 Permit (see page 99), a copy of which should be enclosed in 

the shipping box. It is the responsibility of the person receiving and releasing 

biocontrol agents to secure the required permits, though some vendors will help 

buyers with this process. Confirm in advance that there is a permit in place for 

the species you are acquiring as well as the region in which the release will 

occur. DO NOT purchase or release unapproved or non-permitted biological 

control organisms. Note that before any biocontrol agents can be taken across 

national borders, whether collected or purchased, an importation permit from the 

regulatory agency of the receiving country is required (USDA-APHIS in the USA 

and CFIA in Canada).

Releasing Gorse and Scotch Broom Biological Control Agents

Establish permanent location marker 

Place a steel fence post or plastic/fiberglass pole as a marker at the release point 

(Figure 4-12a). Because gorse and Scotch broom can both grow tall, 13-foot 

(4 m) markers should be used wherever possible. Avoid wooden posts; they are 

vulnerable to weather and decay. Markers should be colorful and conspicuous. 

White, bright orange, pink, and red are preferred over yellow and green, which 

may blend into surrounding vegetation. In addition, white posts will not fade over 

time. Where conspicuous posts may encourage vandalism, mark your release 

sites with short, colorful plastic tent/surveyor’s stakes or steel plates that can 

be tagged with release information and located later with a metal detector and 

GPS. Depending on the land ownership or management status at the release site, 

it may be necessary to attach a sign to the post or pole indicating a biological 

control release has occurred there and that the site should not be sprayed with 

chemicals or be mechanically disturbed (see Figure 4-4 on page 76). Where a 

sign is appropriate, the landowner/land manager and the local weed management 

authority (county, state, federal, and/or provincial) should be notified and given a 

map of the release location.

Record geographical coordinates at release point using GPS 

Map coordinates of the site marker should be determined using a global 

positioning system device (GPS) or a GPS-capable tablet/smartphone. There 
Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program 95

are numerous free apps available for recording GPS coordinates on a tablet/

smartphone (Figure 4-12b). Coordinates should complement but not replace 

a physical marker. Accurate coordinates will help re-locate release points if 

markers are damaged or removed. Along with the coordinates, be sure to record 

what coordinate system and datum you are using, e.g. latitude/longitude in 

WGS 84 or UTM in NAD83.

Prepare map 

The map should be detailed and describe access to the release site, including 

roads, trails, and unique landmarks/terrain features that are not likely to change 

through time (e.g., large rocks or rocky outcrops, creeks, valleys, etc.). Avoid 

using ephemeral landmarks such as “red bush”, “grazing cows”, etc. and 

descriptors which may not be obvious to everyone, such as “the Miller place”, or 

“where the old barn used to be”, etc. Use your vehicle’s trip odometer to measure 

and record mileage between specified locations on your map, e.g., when you 

turn on to a new road, at cattle guards along the route, and where you park. The 

map should complement but not replace a physical marker and GPS coordinates. 

Maps are especially useful for long-term biological control programs in which 

more than one person will be involved or participants are likely to change. Maps 

are often necessary to locate release sites in remote locations or places physically 

difficult or confusing to access.

a b

Figure 4-12. Biocontrol agent release site tools: a. permanent marker; b. smartphone with free weed and biocontrol agent 

mapping app iBioControl. (a. Jennifer Andreas, Washington State University Extension; b. Rachel Winston, MIA Consulting)
96 Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program

Complete relevant paperwork at site 

Your local land management agency/authority may have standard biocontrol 

agent release forms for you to complete. Typically, the information you provide 

includes a description of the site’s physical location, including GPS-derived 

latitude, longitude, and elevation; a summary of its biological and physical 

characteristics and land use; the name(s) of the target weed and biocontrol 

agent(s) released; the number and life cycle stage of the agent(s) released; date 

and time of the release; weather conditions during the release; and the name(s) 

of the person(s) who released the biocontrol agents (see Sample Biological 

Control Agent Release Form in Appendix II). The best time to record this 

information is while you are at the field site. Consider using a smartphone and 

reporting app such as iBioControl. This free application uses EDDMapS (see 

page 100 for more information) to help county, state, and federal agencies track 

releases and occurrences of biological control agents of noxious weeds. Once 

back in the office, submit the information to your local weed control office, land 

management agency, or other relevant authority/database. Always keep a copy 

for your own records.

Set up photo point

A photo point is used to visually document changes in gorse and Scotch broom 

infestations and other components of the plant community over time following 

the release of biocontrol agents. Use a permanent feature in the background as a 

reference point (e.g., a mountain, large rocks, trees, or a permanent structure) and 

make sure each photo includes your release point marker. Pre- and post-release 

photographs should be taken from roughly the same place and at the same time 

of year. Label all photos with the year and location; many smartphone and tablet 

apps such as GrassSnap or Theodolite do this automatically or with minimal 

input. Keep in mind that it may take a long time (e.g., 30 years) to see changes 

in Scotch broom or gorse populations because available biocontrol agents do not 

significantly impact current broom and gorse plants; rather, they impact future 

infestations by reducing seed production.

Release as many biocontrol agents as possible 

As a general rule of thumb, it is better to release many individuals of a biocontrol 

agent species at one gorse or Scotch broom infestation than it is to spread those 

individuals too thinly over multiple gorse or broom infestations. Releasing all 

the biocontrol agents within a release container in one spot will help ensure 

that adequate numbers of males and females are present for reproduction and 

reduce the risks of inbreeding and other genetic problems. Guidelines for a 

minimum release size are uncertain for most biocontrol agents, but releases of 

100-200 individuals of the beetles Bruchidius villosus, Exapion fuscirostre, and 

E. ulicis, 50-100 individuals of the moth Leucoptera spartifoliella, and 200-300 

individuals of the thrips Sericothrips staphylinus (established only in Hawaii) are 

encouraged; releasing more individuals would be advantageous, but should not 

be necessary.
Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program 97

Often, a single release will be sufficient to establish a biocontrol agent 

population, especially if a large number of individuals are released. The only 

way to determine if biocontrol agents have established is to inspect release 

sites annually for up to 5 years (or more) after releases are made. Additional 

biocontrol agent releases may be necessary after a few years if initial releases 

fail to establish. For species or locations where establishment is likely to be slow 

(e.g., sites with high levels of overwintering mortality or large, dense infestations 

where biocontrol agents are easily missed), planning to make releases on the 

same site for 2 or 3 consecutive years may increase successful establishment 

and reduce the time until biocontrol agent impact on target weed populations 

is visible. If more than one release of a biocontrol agent is available in a given 

year, be sure to put some distance between releases; 2/3 mile (1 km) is ideal. If 

possible, make more than one release per drainage or in adjoining drainages; if 

one of your release sites is wiped out by flooding, fire, herbicide application or 

other catastrophic disturbance, then biocontrol agents from adjoining release sites 

can repopulate it. 

In general, you can release biocontrol agents either in open releases or cages. 

For open releases, get to the desired release location and open the release 

container. When releasing adult Bruchidius villosus, Exapion fuscirostre, 

E. ulicis, Leucoptera spartifoliella, or Sericothrips staphylinus (established 

only in Hawaii), gently shake out all biocontrol agents in a dense lower section 

of a single plant. Placing the biocontrol agents on their host plant and a paper 

towel gives them a place to acclimate to their new temperature and surroundings 

without falling to the ground. Take care to dislodge any individuals hiding in 

or clinging to the paper towels in the release containers. Because B. villosus

takes to flight so rapidly, it is beneficial to release in cool temperatures or in the 

shade. When releasing small gorse or broom segments infested with Tetranychus 

lintearius, S. staphylinus, or Agonopterix umbellana larvae (the latter two 

established only in Hawaii), first ensure the segments have no gorse or broom 

root fragments or seeds and that there are no other insect or plant species in the 

release containers. Gently shake out all infested plant segments in one small 

area. Do not scatter biocontrol agents or small plant segments throughout 

the infestation. Do not walk back through the area where you just made a 

release.

When transferring large stem segments infested with Tetranychus lintearius, 

Sericothrips staphylinus, or Agonopterix umbellana larvae (the latter two 

established only in Hawaii), take bundles of 20-50 stems and remove the ties 

on one end of each bundle so that stems can be fanned out at the loose end, 

providing a supportive base. Place the fanned bundles upright within dense 

stands of uninfested gorse or Scotch broom. In less dense infestations or at windy 

locations, tying the fanned bundle against uninfested gorse or broom may aid in 

successful establishment. Four to five bundles should be used per site, though 

more or fewer may be required, depending on the infestation size. Care should 

be taken not to spread gorse seeds to new sites as this may introduce new genetic 

material. Care should also be taken to avoid spreading other plant or insect 

species to new sites as this may inadvertently create future problems.
98 Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program

Caged releases confine biocontrol agents for a period of time so they adjust to the 

site and easily find one another. They may help increase establishment success 

at new locations, but they require you to put up and take down equipment. For 

caged releases, place a mesh bag over a gorse or Scotch broom plant or a small 

area containing multiple plants (Figure 4-13), release biocontrol agents into the 

cage, and secure the bottom of the cage to either the stem or the ground. Cages 

should be removed within a few days (for plants) or weeks (for areas).

Releases of all biocontrol agents should be made under moderate weather 

conditions (mornings or evenings of hot summer days, mid-day for cold season 

releases). Making releases under these conditions reduces the immediate 

dispersal of stressed biocontrol agents when they are dumped out of release 

containers; appropriately timed releases can significantly enhance the probability 

of establishment. Avoid making releases on rainy days. If you encounter an 

extended period of poor weather, it is better to release the biocontrol agents 

than wait three or more days for conditions to improve as the biocontrol agents’ 

vitality may decline with extended storage. Avoid transferring biocontrol agents 

to areas with obvious ant mounds or ground dwelling animals that may prey upon 

some species of biocontrol agents.

Figure 4-13. Caged releases on gorse. (Eric Coombs, Oregon Department of Agriculture, bugwood.org)
Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program 99

Regulations for the Transfer of Gorse and Scotch Broom 

Biological Control Agents

USA, intrastate: Generally, there are few if any restrictions governing the collection 

and shipment of approved biological control agents within the same state; 

however, you should check with your state’s department of agriculture or 

agriculture extension service about regulations governing the release and 

intrastate transport of your specific biological control agent. The state of 

California regulates release permits at the county level. It is illegal to redistribute 

unapproved species in the USA.

USA, interstate: The interstate transportation of biological control agents is 

regulated by the U.S. Department of Agriculture (USDA), and a valid permit 

is required to transport living biological control agents across state lines. You 

should apply for a Plant Protection and Quarantine (PPQ) permit from the 

Animal and Plant Health Inspection Service (APHIS) as early as possible—but 

at least six months before actual delivery date of your biological control agent. 

You can check the current status of regulations governing intrastate shipment 

of weed biological control agents, PPQ Form 526 at the USDA-APHIS-PPQ 

website. The ePermit process can be accessed by doing an internet search 

for “USDA APHIS 526 permit application”. This allows the complete online 

processing of biological control agent permit requests. It is illegal to redistribute 

unapproved species across state lines in the USA.

Canada: Canada requires an import permit for any new biological control agent or 

shipment of previously-released biocontrol agents entering the country. These 

permit requests are reviewed and issued by the Plant Health Division of the 

Canadian Food Inspection Agency. Redistribution within a province (or within 

Canada) of weed biological control agents that have been officially approved for 

release in Canada is not prohibited; however, you should consult with federal 

and provincial authorities and specialists prior to moving any weed biological 

control agent, especially across ecozones (e.g., from the prairies to the interior 

or coast of British Columbia). Similarly, you should consult with appropriate 

experts when considering the movement of adventive biocontrol agents that 

have become established in a region, or native organisms that may feed on a 

weed targeted for control.

The Need for Documentation 

The purpose of monitoring is to evaluate the success of your gorse and broom 

biological control program and to determine if you are meeting your weed 

management goals. Documenting outcomes (both successes and failures) of 

biocontrol release programs will help generate a more complete picture of 

biocontrol impacts, guide future management strategies, and serve education and 

public relations functions. Monitoring can provide critical information for other 

land managers by helping them predict where and when biological control might 

be successful, helping them avoid releasing ineffective biocontrol agents or the 

same biocontrol agent in an area where they were previously released, and/or 

helping them avoid land management activities that would harm local biocontrol 

agent populations or worsen the gorse and broom problem. (See the Code of Best 

Practices for Classical Biological Control of Weeds on page 9.)

