FHTET-2007-05

September 2008

Biology and Biological Control

of Exotic True Thistles

Forest Health Technology

Enterprise Team

Technology 

Transfer Biological

Control

Rachel Winston, Rich Hansen, Mark Schwarzländer,

Eric Coombs, Carol Bell Randall, and Rodney Lym

FHTET Forest

Service

U.S. Department 

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

and does not constitute an endorsement by the U.S. Department of Agriculture.

The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities 

on the basis of race, color, national origin, sex, religion, age, disability, political beliefs, sexual orientation, 

or marital or family status. (Not all prohibited bases apply to all programs.) Persons with disabilities 

who require alternative means for communication of program information (Braille, large print, audiotape, 

etc.) should contact USDA’s TARGET Center at 202-720-2600 (voice and TDD).

To file a complaint of discrimination, write USDA, Director, Office of Civil Rights, Room 326-W, 

Whitten Building, 1400 Independence Avenue, SW, Washington, D.C. 20250-9410 or call 

202-720-5964 (voice and TDD). USDA is an equal opportunity provider and employer.

Federal Recycling Program

Printed on recycled paper.

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.

http://www.fs.fed.us/foresthealth/technology/

On the cover: Italian thistle. Photo: ©Saint Mary’s College of California.
Biology and Biological Control

of Exotic Thistles

For additional copies of this publication, contact:

Rachel Winston

Environmental Consultant

MIA Consulting, LLC

Boise, ID

Dr. Mark Schwarzländer

Associate Professor

Department of Plant, Soil, and 

Entomological Sciences

University of Idaho

Moscow, ID

Carol Bell Randall

Entomologist

USDA Forest Service

Forest Health Protection

Coeur d’Alene, ID

Dr. Rich Hansen

Entomologist

USDA Animal and Plant Health Inspection 

Service

National Weed Management Laboratory

Fort Collins, CO

Eric Coombs

Biological Control Entomologist

Oregon Department of Agriculture–Plant 

Division

Salem, OR

Dr. Rodney G. Lym

Professor

Department of Plant Sciences

North Dakota State University

Fargo, ND

Richard Reardon

USDA Forest Service

180 Canfield Street

Morgantown, WV 26505

(304) 285-1566

rreardon@fs.fed.us

Carol Bell Randall

USDA Forest Service

3815 Schreiber Way

Coeur d’Alene, ID 83815

(208) 765-7343

crandall@fs.fed.us

Mark Schwarzländer

University of Idaho

Moscow, ID 83844

(208) 885-9319

markschw@uidaho.edu
Acknowledgments

We would like to thank all of the county weed superintendents and land managers that 

we have worked with through the years for encouraging us to develop our series of biology and biological control manuals. 

We would like to thank Jennifer Andreas (Washington State University Extension), 

Richard Old (XID Services, Inc.), and other excellent photographers who contributed 

many of the photographs in this manual. We also wish to acknowledge Jennifer Andreas 

for her extensive help in gathering species distribution information. 

Some of the material in this manual was adapted from the manual for the Biology and 

Biological Control of Leafy Spurge. We wish to acknowledge the authors of the adapted material (Drs. Rob Bourchier and Andrew Norton).

Finally, we would like to thank Mark Riffe, ITX/USDA Forest Service-Forest Health 

Technology Enterprise Team (FHTET), for editing, layout, and graphics; and Richard 

Reardon, FHTET, for producing this book. 
__________________________________________Biology and Biological Control of Exotic True Thistles

iii

Contents

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

Overview 1

Classical Biological Control of Weeds.............................................................................. 2

International Code of Best Practices for Classical Biological Control of Weeds................. 4

International Code of Best Practices for Classical Biological Control of Weeds ............ 4

Biological Control of Exotic Thistles ................................................................................. 5

Integrated Weed Management.......................................................................................... 6

Is Biological Control of Exotic Thistles Right For You? ....................................................... 6

About This Manual........................................................................................................... 8

Chapter 2: Getting to Know Exotic True Thistles ...................................9

Plant Life Cycle................................................................................................................ 9

Taxonomic Classification of Exotic True Thistles ............................................................. 13

Species Descriptions of Exotic True Thistles.................................................................... 13

Bull Thistle................................................................................................................. 14

Canada Thistle ........................................................................................................... 16

Italian Thistle ............................................................................................................. 18

Marsh Thistle ............................................................................................................. 20

Milk Thistle................................................................................................................ 22

Musk Thistle............................................................................................................... 24

Plumeless Thistle........................................................................................................ 26

Scotch Thistle............................................................................................................. 28

Slenderflower Thistle.................................................................................................. 30

Native True Thistles ........................................................................................................ 33

Cainville Thistle ......................................................................................................... 36

Clustered Thistle ........................................................................................................ 36

Cobwebby Thistle ...................................................................................................... 37

Edible Thistle ............................................................................................................. 37

Eaton’s Thistle ............................................................................................................ 38

Flodman’s Thistle ....................................................................................................... 39

Jackson Hole Thistle................................................................................................... 39

Gray-Green Thistle..................................................................................................... 40

Meadow Thistle ........................................................................................................ 41

Mountain Thistle........................................................................................................ 41

New Mexico Thistle................................................................................................... 42

Palouse Thistle ........................................................................................................... 42

Prairie Thistle ............................................................................................................. 43

Snowy Cobwebby Thistle........................................................................................... 43

Steen Mountain Thistle............................................................................................... 44

Tall Thistle.................................................................................................................. 44
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Biology and Biological Control of Exotic True Thistles _________________________________________

Wavy-Leaved Thistle .................................................................................................. 45

Yellow Thistle............................................................................................................. 45

Yellowspine Thistle .................................................................................................... 46

Chapter 3: Biology of Exotic True Thistle Biological Control Agents ..47

Basic Insect Biology ....................................................................................................... 47

Basic Rust Biology (Order Uredinales) ........................................................................... 49

Approved Thistle Biological Control Agents.................................................................... 50

Ceutorhynchus litura.................................................................................................. 52

Psylliodes chalcomera................................................................................................ 54

Trichosirocalus horridus............................................................................................. 56

Cheilosia corydon...................................................................................................... 58

Urophora cardui........................................................................................................ 60

Urophora solstitialis................................................................................................... 62

Urophora stylata........................................................................................................ 64

Native Natural Enemies of Exotic Thistles....................................................................... 66

Platyptilia carduidactyla............................................................................................. 66

Vanessa cardui........................................................................................................... 68

Unapproved and Adventive Natural Enemies of Exotic Thistles....................................... 70

Aceria anthocoptes.................................................................................................... 71

Cassida rubiginosa ..................................................................................................... 72

Cleonis pigra.............................................................................................................. 73

Larinus planus............................................................................................................ 74

Puccinia carduorum................................................................................................... 75

Puccinia punctiformis ................................................................................................ 76

Rhinocyllus conicus................................................................................................... 77

Terellia ruficauda ....................................................................................................... 78

Chapter 4: Elements of an Exotic True Thistle Biological Control

Program .......................................................................................79

Defining Your Goals and Objectives............................................................................... 79

Taking Stock: Your Infestation and Your Options ............................................................. 79

Developing, Implementing, and Managing a Biological Control Program....................... 81

Before You Begin........................................................................................................ 81

Selecting Biological Control Agent Release Sites........................................................ 82

Choosing the Appropriate Biological Control Agents for Release................................ 84

Obtaining and Releasing Exotic Thistle Biological Control Agents.............................. 86

Collecting Exotic Thistle Biological Control Agents .................................................... 88

Containers For Exotic Thistle Biological Control Agents.............................................. 90

Transporting Exotic Thistle Biological Control Agents ................................................ 91

Purchasing Exotic Thistle Biological Control Agents ................................................... 94

Releasing Exotic Thistle Biological Control Agents ..................................................... 94
__________________________________________Biology and Biological Control of Exotic True Thistles

v

Documenting, Monitoring, and Evaluating a Biological Control Program ....................... 96

The Need for Documentation .................................................................................... 96

Information Databases............................................................................................... 97

Information Collection............................................................................................... 97

Assessing the Status of Exotic Thistles and Other Plants.............................................. 98

Assessing Exotic Thistle Biological Control Agent Populations.................................... 99

Assessing Impacts on Non-Target Plants................................................................... 101

Chapter 5: An Integrated Exotic Thistle Management Program ...........105

Introduction 105

Integrating Biological Control Methods........................................................................ 106

Weed Control Methods Used to Manage Exotic Thistles............................................... 107

Education and Outreach.......................................................................................... 107

Prevention and Exclusion......................................................................................... 107

Monitoring............................................................................................................... 108

Early Detection and Rapid Response........................................................................ 108

Biological Control ................................................................................................... 108

Physical or Mechanical Treatment............................................................................ 109

Cultural Practices..................................................................................................... 110

Chemical Treatment................................................................................................. 113

General Long-Term Land Management Practices.......................................................... 114

Glossary .........................................................................................117

Selected References...........................................................................123

Chapter 1: Introduction................................................................................................ 123

Chapter 2: Getting to Know Exotic Thistles................................................................... 123

Chapter 3: Biology of Exotic Thistle Biological Control Agents ..................................... 128

Chapter 4: Elements of an Exotic Thistle Biological Control Program............................ 129

Chapter 5: An Integrated Exotic Thistle Management Program...................................... 130

Appendices

Appendix I: Troubleshooting Guide; When Things Go Wrong

Appendix II: PPQ Form 526 Interstate Transport Permit

Appendix III: Sample Biological Control Agent Release Form

Appendix IV: Idaho Statewide Thistle Biological Control Agent 2-Pagers and Monitoring 

Forms

Appendix VI: Exotic Thistle Qualitative Monitoring Forms

Appendix VII: Exotic Thistle Biocontrol-Associated Vegetation Monitoring
vi

Biology and Biological Control of Exotic True Thistles _________________________________________

Figures

Figure 1-1. Exotic thistle infestations...................................................................................... 1

Figure 1-2. Thistle head seed weevil, Rhinocyllus conicus. .................................................... 5

Figure 2-1. Generalized thistle life stages (Canada thistle).................................................... 10

Figure 2-2. Thistle flower head (capitulum) diagram and seeds... ......................................... 10

Figure 2-3. Bull thistle.......................................................................................................... 14

Figure 2-4. Canada thistle.................................................................................................... 18

Figure 2-6. Marsh thistle. ..................................................................................................... 20

Figure 2-7. Milk thistle......................................................................................................... 22

Figure 2-8. Musk thistle. ...................................................................................................... 24

Figure 2-9. Plumeless thistle. ............................................................................................... 26

Figure 2-10. Scotch thistle................................................................................................... 28

Figure 2-11. Slenderflower thistle........................................................................................ 30

Figure 2-12. Cainville thistle................................................................................................ 36

Figure 2-13. Clustered thistle............................................................................................... 36

Figure 2-14. Cobwebby thistle............................................................................................. 37

Figure 2-15. Edible thistle.................................................................................................... 37

Figure 2-16. Eaton’s thistle................................................................................................... 38

Figure 2-17. Flodman’s thistle.............................................................................................. 39

Figure 2-18. Jackson Hole thistle. ........................................................................................ 39

Figure 2-19. Gray-green thistle. ........................................................................................... 40

Figure 2-20. Meadow thistle................................................................................................ 41

Figure 2-21. Mountain thistle. ............................................................................................. 41

Figure 2-22. New Mexico thistle. ........................................................................................ 42

Figure 2-23. Palouse thistle.................................................................................................. 42

Figure 2-24. Prairie thistle. .................................................................................................. 43

Figure 2-25. Snowy cobwebby thistle.................................................................................. 43

Figure 2-26. Steen Mountain thistle..................................................................................... 44

Figure 2-27. Tall thistle........................................................................................................ 44

Figure 2-28. Wavy-leaved thistle. ........................................................................................ 45

Figure 2-29. Yellow thistle................................................................................................... 45

Figure 2-30. Yellowspine thistle. ......................................................................................... 46

Figure 3-1. Complete metamorphosis. ................................................................................. 47

Figure 3-2. Generalized adult insect anatomy. ..................................................................... 48

Figure 3-3. Keys to identification of insect larvae and insect pupae...................................... 49

Figure 3-4. General location of attack for approved biological control agents...................... 51

Figure 3-5. Ceutorhynchus litura adult. ................................................................................ 52

Figure 3-6. Life cycle of C. litura. ........................................................................................ 52