Documenting, 

Monitoring, and 

Evaluating a 

Biological Control 

Program
100 Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program

Monitoring activities utilize standardized procedures over time to assess changes 

in populations of the biocontrol agents, gorse and broom, other plants in the 

community, and other components of the community. Monitoring can help 

determine:

• If the biological control agents have become established at the release site

• If biological control agent populations are increasing or decreasing and 

how far they have spread from the initial release point

• If biological control agent populations are sufficiently abundant to allow 

for collection and redistribution

• If the biological control agents are having an impact on gorse and broom

• If/how the plant community or site factors have changed over time

Monitoring methods can be simple or complex. A single year of monitoring may 

demonstrate whether the biocontrol agents established, while multiple years of 

monitoring may allow you to identify trends in the population of the biocontrol 

agents, changes in the target weed population and plant community, and changes 

in other factors such as climate or soil.

Information Databases

Many federal and state/provincial departments have electronic databases for 

archiving information about weed biological control releases. We have included a 

standardized biological control agent release form that, when completed, should 

provide sufficient information for inclusion in any number of databases (see 

Appendix II).

Biological Control of Weeds: A World Catalogue of Agents 

and their Target Weeds (database)

The USDA Forest Service (in conjunction with the University of Georgia, 

MIA Consulting, University of Idaho, CAB International, and the Queensland 

Government) also maintains a worldwide database for the Biological Control 

of Weeds: A World Catalogue of Agents and their Target Weeds. The database 

includes entries for all weed biocontrol agents released to date, including the 

year of first release within each country, the biocontrol agents’ current overall 

abundance and impact in each country, and more. This database can be accessed 

at www.ibiocontrol.org/catalog/.

EDDMapS

EDDMapS (Early Detection & Distribution MAPping System) is a web-based 

mapping system increasingly being used for documenting invasive species as 

well as biocontrol agent distribution in North America. EDDMapS combines 

data from existing sources (e.g. databases and organizations) while soliciting 

and verifying volunteer observations, creating an inclusive invasive species 

geodatabase that is shared with educators, land managers, conservation 

biologists, and beyond. Information can be added in online forms through 

home computers and/or apps created for smartphones. For more information on 

how to utilize or contribute to these tools, visit www.eddmaps.org/about/ and 

apps.bugwood.org/.
Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program 101

In addition, some states/provinces have county/district weed departments 

or employ weed biocontrol specialists, often affiliated with state/province 

departments of agriculture, county extension offices, or Animal and Plant Health 

Inspection Service Plant Protection and Quarantine (APHIS-PPQ) offices. 

Contact local entities for more information.

Monitoring Methods 

There are three main components to measure in a gorse and broom monitoring 

program: biological control agent populations, gorse and broom populations, 

and the rest of the plant community (including nontarget plants). More detailed 

monitoring might also examine effects on other biotic community components 

(such as other insects, birds, mammals, etc.) or abiotic factors (such as erosion, 

soil chemistry, etc.). Only the three main monitoring components are discussed 

in this manual. While most established biocontrol agents attack either gorse 

or Scotch broom, Bruchidius villosus also attacks French broom. Because the 

monitoring methods described herein could apply to all three weed species, the 

generic “broom” is used throughout this monitoring section to encompass both 

Scotch and French broom.

Assessing biological control agent populations

If you wish to determine whether or not biocontrol agents have established after 

initial release, you simply need to find the biocontrol agents in one or more of 

their life stages, or evidence of their presence (Tables 4-5, 4-6). Begin looking 

for biocontrol agents where they were first released, and then expand to the area 

around the release site.

Populations of some biocontrol agents take two or more years to reach detectable 

levels. Thus if no biocontrol agents are detected a year after release, it does 

not mean they failed to establish. Revisit the site at least once annually for 

three years. If no evidence of biocontrol agents is found, either select another 

site for release or make additional releases at the monitored site. Consult with 

your county extension educator or local biological control of weeds expert for 

assistance.

A systematic monitoring approach is required to determine the changing densities 

of biocontrol agent populations. A systematic yet simplified gorse and broom 

biocontrol agent monitoring form can be found in Appendix III. This may be 

modified to meet the needs of each land manager by adding extra columns, 

descriptive classes, etc.

Assessing the status of gorse and broom and co-occurring plants

The ultimate goal of a gorse and broom management program is to permanently 

reduce the abundance of gorse and broom and enable the recovery of more 

desirable vegetation on the site. To determine the efficacy of biocontrol efforts, 

there must be monitoring of plant community attributes, such as target weed 

distribution and density. Ideally, monitoring begins before biological control 

efforts are started (pre-release) and occurs at regular intervals after release. There 

are many ways to qualitatively (descriptively) or quantitatively (numerically) 

assess weed populations and other plant community attributes at release sites.
102 Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program

Table 4-5. Life stages and damage to look for to determine establishment of gorse and broom biological 

control agents.

Biocontrol Agent

Life 

Stage Where to Look

When to 

Look Damage Appearance

Bruchidius villosus 

Broom seed beetle

Adults

Flowers, foliage, 

on young 

seedpods

Spring to 

summer 

(Apr-Jun)

Do not cause direct damage; eggs 

laid into seedpod wall readily visible; 

attacked seeds are hollowed; tap 

plants on beat sheet to confirm adult 

presence

Larvae Inside 

seedpods

Late springsummer 

(May-Jul)

Feeding damage completely within 

seeds; press thumb nail through 

seed: milky-white juice exudes = 

B. villosus present, no juice = 

B. villosus absent

Exapion fuscirostre 

Scotch broom 

 seed weevil 

Adults

Flowers, foliage, 

on young 

seedpods

Spring to 

summer 

(Apr-Jun)

Stems and flowers with small feeding 

holes or pits, but tap plants on beat 

sheet to confirm adult presence

Larvae Inside 

seedpods

Late 

spring-early 

summer 

(May-Jul)

Feeding damage to external and 

internal seed tissue

Exapion ulicis 

Gorse seed weevil

Adults

Flowers, foliage, 

on young 

seedpods

Early spring 

to summer 

(Mar-Apr)

Stems, flowers, leaves, seedpods 

with small feeding holes, but tap 

plants on beat sheet to confirm adult 

presence

Larvae Inside 

seedpods 

Late 

spring-early 

summer 

(Apr-May)

Feeding damage to external and 

internal seed tissue

Leucoptera 

 spartifoliella 

Scotch broom 

 twig miner

Adults On stems Summer 

(Jun-Aug)

Adults do not cause any direct 

damage to Scotch broom; confirm 

adult presence by using light traps

Larvae Inside stems

SummerSpring 

(Aug-May)

Feeding mines inside and outside 

stems; attacked stems often dead 

above the point of damage

Tetranychus 

 lintearius 

Gorse spider mite

All 

stages

Terminal 

branches

Year-round 

(Aug-Sep 

best)

Extensive webbing covering mite 

colonies; attacked stems are stunted 

and have reduced flowering

Photos and credits: a. B. villosus eggs laid onto seedpod wall; b. B. villosus larva feeding completely within the left seed; c. E. fuscirostre

adult feeding marks on Scotch broom stem (bottom stem); d. E. fuscirostre larva feeding damage in/on seed; e. E. ulicis adult with feeding 

holes on gorse flower; f. E. ulicis larvae and feeding damage on seeds within gorse seedpod; g. L. spartifoliella adult; h. L. spartifoliella cocoon 

and larval mining on Scotch broom stem; i. T. lintearius webbing covering gorse stems with T. lintearius feeding damage. (a.,b. Jennifer 

Andreas, Washington State University Extension; c.,e.,g.,h. Eric Coombs, Oregon Department of Agriculture, bugwood.org; d. Thomas 

Shahan, Oregon Department of Agriculture; f. George Markin, USDA Forest Service, bugwood.org; i. Steven Conaway, Penn State University, 

bugwood.org)

a

b

c

d

e

f

g

h

i
Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program 103

Table 4-6. Life stages/damage to look for to determine establishment of gorse biological control agents 

confirmed present in Hawaii.

Photos and credits: a. A. umbellana adult; b. A. umbellana silken feeding tubes and damage to gorse; c. E. ulicis adult with feeding holes 

on gorse flower; d. E. ulicis larvae and feeding damage on seeds within gorse seedpod; e. S. staphylinus feeding damage on gorse spines; 

f. T. lintearius webbing covering gorse stems with T. lintearius feeding damage. (a. Janet Graham; b. Fritzi Grevstad, Oregon State University; 

c. Eric Coombs, Oregon Department of Agriculture, bugwood.org; d.,e. George Markin, USDA Forest Service, bugwood.org; f. Steven 

Conaway, Penn State University, bugwood.org)

Biocontrol Agent Life Stage Where to Look When to Look

Most Frequently 

Observed Damage Appearance

Agonopterix 

 umbellana 

Gorse soft shoot 

 moth

Adults Among foliage 

at center of shrub

Late summerspring (Jul-Mar)

Adults typically do 

not cause any direct 

damage to gorse

Larvae Stems and foliage Spring 

(Apr-May)

Silken feeding tubes 

covering feeding 

damage to buds, 

shoots, spines

Exapion ulicis 

Gorse seed weevil

Adults

Flowers, foliage, 

on young 

seedpods

Winter 

(Nov-Feb)

Stems, flowers, 

leaves, seedpods 

with small feeding 

holes, but tap plants 

on beat sheet 

to confirm adult 

presence

Larvae Inside seedpods Winter-spring 

(Feb-Mar)

Feeding damage to 

external and internal 

seed tissue

Sericothrips 

 staphylinus 

Gorse thrips

Adults/

Nymphs On stems

Year-round 

(best spring-fall; 

Mar-Nov)

Attacked stem and 

spine tissue mottled 

and blotchy in 

appearance

Tetranychus 

 lintearius 

Gorse spider mite

All stages Terminal branches Year-round

Extensive webbing 

covering mite 

colonies; attacked 

stems are stunted 

and have reduced 

flowering

a

b

c

d

e

f
104 Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program

Qualitative (descriptive) vegetation monitoring

Qualitative monitoring uses subjective measurements to describe gorse and/or 

broom and the rest of the plant community at the management site. Examples 

include listing plant species occurring at the site, estimates of density, age and 

distribution classes, visual infestation mapping (as opposed to mapping with a 

GPS unit), and maintaining a series of photos from designated photo points over 

time. 

Qualitative monitoring provides insight into the status or change of gorse and 

broom populations; however, its descriptive nature does not generally allow for 

detailed statistical analyses. Data obtained in qualitative monitoring may trigger 

more quantitative monitoring later. A qualitative vegetation monitoring example 

applicable to gorse and broom is included in Appendix IV. 

Quantitative (numeric) vegetation monitoring

Quantitative monitoring measures changes in the gorse and/or broom population 

as well as the vegetative community as a whole before and after a biocontrol 

agent release using numbers and statistics. It may be as simple as counting the 

number of gorse and broom plants in a small sample area (Figure 4-15), or as 

complex as measuring gorse and broom plant height and width, flower and seed 

production, biomass, species diversity, and species cover. Quantitative sampling 

data can be more readily analyzed using statistical methods and demonstrate 

significant plant community changes. Pre- and post-release monitoring should 

follow the same protocol and be employed at the same time of year. Post-release 

assessments should be planned annually for at least three to five years after the 

initial biocontrol agent release (and ideally longer than that). 

Figure 4-15. Measuring features of a gorse infestation. (Forest and Kim Starr, Starr 

Environmental, bugwood.org)
Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program 105

Gorse and broom are particularly problematic to monitor for many reasons. 

Their large size and longevity make monitoring individual plants or even patches 

difficult. The most effective approved and established biocontrol agents attack the 

seeds of gorse and broom. This does not hinder existing plants; it only reduces 

their reproductive output. While this may eventually reduce plant recruitment and 

population size, the existing extensive seedbank and the longevity of unattacked 

seeds (30+ years) mean seed reduction impacts might not be visible for 30 years 

or more. Still, diligently monitoring the presence and attack rates of the seedfeeding biocontrol agents for several years (in combination with qualitative gorse 

and broom population monitoring) will help researchers quantify the long-term 

impacts of gorse and broom biological control. See Appendix V for a quantitative 

Scotch broom seedpod monitoring protocol. 

Assessing impacts on nontarget plants

To address possible nontarget attacks on species related to or just growing 

adjacent to gorse and broom, you must become familiar with the plant 

communities present at and around your release sites and be aware of species 

related to gorse and broom. Start by compiling a list of other species in the 

Fabaceae (pea family) that are present at the site. The Fabaceae species most 

closely related to gorse and broom include other exotic broom species (see 

Table 2-1 on page 31 and Table 2-2 on page 32 in addition to other exotic brooms 

in the genera Cytisus and Genista). The most closely related species that are 

native to North America are lupines in the genus Lupinus. There are over 200 

species of lupine, most of which are native to North America. You may need to 

consult with local, state, or regional botanical experts, or review local herbarium 

records for guidance on areas where related nontarget plants might be growing 

and additional information on how you can identify them. 

Care should be taken in the management of your gorse and broom biocontrol 

program to ensure that all closely related native species are identified and 

monitored along with gorse and broom. If lupines are found growing near your 

gorse or broom biocontrol site, wait until most of their flowers have finished 

blooming and their pods begin forming. Place mesh bags over individual lupine 

plants or inflorescences, taking care to stake the bag bottoms to the ground or 

around the stem. When the pods have dehisced, the mesh bags can be searched 

for Bruchidius villosus or Exapion spp. adults. Ensure that all seedpod fragments 

and any seeds recovered are thoroughly checked for feeding damage.

Please be aware that there are many “look-alike” native arthropods that feed on 

related native plants. Correct identification by biocontrol specialists is needed to 

confirm such records. If you observe approved biological control agents feeding 

on and/or developing on nontarget species, collect samples and take them to a 

biocontrol specialist in your area. Alternatively, you may send the specialist the 

site data and/or pictures so he or she can survey the site for nontarget impacts. 

Be sure not to ascribe any damage you observe on nontarget species to any 

specific biocontrol agent/species and thus bias the confirmation of attack and the 

identification of the species causing the attack. 
106

Chapter 5: An Integrated Gorse 

and Scotch Broom Management Program

Introduction The invasion curve (Figure 1-3, repeated here in Figure 5-1) shows that 

eradication of an invasive species such as gorse and Scotch broom becomes 

less likely and control costs increase as an invasive species spreads over time. 

Prevention is the most cost-effective solution, followed by eradication. If a 

species is not detected and removed early, intense and long-term control efforts 

will be unavoidable. Identifying where gorse or Scotch broom are on the 

invasion curve in a particular area is the first step to taking management action. 

Inventorying and mapping current gorse and broom populations, coupled with 

research efforts to predict where gorse and broom are most likely to move, 

enables land managers to concentrate resources in areas which are likely to be 

invaded, and then to treat individual plants and small populations of gorse and 

broom before it is too late to remove them.

Classical biological control has been applied to many invasive plant species, 

but biological control is not appropriate for areas on the left side (species absent 

[prevention] - small number of localized populations [eradication]) of the 

invasion curve. Biological control as a control method is best suited to gorse 

or Scotch broom populations in the later phases of the invasion curve (rapid 

increase in distribution and abundance [containment] - widespread and abundant 

throughout its potential range [asset based protection]). 

There are several examples in which both single- and multiple-biocontrol agent 

introductions have successfully controlled the targeted weeds. Where ideally 

suited, biological control may help maintain gorse and Scotch broom densities 

below economically or ecologically significant levels, enabling land managers to 

live with these weeds; however, at many locations, established biological control 

agents appear to be having little effect on gorse and/or broom. Depending on 

the infestation, integration with other weed control methods or resorting to other 

control measures entirely may be required to attain gorse and Scotch broom 

management objectives. 

A wide variety of successful weed control methods have been developed and 

may be useful for helping meet management goals for gorse and broom. The 

most successful, long-term gorse and Scotch broom management efforts have a 

number of common features, including:
Chapter 4: Elements of a Gorse and Scotch Broom Biological Control Program 107

Figure 5-1. Generalized invasion curve showing actions appropriate to each stage. (© State of Victoria, Department of 

Economic Development, Jobs, Transport and Resources. Reproduced with permission.)

• Education and Outreach

• Inventory and Monitoring

• Prevention 

• Weed Control Activities: A variety of gorse and Scotch broom control 

activities which are selected based on characteristics of the target 

infestation and planned in advance to use the most appropriate method or 

combination of methods at each site, including:

• Biological control

• Physical treatment

• Cultural practices

• Chemical treatment

Integrated Pest Management (IPM) incorporates all efforts noted above, and 

addresses several aspects of land management, not just how to get rid of weed 

populations. Land managers or landowners engaged in IPM take the time to 

educate themselves and others about the threat invasive species pose to the land 

and how management may facilitate invasion. IPM requires land managers to 

regularly inventory and map the land they manage, identifying areas where the 

vegetation is not meeting their management objectives and identifying reasons 
108 Chapter 5: An Integrated Gorse and Scotch Broom Management Program

why. When a weed infestation is found, IPM dictates that land managers map 

and make plans to address it utilizing control methods most appropriate for 

their particular infestation and land use. After initiating control activities, IPM 

encourages land managers to monitor the site to determine if the control activity 

was successful in subsequent years. If re-treatment or additional treatments are 

necessary, these are applied in a timely manner with appropriate post-treatment 

monitoring to ensure that management objectives are being met. 

Integrated Pest Management programs undertaken on a landscape level over 

many years can at times prove logistically difficult, expensive, and timeconsuming. The concept of Cooperative Weed Management Areas (CWMA) 

was created in the western USA in order to erase jurisdictional boundaries as 

an impediment to weed control and make a landscape IPM approach to weed 

management more feasible and successful. CWMAs consist of federal, state 

and local land managers, as well as concerned private landowners, within a 

designated zone who join efforts against exotic plants, pooling and stretching 

limited resources and labor for managing invasive species and protecting/

restoring habitat. Cooperation between neighboring CWMAs helps transfer 

knowledge and experience between heavily treated regions and places not yet as 

impacted by gorse or Scotch broom. Sharing successes and failures in gorse and 

Scotch broom management saves time and funding and reduces the incidence 

of negative impacts from management efforts, such as herbicide resistance. 

Numerous CWMAs exist throughout the western states of the USA and are 

excellent sources of information, experience, and resources for treating gorse 

and Scotch broom infestations using an IPM approach. These groups often hold 

local meetings that landowners can attend to gain access to local programs and 

information.

Though each component of IPM is an important tool for managing gorse and 

Scotch broom, it is important to note that these components work best when 

used in a combined approach. Rather than applying only one tool per site (e.g., 

applying herbicides at one infestation, mowing at another, and using biological 

control at still another), the most effective IPM strategy is to employ as many 

tools as necessary at a single site in order to maximize the efficacy of each tool 

and ultimately reduce gorse and broom infestations. Education, inventorying/

mapping, and prevention are important and applicable across all landscapes, 

whether or not gorse or Scotch broom are already present. When gorse or broom 

are established and control methods are warranted, long-term management 

success is greatly improved when control methods are identified according to 

infested habitat type, land use, ownership, and available resources and then 

integrated where appropriate. As described above, biological control is most 

appropriately used on large infestations where multiple years may be required 

before impacts are realized. During this time, chemical and physical control 

methods are best applied to smaller new or satellite populations where immediate 

eradication is warranted, and to the edges of large infestations to prevent further 

spread. Cultural control methods work to enhance the growth of more desirable 

vegetation and are best applied as complements to all other control methods.

Components 

of Successful 

Integrated Weed 

Management 

Programs to 

Manage Gorse and 

Scotch Broom
Chapter 5: An Integrated Gorse and Scotch Broom Management Program 109

The components of gorse and Scotch broom IPM are described individually 

below. Because the focus of this manual is the biological control of gorse and 

Scotch broom, the potential to integrate biocontrol with other weed control 

methods is described at the end of each control method’s section.

Education and Outreach

Education and outreach activities increase public awareness of noxious weeds, 

the problems they cause, their distribution, and ways to manage them 

(Figure 5-2). Ideally, education and outreach activities also foster cooperation 

and collaboration across land ownership boundaries to facilitate the development 

of a landscape-level weed management response. Education efforts should be 

an important component of any weed management plan, regardless of the target 

weed or weed control method employed. 

Gorse and Scotch broom education and outreach should focus on conveying to 

the public:

• the threats gorse and broom pose

• how to identify gorse and broom in different stages

• ways in which they can help in gorse and broom management

Figure 5-2. Scotch broom 

education brochure. 

(British Columbia Ministry 

of Forests, Lands, and 

Natural Resources)
110 Chapter 5: An Integrated Gorse and Scotch Broom Management Program

By educating land managers and landowners, recreationalists and the public 

about the threat of gorse and Scotch broom, enabling them to identify 

infestations, and enlisting them in mapping and management efforts, it becomes 

possible to cooperatively develop successful weed management responses at the 

landscape level. 

Inventory and Mapping

Inventory and mapping are key elements of a successful weed management 

program. It is imperative to accurately characterize the size and extent of weed 

infestations before control activities are identified, prioritized, and implemented 

because the best treatment methods are often determined by the size and location 

of the infestation. Education and outreach activities that foster collaboration 

between adjacent landowners are particularly useful when developing landscapelevel maps of weed infestations. Once land managers and landowners fully 

understand the threats gorse and Scotch broom pose to their land, they are often 

more willing to participate to ensure that their land is inventoried and accurate 

maps of gorse and broom are developed so the best control activities can be 

implemented. 

Gorse and Scotch broom infestations are often mapped by foot, vehicle, or 

airplane using a global positioning system unit (GPS) and a geographical 

information system (GIS), though hard copy maps made by hand are suitable for 

some locations. For very large infestations, GoogleEarth can be a useful tool for 

providing birds-eye-view imagery that allows for visual delineation of broom/

gorse patch boundaries; however, image quality and timing can make it difficult 

to make comparisons. An increasing number of free smartphone and tablet apps 

help make accurate, detailed, and versatile weed mapping available to anyone 

(e.g., the apps available from EDDMapS, see page 100 for more information). 

Inventory efforts should document the following for each infestation: location 

coordinates, boundaries, estimated density (number of plants of target weed per 

area, e.g. 10 m2 or yd2), land usage, treatment history, disturbance history (e.g., 

fire, flooding, grazing, logging), habitat type (coastal bluff, upland, shrubland, 

grassland, forest margin), and date. Photos of the infestation and a list of cooccurring species are also very useful. Documenting inventory and mapping 

efforts enables land managers to determine if all known gorse and Scotch broom 

infestations have been treated, and facilitates post-treatment monitoring. In 

turn, this allows land managers to judge the effectiveness of various treatment 

methods. See Chapter 4 for suggested techniques of monitoring infestations. 

Prevention

Prevention activities focus on areas not currently infested by gorse or Scotch 

broom with the goal of keeping these areas weed-free. Though gorse and Scotch 

broom are already present throughout much of western North America, there are 

many sites where they are absent or remain at low densities. Inventory efforts 

help identify the precise borders of these locations. Preventing introduction and 

spread of gorse and broom to uninfested areas is more environmentally desirable 

and cost-effective than treating large-scale infestations.
Chapter 5: An Integrated Gorse and Scotch Broom Management Program 111

Gorse and Scotch broom are spread by the movement of seeds, which are 

usually transported by contaminated sand/gravel, vehicles/equipment, ants, 

birds, humans, other animals, and waterways. Preventing the spread of gorse and 

Scotch broom requires cooperation among all landowners and land managers. 

In areas where gorse and broom are not yet present, it is important to ensure that 

possible invasion avenues are identified and management actions taken to reduce 

the risk of spread. This includes minimizing soil disturbances and regularly 

monitoring uninfested sites to confirm that they have remained uninfested. 

Cultivation, soil erosion (especially following flooding events and prescribed 

or wild fire), road grading, recreational activities (e.g., riding dirt bikes or four 

wheelers), and overgrazing all weaken existing plant communities, decrease 

plant cover, and cause disturbance, conditions that favor gorse and Scotch broom 

establishment, spread, and persistence (Figure 5-3). Because such activities are 

also potential ways of spreading gorse and Scotch broom seeds, they should 

either be avoided or closely monitored in invasion-prone areas. Where grazing 

does occur, proper livestock management (such as strategic timing and stocking 

rates) will allow grazed vegetation to recover and competitive plants to increase 

which, in turn, will help prevent the establishment of gorse and Scotch broom. 

If possible, livestock should be kept off weed-infested land when they are most 

likely to spread viable seeds (e.g., after seed formation). If it is not possible to 

avoid driving vehicles and machinery (e.g., logging, construction, or rangeland 

fire-fighting equipment) through gorse and Scotch broom infestations, it 

is crucial that a thorough cleaning take place before equipment leaves the 

contaminated area. 

Figure 5-3. Gorse covering an overgrazed hillside. (George Markin, U.S. Forest Service, 

bugwood.org)
112 Chapter 5: An Integrated Gorse and Scotch Broom Management Program

Prevention and exclusion activities are typically paired with education efforts. 

Examples of exclusion efforts include weed-free forage programs, state and 

provincial seed laws, and mandatory equipment cleaning when leaving infested 

sites and before entering uninfested sites. 