Figure 3-7. Ceutorhynchus litura larvae and damage............................................................ 53

Figure 3-8. Psylliodes chalcomera adult. .............................................................................. 54

Figure 3-9. Life cycle of P. chalcomera. ............................................................................... 54

Figure 3-10. Trichosirocalus horridus adult. ......................................................................... 56
__________________________________________Biology and Biological Control of Exotic True Thistles

vii

Figure 3-11. Life cycle of T. horridulus. ............................................................................... 56

Figure 3-12. Trichosirocalus horridus larva and damage. ..................................................... 57

Figure 3-13. Cheilosia corydon adult................................................................................... 58

Figure 3-14. Life cycle of C. corydon. ................................................................................. 58

Figure 3-15. Chelosia corydon larval mining damage.......................................................... 59

Figure 3-16. Urophora cardui adult..................................................................................... 60

Figure 3-17. Life cycle of U. cardui. .................................................................................... 60

Figure 3-18. Urophora cardui gall. ...................................................................................... 61

Figure 3-19. Urophora cardui pupa in multi-chambered gall............................................... 61

Figure 3-20. Urophora solstitialis adult. ............................................................................... 62

Figure 3-21. Life cycle of U. solstitialis. ............................................................................... 62

Figure 3-22. Urophora solstitialis larvae in a flower head..................................................... 63

Figure 3-23. Urophora stylata adult..................................................................................... 64

Figure 3-24. Life cycle of U. stylata. .................................................................................... 64

Figure 3-25. Urophora stylata larvae in a flower head. ........................................................ 65

Figure 3-26. Platyptilia carduidactyla adult.......................................................................... 66

Figure 3-27. Life cycle of P. carduidactyla............................................................................ 67

Figure 3-28. Platyptilia carduidactyla larva.......................................................................... 67

Figure 3-29. Vanessa cardui adult........................................................................................ 68

Figure 3-30. Vanessa cardui larva and pupa......................................................................... 68

Figure 3-31. Life cycle of V. cardui. .................................................................................... 69

Figure 3-32. Vanessa cardui larva and damage. ................................................................... 69

Figure 3-33. Aceria anthocoptes.......................................................................................... 71

Figure 3-34. Cassida rubiginosa adult. ................................................................................. 72

Figure 3-35. Cleonis pigra adult........................................................................................... 73

Figure 3-36. Larinus planus adult......................................................................................... 74

Figure 3-37. Puccinia carduorum. ....................................................................................... 75

Figure 3-38. Puccinia punctiformis. ..................................................................................... 76

Figure 3-39. Rhinocyllus conicus adult. ............................................................................... 77

Figure 3-40. Terellia ruficauda adult. ................................................................................... 78

Figure 4-1. Hypothetical weed infestation maps for Idaho Forest Service land and

Snake River Birds of Prey National Conservation Area........................................ 80

Figure 4-2. Canada thistle infestation suitable for a biological control program.................... 83

Figure 4-3. Typical musk thistle release site. ....................................................................... 86

Figure 4-4. Participants in a field day in New Mexico. ......................................................... 87

Figure 4-5. Canvas sweep net. ............................................................................................. 88

Figure 4-6. Aspirator. ........................................................................................................... 90

Figure 4-7. Cardboard containers......................................................................................... 90

Figure 4-8. Commercially made shipping container. ............................................................ 92

Figure 4-9. Permanent marker for biological control agent release site................................. 94

Figure 4-10. Establishment of a Canada thistle biological control monitoring site................ 97

Figure 4-11. Cheilosia corydon larval mining damage. ...................................................... 101

Figure 4-12. Native Cirsium undulatum............................................................................. 102
viii

Biology and Biological Control of Exotic True Thistles _________________________________________

Figure 5-1. A pasture before and five years after the release of Rhinocyllus conicus. .......... 106

Figure 5-2. Various habitats where exotic thistles can thrive............................................... 107

Figure 5-3. Weed education sign erected by Sublette County, Wyoming............................ 108

Figure 5-4. Mechanical treatment. ..................................................................................... 109

Figure 5-5. Prescribed fire.................................................................................................. 111

Figure 5-6. Cattle and sheep grazing.................................................................................. 112

Figure 5-7. Applying herbicides. ....................................................................................... 113

Tables

Table 1-1. Exotic thistles discussed in this manual.................................................................. 2

Table 1-2. Advantages and disadvantages of classical biological control as a weed 

management tool. ................................................................................................. 3

Table 2-1. Taxonomic classification of exotic true thistles. ................................................... 13

Table 2-2. Trait comparisons of nine exotic thistles in western North America...................... 32

Table 2-3. Native true thistle species in the northwestern U.S. ............................................. 33

Table 3-1. Biological control agents approved for release in the U.S. against exotic thistles. 50

Table 3-2. Selected insects native to North America that attack exotic thistles...................... 66

Table 3-3. Thistle biological control agents and adventive natural enemies present in North 

America but not approved................................................................................... 70

Table 4-1. Summary of general characteristics and site preferences of exotic thistle biological 

control agents released in the United States (through 2007)................................. 85

Table 4-2. Recommended timetable and methods for collecting approved exotic thistle 

biological control agents in the U.S..................................................................... 89

Table 4-3. Life stages/damage to look for to determine establishment of exotic thistle 

biological control agents. .................................................................................. 100

Table 5-1. Comparison of exotic thistle management options............................................. 115

Simple Key to Nine Exotic True Thistle Species Commonly Found in Western 

North America..................................................................................................... 11
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 1: Introduction 1

Chapter 1: Introduction

Carol Bell Randall and Rachel Winston

Overview

“Thistle” is an old English name for a large variety of weedy, prickly plants that grow 

throughout the world. The most notable characteristics of thistles are the prickly stems and 

leaves and the bracts around the flower head. While many different plants have “thistle” 

in their common name, only certain plant species fit the taxonomic requirements of being considered “true thistles.” True thistle species fall within the family Asteraceae, the 

tribe Cardueae, and the subtribe Carduinae. Examples of plants that are not true thistles 

include yellow starthistle (subtribe Centaureinae), sow thistle (subtribe Sonchinae), and 

Russian thistle (family Chenopodiaceae). Only true thistles in the subtribe Carduinae are 

discussed in this manual.

The North American flora includes many true native thistles that are important components 

of healthy, natural ecosystems. In addition to native thistle species, there are true thistle 

species that are exotic and invasive in the United States and Canada (Figure 1-1.). This 

manual addresses the biological control of nine exotic thistles present in western North 

America (Table 1-1). 

 

Figure 1-1. Exotic thistle infestations. Photos: left, Mark Schwarzländer (University of Idaho); right, 

Thurston County Noxious Weed Board.
2 Chapter 1: Introduction

Biology and Biological Control of Exotic True Thistles _________________________________________

Table 1-1. Exotic thistles discussed in this manual.

Scientific name Common name Duration

Carduus acanthoides Plumeless thistle Annual

Carduus nutans Musk thistle Biennial

Carduus pycnocephalus Italian thistle Annual

Carduus tenuiflorus Slenderflower thistle Annual

Cirsium arvense Canada thistle Perennial

Cirsium palustre Marsh thistle Biennial

Cirsium vulgare Bull thistle Biennial

Onopordum acanthium Scotch thistle Biennial

Silybum marianum Milk thistle Annual

Exotic thistles currently infest millions of acres across the United States, primarily occurring 

along roadways, waterways, agricultural fields, and in rangelands, pastures, forests, and 

disturbed areas. They are responsible for millions of dollars of damage annually in reduced 

agricultural yields and lowered forage value in grazing systems. Exotic invasive thistles can 

also displace native vegetation, negatively impacting wildlife and threatening the delicate 

ecological balance within many habitats. 

Herbicides have traditionally been the most popular weed control method used to manage 

exotic thistles, and some long-term herbicide programs have been relatively successful. 

However, herbicides are not always the appropriate management tool due to their high 

costs, potential for environmental contamination, plant resistance, non-target effects, and 

prohibition in environmentally sensitive areas. Non-chemical weed control methods are 

becoming increasingly important component alternatives in integrated exotic thistle management programs, including the introduction of biological control agents.

Classical Biological Control of Weeds

Most invasive plants in North America are not native; 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 the plant in its native range. The lack of natural enemies is one reason non-native plant 

species become invasive pests when introduced in areas outside of their native range. 

Biological control (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 host-specific natural enemies from 

the weed’s native range with the target weed. Biological control agents feed on a weed’s 

flowers, seeds, roots, foliage, or stems. This damage may kill the weed outright, reduce its 

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

which reduce the weed’s ability to compete with other plants. 
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 1: Introduction 3

Natural enemies used in classical biological control of weeds include a variety of organisms, 

such as insects, mites, nematodes, and fungi. In the U.S., most weed biological control 

agents are plant-feeding insects. Beetles, flies, and moths are among the most commonly 

used insects. To be considered for release in the United States, biological control agents 

must feed and develop only on the target weed and, in some cases, on a few closely related 

plant species. A potential biological control agent’s life cycle should be closely matched, or 

synchronized, with the development of the target weed. If properly synchronized, foliagefeeding insects would be in the feeding stage when the weeds are actively growing and 

seed-feeding insects would be in the feeding stage when the plant is actively developing 

seeds. 

The most effective biological control agents tend to be those that damage the most vulnerable or most problematic and persistent parts of the host plant. Root- and stem-feeding 

biological control agents are usually more effective against perennial plants that primarily 

spread by root buds. Alternatively, flower- and seed-feeding biological control insects are 

typically more useful against annual or biennial species that only spread by seeds. 

There are advantages and disadvantages to biological control of weeds. Some of these are 

listed in Table 1-2. 

Table 1-2. Advantages and disadvantages of classical biological control as a weed management 

tool.

Advantages Disadvantages

Target specificity Protracted time until impact is likely

Continuous action Unpredictable level of control

Long-term cost-effective Uncertain “non-target” effects in the ecosystem

Gradual in effect Irreversible

Generally environmentally benign Not all exotic weeds are appropriate targets

Self dispersing, even into difficult terrain Will not work on every weed in every setting

Host specificity is a crucial point of consideration for a natural enemy to be released as a 

biological control agent. Host specificity is the extent to which a biological control agent 

can survive only on the target weed. Potential biological control agents often undergo 

more than five years of rigorous testing to ensure that host specificity requirements are 

met. These tests are necessary in order to ensure that the biological control agents are 

effective and that they will damage only the target weed.

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

-Plant Protection and Quarantine (USDA-APHIS-PPQ) is the federal agency responsible 

for authorizing the importation of biological control agents into the United States. The 

Canadian Food Inspection Agency (CFIA) serves the same role in Canada. Federal laws 

and regulations are in place to minimize the risks to native plant and animal communities 

associated with introduction of exotic organisms to manage weeds. The Technical Advisory 
4 Chapter 1: Introduction

Biology and Biological Control of Exotic True Thistles _________________________________________

Group (TAG) for Biological Control Agents of Weeds is an expert committee with representatives from regulatory agencies, federal land management offices, and environmental 

protection agencies from the U.S. and representatives from Canada and Mexico. TAG 

reviews all petitions to import new biological control agents into the United States and 

makes recommendations to USDA-APHIS-PPQ about the safety and potential impact of 

prospective biological control agents. Weed biological control researchers work closely with 

USDA-APHIS-PPQ and TAG to accurately assess the environmental safety of potential 

weed biological control agents and programs. The Canadian counterpart to TAG is the 

Biological Control Review Committee (BCRC). 

In addition, each state in the United States has its own approval process to permit field 

release of weed biological control agents.

International Code of Best Practices for Classical Biological Control 

of Weeds

Biological control practitioners have adopted the International Code of Best Practices for 

Biological Control of Weeds. The Code was developed by delegates and participants to the 

Tenth International Symposium for Biological Control of Weeds to reduce the potential 

for negative impacts from biological control of noxious weed activities By following the 

Code, practitioners reduce the potential for causing environmental damage through the 

use of biological control by voluntarily restricting biological control activities to those 

most likely to result in success. 

International Code of Best Practices for Classical Biological 

Control of Weeds

1. Ensure that thetarget weed’s potential impact justifies release of non-endemic 

agents

2. Obtain multi-agency approval for target

3. Select agents with potential to control target

4. Release safe and approved agents

5. Ensure that only the intended agent is released

6. Use appropriate protocols for release and documentation

7. Monitor impact on the target

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

9. Monitor impacts on potential non-targets

10. Encourage assessment of changes in plant and animal communities

11. Monitor interaction among agents

12. Communicate results to public

Although weed biological control is an effective and important weed management tool, it 

does not work in all cases and is not expected to eradicate or completely remove the target 

weed. Ideally, biological control should be integrated with other chemical, mechanical, or 

cultural methods of weed control to improve overall weed control success.
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 1: Introduction 5

Biological Control of Exotic Thistles

Developing a biological control program for invasive thistles has a number of unique 

challenges. Thistles are a diverse group of plants exhibiting different life cycles (annual, 

biennial, and perennial) and occurring in many habitats. 

One of the key characteristics of a successful biological control agent is host specificity. 

There are over 100 species and subspecies of closely related native thistles in the U.S. that 

may potentially be impacted by exotic thistle biological control agents. In order for a biological control agent to be approved for release in the United States or Canada, biological 

control researchers must demonstrate that the agent will not feed and develop on native 

thistles. Because of their large number, host specificity testing for native thistle species is 

complicated and time-consuming.

Seed-feeding insects are often chosen for biological control programs because, as a group, 

they often have relatively high levels of host specificity. The exotic thistles described in 

this manual are, for the most part, not seed-limited, so they are not particularly vulnerable 

to seed-feeding biological control agents. To increase the chances of success, researchers 

must identify biological control agents from other feeding guilds, such as stem-feeding or 

root-feeding organisms.

Some of the earliest biological control programs in 

the United States and Canada targeted exotic thistles. 

Many biological control agents have been intentionally 

and unintentionally released for exotic thistle control. 

In 1969, Rhinocyllus conicus (Figure 1-2) became the 

first approved exotic thistle biological control agent to 

successfully establish on musk thistle in Montana and 

Virginia. When this agent was approved, it was known 

to feed on some native North American thistles. Our 

understanding of the importance of native thistles to 

ecosystem function was not well-developed; many 

states and land management agencies even considered 

some native thistles to be weedy pests. Consequently, 

the non-target feeding was not considered to be a 

problem. In the 1990s, once scientists recognized 

that this weevil was causing unacceptable damage to 

native thistles and habitats, its release permits were revoked.

Research is currently underway to assess the control level that individual biological control 

agents or combinations of biological control agents have on exotic thistles. As of 2008, 

a total of seven insect species have been approved for release in the United States and 

Canada as classical biological control agents of exotic thistles. The biology of these agents 

is presented in Chapter 3. It should be noted that although widely established, none of 

these approved exotic thistle biological control agents have yet built up to population levels 

that would be similar to those of a plant pest.

Figure 1-2. Thistle head seed weevil, 

Rhinocyllus conicus. 

Photo: Laura Parsons 

(University of Idaho).
6 Chapter 1: Introduction

Biology and Biological Control of Exotic True Thistles _________________________________________

Integrated Weed Management

The successful management of noxious weeds usually incorporates several control methods 

and activities over a number of years in order to reach the land manager’s weed management objectives. The use of multiple weed control methods is called Integrated Weed 

Management. A successful weed management program relies on realistic management 

objectives, accurate weed identification and mapping, and post-treatment monitoring to 

answer the question: “Are current weed management activities enabling me to meet my 

weed management objectives?” Most successful weed management programs incorporate 

the following weed control methods: chemical (herbicides), mechanical, and cultural treatments and biological control. 

Land managers choose weed control methods that will enable them to achieve their goal in 

the most cost-effective manner. The control method(s) employed in a weed management 

strategy will depend on the size and location of the infested area, the target thistle species, 

and specific management goals (e.g., eradication vs. weed density reduction). Small patches 

of exotic thistles may be eliminated with a persistent herbicide program, but large areas 

will often require that managers employ additional control methods. 

No single control method will enable managers to meet their exotic thistle management 

objectives in all environments. A combination of control methods, such as biological control with supplemental cultural practices or chemical controls, consistently applied through 

time is usually necessary to attain management objectives for exotic thistles, especially when 

they infest large acreages. 

Is Biological Control of Exotic Thistles Right For You?

When biological control is successful, biological control agents behave like a pest species 

of the target weed: they increase in abundance until they suppress the target weed. As local weed populations are reduced, biological control agent populations decline with them 

due to starvation and/or dispersal. 

Some factors to be aware of before starting biological control activities include the following: 

•	 The efficacy of biological control agents cannot be guaranteed.

•	 Biological control will not work every time in every situation.

•	 Biological control will not eradicate the weed.

•	 Biological control may not, by itself, provide the desired level of control.

•	 It might take years before you can see biological control impacts.

For these reasons, we recommend that you develop an integrated weed management 

program in which biological control is one of many weed control methods used. Here are 

some questions you should ask before you begin a biological control program.
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 1: Introduction 7

What are my weed management goals: to eradicate the weed or reduce weed abundance?

Biological control does not eradicate target weeds, so it is not a good fit with an eradication goal. Depending on the target weed, biological control agent used, and land use, 

biological control can be effective at reducing the abundance of a target weed. If your 

goal is to reduce weed abundance, then biological control may help you achieve it.

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

Biological control takes time to work, so another weed management method may be a 

better choice if you need to show immediate results. 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 agents to cause significant impacts to the target weed. Biological 

control may not be your best choice if you are looking for results within one to two 

seasons. In some weed infestations, more than five years may be needed for biological 

control to reach its weed-management goal. 

What resources can I devote to my weed problem? 

If you have a small weed problem (small infested area), weed control methods such 

as herbicides or hand pulling, followed by annual monitoring for re-growth, may be 

most effective in terms of reaching weed management goals with the lowest cover 

over time. These intensive control methods may allow you to achieve rapid control 

and prevent the weed from infesting more area. However, if an invasive weed is wellestablished over a large area, and resources are limited, biological control may be your 

most economical weed control option. 

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

Invasive plant infestations often occur where desirable plant communities have been 

disturbed. If the disturbance continues without restoration of a desirable, resilient 

plant community, biological control may not enable you to successfully deal with your 

weed problems.

The ideal biological control program:

1. is based upon an understanding of weed, habitat, and land use conditions,

2. is part of a broader integrated weed management program,

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

is the best option based on available resources and weed management objectives, 

and

4. has realistic goals and timetables and includes adequate monitoring. 
8 Chapter 1: Introduction

Biology and Biological Control of Exotic True Thistles _________________________________________

About This Manual

This manual provides information on nine species of exotic thistles commonly found 

in western North America and each of their biological control agents. It also presents 

guidelines to establish and manage biological control agents as part of an exotic thistle 

management program.

Chapter 1: Introduction provides introductory information on exotic thistles, and biological control. 

Chapter 2: Getting to Know Exotic Thistles provides detailed descriptions of taxonomy, 

growth characteristics and features, habitat, and occurrence in the United States of nine 

exotic thistles. It also presents general information on 19 native thistle species with which 

some of the exotic species may be confused. Photographs and drawings are provided. 

Chapter 3: Biology of Thistle Biological Control Agents describes the biological control agents of exotic thistles. Included is information on each biological control agent’s 

native range, original source of North American releases, part of plant attacked, life cycle, 

description, destructive stages, host specificity, known non-target effects, habitat preferences, and availability. This chapter is particularly useful for identifying biological control 

agents in the field.

Chapter 4: Elements of a Thistle Biological Control Program includes detailed information and guidelines on how to plan, implement, monitor, and evaluate an effective exotic 

thistle biological control program. Included are guidelines and methods for:

•	 Selecting and preparing release sites,

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

agents, and

•	 Monitoring biological control agents and vegetation.

Chapter 5: An Integrated Exotic Thistle Management Program discusses the role of 

exotic thistle biological control in the context of an integrated exotic thistle management 

plan. 

The Glossary defines technical terms frequently used by those involved in exotic thistle 

biological control. 

The Appendices are:

I Troubleshooting Guide: When Things Go Wrong

II PPQ Form 526: Interstate Transport Permit

II Sample Biological Control Agent Release Form

IV Idaho Statewide Thistle Biological Control Agent 2-Pagers and Monitoring 

Forms 

V General Biological Control Agent Monitoring Form

VI Exotic Thistle Qualitative Monitoring Form

VII Exotic Thistle Biocontrol-Associated Vegetation Monitoring
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 2: Getting to Know Exotic True Thistles 9

Chapter 2: Getting to Know Exotic True Thistles

Rachel Winston and Eric Coombs

Exotic true thistles in North America are represented by a number of different genera, all 

of which are in the sunflower family (Asteraceae). This family is large and very diverse and 

includes dandelions, sunflowers, and daisies. There are 20 species (and additional subspecies 

and varieties) of exotic thistles in the United States. Nine species, occurring primarily in the 

western states, are described in detail within this manual. Under favorable conditions, all 

nine are capable of forming large infestations and having serious environmental impacts. 

Plant Life Cycle

Thistles may be annuals, biennials, or perennials, which means they may take 1, 2, or more 

years, respectively, to complete their life cycle (Figure 2-1). The life cycle of all thistles 

begins as a seedling that develops into a rosette with 5 to 12 leaves. The leaves of all true 

thistles are characterized by sharp spines along the margins. Biennial and perennial thistles 

remain as rosettes their entire first year; annual thistles germinate and flower within the 

same growing season. 

Like all other members of the sunflower family, thistles produce flower heads, or capitula, 

that are an aggregation of many individual flowers (Figure 2-2). These flowers, called 

florets, are clustered together and attached to a receptacle. Both the receptacle and florets 

are enclosed by modified leaves called involucral bracts. Each floret produces one seed 

(achene) from mid- to late-summer. Some species produce seeds with a tuft of whitish 

hairs (pappus) on one end, similar to those on seeds of dandelions. 

Seeds are 4-5 mm long and dispersed primarily by wind. The seeds of most thistle species 

may remain viable for 20 years, though the majority of seeds germinate within one year. 

Flowering stems may be unbranched or branched and have one to many flower heads on 

each stem. The color, shape, and texture of thistle foliage, color of flowers, capitulum size, 

bract shape and texture, seed color and shape, and branching habit are important traits for 

distinguishing thistle species. 
10 Chapter 2: Getting to Know Exotic True Thistles

Biology and Biological Control of Exotic True Thistles _________________________________________

Figure 2-1. Generalized thistle life stages (Canada thistle): a. seedling, b. rosette, c. bolting, d. bud, 

e. flowering, f. senescence. Photos: a, d, and e, Richard Old (XID Services, Inc., www.

xidservices.com); b, Jennifer Andreas (Washington State University Extension); c and f, 

Rachel Winston (MIA Consulting).

Figure 2-2. Thistle flower head (capitulum) diagram and 

seeds. Photos: flower head, Rachel Winston 

(MIA Consulting); seeds, Richard Hansen (USDAAPHIS-PPQ-CPHST)..

The taxonomic key depicted on the following pages can 

be used to differentiate the nine exotic true thistle species 

described in this manual. Following the key, each of the 

nine exotic true thistle species are described. Taxonomic 

classification and distribution information was taken from 

the USDA PLANTS database. (PLANTS is a collaborative 

effort of the USDA NRCS National Plant Data Center, the 

USDA NRCS Information Technology Center, and many 

other partners. See www.plants.usda.gov.)
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 2: Getting to Know Exotic True Thistles 11

Simple Key to Nine Exotic True Thistle Species Commonly Found in 

Western North America

Rachel Winston (MIA Consulting)

Parts not drawn to scale.