EDRR

An early detection and rapid response (EDRR) program is a specific protocol 

for tracking and responding to new infestations. It relies heavily on education 

and outreach activities to be effective. An EDRR program targets areas where 

gorse and Scotch broom may spread. It consists of three complementary 

activities: 1) educating land managers and the public on weed identification and 

mapping techniques, 2) enlisting their aid in immediate and thorough detection 

of the weed(s), and 3) initiating rapid response eradication efforts at all verified 

locations of the weeds.

Weed Control Activities

Biological Control

Biological control involves the use of living organisms, usually insects, mites, or 

pathogens, to control a weed infestation and regain the balance among coexisting 

plant species. Classical biological control focuses on the introduction of host 

specific natural enemies from the invasive weed’s native range. This method of 

gorse and Scotch broom management is the most economical and suitable for 

larger infestations (tens to hundreds of acres). For small patches (less than 2 

acres or 0.8 hectares) of new satellite (those growing outside of well-established) 

gorse and Scotch broom infestations, more aggressive control methods should 

be utilized (e.g. physical control or herbicides). Refer to Chapter 3 for detailed 

descriptions of the biological control agents currently approved for use on gorse 

and Scotch broom and Chapter 4 for how to implement a gorse and broom 

biological control program in your area.

Physical Treatment

Physical treatment utilizes pulling, cutting, or mowing to remove or disrupt the 

growth of weeds and is the oldest method of weed control. Physical methods 

have had variable success in controlling gorse and Scotch broom, depending 

on specific site and infestation characteristics. All physical control methods 

are labor-intensive and not suitable for the more rugged and inaccessible sites 

where both weeds have invaded. Due to the ability of gorse and Scotch broom 

to regenerate from severed roots and from how long-lived their seeds can be, 

all physical treatments require repeat monitoring and treatment. Integrating 

physical methods with other control methods may increase success (e.g., see 

cutting below). Physical control activities should be planned to minimize ground 

disturbance to reduce recruitment from the seedbank. Regardless of the physical 

method employed, it is imperative that all equipment be thoroughly cleaned 

following use to prevent the spread of gorse and Scotch broom seeds.
Chapter 5: An Integrated Gorse and Scotch Broom Management Program 113

Pulling

Pulling is most appropriate in the EDRR stage of a gorse or Scotch broom 

infestation or on satellite populations occurring outside larger containment areas. 

Pulling can provide successful control of small infestations (under 1 acre or 

0.4 ha) if applied persistently. It is especially effective in coarse-textured and 

moist soils so that the entire root system is removed. Small plants less than 

3.2 feet (1 m) tall can be pulled by hand (Figure 5-4a), while larger plants are 

removed most effectively with the use of a weed wrench (Figure 5-4b,c). Weed 

wrenches are most effective on plants with a basal stem diameter of 0.5 inches 

(1.25 cm) or less, since pulling plants with a larger stem diameter can lead 

to considerable soil disturbance. If gorse or Scotch broom plants are densely 

branched, loppers or pruning tools can be used to remove the lower limbs before 

pulling. When gorse or Scotch broom plants have seedpods present, cut off and 

bag all seeding stems prior to pulling. Otherwise, the jarring action of pulling 

may dislodge and distribute seeds at the site. All plant parts containing seeds 

should be securely bagged and taken to the trash or a transfer site to prevent 

possible gorse or Scotch broom seed dispersal from pulled material.

When root sections of pulled plants remain in the soil, some may re-sprout new 

stems. Re-sprouting occurs less frequently when plants are pulled during times 

of moisture stress (typically July through September, depending on location). 

Pulling creates soil disturbance, which is ideal for the germination of gorse 

and Scotch broom seeds. While this may increase the gorse or Scotch broom 

problem, it can also reduce the gorse/broom seedbank more quickly and help 

lead to population decreases if germinating plants are regularly pulled. Caution 

should be used because pulling actions frequently damage more desirable 

species growing around gorse and Scotch broom, again favoring re-invasion by 

gorse, broom, or other weedy species. Re-seeding the open space resulting from 

gorse and Scotch broom removal with seeds of desirable vegetation can provide 

competition to decrease gorse and broom seedling germination and persistence. 

Figure 5-4. Physical control with pulling: a. gorse seedlings suitable for hand pulling; b. weed wrench on gorse stem; c. large 

gorse plant being pulled with weed wrench. (a. John M. Randall, The Nature Conservancy, bugwood.org; b. Nancy Ness, 

Grays Harbor Noxious Weed Control Board; c. Snohomish County Noxious Weed Control Board)

a b c
114 Chapter 5: An Integrated Gorse and Scotch Broom Management Program

Any site where pulling is utilized on gorse or Scotch broom should be monitored 

repeatedly for multiple years for stem re-sprouting and seedling germination, and 

should be re-treated as needed.

Due to the destructive nature of pulling, this control method is not compatible 

with biological control. Pulling is most appropriate for small infestations where 

immediate eradication is feasible, while biological control is more appropriate for 

much larger, established infestations. One way to successfully combine these two 

methods is to release biological control agents in a large, main infestation while 

employing pulling to remove individual plants and to control small, satellite 

patches arising outside of the main gorse or Scotch broom infestation.

Cutting

Individually cutting gorse or Scotch broom plants (Figure 5-5a) may reduce seed 

production and may, under some circumstances, result in plant death. However, 

cutting frequently results in stems re-sprouting from remaining stumps, which 

may make future physical removal challenging and must be repeated regularly. 

Cutting is most effective when done during times of moisture stress (typically 

July through September, depending on location) and when plants are cut just 

above or below ground level. Large, old plants are less likely to re-sprout after 

cutting during this time. Weed control can be increased if cutting is immediately 

followed by herbicide applications made to remaining stumps. When gorse or 

Scotch broom plants have seedpods present, cut off and bag all seeding stems. 

Otherwise, the jarring action of cutting and removing larger plant sections 

may dislodge and distribute seeds at the site. All plant parts containing seeds 

should be securely bagged and taken to the trash or a transfer site to prevent 

possible gorse or Scotch broom seed dispersal from cut material. 

Due to the destructive nature of cutting and removing all aboveground growth 

of gorse and Scotch broom, this control method is not compatible with the 

biological control agents presently established in North America and Hawaii. 

Cutting is most appropriate for small infestations where follow-up treatments 

and eradication are feasible, while biological control is more appropriate for 

much larger, established infestations. One way to successfully combine these two 

methods is to release biological control agents in a large, main infestation while 

employing cutting to remove individual plants and to control small, satellite 

patches arising outside of the main gorse or Scotch broom infestation.

Mowing

Mowing gorse and Scotch broom infestations (Figure 5-5b,c) is typically not 

effective in the long term as mowing usually stimulates stem re-growth (and 

subsequent flowering), increases seed germination, and reduces competition 

from surrounding vegetation. Regular mowing throughout the year utilizes much 

of gorse and broom’s stored root reserves, and over time decreases their root 

regenerative capacity and subsequent seed production. Frequent mowing of gorse 

and Scotch broom is not feasible at remote, rugged, or rangeland sites where 

these weeds have readily invaded, but it may provide control to gorse and broom 

along roadsides and rights-of-way. Alternatively, mowing can be used to reduce 
Chapter 5: An Integrated Gorse and Scotch Broom Management Program 115

nontarget plant cover and litter prior to herbicide applications. When mowing is 

used as a form of gorse or Scotch broom control, it is important that treatments 

occur before seed maturation (May-September for gorse, July-September for 

Scotch broom) because mowing can facilitate seed dispersal.

The destructive nature of mowing is damaging to the seed beetles Bruchidius 

villosus, Exapion fuscirostre, and E. ulicis as well as the Scotch broom twig 

miner Leucoptera spartifoliella and the gorse soft shoot moth Agonopterix 

umbellana (established only in Hawaii). Mowing may actually help distribute 

the gorse spider mite Tetranychus lintearius, and this biocontrol agent can 

re-establish on gorse plants recovering from mowing efforts. Mowing is likely 

damaging to the gorse thrips Sericothrips staphylinus, though it may aid in thrips 

dispersal in some situations.

Cultural Practices

Cultural methods of weed control (including flooding, burning, grazing, and 

seeding with competitive species) can enhance the growth of desired vegetation, 

which may slow the invasion of noxious weeds onto a site. For gorse and Scotch 

broom management, flooding is typically not applicable due to the non-wetland 

locations gorse and broom frequently infest. Regardless of which method is used, 

all cultural control techniques are more successful when combined with other 

control methods, such as pulling or cutting gorse or Scotch broom prior to reseeding. 

Burning

Burning has yielded mixed results when used as a form of controlling gorse 

and Scotch broom (Figure 5-6a). Both species contain flammable oils, and 

older individuals have large amounts of dead growth/litter at their centers. The 

combination of these traits makes both gorse and Scotch broom extreme fire 

hazards. While the aboveground biomass of gorse and broom burns readily in 

very hot fires, some plants recover and re-sprout from their roots post-fire. 

When fires burn sufficiently long or hot enough to remove aboveground biomass, 

the bare soil left from fire events is ideal for sprouting new gorse and Scotch 

broom plants (Figure 5-6b).

Figure 5-5. Physical weed treatments: a. cutting a single-stemmed Scotch broom plant; b. mowing Scotch broom; c. gorse 

plants following mowing. (a. Jennifer Andreas, Washington State University Extension; b. Ray Willard, Washington State 

Department of Transportation; c. Nancy Ness, Grays Harbor Noxious Weed Control Board)

a b c
116 Chapter 5: An Integrated Gorse and Scotch Broom Management Program

While fire sometimes exacerbates the problem, it has been used intentionally in 

some locations to burn off plant litter in order to make re-sprouting gorse and 

Scotch broom easier to see when applying herbicides and to help reduce the 

seedbank by causing mass germination. When prescribed fire kills off competing 

vegetation, it will only increase the gorse or Scotch broom problem, even 

with subsequent herbicide applications. Revegetation with desired vegetation 

is recommended wherever fire is utilized to aid in gorse and Scotch broom 

chemical control.

The destructive nature of fire makes it incompatible with all biological control 

agents currently established on gorse or Scotch broom in North America and 

Hawaii.

Grazing

Both gorse and Scotch broom are considered largely unpalatable to cattle. Scotch 

broom may be mildly toxic to cattle and sheep. Domestic goats and sheep have 

been used for both gorse and broom control (Figure 5-7) with varying success. 

Gorse is generally more readily fed upon when leaves are present and stems 

are young and soft. Once leaves give way to hardened spines, grazing is less 

effective. Some studies have found goats eradicate even old growth of gorse 

and Scotch broom, and that sheep help maintain this control. Other studies 

have found no control with either grazing species, even at high stocking rates. 

Overgrazing infested pastures reduces gorse and Scotch broom competition 

and increases soil disturbance, enhancing the establishment and spread of 

gorse and broom. Consequently, utilizing livestock for gorse and Scotch broom 

management must be done with caution, close observation, and only under the 

right circumstances. Where it is feasible to utilize livestock to manage gorse and 

Scotch broom, it’s important that the animals do not graze during seed set, as this 

can assist in the distribution of gorse and broom seeds. 

Figure 5-6. Fire used for the control of gorse: a. in a thick hillside infestation; b. resulting in a flush of new gorse seedlings. 

(a. Whitney Cranshaw, bugwood.org; b. Eric Coombs, Oregon Department of Agriculture, bugwood.org)

a b
Chapter 5: An Integrated Gorse and Scotch Broom Management Program 117

The combination of grazing with biological control is largely unknown, though 

it can be assumed that feeding on gorse and Scotch broom stems, leaves, and 

seedpods infested with currently established biocontrol agents would destroy 

populations of the insects and mites. 

Seeding competitive species

Where gorse or Scotch broom are established and then suppressed by one or 

more control methods, reinvasion by gorse, broom, or other undesirable species 

is likely if the ecological niche they occupied remains unfilled. Successful 

long-term management requires the establishment and maintenance of desirable 

competitive species to avoid reinvasions. 

Both gorse and Scotch broom are very sensitive to competition for light and 

resources during all growth stages. Planting rapidly growing grasses and 

overstory trees have been proven effective at some sites where the competitive 

grasses impede gorse and broom spread, and the trees provide shade detrimental 

to gorse and Scotch broom. The most suitable plant species to use for 

competition with gorse and Scotch broom depends on habitat, site conditions, 

climate, management goals, and future land use. Ideally, planted seeds should 

contain a mix of species, some of which should be quick to germinate and others 

to provide more long-term competition to gorse and Scotch broom seedlings. 

Utilizing ecologically equivalent species (those with root and growth patterns 

similar to gorse and broom) may provide the best competition. Because both 

gorse and Scotch broom are nitrogen-fixing species, re-seeding with other, more 

desirable nitrogen-fixing species (e.g. lupines or clovers) can provide significant 

competition for gorse and broom seedlings growing in sandy and/or low nutrient 

soil. Inventorying nearby sites that are uninvaded by gorse and Scotch broom 

may provide insight into the best replacement species. Consult your local 

county extension educator or Natural Resource Conservation Service (NRCS) 

representative for additional help in determining the best alternatives in your 

Figure 5-7. Cultural control method: goats grazing Scotch broom. (San Juan County 

Noxious Weed Control Board)
118 Chapter 5: An Integrated Gorse and Scotch Broom Management Program

area. Further suggestions for ecoregions throughout the United States may be 

found on the Native Seed Network website (please see Chapter 5 References for 

the URL). Likewise, the “links” section of the USDA PLANTS website offers 

numerous revegetation guideline manuals specific to different regions of both 

the United States and Canada. This site also provides access to a program and 

fact sheets that utilize soil, plant, and climate data to select plant species that are 

site-specifically adapted, suitable for the selected practice, and appropriate for the 

goals and objectives of the revegetation project.

Control of gorse and Scotch broom prior to seeding more desirable species is 

important because established gorse and broom plants are highly competitive. 

Seeding of competitors should take place immediately following exposure of 

soil to maximize their competitive abilities (Figure 5-8). For example, seeding 

should occur in bare soil following burning or after young gorse or broom 

plants have been pulled or killed with herbicides. Some herbicides have residual 

activity which could injure or kill seedlings of some desirable plant species, 

so care should be taken if seeding/transplanting is done on herbicide-treated 

sites. Because high populations of rodents can reduce the success of re-seeding, 

erecting a raptor perch/pole may discourage rodent habitation and help ensure 

seeded species successfully germinate and establish. 

Scotch broom, in particular, has been shown to have negative effects on 

revegetation efforts long after it has been removed. This has been attributed to 

allelopathy and increased soil nitrogen levels, as well as to negative impacts on 

ectomycorrhizal fungi. Applying activated carbon and sucrose to soils previously 

Figure 5-8. Cultural control method: revegetation efforts following soil disturbance 

(Lassen Volcanic National Park)
Chapter 5: An Integrated Gorse and Scotch Broom Management Program 119

invaded by Scotch broom may help alleviate the allelopathic and nitrogen effects, 

although these results are still experimental and in need of further confirmation.

Incorporating biocontrol agents with re-seeding has not been studied explicitly 

for gorse or Scotch broom, but could be difficult, primarily because the methods 

used to establish a productive stand of competitive species are not always 

compatible with the establishment and survival of biological control agents. Any 

method used to initially reduce gorse or broom stems and foliage to promote 

the growth of competitive species would hinder the survival of all gorse and 

broom biocontrol agents currently established in North America and Hawaii. 

Consequently, many successful revegetation programs establish competitive 

plant species first, using biological control agents after the seeded species have 

become established and gorse or Scotch broom begin to reappear. Alternatively, 

revegetation projects can target only a small portion of the infestation annually, 

leaving a reservoir of gorse or Scotch broom plants to support biocontrol agent 

populations. In some settings, it may be the biological control agents that open up 

the competing plant canopy, allowing for subsequent re-seeding to occur. 

Chemical Control

Many herbicides are registered for use on gorse and Scotch broom growing 

in a variety of locations. Herbicides are most effective when applied to small 

infestations, including newly established populations and recently established 

satellite patches arising from nearby older, larger gorse and broom infestations. 

If utilized appropriately, herbicides are also useful on the leading edge of large, 

advancing gorse and Scotch broom infestations. 

Herbicides may be too costly to be of practical use in treating extensive 

infestations of gorse and broom and, similar to physical and cultural control 

methods, are also impractical in hard-to-access and environmentally sensitive 

areas. Repeated herbicide applications are often required over time as gorse 

and Scotch broom stems can re-sprout from their root system if not completely 

killed (Figure 5-9a), and new gorse and Scotch broom plants can germinate from 

the seedbank. Potential nontarget damage to associated vegetation must also be 

considered when using herbicides. For these reasons, herbicides are best used as 

part of a larger, integrated weed management program that employs other weed 

control methods in areas where herbicides are less likely to be cost effective or 

the most appropriate control choice. 

Herbicides are generally applied in one of two ways: spot or broadcast 

applications. Spot treatments are used for individual gorse and Scotch broom 

plants or small patches. In spot applications, herbicide is applied to the foliage or 

stems of target plants only, thus reducing nontarget effects. Broadcast treatments 

are when herbicides are applied to an entire weed infestation rather than to single 

plants. Broadcast treatments should be used with caution as many herbicides may 

also impact plants that land managers may want to retain. If a broadcast treatment 

kills all plants in a treated area, the resulting bare soil may allow gorse and 

Scotch broom to reinvade from the seedbank, creating a denser infestation than 

was there originally. Selective herbicides are those that target selected species 
120 Chapter 5: An Integrated Gorse and Scotch Broom Management Program

(e.g., broadleaf forbs vs. grasses) while leaving other species virtually unharmed. 

The herbicide label should always be referenced to help determine the chance of 

nontarget species damage. Utilizing selective herbicides in spot treatments helps 

reduce the nontarget impacts of herbicide applications, and is the recommended 

approach for treating gorse and Scotch broom infestations with herbicides. 

Some of the best results (with minimal nontarget effects) have been achieved by 

applying selective herbicides to stumps of gorse or Scotch broom immediately 

after the stems are cut (Figure 5-9b).

The optimal timing for herbicide application depends on site-specific variables 

including the stage of growth of the gorse or Scotch broom plants at the time of 

application and the climatic conditions present at the site. Most herbicides used 

for foliar treatments have the highest efficacy when gorse and broom are rapidly 

growing (from spring through early fall). Cut stump herbicide treatments can 

be useful most times of the year, but are particularly effective from late summer 

through the dormant season, provided the herbicides are applied immediately 

after gorse and broom stems are cut. Most herbicides currently registered for use 

on gorse and Scotch broom work best when applied with a surfactant. For both 

species, repeat applications and careful attention to the timing of application are 

typically required.

Some of the most widely used herbicides to combat gorse and Scotch broom in 

North America include:

Broadleaf selective herbicides:

• Aminocyclopyrachlor + metsulfuron methyl. Aminocyclopyrachlor 

is a relatively new broadleaf selective herbicide which is currently 

being packaged for sale with metsulfuron methyl in uncultivated nonagricultural land, industrial sites, and natural areas. Aminocyclopyrachlor 

+ metsulfuron should be applied to foliage of actively growing gorse and 

Figure 5-9. Chemical control: a. re-growth and missed growth in Scotch broom infestation 

treated with herbicides; b. Scotch broom stump treated with herbicide and blue dye 

immediately after cutting. (a.,b. Jennifer Andreas, Washington State University Extension)

a b
Chapter 5: An Integrated Gorse and Scotch Broom Management Program 121

Scotch broom from early bloom to post bloom. It should be applied with 

a surfactant to improve herbicide uptake. This product has the potential to 

be mobile in the soil and may demonstrate residual activity several years 

after application, which will reduce re-growth from remaining gorse roots 

or seedlings. Low rates of aminocyclopyrachlor can kill nontarget 

tree and shrub species, so do not apply within the dripline of trees 

or shrubs, to a distance equal to the height of the species of concern. 

Aminocyclopyrachlor may also injure a number of desirable grass 

species depending on the product rate. 

• Dicamba. Dicamba should be applied to foliage of actively growing gorse 

from spring to early fall. It should be applied with a surfactant to improve 

herbicide uptake. Dicamba used alone is usually not the most effective 

herbicide for the control of gorse because although it kills aboveground 

growth, plants re-sprout from the roots, and repeated applications are 

required. There is slight residual activity of dicamba that may give shortterm control of germinating gorse seeds. Dicamba is often mixed with 

other herbicides (such as picloram or diflufenzopyr) to improve efficacy on 

many perennial weed species. When mixed with diflufenzopyr, dicamba is 

accumulated in the plant and is more effective on the root system, although 

data are lacking for this combination on gorse or Scotch broom. Dicamba 

will likely kill desirable broadleaf species, including legumes. Alone, 

it does not kill grasses, sedges, or other monocots, although combination 

treatments may cause injury to nontarget vegetation.

• Metsulfuron. Metsulfuron should be applied to foliage of actively 

growing gorse from early bloom to post bloom. Foliar treatments should 

be applied with a surfactant to improve herbicide uptake. Metsulfuron has 

a soil residual activity, which will reduce re-growth from remaining gorse 

roots or seedlings. Metsulfuron is not as active on Scotch broom as on 

gorse. While some formulations may be broadleaf-specific, there are 

restrictions on its use on some grasses, and it can kill desirable legume 

species.

• Picloram. Picloram should be applied to foliage of actively growing gorse 

and Scotch broom from early bloom to post bloom. It should be applied 

with a surfactant to improve herbicide uptake. Picloram has a long soil 

residual, which will reduce re-growth from remaining gorse and broom 

roots or seedlings for a few years following application. Picloram is 

mostly safe on grasses (young monocots may be affected), but it can 

kill many desirable legume and other broadleaf species. Picloram is less 

useful in hot, sunny conditions or in sandy soil because it is degraded by 

sunlight and can leach below the root zone in sandy soils.

• Triclopyr amine. Triclopyr amine should be applied to foliage of actively 

growing gorse and Scotch broom from early bloom to post bloom. It 

should be applied with a surfactant to improve herbicide uptake. Cut 

stump treatments with triclopyr ester can be made anytime the ground is 

not frozen, but are best used in late summer to early fall immediately after 
122 Chapter 5: An Integrated Gorse and Scotch Broom Management Program

stems are cut. Cut stump treatments should be applied as a mixture with 

a crop oil or methylated seed oil concentrate as directed by the product 

label. Triclopyr is a broadleaf herbicide so it will not harm grasses 

or other monocots, but it can kill many desirable legume and other 

broadleaf species. Triclopyr is often combined with 2,4-D as a foliar 

treatment used on actively growing gorse and Scotch broom; this mixture 

should be agitated continuously to prevent herbicide separation in the 

spray tank. 2,4-D is a broadleaf herbicide with a short soil residual. 2,4-D 

used alone generally does not provide full control, as treated plants often 

re-sprout from the roots and from the soil seedbank. Consequently, 2,4-D 

is best used on gorse and broom when combined with other herbicides. 

Triclopyr is sometimes combined in a premix with aminopyralid as a foliar 

treatment for Scotch broom. Aminopyralid is a broadleaf herbicide with 

a moderate soil residual period that will kill germinating seedlings and 

reduce re-growth from roots of treated Scotch broom plants for several 

months after application.

Non-selective herbicides:

• Glyphosate. Glyphosate should be applied to foliage of actively growing 

gorse and Scotch broom from spring to early fall. If an aquatically labelled 

glyphosate formulation is used, a surfactant should be mixed with spray 

solution prior to application to improve herbicide uptake. Glyphosate can 

also be used for cut stump treatments, which are most effective in late 

summer, early fall, or the dormant season; it should be applied immediately 

after stems are cut. Glyphosate has no residual activity in the soil, and 

repeated applications are often required. It is a non-selective herbicide 

and will create bare ground. It should only be used in spot treatments 

and in situations where loss of nontarget vegetation is acceptable.

Glyphosate use should be accompanied by revegetation of desirable 

species.

• Imazapyr. Imazapyr should be applied to foliage of actively growing 

Scotch broom plants; best results are achieved in late summer or early 

fall. Imazapyr can also be used for cut stump treatments on Scotch broom 

in late summer, early fall, or the dormant season; it should be applied 

immediately after stems are cut. It should be applied with a surfactant to 

improve herbicide uptake. It is a non-selective herbicide and should only 

be used in spot treatments and in situations where loss of nontarget 

vegetation is acceptable. It is soil-active with moderate residual activity, 

so it may persist to kill germinating seedlings; however, it can injure 

other plants rooted in, and downhill of, the treated areas and may 

persist to interfere with revegetation efforts.
Chapter 5: An Integrated Gorse and Scotch Broom Management Program 123

When herbicides are used for the control of gorse or Scotch broom, it is 

important that the applicator adheres to individual jurisdiction’s legislation and 

to all label instructions to ensure the usage, surfactant requirement, application 

rate, application timing and location/site of herbicide application fall within 

label recommendations. Not all herbicides are registered for use on these plants 

in all settings (including on or near water), or for use in each US state or in 

Canada. Some herbicides are restricted use and can only be applied by a certified 

and licensed applicator, and then only under specific conditions. Herbicide 

effectiveness can vary depending upon geographic location, climatic conditions, 

and rate of application. Please consult your local weed control authority, county 

agricultural extension educator, forest invasive species coordinator, or invasive 

plant specialist to learn which herbicides work best for gorse and broom control 

and when to apply them in your area.