1a) Spines extend along entire length of stems

2a) Flower heads in clusters, each head measuring less than (or up to) 1 inch 

(2.5 cm) in diameter

3a) Bracts not spiny

Marsh thistle, Cirsium palustre

3b) Bracts spine-tipped

4a) Bracts needlelike, not broadly triangular in shape

Plumeless thistle, Carduus acanthoides

4b) Bracts not needlelike, broadly triangular

5a) Bracts with stiff, forward-pointing hairs

Italian thistle, Carduus pycnocephalus

5b) Bracts without stiff, forward-pointing hairs

Slenderflower thistle, Carduus tenuiflorus
12 Chapter 2: Getting to Know Exotic True Thistles

Biology and Biological Control of Exotic True Thistles _________________________________________

2b) Flower heads usually solitary, each head measuring greater than 1 inch 

(2.5 cm) in diameter

6a) Leaves and foliage gray in appearance, plant growing up to 12 feet 

(3.6 m) in height

Scotch thistle, Onopordum acanthium

6b) Leaves and foliage not gray in appearance, plant never growing 

beyond 6 feet (1.8 m) in height

Bull thistle, Cirsium vulgare

1b) Spines do not extend along the entire length of stems (there is a smooth 

expanse beneath flower heads at least) (Insert non spiny stem)

7a) Flower heads in clusters, each head measuring less than 1 inch (2.5 cm) 

in diameter

Canada thistle, Cirsium arvense

7b) Flower heads usually solitary, each head measuring greater than 1 inch 

(2.5 cm) in diameter

8a) Bracts not thick, less than 1⁄2 inch (1.25 cm) long, not fringed with 

smaller spines, purple at maturity

Musk thistle, Carduus nutans

8b) Bracts thick, greater than 1⁄2 inch (1.25 cm) long, fringed with smaller 

spines, green at maturity

Milk thistle, Silybum marianum
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 2: Getting to Know Exotic True Thistles 13

Taxonomic Classification of Exotic True Thistles

All exotic thistles described in this manual are true thistles. As such, they fall within the 

same Subtribe of the Asteraceae (Table 2-1). True thistles differ only in their classification 

at the level of Genus or Species.

Table 2-1. Taxonomic classification of exotic true thistles.

Superdivision Spermatophyta

Division Magnoliophyta

Class Magnoliopsida

Subclass Asteridae

Order Asterales

Family Asteraceae

Tribe Cardueae

Subtribe Carduinae

Species Descriptions of Exotic True Thistles

The following pages contain descriptions of the exotic thistles. The thistles are compared 

to each other in Table 2-2 on page 38.
14 Chapter 2: Getting to Know Exotic True Thistles

Biology and Biological Control of Exotic True Thistles _________________________________________

Bull Thistle

Scientific name: Cirsium vulgare (Savi) Tenore

Synonyms: Spear thistle

Carduus lanceolatus L.

Carduus vulgaris Savi

Cirsium lanceolatum (L.) Scop., non Hill, Scop. var. hypoleucum DC

Bull thistle is native to Europe, western Asia, and northern 

Africa; it was introduced to the United States in the late 

1800s. It is mostly a biennial but can also behave as an 

annual or very short-lived perennial. This species grows 

3 to 4 feet (0.9 to 1.2 m) tall on average and reproduces 

only by seed. Seeds germinate and form rosettes whenever 

moisture is sufficient, but the majority of rosettes form 

during spring. Bolting occurs in May. Flowering occurs 

from June to July.

Leaves: Basal leaves are long (3 to 12 inches or 7.6 to 30 

cm), lance-shaped, and lobed. Coarse hairs cover the 

upper surface of the leaf blade, while the lower surface 

is woolly. Long, yellow spines extend from the midrib 

and at each lobe, creating dagger-like tips. Terminal 

spines from the midrib grow longer than the others. 

Basal leaves form rosettes that grow up to 3 feet (0.9 

m) in diameter. The bases of stem leaves attach to the 

stem in such a manner as to form long wings. Leaves 

grow alternately along the stems. 

Stems: Stems are erect, stout, and hairy and can have 

many branches, though they sometimes appear solitary on the plant. Stems are spiny along their entire 

length. 

Flowers: Bull thistle capitula are 11⁄2 to 2 inches (3.8 to 

5 cm) in diameter, 1 to 2 inches (2.5 to 5 cm) long, 

and are usually solitary at the ends of shoots and 

branches but may form small clusters at stem ends. 

Receptacles have rows of narrow, spiny bracts tipped 

in yellow. A single capitulum can produce more than 

200 florets ranging from light pink to deep purple 

in color. All florets are hermaphroditic, having both 

male and female reproductive parts. 

Figure 2-3. Bull thistle. a. plant (Marianna Szucs, University of Idaho), b. flower (Jennifer Andreas, Washington 

State University Extension), c. stem (Jennifer Andreas, Washington State University Extension), d. 

U.S. distribution.
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 2: Getting to Know Exotic True Thistles 15

Seeds: Achenes of bull thistle are 1/8 inch (3 mm) long and 1/16 inch (1.5 mm) wide. 

They are yellow in color, streaked with black, and have long white feathered pappus 

(3/4 inch or 1.9 cm long) that aids in wind dispersal over long distances. Mature plants 

can produce up to 4,000 seeds per plant.

Roots: The root system consists of one long taproot that grows up to 28 inches (71 cm) 

long. This plant does not reproduce vegetatively. 

Habitat and Ecology: Bull thistle grows best on neutral soils rich in nitrogen and with 

moderate moisture. It is not typically found in sand, pure clay, or in soils with high 

humus content, nor does it grow well in shade or drought conditions. It can be found 

in almost any type of disturbed area, including forest clearcuts, riparian areas, and 

pastures, where it can form dense thickets that displace other vegetation. Overgrazing 

pastures makes infestations worse. 

Noxious Weed Status: This species is listed as noxious in nine states: Arkansas, Colorado, 

Iowa, Maryland, Minnesota, New Mexico, Oregon, Pennsylvania, and Washington. 

Notes: Phenolic acids produced by the thistle have allelopathic effects against competing 

plants and (along with spines) serve as a defense against herbivory.

Approved Biological Control Agents: Bull thistle is attacked by Cheilosia corydon (a fly), 

Trichosirocalus horridus (a weevil), and Urophora stylata (a fly). 
16 Chapter 2: Getting to Know Exotic True Thistles

Biology and Biological Control of Exotic True Thistles _________________________________________

Canada Thistle

Scientific name: Cirsium arvense (L.) Skopoli

Synonyms: Creeping thistle

Field thistle

Breea arvensis Less. 

Breea incana (Gmel.) W.A. Weber 

Carduus arvensis (L.) Robson 

Cirsium arvense (L.) Scop. varieties: argenteum (Vest) Fiori, 

horridum Wimmer & Grab., integrifolium Wimmer & Grab., 

mite Wimmer & Grab., vestitum Wimmer & Grab.

Cirsium incanum (Gmel.) Fisch.

Cirsium setosum (Willd.) Bess. ex Bieb.

Serratula arvensis L.

Canada thistle is native to Europe, parts of northern 

Africa, and Asia south to Afghanistan, Iran and Pakistan. 

It was introduced to the United States in the 1600s. It 

is a perennial that grows to 4 feet (1.2 m) tall on average 

and reproduces both vegetatively through its roots and 

by seed. Seeds germinate and form rosettes whenever 

moisture is sufficient, though the majority of rosettes 

are formed in spring. Flowering stems bolt in late spring. 

Flowering occurs from June to September.

Leaves: Leaves are irregularly lobed and have very prickly 

and ruffled margins. They are green on both sides 

and have a slightly downy lower surface. Basal leaves 

are less than 5 inches long. Along the stems, leaves 

grow alternately. 

Figure 2-4. Canada thistle. a. plant (Steve Dewey, Utah State University, Bugwood.org) b. male flower (Richard 

Old, XID Services, Inc., www.xidservices.com), c. stem and leaf (Jennifer Andreas, Washington State 

University Extension), d. U.S. distribution.
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 2: Getting to Know Exotic True Thistles 17

Stems: Stems are slender and grooved and can be highly branched. They are slightly hairy 

when young but become covered with fine hair as the plant grows. Stems are not spiny. 

Stems die back in the fall and winter, with new shoots produced each spring from old 

stem bases or root buds. Because of its ability to vegetatively reproduce, what appear to 

be individual Canada thistle plants in fact may be individual flowering shoots (ramets) 

of the same plant. 

Flowers: Canada thistle produces numerous small, compact (1/2 inch or 1.25 cm in diameter) capitula in clusters along upper stems. Bracts on the receptacle are not spiny. 

This species is dioecious: the florets on all flowering shoots of a single clonal plant 

are either male or female. This trait is unique among North American exotic thistles. 

At some sites, infestations consist of plants of only one sex. Florets vary in color from 

white to deep lavender. 

Seeds: Achenes of Canada thistle are small (1/6 to 1/5 inches or 4 to 5 mm long), light 

brown, smooth, slightly tapered, and with a tuft of tan pappus loosely attached to the 

tip. A single flowering shoot produces 1,500 seeds on average. Male plants can produce 

copious amounts of pappus.

Roots: The root system of Canada thistle is rhizomatous, allowing for the vegetative reproduction of this species. It consists of a series of lateral shoots spreading horizontally 

from a central taproot and extending up to 3 feet (0.9 m) deep into the ground. New 

flowering shoots arise every 2 to 6 inches (5 to 15 cm) along the lateral roots. Root 

fragments 1/4 inch (6 mm) long by 1/8 inch (3 mm) in diameter have enough stored 

energy to develop new plants and can survive at least 100 days without nutrient replenishment from photosynthesis.

Habitat and Ecology: Canada thistle grows best in disturbed, moist sites, including prairies, 

meadows, ditches, streambanks, lawns, and agricultural fields. It does not grow well 

in shade and is not common in undisturbed areas. Plowing, disking, or overgrazing 

pastures makes infestations worse.

Noxious Weed Status: This species is listed as noxious in 32 states: Alaska, Arkansas, Arizona, California, Colorado, Delaware, Hawaii, Idaho, Illinois, Indiana, Iowa, Kansas, 

Kentucky, Maryland, Michigan, Minnesota, Missouri, Montana, Nebraska, Nevada, 

New Mexico, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, 

South Dakota, Utah, Washington, Wisconsin, and Wyoming.

Notes: This plant has possible allelopathic effects against competing species. Because of 

its ability to regrow from very small root fragments, cultivation often exacerbates the 

problem in most areas, aiding infestations in spreading further. 

Approved Biological Control Agents: Canada thistle is attacked by Ceutorhynchus litura

(a weevil) and Urophora cardui (a fly) and less often by Trichosirocalus horridus (a 

weevil).
18 Chapter 2: Getting to Know Exotic True Thistles

Biology and Biological Control of Exotic True Thistles _________________________________________

Italian Thistle

Scientific name: Carduus pycnocephalus L.

Italian thistle is native to the Mediterranean region, 

southern Europe, northern Africa, and western Asia; it 

was introduced to the United States in the early 1900s. 

It is a winter annual that occasionally grows as a biennial. 

This species grows to 4 feet (1.2 m) tall on average and reproduces only by seed. Seeds germinate and form rosettes 

in the fall following autumn rains. Flowering stems bolt 

in early spring. Flowering occurs from April to July.

Leaves: Leaves are lance-shaped, green, and hairless above 

and covered with short matted hairs on the undersides. Basal leaves grow up to 5 inches (13 cm) long 

and 21⁄2 inches (6.5 cm) wide and have four to eight 

lobes and spines on lobe tips. Terminal spines grow 

larger than the others. Stem leaves grow alternately 

and lightly clasp the stem. 

Stems: Stems are slender, slightly winged, and spiny 

along their entire length. They may be branched or 

unbranched. 

Flowers: Italian thistle produces numerous small, 

thimble-sized capitula (up to 1 inch or 2.5 cm long) 

in clusters of two to five at branch tips. Receptacles 

have very long, triangular bracts that partially cover 

the florets with their tips. Bracts have many tiny, firm, 

forward-pointing hairs, especially along the midrib. 

The bases of bracts are covered with cobwebby hair. 

Florets vary in color from pink to purple and are 

hermaphroditic, containing both male and female 

reproductive parts. 

Seeds: Achenes of Italian thistle are small (1/5 inch or 5 

mm long) and have bristly pappus measuring 1/2 inch 

(1.25 cm) long. Two different forms of seeds are 

produced: brown and silver. Outer brown achenes 

are covered in mucous and generally remain in the 

flower heads, falling with them to the ground at the 

end of the season. These seeds can germinate at lower 

temperatures than the inner silver achenes, which are 

dispersed by wind. Silver achenes can remain dormant 

in the soil longer than brown achenes.

Figure 2-5. Italian thistle. a. plant (© 1995 

Saint Mary’s College of California), 

b. flower (Carol Witham), c. stem 

(© 1995 Saint Mary’s College of 

California), d. U.S. distribution
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 2: Getting to Know Exotic True Thistles 19

Roots: The root system consists of one long taproot with several smaller lateral roots. This 

plant does not reproduce vegetatively. 