If treated areas are to be grazed by livestock, consult the herbicide label for any 

grazing restrictions, including re-entry periods, that might be applicable.

Heavy herbicide use will reduce the gorse and Scotch broom stems and leaves on 

which currently established biocontrol agents rely, thus hindering establishment 

of these species. In order to guarantee that biological control agent populations 

remain viable as the gorse or Scotch broom infestations are reduced, plants 

should either be sprayed late in the growing season when adults of the seed 

beetles (Bruchidius villosus, Exapion fuscirostre, and E. ulicis) are overwintering 

in soil and plant litter, or some of the infested area should not be treated with 

herbicides to serve as “refuges” for biological control agents. 

The advantages and disadvantages of the most gorse and broom control methods 

are summarized in Table 5-1. 

Use Herbicides Safely!

Read the herbicide label, even if you have used the herbicide before. Follow all 

instructions on the label.

Wear protective clothing and safety devices as recommended on the label.

Bathe or shower after each herbicide application.

Be cautious when you apply herbicides. Know your legal responsibility as an 

herbicide applicator. You may be liable for injury or damage resulting from 

herbicide use.

Follow all storage and disposal instructions on the herbicide label.
124 Chapter 5: An Integrated Gorse and Scotch Broom Management Program

Control Method Advantage Disadvantage Compatibility with Biocontrol

Biological Control

Sustainable – biocontrol 

agents generally do not 

have to be reintroduced 

once established

Measurable changes in 

weed densities may take 

many years (eradication is 

not the goal)

Gorse: high populations of Tetranychus 

lintearius reduce food available 

for Exapion ulicis. Currently low T. 

lintearius populations likely make this 

impact minimal. In Hawaii, Agonopterix 

umbellana temporarily reduces food 

available to all established biocontrol 

agents, however plants recover when 

the moths are no longer active. High 

populations of Sericothrips staphylinus

reduce food available for other species; 

however populations are currently too 

low for impact. 

Scotch broom: Leucoptera spartifoliella 

has little if any impact on Bruchidius 

villosus and Exapion fuscirostre. 

B. villosus appears to outcompete 

E. fuscirostre.

Most economical option for 

large infestations

Some risk of adverse effects 

on nontarget plants

Public acceptance is 

generally higher than 

with other weed control 

methods

Permanent; cannot be 

undone

Selective Not successful in all 

situations

Physical Control 

 (Pulling & Cutting)

Reduces seed production Expensive and time 

consuming 

Applicable only to very small 

infestations where biocontrol is not 

recommended. Pulling and cutting 

are not directly compatible with any 

biocontrol agent; however, biocontrol 

can be applied to large, main 

infestations while pulling and cutting can 

be used on surrounding small, satellite 

populations.

Useful for small 

infestations that must be 

quickly eradicated

Must be repeated regularly 

to prevent re-establishment 

from seedbank or plants regrowing from root sections

Physical Control 

 (Mowing)

Repeated mowing 

may reduce seed 

production, and reducing 

photosynthesis impedes 

root carbohydrate storage 

essential for plant 

persistence and vigor

May spread gorse and 

broom if done during 

flowering or seeding; or lead 

to compensatory growth if 

done infrequently

Not compatible with B. villosus, E. 

fuscirostre, E. ulicis, L. spartifoliella, or 

A. umbellana (Hawaii only). Mowing 

likely aids in the dispersal of T. 

lintearius. Unknown if mowing hinders 

or aids dispersal of S. staphylinus

(Hawaii only).

Expensive and time 

consuming; requires proper 

timing and equipment

Cultural Control 

 (Flooding) Not recommended for gorse or Scotch broom management

Cultural Control 

 (Burning)

May kill gorse or broom if 

done repeatedly and/or the 

right conditions

Expensive and time 

consuming; requires proper 

timing and equipment

Incompatible with all biocontrol agents 

established on gorse or Scotch broom 

in North America or Hawaii

Causes flush of gorse and 

broom seed germination, 

helping reduce seedbank

Causes environmental and 

health hazards; especially 

problematic due to high 

flammable oil content in 

gorse/broom foliage

May make infestations 

worse due to flush of gorse 

and broom seed germination

Removes plant litter, 

making re-seeding or 

herbicide treatments more 

effective

Nonselective; can 

exacerbate the problem 

by reducing competing 

vegetation

Table 5-1. Comparison of gorse and Scotch broom management options
Chapter 5: An Integrated Gorse and Scotch Broom Management Program 125

Control Method Advantage Disadvantage Compatibility with Biocontrol

Cultural Control 

 (Grazing)

Allows use of the land 

even with heavy gorse and 

broom infestations

Cannot be used in many 

natural areas such 

as national parks and 

wilderness areas Compatibility with biocontrol largely 

unknown. Livestock would likely wish 

to avoid gorse plants infested with 

the mite T. lintearius. Grazing stems 

and seedpods infested with all other 

established biocontrol species would 

destroy the insects. 

Nonselective; can 

exacerbate the problem

Can be used (under 

the right conditions) in 

combination with biological 

or chemical control 

methods

Can be expensive

Kills only above-ground 

growth; gorse and broom 

can recover rapidly postgrazing

Cultural Control 

 (Re-seeding)

Can be used to restore 

native or more desirable 

species

Expensive for large 

areas; requires regular 

maintenance initially

Compatible if biocontrol agents are 

introduced after competitive species 

are established. Also compatible if 

re-seeding is done only on small 

sections of the infestation annually, 

leaving gorse and broom “refuges” 

for the biocontrol agents. In some 

settings, it is biocontrol that may make 

re-seeding feasible.

Can be self-perpetuating 

May be ineffective if existing 

gorse and Scotch broom 

stand is dense

Chemical Control

Fast acting

Expensive for large areas; 

repeat applications and 

monitoring often required

Herbicides are applicable to small 

infestations, which are typically 

unsuitable for biocontrol. Compatible 

when using biocontrol on a main 

infestation and herbicides on 

surrounding small, satellite infestations. 

Somewhat compatible if herbicides are 

applied late in the growing season when 

most biocontrol species are inactive 

and/or overwintering away from host 

plants. 

Successful for reducing 

gorse and broom 

densities in some settings, 

especially in combination 

with other control methods

May harm existing desirable 

vegetation, or impede 

revegetation efforts

If applied correctly and 

repeatedly, has the 

potential to eradicate 

some populations of gorse 

and broom

Public resistance to 

chemical controls

Useful along transportation 

corridors (roads, trails, 

occasionally waterways)

Regulations or policies may 

prohibit use in some areas

Table 5-1 (continued). Comparison of gorse and Scotch broom management options
126

Glossary

abdomen The last of the three insect body regions; usually containing the digestive and 

reproductive organs

adventive Species that arrived in the geographical area from elsewhere by any means

alternate Where leaves appear singly at stem nodes, on alternate sides of the stem

annual A plant that sprouts, flowers, and dies all in the same year 

antenna (pl. antennae) In arthropods, one of a pair of appendages on the head, normally many jointed and of 

sensory function

app (application) A self-contained program or piece of software designed to fulfill a particular purpose; 

an application, especially as downloaded by a user to a mobile device

biennial A plant that flowers and dies between its first and second years and does not flower 

in its first year

biological control The reduction in the abundance of a pest through intentional use of its natural 

enemies (predators, parasitoids, and pathogens)

bolting Plant stage at which the flower stalk begins to grow

caterpillar The larval stage of a moth or butterfly

chrysalis The pupal stage of a moth or butterfly

cocoon A silk case that moth or butterfly larvae spin to contain the chrysalis

community A naturally-occurring group of different species of organisms that live together and 

interact as a more or less self-contained ‘unit’

complete metamorphosis An insect life cycle with four distinct stages (egg, larva, pupa, adult)

compound eyes Paired eyes consisting of many facets, or ommatidia, in most adult Arthropoda

coordinates A set of numbers used to specify a location 

deciduous Plant that sheds its leaves annually

dehisce When seedpods dry out at maturity and burst open to scatter seeds 

density Number of individuals per unit area

dicot Plant with two seed leaves upon germination, including most common flowering 

species, excluding grasses, sedges, cattails, lilies and orchids
Glossary 127

dissemination Dispersal. Can be applied to seeds or insects

emergence (insect) Act of adult insect leaving the pupal exoskeleton, or leaving winter or summer 

dormancy

eradicate To get rid of something completely, as in eliminate a weed population

erect Grows upright and vertical as opposed to prostrate (spreading on the ground)

evergreen A plant that retains green leaves (or green photosynthesizing parts) throughout the 

year

exoskeleton Hard, external skeleton of the body of arthropods, including insects and mites

exotic Originating in a distant foreign country; not native 

field insectary An area where host plants or animals are abundant and biological control agents are 

released and propagated with or without additional human manipulation

forb Herbaceous plant (does not have solid woody stems)

gall A plant tumor; a localized proliferation of abnormal plant tissue that is induced by an 

insect, nematode, fungus or other organism and usually exhibits a characteristic shape 

and color; gall-causing insects and mites usually live and feed within the gall

genus (pl. genera) A taxonomic category ranking below family and above species and consisting of a 

group of species exhibiting similar characteristics. The genus name is followed by a 

Latin adjective or epithet to form the name of a species

GPS Global Positioning System; a space-based navigational system providing location and 

time information by using four or more satellites

habit The form or structure in which a plant grows (examples include shrubby, dwarf, and 

tree)

head Insect body region with the mouthparts, antennae, and eyes

head capsule Hardened covering of the head of an immature insect. They are especially prominent 

on larvae of some beetles and moths and noticeably reduced on larvae of many flies 

and wasps.

herbivory Feeding on plants

host The plant or animal on which an organism feeds; the organism utilized by a 

parasitoid; a plant or animal susceptible to attack by a pathogen

host specificity The highly-evolved, often obligatory association between an insect and its host (i.e. 

weed). A highly host-specific insect feeds only on its host and on no other species

incomplete metamorphosis An insect life cycle with three distinct stages (egg, nymph, adult)

instar The phase of an arthropod’s nymphal or larval development between molts

invasive Tending to spread prolifically and undesirably or harmfully

larva (pl. larvae) Immature stage of some animals, including insects and mites. In insects with 

complete metamorphosis, it is the stage between the egg and pupa (examples include 

grubs, caterpillars, and maggots)

leaf axils Where a leaf connects to the stem of a plant
128 Glossary

leaflets Leaf-like part of a compound leaf. Though it resembles an entire leaf, a leaflet is not 

borne on a main plant stem or branch as a leaf is, but rather on a branch of the leaf

legume A plant in the family Fabaceae (pea family or Leguminosae), or the fruit or seed of 

such a plant

litter Dead plant material, such as leaves, bark, needles, and twigs, that has fallen to the 

ground

margin (of leaf) The edge of a leaf. Margins typically fall within a handful of categories and are 

useful in plant identification 

membranous Thin and transparent

molting Process of arthropod development that involves shedding its exoskeleton and 

producing another as an arthropod grows

monocot Plant with only one seed leaf upon germination, including grasses, sedges, cattails, 

lilies, and orchids

monoculture An area vegetated by a single plant species

NAD 83 North American Datum, the official datum used for the UTM geographic coordinate 

system in North America

native Of indigenous origin

node Part of the stem of a plant from which a leaf, branch, or root grows

nontarget effect When control efforts affect a species other than the species they were enacted to 

control (can be positive or negative)

noxious weed A weed whose control is mandated, and whose movement is regulated by federal, 

state, provincial, or county law

nymph Immature form of invertebrates, including mites and insects that undergo incomplete 

metamorphosis. Resembles adults

oviposit To lay or deposit eggs

parasitoid An insect (e.g. a wasp) whose larvae live as parasites, eventually killing their hosts 

(typically other insects)

perennial A plant that lives for more than two years

photosynthesis Process used by plants and other organisms to convert light energy, normally from the 

sun, into chemical energy that can be later released to fuel the organisms’ activities

plant cover The portion of the vegetative canopy in a fixed area attributable to an individual or a 

single plant species

pupa (pl. pupae; v. pupate) Non-feeding, inactive stage between larva and adult for an insect with complete 

metamorphosis

qualitative Measurement of descriptive elements (e.g. age class, distribution)

quantitative Measurement of quantity; the number or amount (e.g. seeds per capitula)

root crown Part of a root system from which a stem arises; where a plant’s stem meets the roots

scarification Cutting the seed coat using abrasion, thermal stress, or chemicals to encourage 

germination
Glossary 129

senescence Final stage in a plant’s life cycle

species A fundamental category of taxonomic classification, ranking below a genus or 

subgenus and consisting of related organisms capable of interbreeding

surfactant An additive often applied with an herbicide mix to help bring the herbicide into 

closer contact with the leaf surface in order to aid absorption

synchrony Occurring at the same time (e.g. plant flowering and insect oviposition)

taxonomy The classification of organisms in an ordered system that indicates natural 

relationships. The science, laws, or principles of classification; systematics

thorax Body region of an insect behind the head and abdomen, bearing the legs and wings

toothed Leaf margin that is regularly incised, similar to a saw

transect A straight line of varying length along which plants are periodically sampled 

individually or in quadrants

UTM Universal Transverse Mercator, a grid-based geographic coordinate system 

viability The proportion of propagules (e.g. seeds) that are alive and can germinate

weed A plant growing where it is not wanted

WGS 84 The World Geodetic System, a datum for latitude/longitude geographic coordinate 

systems
130

Selected References

Chapter 1: 

Introduction

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Control in the Western United States: Accomplishments and Benefits of 

Regional Research Project W-84, 1964-1989. Publication 3361. University 

of California, Division of Agriculture and Natural Resources, Oakland, 

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Balciunas, J.K. 1999. Code of best practices for classical biocontrol of weeds. 

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USA.

British Columbia IAPP. 2013. British Columbia Ministry of Forests, Lands, and 

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Center for Invasive Species Management. 2016. State and Province Noxious 

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Clements, D.R., D.J. Peterson, and R. Prasad. 2001. The biology of Canadian 

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Community Attributes Inc. 2017. Economic Impact of Invasive Species: Direct 

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Coombs, E.M., J.K. Clark, G.L. Piper, and A.F. Cofrancesco, Jr. (eds.). 2004. 

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Selected References 131

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15 June 2016). National Plant Data Center, Baton Rouge, LA 70874-4490 

USA.

Winston, R.L., M. Schwarzländer, H.L. Hinz, M.D. Day, M.J.W. Cock and M.H. 

Julien, eds. 2014. Biological Control of Weeds: A World Catalogue of Agents 

and Their Target Weeds, 5th edition. USDA Forest Service, Forest Health 

Technology Enterprise Team, Morgantown, West Virginia. FHTET-2014-04. 

838 pp.

Zouhar, K. 2005. Cytisus scoparius, C. striatus. In: Fire Effects Information 

System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky 

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(www.fs.fed.us/database/feis/, accessed 10 December 2015). 

Zouhar, K. 2005. Ulex europaeus. In: Fire Effects Information System [online]. 

U.S. Department of Agriculture, Forest Service, Rocky Mountain Research 

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accessed 10 December 2015).
132 Selected References

Andres, L.A. and E. Coombs. 1995. Scotch broom. In: J.R. Nechols, L.A. 

Andres, J.W. Beardsley, R.D. Goeden, and C.G. Jackson, eds. Biological 

Control in the Western United States: Accomplishments and Benefits of 

Regional Research Project W-84, 1964-1989. Publication 3361. University 

of California, Division of Agriculture and Natural Resources, Oakland, 

California. pp. 303-305. 

Coombs, E.M., J.K. Clark, G.L. Piper, and A.F. Cofrancesco, Jr. (eds.). 2004. 

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University Press, Corvallis. 467 pp.

Cristofolini, G. and A. Troìa. 2006. A reassessment of the sections of the genus 

Cytisus Desf. (Cytiseae, Leguminosae). Taxon 55(3): 733-746. 

Cronquist, A., N.H. Holmgren, and P.K. Holmgren. 1997. Intermountain Flora, 

Vol. 3. New York Botanical Society, New York.

DiTomaso, J.M. 1998. The biology and ecology of brooms and gorse. 

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Chapter 2: 

Getting to Know 

Gorse and Broom
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Biology of 

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Biological Control 

Agents
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of California, Division of Agriculture and Natural Resources, Oakland, 

California, USA. pp. 299-302. 

Markin, G.P., E.R. Yoshioka, and P. Conant. 1996. Biological control of gorse 

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Winston, R.L., M. Schwarzländer, H.L. Hinz, M.D. Day, M.J.W. Cock and M.H. 

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Andreas, J. 2016. Pest Watch: Biological Control of Scotch Broom, FS203E. 

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Andreas, J.E., E.M. Coombs, J. Milan, and M. Schwarzländer. 2015. Biological 

Control. In: E. Peachey, ed. Pacific Northwest Weed Management Handbook. 

Oregon State University, Corvallis, Oregon. pp. B1-B6. 

Andres, L.A. and E. Coombs. 1995. Scotch broom. In: J.R. Nechols, L.A. 

Andres, J.W. Beardsley, R.D. Goeden, and C.G. Jackson, eds. Biological 

Control in the Western United States: Accomplishments and Benefits of 

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of California, Division of Agriculture and Natural Resources, Oakland, 

California. pp. 303-305. 

Coombs, E.M. 2012. Biological control of weeds in Oregon. Annual Report 

2012. Oregon Department of Agriculture, Noxious Weed Control Program, 

Salem, Oregon, USA. 35 pp. 

Coombs, E.M., J.K. Clark, G.L. Piper, and A.F. Cofrancesco, Jr., eds. 2004. 

Biological Control of Invasive Plants in the United States. Oregon State 

University Press, Corvallis. 467 pp.

Conant, P., J.N. Garcia, M.T. Johnson, W.T. Nagamine, C.K. Hirayama, G.P. 

Markin, and R.L. Hill. 2013. Releases of natural enemies made in Hawaii 

since 1980 for classical biological control of weeds. In: Y. Wu, T. Johnson, 

S. Sing, S. Raghu, G. Wheeler, P. Pratt, K. Warner, T. Center, J. Goolsby, 

and R. Reardon, eds. Proceedings of the XIII International Symposium on 

Biological Control of Weeds. 11-16 September 2011, Kohala Coast, Hawaii, 

USA; Forest Health Technology Enterprise Team, Morgantown, WV, USA. 

pp. 230-242.

Hill, R.L., J. Ireson, A.W. Sheppard, A.H. Gourlay, H. Norambuena, G.P. Markin, 

R. Kwong, and E.M. Coombs. 2008. A global view of the future for biological 

control of gorse, Ulex europaeus L. In: M.H. Julien, R. Sforza, M.C. Bon, 

H.C. Evans, P.E. Hatcher, H.L. Hinz, and B.G. Rector, eds. Proceedings of the 

XII International Symposium on Biological Control of Weeds. 22-27 April 

2007, La Grande Motte, France; CAB International. pp. 680-686. 

Hogg, B.N., L. Smith, and K.M. Daane. 2016. Impacts of the adventive 

psyllid Arytainilla spartiophila on growth of the invasive weed Cytisus 

scoparius under controlled and field conditions in California. Environmental 

Entomology 45: 109-116.

Ireson, J.E., A.H. Gourlay, R.J. Holloway, W.S. Chatterton, S.D. Foster, and R.M. 

Kwong. 2008. Host specificity, establishment and dispersal of the gorse thrips, 

Sericothrips staphylinus Haliday (Thysanoptera: Thripidae), a biological 

control agent for gorse, Ulex europaeus L. (Fabaceae), in Australia. Biological 

Control 45: 460-471.

King, S., T. Drlik, L. Simon, and W. Quarles. 1996. Integrated weed management 

of gorse. The IPM Practitioner 18: 1-9. 

Chapter 4: 

Elements of a 

Gorse and Broom 

Biological Control 

Program
138 Selected References

Markin, G.P., E.R. Yoshioka, and R.E. Brown. 1995. Gorse. In: J.R. Nechols, 

L.A. Andres, J.W. Beardsley, R.D. Goeden, and C.G. Jackson, eds. Biological 

Control in the Western United States: Accomplishments and Benefits of 

Regional Research Project W-84, 1964-1989. Publication 3361. University 

of California, Division of Agriculture and Natural Resources, Oakland, 

California, USA. pp. 299-302.

Markin, G.P., E.R. Yoshioka, and P. Conant. 1996. Biological control of gorse 

in Hawaii. In: V.C. Moran and J.H. Hoffmann, eds. Proceedings of the IX 

International Symposium on Biological Control of Weeds. 19-26 January 

1996, Stellenbosch, South Africa; University of Cape Town Press. 

pp. 371-375.

Markin, G.P. and P. Conant. 2013. Status of biological control of the shrub gorse 

(Ulex europaeus) on the island of Hawaii. In: Y. Wu, T. Johnson, S. Sing, S. 

Raghu, G. Wheeler, P. Pratt, K. Warner, T. Center, J. Goolsby, and R. Reardon, 

eds. Proceedings of the XIII International Symposium on Biological Control 

of Weeds. 11-16 September 2011, Kohala Coast, Hawaii, USA; Forest Health 

Technology Enterprise Team, Morgantown, WV, USA. pp. 429-434.

McFadyen, R.E.C. 1998. Biological control of weeds. Annual Review of 

Entomology 43:369- 393. 

DiTomaso, J.M., G.B. Kyser et al. 2013. Weed Control in Natural Areas in the 

Western United States. Weed Research and Information Center, University of 

California. 544 pp.

Bossard, C., C. Randall, John M. and M.C. Hoshovsky. 2000. Invasive Plants of 

California’s Wildlands. University of California Press, CA. 

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following removal of a nitrogen-fixing invader. Biological Invasions 17(9): 

2621-2631.

Grove, S., K.A. Haubensak, and I.M. Parker. 2012. Direct and indirect effects 

of allelopathy in the soil legacy of an exotic plant invasion. Plant Ecology 

213(12): 1869-1882.

Harrington, T.B. 2014. Synthetic auxin herbicides control germinating Scotch 

broom (Cytisus scoparius). Weed Technology 28: 435-442.

Herrera-Reddy, A.M., R.I. Carruthers, and N.J. Mills. 2012. Integrated 

management of Scotch broom (Cytisus scoparius) using biological control. 

Invasive Plant Science and Management 5: 69-82.

Hermann, R.K. and M. Newton. 1968. Tree planting for control of gorse on the 

Oregon Coast. Research Paper 9. Corvallis, OR: Oregon State University, 

School of Forestry, Forest Research Laboratory. 12 pp.

Chapter 5: 

An Integrated 

Gorse and Broom 

Management 

Program
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Northwest. The Nature Conservancy. 13 pp.

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2016. 

Oneto, S.R., G.B. Kyser, and J.M. DiTomaso. 2010. Efficacy of mechanical and 

herbicide control methods for Scotch broom (Cytisus scoparius) and cost 

analysis of chemical control options. Invasive Plant Science and Management 

3(4): 421-428.

Pacific Northwest Weed Management Handbook. 2014. Compiled by E. Peachey, 

D. Ball, R. Parker, J.P. Yenish, T.W. Miller, D.W. Morishita, and P.J.S. 

Hutchinson. Oregon State University Agricultural, Corvallis, OR. 451 pp. 

http://pnwhandbooks.org/weed/.

Prasad, R. 2003. Management and control of gorse and Scotch broom in British 

Columbia. Technology Transfer Note Number 30. Victoria, BC: Canadian 

Forest Service, Natural Resources Canada, Pacific Forestry Centre, Forestry 

Research Applications. 6 pp.

USDA, NRCS. 2016. The PLANTS Database. (http://plants.usda.gov, accessed 

15 June 2016). National Plant Data Center, Baton Rouge, LA 70874-4490 

USA.

Ussery, J.G. and P.G. Krannitz. 1998. Control of Scot’s broom (Cytisus scoparius

(L.) Link.): the relative conservation merits of pulling versus cutting. 

Northwest Science. 72(4): 268-273.

VanBebber, R. 2003. CWMA Cookbook: A recipe for success. Idaho State 

Department of Agriculture, Boise, ID. 22 pp.

Zouhar, K. 2005. Cytisus scoparius, C. striatus. In: Fire Effects Information 

System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky 

Mountain Research Station, Fire Sciences Laboratory (producer). 

(www.fs.fed.us/database/feis/, accessed 10 December 2015). 

Zouhar, K. 2005. Ulex europaeus. In: Fire Effects Information System, [Online]. 

U.S. Department of Agriculture, Forest Service, Rocky Mountain Research 

Station, Fire Sciences Laboratory (producer). (www.fs.fed.us/database/feis/, 

accessed 10 December 2015).
140 Appendices

Appendix I. Troubleshooting Guide: When Things Go Wrong

This guide is intended to assist those who encounter problems when establishing biological control agent 

populations. It identifies the probable cause of typical problems and offers solutions.

Problem Probable Cause Solution

Biological control 

agents unhealthy 

or dead when 

received

Physical damage to biocontrol 

agents in transport

Provide adequate packing material to minimize movement of 

containers and ice packs.