Habitat and Ecology: Italian thistle grows best in warm, dry, Mediterranean climate areas 

in basalt soils or decomposing granitic soils with high nitrogen content and relatively 

high pH. It occurs in open areas such as pastures, rangelands, rights-of-way, and waste 

areas, capitalizing on disturbance of the soil. Overgrazing and annual burning of pastures infested with Italian thistle make the problem worse.

Noxious Weed Status: This species is listed as noxious in five states: Arkansas, California, 

Iowa, Oregon, and Washington.

Notes: This species is very similar in appearance to slenderflower thistle but is not as stout 

and has greener leaves.

Approved Biological Control Agents: Italian thistle is attacked by Cheilosia corydon (a 

fly), Psylliodes chalcomera (a beetle not established in the U.S.), and Trichosirocalus 

horridus (a weevil). 
20 Chapter 2: Getting to Know Exotic True Thistles

Biology and Biological Control of Exotic True Thistles _________________________________________

Marsh Thistle

Scientific name: Cirsium palustre (L.) Skopoli

Marsh thistle is native to Europe and western Asia; it was 

introduced to the United States in the early 1900s. It is a 

biennial that can sometimes grow as a short-lived perennial. This species grows to 4 feet (1.2 m) tall on average 

and reproduces by seed only. Seeds germinate and form 

rosettes during spring. Flowering stems bolt in spring. 

Flowering occurs from June to July.

Leaves: Leaves are deeply lobed, toothed, and have a 

strong midvein. The undersides of leaves are hairy 

while the upper surfaces only have scattered hairs. 

Prominent spines are found along margins but not 

on leaf surfaces. Basal leaves grow as long as 8 inches 

(20 cm). Stem leaves are smaller and grow alternately 

up the stem. 

Stems: Stems are slender and have spiny wings along their 

entire length. They are generally unbranched until the 

very upper parts of the stem. Branches and stem tips 

nod occasionally as flower heads develop.

Flowers: Marsh thistle capitula are typically 1/2 inch (1.25 

cm) in diameter and appear in clusters of 3 to 12 at 

the ends of shoots and branches. The receptacle has 

rows of purplish bracts that are not very spiny. Florets 

are usually purple in color and are hermaphroditic, 

having both male and female reproductive parts. 

Seeds: Achenes of marsh thistle are small, hard, and 

elongated (1/6 inch or 4 mm long), with a pappus of 

feathery bristles. 

Roots: The root system of marsh thistle is fibrous. This 

plant does not reproduce vegetatively. 

Habitat and Ecology: Marsh thistle grows best in moist, 

acidic soils and is found along roadsides and ditches 

and in wetlands, forest edges, and fields. This plant 

tolerates shade and can be found in woodlands. Both 

disturbed and undisturbed soils are susceptible to 

marsh thistle invasion. Overgrazing of infested pastures makes the problem worse. 

Figure 2-6. Marsh thistle. a. plant, b. flowers, c. stem (Malcolm Storey, www.bioimages.org.uk), d. U.S. 

distribution.
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 2: Getting to Know Exotic True Thistles 21

Noxious Weed Status: This species is listed as noxious in two states: Arkansas and 

Iowa.

Notes: This species currently does not occur in the western U.S.; however, it was included 

because of its presence in nearby British Columbia, Canada.

Approved Biological Control Agents: Marsh thistle is attacked only occasionally by 

Cheilosia corydon (a fly).
22 Chapter 2: Getting to Know Exotic True Thistles

Biology and Biological Control of Exotic True Thistles _________________________________________

Milk Thistle

Scientific name: Silybum marianum (L.) Gaertn.

Synonyms: Blessed milkthistle

Carduus marianus L.

Mariana mariana (L.) Hill

Milk thistle is native to southern Europe, the Mediterranean, and Northern Africa; it was introduced to the 

United States in the 1800s. It is a winter annual that 

occasionally grows as a biennial. This species grows 4 to 

6 feet (1.2 to 1.8 m) tall on average and reproduces by 

seed only. Seeds germinate and form rosettes in the fall 

following autumn rains. Flowering stems bolt in early 

spring. Flowering occurs from May to August.

Leaves: Leaves are broad with yellowish margins tipped 

with woody spines that grow up to 1/2 inch (1.25 cm) 

long. Milk thistle leaves are easily distinguished by the 

white coloration (marbling) found along the veins of 

the dark green leaves, often resembling milk poured 

across their surfaces. Basal leaves are deeply lobed and 

can be 20 inches (51 cm) long and up to 10 inches 

(25 cm) wide. Stem leaves are smaller, not quite as 

lobed as basal leaves, and clasp the stem. They grow 

alternately up the stem. 

Stems: Stems are stout, rigid, and not spiny along their 

length. They may be branched or unbranched. 

Flowers: Milk thistle flower heads are up to 2 inches (5 

cm) in diameter and are usually solitary at the ends 

of shoots and branches. Receptacles have rows of 

broad, leathery bracts that are tipped with very stiff 

spines 3⁄4 to 2 inches (1.9 to 5 cm) long and fringed 

with smaller spines. A single capitulum can produce 

up to 200 florets ranging from magenta to purple. 

All florets are hermaphroditic, having both male and 

female reproductive parts. 

Seeds: Achenes of milk thistle are heavy, 1/4 inch (6 mm) 

long, flat, smooth, and shiny and their color ranges 

from a mottled brown to black. They have a tuft of 

minutely barbed bristles that falls off in a ring when 

the seeds mature. A typical capitulum produces 150 

achenes; a mature plant produces 6,000 achenes on 

average.

Figure 2-7. Milk thistle. a. plant (Eric Coombs 

Department of Agriculture), b. flower 

(Eric Coombs, Oregon Department 

of Agriculture), c. leaves (Thurston 

County Noxious Weed Board), d. 

U.S. distribution.
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 2: Getting to Know Exotic True Thistles 23

Roots: The root system consists of one long taproot. This plant does not reproduce vegetatively. 

Habitat and Ecology: Milk thistle grows best in soils with high fertility and disturbance, 

including overgrazed pastures, roadsides, ditches, waste areas, and stockyards. 

Noxious Weed Status: This species is listed as noxious in three states: Arkansas, Oregon, 

and Washington.

Notes: This plant is sometimes used in site remediation projects as it is capable of removing large amounts of lead, nitrates, and zinc from polluted soil. While all milk thistle 

plants are considered toxic to livestock, plants growing in polluted areas are especially 

toxic. Milk thistle is an ancient medicinal plant used in the treatment of liver disorders 

and is still sold by a number of garden seed suppliers in the U.S.

Approved Biological Control Agents: There are currently no biological control agents 

approved for use against milk thistle. 
24 Chapter 2: Getting to Know Exotic True Thistles

Biology and Biological Control of Exotic True Thistles _________________________________________

Musk thistle is native to southern Europe, central Asia, 

and eastern Africa; it was introduced to the United States 

in the mid-1800s. It is a biennial that occasionally grows 

as a winter annual. This species grows 5 to 6 feet (1.5 

to 1.8 m) tall on average and reproduces only by seed. 

Rosettes usually form in spring, though autumn rains 

stimulate germination as well. Bolting occurs in late 

spring. Flowering occurs from May to August. 

Leaves: Leaves are dark green, hairless, and waxy and 

have characteristic white margins. They are coarsely 

lobed, with white spines along margins and at lobe 

tips. Basal leaves are up to 20 inches (51 cm) long 

and 6 inches (15 cm) wide. Stem leaves are smaller 

than basal leaves, grow alternately, and lightly clasp 

the stem. 

Stems: Stems are stout and highly branched and have 

spiny wings along their lower sections but not on 

their upper portions. 

Flowers: Musk thistle flower heads are up to 3 inches 

(7.5 cm) in diameter and are solitary at the ends of 

stems and branches. Receptacles have rows of wide, 

triangular bracts that end in small spines and are 

purple at maturity. A single capitulum can produce 

up to 1,500 magenta florets. When capitula mature, 

they nod (droop) at a 90-degree angle from the stem. 

All florets are hermaphroditic, having both male and 

female reproductive parts. 

Musk Thistle

Scientific name: Carduus nutans L.

Synonyms: Nodding thistle

Carduus macrocephalus Desf.

Carduus macrolepis Peterm.

Carduus nutans L. subspecies: leiophyllus (Petrovic) Stojanov & Stef.,

macrocephalus (Desf.) Nyman, macrolepis (Peterm.) Kazmi, nutans L. 

Carduus nutans L. varieties: leiophyllus (Petrovic) Arènes,

macrocephalus (Desf.) Boivin, vestitus (Hallier) Boivin

Figure 2-8. Musk thistle. a. plant (Eric Coombs, Oregon Department of Agriculture), b. flower (Becca Winston, 

MIA Consulting), c. leaf (Rachel Winston, MIA Consulting), d. stem (Richard Old, XID Services, Inc., 

www.xidservices.com), e. U.S. distribution.
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Chapter 2: Getting to Know Exotic True Thistles 25

Seeds: Achenes of musk thistle are yellowish-brown (1/8 inch or 3 mm long), with one 

edge straight and the other curved. The pappus consists of white bristles 3/4 inch (1.9 

cm) long. Capitula can produce up to 1,000 achenes each but produce only 250 on 

average. A mature plant can produce 10,000 achenes or more.

Roots: The root system consists of one large, fleshy taproot that is hollow near the root 

crown. This plant does not reproduce vegetatively. 

Habitat and Ecology: Musk thistle grows best in neutral to acidic soils with moist conditions and a good deal of disturbance. It readily invades pastures, roadsides, ditches, and 

meadows. This species does not grow well in excessively dry, wet, or shady conditions. 

Overgrazing of infested pastures increases the problem. 

Noxious Weed Status: This species is listed as noxious in 25 states: Arkansas, California, 

Colorado, Idaho, Illinois, Iowa, Kansas, Kentucky, Maryland, Minnesota, Missouri, 

Nebraska, Nevada, New Mexico, North Carolina, North Dakota, Ohio, Oklahoma, 

Oregon, Pennsylvania, South Dakota, Utah, Washington, West Virginia, and Wyoming.

Notes: Musk thistle was the first thistle species targeted for biological control in the 

U.S.

Approved Biological Control Agents: Musk thistle is attacked by Cheilosia corydon (a 

fly), Psylliodes chalcomera (a beetle not established in the U.S.), Trichosirocalus horridus, 

(a weevil), and Urophora solstitialis (a fly not established in the U.S.). 
26 Chapter 2: Getting to Know Exotic True Thistles

Biology and Biological Control of Exotic True Thistles _________________________________________

Plumeless Thistle

Scientific name: Carduus acanthoides L.

Plumeless thistle is native to eastern Europe and Asia; it 

was introduced to the United States in the 1800s. It is 

an annual that occasionally grows as a biennial. This species grows 3 to 4 feet (0.9 to 1.2 m) tall on average and 

reproduces only by seed. Rosettes usually form in spring, 

though autumn rains stimulate germination as well. Bolting occurs in early summer.

Leaves: Leaves are hairy on the undersides, narrow, deeply 

lobed (almost to the midrib), and finely divided. Each 

lobe has one to three pointed spines on the margins 

that are short but very sharp. Basal leaves have white 

margins and are up to 8 inches (20 cm) long and 2 

inches (5 cm) wide. Stem leaves are smaller than basal 

leaves and lightly clasp the stem. Flowering occurs 

from June to October.

Stems: Stems are highly branched but all stand erect. 

They have leaf-like spines that cover stems, extending 

along their entire length. 

Flowers: Plumeless thistle capitula grow up to 1 inch (2.5 

cm) in diameter and are either solitary or form small 

clusters of up to five capitula at the ends of stems and 

branches. Receptacles have rows of needlelike bracts 

tipped with sharp spines. A single capitulum can 

produce up to 100 pink or purple florets. All florets 

are hermaphroditic, having both male and female 

reproductive parts. 

Seeds: Achenes of plumeless thistle are small, 1/16 inch 

(1.5 mm) long, yellow to brown, slightly curved, and 

have a distinct light collar near the top. The pappus 

consists of small white bristles. Capitula can produce 

approximately 75 achenes each; a mature plant produces 3,750 achenes on average.

Roots: The root system consists of one large, fleshy taproot that is hollow near the root crown. This plant 

does not reproduce vegetatively. 