Drowning Do not put water in containers during transport; prevent accumulation 

of excess moisture; too much plant material causes condensation.

Excess or prolonged heat or 

cold

Keep containers cool at all times; use coolers and ice packs; avoid 

exposure to direct sunlight while in transit.

Starvation Put gorse and Scotch broom foliage (no flowers, seeds, or roots) in 

containers.

Release delay Transport/ship biocontrol agents immediately after collection/sorting.

Release biocontrol agents at new site immediately upon arrival/receipt.

Parasitism and/or disease Check source biocontrol agents. Ensure the insect population is 

disease-free when collecting or receiving shipment.

Reproductive 

problems

Biocontrol agents past 

reproductive stage

Collect at peak activity (i.e. insects are mating and ovipositing).

Sex ratio: not enough males 

or females

Collect at peak activity; observe mating among target biocontrol 

agents before collecting; males often emerge earlier than females.

Biocontrol agents not 

synchronized with the gorse 

and broom growth stage

Biological control agents require the weed to be at specific growth 

stage for optimal oviposition; collect biocontrol agents from sites with 

plants in similar stages.

Few biological 

control agents 

collected

Collection at wrong time Refer to Tables 4-3, 4-4 for collection time and technique.

Collection technique Biological control agents can be killed/damaged during sweeping or 

aspirating so sweep lightly; avoid debris.

Conditions at time of 

collection wrong

Refer to the Chapter 4 section “Collecting Gorse and Broom Biological 

Control Agents” for guidelines on desirable weather conditions.

Population insufficient Only collect from well-established populations.

Biological agents 

not found after 

release

Site is unsuitable or too small Refer to the Chapter 4 section “Selecting Biological Control Agent 

Release Sites.”

Not enough biocontrol agents 

released

Release at 100-200 adults of the gorse and broom seed beetles, 

50-100 L. spartifoliella, 200-300 S. staphylinus; transfer as many 

stems as is possible infested with T. lintearius, A. umbellana.

Pesticide use/mowing in area Select sites where land use and management practices do not 

interfere with biological control agent life cycles.

Released on wrong species Ensure gorse and Scotch broom are targeted, and the correct 

biocontrol agent is used.

Released at wrong time Release only during the correct plant stage and in the cool hours of 

the day. Refer to Tables 4-3, 4-4 for guidelines.

Insufficient time has passed 

since release

Populations of some biocontrol agents take two or more years to reach 

detectable levels

Biocontrol agents not well 

adapted to conditions

Release field-collected biocontrol agents from local sources wherever 

possible rather than greenhouse-reared adults or insects collected 

from distant locations.

Ants or other predators 

preyed upon biocontrol agents

Release only at sites with no obvious ant mounds or high insect 

predator populations (e.g. mice, voles, spiders).

Cannot locate 

release site

Location marker not obvious Use bright-colored wooden, metal, or plastic stake.

Site destroyed Communicate with all direct and neighboring land users.

Map poorly/incorrectly drawn Check map; redraw with more detail or add landmarks; GPS.
Appendices 141

Appendix II. Sample Biological Control Agent Release Form
142 Appendices

Appendix II. Sample Biological Control Agent Release Form (Side 2)
Appendices 143

Appendix III. Gorse/Broom Biological Control Agent Monitoring Form
144 Appendices

Appendix IV. Gorse/Broom Qualitative Monitoring Form
Appendices 145

Appendix V: Scotch Broom Seedpod Quantitative Monitoring Form
146 Appendices

Appendix V: Scotch Broom Seedpod Quantitative Monitoring Form 

 Instructions

General: 

The purpose of this activity is to estimate the abundance and attack rates of Scotch broom seed beetles. Seedpods 

can be dissected in the field; however, this may result in errors due to seed or beetle counts being missed or 

duplicated. It is recommended dissections occur in an office or lab setting to protect samples from inclement 

weather and to ensure all information is recorded accurately per pod. Data collection is easier with two people, 

one to make the observations and the other to record data.

Timing: 

It is important to collect pods before they dry and crack open. Data will be lost if pods open, releasing seeds 

and/or beetles. Visit Scotch broom sites when seedpods are nearly entirely brown or black. They should not 

be green as the seeds and biocontrol agents are still developing, and pod dissections will be more difficult. 

In Washington, this corresponds to roughly early July, but watch local phenology as the pods will ripen and 

dehisce quickly, within a matter of days. Collect pods that are fully intact and completely closed. The best 

time to collect is the morning. If possible, avoid collecting pods during periods of rain or when the pods are 

wet. If pods are stored for later dissections and they are wet, add a paper towel to the collection bags to soak up 

moisture; remove it within 1-2 days to avoid mold issues. Keep pods refrigerated if dissection cannot be done 

immediately.

Supplies needed: 

Seedpod collection: datasheet, clipboard, pen/pencil, compass, 100m tape measure (optional), flagging (optional)

If storing seedpods for later dissections (highly recommended), additional supplies needed include: zip-top bags, 

paper towels, permanent marker for writing on bags or, alternatively, small pieces of paper to write on and insert 

in bags. 

Seedpod dissection: datasheet (partially filled out after seedpod collections); tub or container to hold the seedpod 

being dissected; thumb nail, scalpel, or other sharp blade; bright lighting; microscope (if available).

Seedpod collection protocol:

At each site: 

1) Fill out the site and collector information at the top of the datasheet.

2) Assess the Scotch broom infestation. Scope out a straight line transect at least 100 m long that crosses 

through the densest part of the population. For example, if the infestation is along a powerline corridor, the 

transect would follow the corridor through the middle of the infestation. Do your best to follow a straight 

line through the infestation, even if the Scotch broom is very dense. A tape measure or flagging can be used 

to help. Avoid collecting from the edge of an infestation even if it is easier, because this may skew the data. 

3) Sample 10 Scotch broom plants at each site. Start at the beginning of the transect. This is plant #1. From 

there, walk 10 normal paces (or 10 m if using a tape measure) in a straight line along the transect and 

sample the closest plant to the pace line. This is plant #2. Repeat until 10 plants are chosen. 
Appendices 147

4) At each plant, and using a compass to help orientation, collect 3 pods in the following manner: 

a. On the north side of the plant (0° or 360°N), select one pod from the bottom of the canopy 

(0-33% total plant height). Try to be as random as possible when selecting individual pods. Select pods 

that are unopened but dark. Try not to select pods that are overly large or small or that look like they 

have been over- or under-attacked. 

b. Move to the southeast side of the plant (120°SE), and select one pod from the middle of the canopy 

(33-66% total plant height). 

c. Move to the southwest side of the plant (240°SW), and select one pod from the top of the canopy 

(66-100% total plant height). 

d. If you are dissecting the seedpods in the field, skip to the instructions below for Seedpod Dissection 

Protocol. If you are storing the pods for later dissection in a controlled setting (highly recommended), 

put all 3 pods in a zip top bag. Label the bag with the site name, plant #, collector name, and date by 

writing this directly on the bag or on a small piece of paper to insert into the bag. If the pods are wet, 

place a small amount of paper towel in the bag to help absorb moisture.

5) If storing the seedpods for later dissection, place all 10 bags (of 3 pods each) from a single site into one 

large bag. Label the large bag with site name, collection day, and collector. Keep the seedpods in a cooler 

with icepacks until returning to the dissection location; store in a refrigerator until dissections are 

completed.

Seedpod dissection protocol:

Record information for all steps below on the same site datasheet used during the seedpod collection process. 

If possible, take bags of pods/site out one at a time. A small cooler with an ice pack can be used to keep the 

remainder of the pods cool and moist. Hold each pod being dissected in or over a tub or container to ensure the 

container catches all pod parts that may come away during dissection. Bruchidius villosus = BRVI; 

Exapion fuscirostre = EXFU.

Note on Parasitoids: Part of this monitoring protocol includes observing parasitoids, which are insects 

(e.g., wasps) whose larvae live as parasites, eventually killing their hosts (typically other insects). Scotch broom 

parasitoids deposit their eggs directly onto their BRVI or EXFU hosts. Parasitoid larvae feed externally on their 

hosts until host death, and adult parasitoids emerge from seedpods through holes they must cut in the seedpod 

walls. Because EXFU feeds externally on Scotch broom seeds, the parasitoids don’t have to create an emergence 

hole through the seed to get out; they only must cut a hole through the seedpod wall. Because BRVI feeds 

internally within a seed, the parasitoids must cut emergence holes through both the seed coat and the seedpod wall 

to emerge. Observing emergence holes in Scotch broom seeds and seedpods can help determine if parasitoids are 

present, and what species of Scotch broom beetles they may have attacked.

1) Measure the length of the pod (cm) from tip to tip and note in the pod length column (image 1, page 149).

2) Look for BRVI eggs on both sides of the pod and note presence/absence in the BRVI eggs column 

(image 2).

3) Hold the pod firmly but gently so it won’t burst apart once you have started to open it. Split the pod open 

on the inner-curved side of the pod using a thumb nail, scalpel, or other sharp blade. Some pods are tightly 

fused on one end and will require a little nudging with the scalpel.
148 Appendices

4) Once the pod is open, look for parasitic wasps and/or their damage. Look for any small, circular parasitoid 

emergence holes in the seedpod wall (images 3 and 4) and/or parasitoids present in the pod as either larvae 

(images 5 and 6), pupae (image 7), or adults (images 8 and 9). Note presence (Y/N) in the Parasitoid 

presence column. Note: Parasitoid larvae are translucent and do not have a visible head capsule while the 

biocontrol agents are more opaque and have a brown head capsule.

5) When seed pods open, the seeds will lay to one side or the other, so seeds are most easily counted by lining 

the two pod halves side by side as if they were still attached. 

6) Count the number of total seeds, regardless of attacked/not attacked (but DO NOT include aborted seeds 

since these are not viable) and write this in the Total Seeds column. Aborted seeds are shriveled and small 

and do not have biocontrol larvae in them when cut in half with a blade or fingernail (image 10, red arrows).

7) There are three types of normal seeds: healthy unattacked by biocontrol agents, attacked by EXFU, and 

attacked by BRVI. Write the number of each type in the appropriate column.

a. Healthy seeds are hard, smooth, and typically brown in color (image 10, green arrows; image 11).

b. EXFU-attacked seeds always show feeding damage on the outside of the seed coat. Some or all of the 

seed may also be eaten, and a larva or pupa may be present (image 12, left seed). Note the number of 

EXFU-attacked seeds in the column under EXFU.

c. BRVI-attacked seeds do not have external feeding damage unless the adult has already emerged. Seeds 

with BRVI larvae or pupae inside will be smooth but often discolored (image 12, right seed). They are 

easily squished as opposed to hard, unattacked seeds, and they exude a milky juice when squeezed or 

cut with a blade or fingernail. Sometimes larvae are small and multiple cuts are required to reveal their 

presence in a seed. If BRVI has already emerged from the seed, there will be an oval hole at the bottom 

half of the seed (image 13). Note the number of BRVI-attacked seeds in the column under BRVI. 

d. If adult beetles have already emerged within seedpods, they can be used to confirm the number of 

healthy versus destroyed seeds and differentiate the seed damage between the two biocontrol agent 

species (image 14). The number of beetle-damaged seeds should correspond to the number of adults 

emerged, and this number(s) can be written in the appropriate species column(s). Ensure the numbers in 

the three categories of seeds add up to the number you wrote in the Total Seeds column. 

8) Dump and wipe your dissection container after each pod to ensure that you only count the material once. If 

working in a lab or office, keep a neat working area. If a seed goes astray it is helpful to not have too many 

other seed pieces around that will make it harder to find the missing seed.

9) Once all pods have been dissected, make a copy of the data sheet; keep one copy for your files and send the 

other copy to either your state biocontrol specialist, entomologist, extension office, or county noxious weed 

program.
Appendices 149

Seedpod monitoring images: 1. pod length; 2. BRVI eggs; 3., 4. parasitoid emergence holes; 5., 6. parasitoid larvae; 

7. parasitoid pupa; 8., 9. parasitoid adults; 10. aborted (red arrow) vs. healthy (green arrow) seeds; 11. healthy seed cut 

in half; 12. EXFU-attacked seed (left), BRVI-attacked seed (right); 13. BRVI adult emerged from BRVI-attacked seed; 

14. EXFU adult (left), BRVI adult (right). (Photo credits: 1.,13.,14. Jennifer Andreas, Washington State University Extension; 

3.,4. Jessica Orr, Washington State Department of Agriculture; 7. James Moore, Washington State University Extension; 

2.,5.,6.,10.,11.,12. Thomas Shanan, Oregon Department of Agriculture)