Figure 2-9. Plumeless thistle. a. plant (Becca Schneiderhan, MIA Consulting), b. flower (Becca Schneiderhan, 

MIA Consulting) c. stem (Richard Old, XID Services, Inc., www.xidservices.com), d. U.S. 

distribution.
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Chapter 2: Getting to Know Exotic True Thistles 27

Habitat and Ecology: Plumeless thistle grows best in well-drained soil with moist conditions and a good deal of disturbance. It readily invades pastures, roadsides, ditches, 

and meadows.

Noxious Weed Status: This species is listed as noxious in 14 states: Arizona, Arkansas, 

California, Colorado, Iowa, Maryland, Minnesota, Nebraska, North Carolina, Oregon, 

South Dakota, Washington, West Virginia, and Wyoming.

Approved Biological Control Agents: Plumeless thistle is attacked by Cheilosia corydon

(a fly), Psylliodes chalcomera (a beetle not established in the U.S.), Trichosirocalus horridus (a weevil), and Urophora solstitialis (a fly not established in the U.S.).
28 Chapter 2: Getting to Know Exotic True Thistles

Biology and Biological Control of Exotic True Thistles _________________________________________

Scotch Thistle

Scientific name: Onopordum acanthium L.

Synonyms: Scotch cottonthistle

Onopordum acanthium L. ssp. acanthium L.

Scotch thistle is native to the Mediterranean region of 

Europe and western Asia; it was introduced to the United 

States in the 1800s. It is a biennial that occasionally grows 

as a short-lived perennial or even an annual. This species 

grows to 7 or 8 feet (2.1 to 2.4 m) tall and 4 feet (1.2 m) 

wide on average and reproduces only by seed. Rosettes 

usually form in spring and can grow on average to 3 feet 

(0.9 m) in diameter; autumn rains stimulate germination 

and rosette formation as well. Bolting occurs in early summer. Flowering occurs from July to October.

Leaves: Leaves are wider in the center than at the base 

and have wavy, toothed margins armed with sharp yellow spines. Upper and lower leaf surfaces are covered 

with a thick mat of woolly hairs that give the foliage 

a gray-green or silvery appearance. Basal leaves can 

grow up to 2 feet (61 cm) long and 1 foot (30 cm) 

wide. Stem leaves are smaller than basal leaves, grow 

in an alternate pattern, and extend onto the stems 

as wings. 

Stems: Scotch thistle stems are robust, have numerous 

branches covered in woolly hair, and have spiny wings 

extending along their entire length. 

Flowers: Scotch thistle capitula are globe-shaped and 

grow to 2 inches (5 cm) in diameter. They are usually solitary or form small groups of three capitula 

at the ends of stems and branches. The receptacle is 

honeycombed within and has rows of long, needlelike 

bracts tipped with orange-colored spines. A single capitulum can produce up to 150 pink or purple florets. 

All florets are hermaphroditic, having both male and 

female reproductive parts. 

Seeds: Achenes of Scotch thistle are 1/5 inch (5 mm) 

long, gray with dark mottling, and attached to a 

brown-colored pappus of toothed bristles that can be 

twice as long as the seed. Capitula can produce over 

100 achenes each; a mature plant produces 10,000 

achenes on average.

Figure 2-10. Scotch thistle. a. plant, b. flower, c. 

leaf, d. stem (all Rachel Winston, 

MIA Consulting), e. U.S. distribution
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Chapter 2: Getting to Know Exotic True Thistles 29

Roots: The root system consists of one large, fleshy taproot. This plant does not reproduce 

vegetatively. 

Habitat and Ecology: Scotch thistle grows best in light, well-drained, and sandy or 

stony soils. It readily invades dry pastures, fields, and rangeland but is also found in 

waste places and along rivers, streams, canals, or other waterways. Heavy disturbance 

increases infestations. 

Noxious Weed Status: This species is listed as noxious in 14 states: Arizona, Arkansas, 

California, Colorado, Connecticut, Idaho, Missouri, Nevada, New Mexico, Oklahoma, 

Oregon, Utah, Washington, and Wyoming.

Notes: Scotch thistle is the national emblem of Scotland. It is sometimes sold as an ornamental and has some medicinal and practical uses.

Illyrian and Taurian thistle (Onopordum illyricum and Onopordum tauricum, respectively) are closely related to Scotch thistle and are very similar in biology and suitability 

for biological control agents. They are not specifically addressed in this manual. These 

species are most problematic in California, though they are also becoming nuisances 

elsewhere. 

Approved Biological Control Agents: There are currently no biological control agents 

approved for use against Scotch thistle. 
30 Chapter 2: Getting to Know Exotic True Thistles

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Slenderflower Thistle

Scientific name: Carduus tenuiflorus W. Curtis

Slenderflower thistle is native to western and southern 

Europe and the Mediterranean region, northward to 

Scandinavia. It was introduced to the United States in 

the 1900s. It is a winter annual that occasionally grows 

as a biennial. This species grows 3 to 4 feet (0.9 to 1.2 

m) tall and reproduces only by seed. Seeds germinate 

and form rosettes in the fall following autumn rains. 

Flowering stems bolt in early spring. Flowering occurs 

from April to June.

Leaves: Leaves are lance-shaped and gray-green with light 

colored veins. They are hairless and spiny above and 

covered with short matted hairs on the undersides. 

Basal leaves grow up to 5 inches (13 cm) long and 2 1⁄2 

inches (6 cm) wide, have 4 to 20 lobes, and spines on 

lobe tips. Stem leaves grow alternately along stems.

Stems: Stems are slender and may be branched or unbranched. They have triangular-shaped stem wings 

that are up to 1/3 inch (0.8 cm) wide and are tipped 

with spines; wings extend over the entire length of 

the stem. 

Flowers: Slenderflower thistle produces numerous, small 

capitula less than 1 inch (2.5 cm) long. They grow in 

clusters of 5 to 20 at the ends of stems and branches. 

Receptacles have narrow, triangular bracts tipped with 

spines. Florets, averaging 20 per capitulum, vary in 

color from pink to purple and are hermaphroditic, 

having both male and female reproductive parts. 

Seeds: Achenes of slenderflower thistle are small (1/6 inch 

or 4 mm long) and have bristly pappus measuring 1/2

inch (1.25 cm) long. Two different forms of seeds are 

produced: brown and silver. Outer brown achenes are 

covered in mucus and generally remain in the flower 

heads, falling with them to the ground at the end 

of the season. These seeds can germinate at lower 

temperatures than the inner silver achenes, which are 

dispersed by wind. Silver achenes can remain dormant 

in the soil longer than the brown achenes.

Figure 2-11. Slenderflower thistle. a. plant 

(Washington State Noxious 

Weed Control Board), b. flowers 

(Washington State Noxious Weed 

Control Board), c. leaf and stem 

(Thurston County Noxious Weed 

Board), d. U.S. distribution.
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Chapter 2: Getting to Know Exotic True Thistles 31

Roots: The root system consists of a shallow, branched, and slender taproot. This plant 

does not reproduce vegetatively. 

Habitat and Ecology: Slenderflower thistle grows best in dry, open areas with highly 

fertile soils. It occurs in pastures, rangelands, rights-of-way, and waste areas, capitalizing on disturbance. 

Noxious Weed Status: This species is listed as noxious in five states: Arkansas, California, 

Iowa, Oregon, and Washington.

Notes: Slenderflower thistle is very similar to Italian thistle but has lighter colored leaf 

veins and stouter, more strongly winged stems.

Approved Biological Control Agents: Slenderflower thistle is attacked by Cheilosia 

corydon (a fly) and Trichosirocalus horridus (a weevil).
32 Chapter 2: Getting to Know Exotic True Thistles

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BULL

CANADA ITALIAN

MARSH

MILK

MUSK

PLUMELESS

S

COTCH

SLENDERFLOWER

Life history Biennial Perennial

Winter 

annual/ 

biennial

Biennial

Winter 

annual/ 

biennial

Biennial Annual/ biennial Biennial/ short lived perennial Winter annual

Reproduction Seed Seed and rhizomatous roots Seed Seed Seed Seed Seed Seed SeedAverage 

height

3 1/2’ 

1 m

4’ 

1.2 m

4’ 

1.2 m

4’ 

1.2 m

5 

1.5 m’

5 1/2’ 

1.7 m

3 1/2’ 

1 m

7’ 

2 m

3 1/2’ 

1 m

Capitulum 

diameter

1 1/2 to 2” 

3.8 to 5 cm

1/2” 

1.25 cm

<1” 

<2.5 cm

1/2” 

1.25 cm

2” 

5 cm

3” 

7.5 cm

Up to 1” 

Up to 2.5 

cm

2” 

5 cm

<1” 

<2.5 cm

Capitula 

cluster # 1 1 to several 2 to 5 3 to 12 1 1 Up to 5 1 5 to 20

Bract 

description

Narrow; 

needlelike; 

tipped in 

yellow

Triangular; 

close to 

receptacle; 

not spiny

Triangular; 

cobwebby 

hairs; tiny, 

firm hairs on 

midrib

Triangular; 

close to 

receptacle; 

not spiny

Broad; 

leathery; up 

to 2” long; 

fringed with 

smaller 

spines

Wide; 

triangular; 

purple at 

maturity

Needlelike; 

slightly 

triangular

Needlelike; 

tipped with 

orange 

colored 

spines

Triangular; not 

hairy

Stems spiny 

along entire length?

Yes No Yes Yes No

No: not 

upper 

portions

Yes Yes Yes

General leaf 

color Green Green Green Green

Green 

with white 

marbling

Green 

with white 

margins

Green Gray green Gray green

Table 2-2. Trait comparisons of nine exotic thistles in western North America.
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Chapter 2: Getting to Know Exotic True Thistles 33

Native True Thistles

Even though the taxonomy of the group remains unsettled, there are at least 86 species 

(and numerous subspecies) of native true thistles in the U.S. All are in the genus Cirsium. 

This manual is focused on the northwestern United States and, more specifically, the states 

of Colorado, Idaho, Montana, Oregon, Utah, Washington, and Wyoming. Consequently, 

only the native thistle species (47 total) occurring in these regions are included in Table 

2-3. 

Table 2-3. Native true thistle species in the northwestern U.S.

Scientific Name

(all Cirsium) Common Name Life Strategy Distribution

C. altissimum Tall thistle Biennial AL, AR, DC, DE, FL, GA, 

IA, IL, IN, KS, KY, LA, MI, 

MN, MO, MS, NC, ND, 

NE, NY, OH, OK, PA, SC, 

SD, TN, TX, VA, WI, WV

C. andersonii Rose thistle Biennial CA, NE, OR

C. araneans Jeweled thistle Perennial CO

C. aridum Desert thistle Perennial WY

C. barnebyi Barneby’s thistle Perennial CO, UT, WY

C. brevifolium Palouse thistle Perennial IH, OR, WA

C. brevistylum Clustered, short-styled 

thistle

Annual 

Biennial 

Perennial

CA, IH, MT, OR, WA

C. calcareum Cainville thistle Biennial 

Perennial

CO, NM, UT

C. canescens Prairie thistle Biennial CO, IA, ID, MO, MT, ND, 

NE, NM, OH, SD, WI, 

WY

C. canovirens Gray-green thistle Biennial 

Perennial

CA, ID, MT, NV, OR, UT, 

WY

C. centaureae Fringed thistle Perennial CO, UT, WY

C. ciliolatum Ashland thistle Perennial CA, OR

C. clavatum Fish Lake thistle Perennial CO, UT

C. cymosum Peregrine thistle Perennial CA, OR

C. douglasii Douglas’ thistle Biennial 

Perennial

CA, NV, OR

C. drummondii Dwarf thistle Perennial SD, WY

C. eatonii Eaton’s thistle Biennial 

Perennial

CO, ID, MT, NV, UT, WY

C. edule Edible thistle Biennial 

Perennial

ID, OR, WA

C. flodmanii Flodman’s thistle Perennial CO, IA, ID, KS, MI, MN, 

MT, ND, NE, SD, UT, VT, 

WI, WY
34 Chapter 2: Getting to Know Exotic True Thistles

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C. foliosum Elk thistle Perennial ID, MT, WA, WY

C. hookerianum Hooker’s, white thistle Perennial ID, MT, WA, WY

C. horridulum Yellow thistle Annual Biennial AL, AR, CT, DE, FL, GA, 

LA, MA, MD, ME, MS, 

NC, NH, NJ, NY, OK, PA, 

RI, SC, TN, TX, VA

C. laterifolium Porcupine thistle Perennial CO

C. longistylum Longstyle thistle Perennial MT

C. modestum Lacy thistle Perennial CO

C. murdockii Murdock’s thistle Biennial 

Perennial

UT

C. neomexicanum New Mexico thistle Biennial 

Perennial

AZ, CA, CO, NM, NV, UT

C. occidentale Cobwebby thistle Biennial 

Perennial

CA, NV, OR

C. ochrocentrum Yellowspine thistle Biennial 

Perennial

AZ, CA, CO, KS, NE, NM, 

OK, SD, TX, UT, WI, WY

C. osterhoutii Osterhout’s thistle Perennial CO

C. ownbeyi Ownbey’s thistle Perennial CO, UT, WY

C. pallidum Pale thistle Perennial CO, NM

C. peckii Steen Mountain thistle Perennial OR

C. perplexans Rocky Mounain thistle Perennial CO

C. pulcherrimum Wyoming thistle Perennial CO, NE, UT, WY

C. pyrenaicum Pyrenean thistle Perennial OR

C. remotifolium Fewleaf thistle Biennial 

Perennial

CA, OR, WA

C. rothrockii Rothrock’s thistle Biennial AZ, UT

C. rydbergii Rydberg’s thistle Perennial AZ, UT

C. scapanolepis Mountain slope thistle Perennial CO

C. scariosum Meadow thistle (previously Elk thistle)

Biennial 

Perennial

AZ, CA, CO, ID, MT, NM, 

NV, OR, UT, WA, WY

C. scopulorum Mountain thistle Biennial CO, NM, UT

C. subniveum Jackson Hole thistle Perennial CA, ID, MT, NE, OR, UT, 

WY

C. undulatum Wavy-leaved thistle Biennial 

Perennial

AZ, CA, CO, GA, IA, ID, 

IL, IN, KS, MI, MN, MO, 

MT, ND, NE, NM, NE, 

OK, OR, PA, SD, TX, UT, 

WA, WI, WY

C. vernale Spring thistle Perennial CO

C. virginense Virgin thistle Perennial AZ, NE, UT

C. wheeleri Wheeler’s thistle Perennial AZ, CO, NM, NE, UT
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Chapter 2: Getting to Know Exotic True Thistles 35

It is often difficult to differentiate exotic and native thistle species because of their similar 

appearance, growth traits, preferred habitat, and distribution. However, there are a few 

“rules of thumb” or questions that can aid in the identification process. 

•	 Are the stems of the species in question covered in spines or sharp wings along 

their entire length? 

YES: The species is not native. Stems of native thistles are not spiny along 

their entire length, nor are they winged; they have bare or woolly (but 

not spiny) expanses between leaves, unless the leaves overlap profusely 

which does not occur on exotic species in this manual. 

NO: Refer to other characteristics to further differentiate.

•	 Are the bracts of the species in question spine-tipped, firm, and broad-triangular 

or wedge-shaped? 

YES: The species is not native. Bracts of native species are either firm and 

needle-shaped or soft and woolly.

NO: Refer to other characteristics to further differentiate.

•	 Are the bracts of the species in question leathery and/or jagged along the margins?

YES: The species is not native. Bracts of native species are either firm and 

needle-shaped or soft and woolly.

NO: Refer to other characteristics to further differentiate.

•	 Is the root system of the species in question rhizomatous?

YES: The species is not native. Native species are not rhizomatous.

NO: Refer to other characteristics to further differentiate.

•	 Is the species in question dioecious (male and female florets are on separate 

plants)?

YES: The species is not native. Native thistles are not dioecious.

NO: Refer to other characteristics to further differentiate.

•	 Are the bracts of the species in question glandular (sticky and often aromatic)?

YES: The species is native. None of the exotic thistles discussed in this manual 

have glandular bracts.

NO: Refer to other characteristics to further differentiate.

Selected native species are highlighted on the following pages, with special emphasis placed 

on distinguishing each species from the exotic thistles targeted in this manual.
36 Chapter 2: Getting to Know Exotic True Thistles

Biology and Biological Control of Exotic True Thistles _________________________________________

Cainville Thistle (Cirsium calcareum)

Seed heads resemble slenderflower, Canada, and Italian 

thistles. Stems are not spiny or winged. There is an overall 

grayish cast to foliage

Figure 2-12. Cainville thistle. Photos: plant, Mary Ellen 

Harte (Bugwood.org); flowers, Rich Hansen 

(USDA-APHIS-PPQ-CPHST); leaf and 

stem, Eric Coombs (Oregon Department of 

Agriculture).

Clustered Thistle (Cirsium brevistylum)

Resembles bull and Canada thistles. Stems are soft and 

not spiny or winged, and leaves do not have long, woody 

spines.

Figure 2-13. Clustered thistle. Photos: Jennifer Andreas 

(Washington State University Extension).
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Chapter 2: Getting to Know Exotic True Thistles 37

Cobwebby Thistle (Cirsium occidentale var. venustum)

Flower head most closely resembles that of Italian and 

slenderflower thistles but is much more cobwebby. Stems 

are not spiny or winged. Foliage has grayish cast.

Figure 2-14. Cobwebby thistle. Photos: plant and leaves, 

Mark Schwarländer (University of Idaho); 

flower head, Rich Hansen (USDA-APHIS-PPQCPHST).

Edible Thistle (Cirsium edule)

Flower head most closely resembles that of bull, Scotch, 

and plumeless thistles. Stems are not spiny or winged, 

and leaves do not have long, woody spines.

Figure 2-15. Edible thistle. Photos: Richard Old (XID 

Services, Inc., www.xidservices.com).
38 Chapter 2: Getting to Know Exotic True Thistles

Biology and Biological Control of Exotic True Thistles _________________________________________

Eaton’s Thistle (Cirsium eatonii)

Resembles marsh thistle with its erect and solitary form 

and cluster of flower heads subtended by leaves that 

extend beyond the cluster. Stems are soft, not spiny or 

winged.

Figure 2-16. Eaton’s thistle. Photos: Mary Ellen Harte 

(Bugwood.org).
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 2: Getting to Know Exotic True Thistles 39

Flodman’s Thistle (Cirsium flodmanii)

Flower head most closely resembles that of bull, Canada, 

and Scotch thistles, and overall grayish cast resembles 

Scotch thistle. Stems are not spiny or winged. Leaves are 

highly lobed and wavy-margined.

Figure 2-17. Flodman’s thistle. Photos: Mark 

Schwarzländer (University of Idaho).

Jackson Hole Thistle (Cirsium subniveum)

Most closely resembles Canada thistle. Leaves are more 

slender and not as wavy. Plant has overall busy appearance 

with a gray-green hue.

Figure 2-18. Jackson Hole thistle. Photos: Eric Coombs 

(Oregon Department of Agriculture).
40 Chapter 2: Getting to Know Exotic True Thistles

Biology and Biological Control of Exotic True Thistles _________________________________________

Gray-Green Thistle (Cirsium canovirens)

Most closely resembles white-flowered (uncommon) 

Canada thistle. Stems are not spiny or winged. Leaves are 

very spiny, and plant has overall grayish cast.

Figure 2-19. Gray-green thistle. Photos: plant, Eric Coombs 

(Oregon Department of Agriculture); stem/

leaves and flower head, Steve Matson.
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Chapter 2: Getting to Know Exotic True Thistles 41

Meadow Thistle (Cirsium scariosum) 

Resembles marsh thistle with its erect and solitary form 

and cluster of flower heads subtended by leaves that extend beyond the cluster. Stems are soft, not spiny, and 

are covered by overlapping leaves. 

Figure 2-20. Meadow thistle. Photos: plant, Richard Old 

(XID Services, Inc., www.xidservices.com); 

leaves, Rachel Winston (MIA Consulting); 

flower head, Richard Hansen (USDA-APHISPPQ-CPHST).

Mountain Thistle (Cirsium scopulorum)

Resembles marsh thistle somewhat with its solitary main 

stem and cluster of flower heads subtended by leaves that 

extend beyond the cluster. Stems are very woolly, not 

spiny, and are covered by overlapping leaves. Basal leaves 

are long. Flowers are yellow.

Figure 2-21. Mountain thistle. Photos: plant, Richard 

Hansen (USDA-APHIS-PPQ-CPHST); flower 

head, Dave Powell (USDA Forest Service, 

Bugwood.org).
42 Chapter 2: Getting to Know Exotic True Thistles

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New Mexico Thistle (Cirsium neomexicanum)

Flower head most closely resembles that of bull, Scotch, 

and plumeless thistles. Stems are not spiny or winged, 

and leaves do not have long, woody spines. Foliage has 

grayish cast.

Figure 2-22. New Mexico thistle. Photos: Steve Dewey 

(Utah State University, Bugwood.org).

Palouse Thistle (Cirsium brevifolium)

Most closely resembles white-flowered (uncommon) Canada 

thistle. Stems are not spiny, and leaves are not crinkled or very 

spiny. Leaves are also lighter beneath. Has glandular bracts.

Figure 2-23. Palouse thistle. Photos: Richard Old (XID Services, 

Inc., www.xidservices.com).
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 2: Getting to Know Exotic True Thistles 43

Prairie Thistle (Cirsium canescens)

Flower head most closely resembles that of Italian, plumeless and slenderflower thistles. Stems are not spiny or 

winged.

Figure 2-24. Prairie thistle. Photos: Rich Hansen (USDAAPHIS-PPQ-CPHST).

Snowy Cobwebby Thistle (Cirsium occidentale var. candidissimum)

The cobwebby texture and white color 

of the bracts combined with the reddish 

magenta florets are unlike any of the exotic thistles in this manual. 

Figure 2-25. Snowy cobwebby thistle. Photos: Richard 

Old (XID Services, Inc., www.xidservices.

com).
44 Chapter 2: Getting to Know Exotic True Thistles

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Tall Thistle (Cirsium altissimum)

Flower head most closely resembles that of bull and 

Scotch thistles. Stems are not spiny or winged, and leaves 

do not have long, woody spines.

Steen Mountain Thistle (Cirsium peckii)

Flower heads resemble Scotch, bull, and musk thistles. 

Leaves resemble Canada thistle. Stems are not spiny or 

winged but are covered in fine hair. 

Figure 2-27. Tall thistle. Photos: Richard Old (XID Services, 

Inc., www.xidservices.com).

Figure 2-26. Steen Mountain thistle. Photos: Eric Coombs 

(Oregon Department of Agriculture).
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Chapter 2: Getting to Know Exotic True Thistles 45

Wavy-Leaved Thistle (Cirsium undulatum)

Flower heads resemble Scotch and bull thistles, and overall 

grayish cast resembles Scotch thistle. Stems are not spiny 

or winged. Leaves are highly lobed and wavy-margined. 

Has glandular bracts.

Figure 2-28. Wavy-leaved thistle. Photos: plant and flower, 

Rachel Winston (MIA Consulting); leaves, 

Richard Old (XID Services, Inc., www.

xidservices.com).

Yellow Thistle (Cirsium horridulum)

Flower head most closely resembles that of bull and 

Scotch thistles. Stems are not spiny or winged, and leaves 

do not have long, woody spines.

Figure 2-29. Yellow thistle. Photos: Richard Old (XID 

Services, Inc., www.xidservices.com).
46 Chapter 2: Getting to Know Exotic True Thistles

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Yellowspine Thistle (Cirsium ochrocentrum)

Resembles bull and Scotch thistles but does not have spiny 

or completely winged stems. The leaves of this native are 

more deeply lobed and wavy-margined than the exotics 

in this manual

Figure 2-30. Yellowspine thistle. Photos: plant and leaves, 

Richard Old (XID Services, Inc., www.

xidservices.com); flower head, Rich Hansen 

(USDA-APHIS-PPQ-CPHST).
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 3: Biology of Biological Control Agents 47

Chapter 3: Biology of Exotic True Thistle Biological Control Agents

Rachel Winston, Eric Coombs, Carol Bell Randall, and Rich Hansen

Biological control of exotic true thistles is one of the oldest classical biological control 

programs in the United States and Canada. It began in 1969 with the approval and release 

of the seed head weevil Rhinocyllus conicus in Montana and Virginia. Unfortunately, this 

weevil was later found to attack native thistle species, and its release permit was revoked 

in the late 1990s. To date, a total of seven insect species are approved for release in the 

United States and Canada as classical biological control agents of exotic thistles. These 

include three beetles and four flies. In addition, there are two species of native butterflies 

(not approved biological control agents) that feed on exotic thistles and a number of accidentally introduced and/or unapproved natural enemies. It is unlikely that any one of the 

currently established species could successfully control exotic thistles alone. Many exotic 

true thistle biological control programs use a combination of approved biological control 

agents that together may sometimes put severe stress on the plant and have a greater chance 

of contributing to weed suppression. 

Basic Insect Biology

Insects are the largest and most diverse class of animals. An understanding of basic insect 

biology and anatomy will help land managers recognize and identify the insects used as 

biological control agents of exotic true thistles.

The insects used as biological control agents of thistles 

have complete metamorphosis, which means they exhibit a life cycle with four distinct stages: egg, larva, 

pupa, and adult (Figure 3-1). Adult insects have an exoskeleton (a hard external skeleton), a segmented body 

divided into three regions (head, thorax, and abdomen), 

three pairs of segmented legs, and one or two pairs of 

wings (Figure 3-2). The head of the adult insect has 

one pair each of compound eyes and antennae. 

Figure 3-1. Complete metamorphosis. 

Image: Bugwood.org.
48 Chapter 3: Biology of Biological Control Agents

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Immature insects have an exoskeleton that must be shed for immature insects to grow to 

the next stage. The process of an insect shedding its “skin” in order to grow is called molting, and larval stages between molts are called “instars.” Larvae generally complete three 

to five instars before they molt into the pupal stage (Figure 3-3). During the pupal stage, 

the insect changes from a larva to an adult. Insects do not feed during the pupal stage. 

Beetles (Order Coleoptera)

Most adult beetles are hard-bodied with tough exoskeletons. They have two pairs of 

wings. The two front wings, called elytra, are thickened and meet in a straight line down 

the abdomen of the adult insect, forming a hard, shell-like, protective covering. The two 

hind wings are membranous and used for flight. These are larger than the elytra and are 

folded under the elytra when not in use. Beetle larvae are grub or wormlike with three 

small pairs of legs. Most are pale white with a brown or black head.

Flies (Order Diptera)

Many insects have the word “fly” in their common name though they may not be true flies. 

In the common names of true flies, “fly” is written as a separate word (e.g., house fly) to 

distinguish them from other orders of insects that use “fly” in their common name (e.g., 

butterfly in the order Lepidoptera and mayfly in the order Ephemeroptera). Adult true flies 

are easily distinguished from other groups of insects by their single pair of membranous 

wings and typically small, soft-bodies. Larvae of most true flies are legless and wormlike 

and are called maggots. 

Butterflies and Moths (Order Lepidoptera)

Adult Lepidoptera have two pair of membranous wings, covered (usually completely) by 

minute powderlike scales. Antennae are prominent. The larvae (caterpillars) have a toughened head capsule, chewing mouthparts, and a soft body that may have hair-like or other 

projections, three pairs of true legs, and up to five pairs of additional prolegs. The pupal 

stage is known as a chrysalis or cocoon. 

Figure 3-2. Generalized adult insect anatomy. Image: Bugwood.org.
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Chapter 3: Biology of Biological Control Agents 49

Basic Rust Biology (Order Uredinales)

Rusts are plant diseases caused by fungi. Rust diseases typically attack leaves and stems of 

the host plant. Rust infections usually appear as numerous rusty, orange, yellow, or even 

white colored spots that rupture the leaf surface. 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 aide in the transmission and 

infection process. 

The life cycle of rust fungi can be very complicated: the following is a simplified description. Rust fungi produce four distinctive spores: teliospores, basidiospores, aeciospores, 

and urediospores. Teliospores serve as the overwintering stage of rust fungi; these germinate in spring and, following a number of steps, produce basidiospores. Basidiospores 

eventually give rise to aeciospores, which in turn give rise to urediospores. Urediospores 

are frequently the most recognizable stage of rust diseases and are easily blown to other 

plants to repeat the active cycle or produce teliospores for overwintering. Basidiospores, 

aeciospores, and urediospores can all attack and infect host plants.

Figure 3-3. Keys to identification of insect larvae (left) and insect pupae (right). Image: Bugwood.org.
50 Chapter 3: Biology of Biological Control Agents

Biology and Biological Control of Exotic True Thistles _________________________________________

Approved Thistle Biological Control Agents

Seven exotic true thistle biological control species (three beetles and four flies) are permitted for release in the U.S. (Table 3-1). These insects attack six of the nine species of 

exotic true thistles described in this manual. The remaining three exotic thistles within this 

manual have no approved biological control agents. 

The seven approved biological control agents attack four distinct parts of thistle plants: 

two of the biological control agents are stem/root miners and gallers, two are seed feeders, 

one is a bud/leaf feeder, and one is a root crown feeder (Figure 3-4).

Table 3-1. Biological control agents approved for release in the U.S. against exotic thistles. PL= Plumeless 

thistle, SF= Slenderflower thistle.

Type of

Insect

Scientific Name Established

Thistle Species Attacked

Bull Canada Italian Marsh Musk PL SF

Beetle

Ceutrorhynchus litura X X

Psylliodes chalcomera X X X

Trichosirocalus horridus X X X X X X X 

Fly

Cheilosia corydon X X X X X X X

Urophora cardui X X

Urophora soltitialis X X

Urophora stylata X X
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Chapter 3: Biology of Biological Control Agents 51

Figure 3-4. General location of attack for approved biological control agents for exotic thistle insects on a 

thistle plant. Plant drawing: Rachel Winston (MIA Consulting). Photos: a, c, and g, Laura Parsons 

(University of Idaho); b and d, Eric Coombs (Oregon Department of Agriculture); e, Peter Harris 

(Agriculture and Agri-Food Canada, Bugwood.org); f, USDA-APHIS, Bugwood.org.
52 Chapter 3: Biology of Biological Control Agents

Biology and Biological Control of Exotic True Thistles _________________________________________

Ceutorhynchus litura

Canada thistle stem weevil

Figure 3-5. Ceutorhynchus litura

adult. Photo: Laura Parsons 

(University of Idaho).

Order Coleoptera

Family Curculionidae

Native distribution Europe

Original sources Germany

First U.S. release 1972: Montana

Species attacked Canada thistle

Nontarget effects None reported

Establishment ID, MT, NE, ND, OR, SK, UT, 

VA, WA, WY

Description

Larvae are white, grub-like, and are pointed in the front end. They complete three instars 

and measure up to 3 mm (1/8 inch) long at maturity. Adults are mottled black and white 

with a ‘T’-shaped marking on their back. Adults have long snouts and are up to 4 mm 

(1/6 inch) long. 

Life Cycle

Overwintering adults emerge from soil litter and feed on leaf and stem tissue in early spring. 

Eggs are laid in spring in the mid-vein on the underside of a new rosette leaf. Emerging 

larvae mine leaf veins, stems, and root crowns of target plants throughout spring and 

summer. Pupation occurs in the soil; emerging adults overwinter in soil litter. There is 

one generation per year.

Egg            

Larva

Pupa

Adult

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Figure 3-6. Life cycle of C. litura. Bars indicate the approximate length of activity for each 

of the life stages. Black bars represent the overwintering period. 
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Chapter 3: Biology of Biological Control Agents 53

Impact

Larval and adult feeding usually does not significantly impact populations of Canada thistle 

directly, though some stunting of and stand reduction has been observed in eastern Oregon. 

Feeding does cause secondary damage as pathogens and other organisms enter the stems of 

Canada thistle via holes made by exiting larvae. Canada thistle is difficult to control given 

its ability to reproduce both vegetatively through its roots and by seed. Studies conducted 

to determine the effectiveness of this insect have shown varying results. Most conclude 

that impacts on Canada thistle would be greatest if this species were used in conjunction 

with other biological control agents. 

Figure 3-7. Ceutorhynchus litura larvae and 

damage. Photo: Eric Coombs 

(Oregon Department of Agriculture).

Habitat Preference

This species does well in moist, disturbed areas where Canada thistle is dense and not 

stressed by drought, grazing, or other control methods. 

Availability

This insect is readily available for collection in many states. It is best collected in the adult 

stage using a sweep net and aspirator during spring when host plants are beginning to 

bolt.
54 Chapter 3: Biology of Biological Control Agents

Biology and Biological Control of Exotic True Thistles _________________________________________

Psylliodes chalcomera

Musk thistle leaf beetle

Figure 3-8. Psylliodes chalcomera

adult. Photo: USDA-ARS, 

Bugwood.org.

Order Coleoptera

Family Chrysomelidae

Native distribution Europe and Asia

Original sources Italy

First U.S. release 1997: Kansas, Maryland, 

Texas

Species attacked Italian, musk, and plumeless 

thistles

Nontarget effects None reported

Establishment Not established

Description

Larvae are slender, grub-like, and white with brown head plates. They complete three 

instars and are up to 3 mm (1/8 inch) long at maturity. Adults are small (up to 3 mm or 

1/8 inch long), shiny dark, and with a metallic blue-green sheen.

Life Cycle

Overwintering adults emerge in spring and oviposit at the bases of plants when host plants 

are bolting or in the soil near host plants. Emerging larvae feed on leaves, buds, and flowers throughout the growing season. Pupation occurs in the soil litter; emerging adults 

overwinter in soil litter as well. There is one generation per year. 

Egg            

Larva

Pupa

Adult

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Figure 3-9. Life cycle of P. chalcomera. Bars indicate the approximate length of activity for 

each of the life stages. Black bars represent the overwintering period. 

Impact 

Larval feeding stunts the growth and reduces the seed production of Italian, musk, and 

plumeless thistles, decreasing their rates of spread. 
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Chapter 3: Biology of Biological Control Agents 55

Habitat Preference

Little is known about the habitat preferences of P. chalcomera beyond the observation that 

it survives both cold and hot temperatures.

Availability

This species is currently not established in North America despite attempted introductions. 

Consequently, it is unavailable for redistribution.

Comments

Psylliodes chalcomera is not widespread in its native range; as a result, it has been difficult 

to collect large enough populations for release and establishment in North America.
56 Chapter 3: Biology of Biological Control Agents

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Trichosirocalus horridus

Musk thistle crown weevil

Figure 3-10. Trichosirocalus horridus 

adult. Photo: Laura Parsons 

(University of Idaho).

Description

Larvae are white with dark brown head capsules. They complete three instars and reach up 

to 3 mm (1/8 inch) at maturity. Adults are small (approximately 4 mm or 1/6 inch long), 

round, brown with white mottling, have obvious spines on their thorax, and have long 

snouts. 

Life Cycle

Overwintering adults emerge in spring and feed on rosettes of host plants. Eggs are deposited on the undersides of leaves along the midrib and primary veins of young plants. 

Hatching larvae move down and feed on the tissue at the root-stem junction. Pupation 

occurs in the soil in early summer. Adults emerge over the summer but are inactive until 

fall, when they feed superficially on host plant foliage. Adults overwinter in soil litter, 

emerging again in spring and often living until the next generation of adults begins to 

emerge. There is only one generation per year.

Egg            

Larva

Pupa

Adult not active

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Figure 3-11. Life cycle of T. horridulus. Bars indicate the approximate length of activity for 

each of the life stages. Black bars represent the overwintering period. 

Impact

Adult feeding in the spring has minimal impacts, though larval feeding can weaken, reduce 

the seed production, and alter the growth of bull, Canada, Italian, musk, plumeless, and 

Order Coleoptera

Family Curculionidae

Native distribution Europe

Original sources Italy

First U.S. release 1974: Virginia

Species attacked bull, Canada, Italian, 

musk, plumeless, and 

slenderflower thistles

Nontarget effects Other species in the tribe 

Cardueae might be 

marginal hosts

Establishment CO, ID, KS, MD, MO, MT, 

OR, VA, WA,WY
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Chapter 3: Biology of Biological Control Agents 57

slenderflower thistles. This species is most effective in combination with other biological 

control agents.

Figure 3-12. Trichosirocalus horridus larva and 

damage. Photo: Eric Coombs 

(Oregon Department of Agriculture).

Habitat Preference

This insect does best in open infestations of the target weed and on the perimeter where 

stems do not grow as tall. It is widely distributed but does not do as well at high elevations 

or under marshy conditions.

Availability

This species is eadily available for collection in the western states. Overwintering adults 

of this species can be hand collected in spring from rosettes of host plants. Alternatively, 

the summer generation can be collected in the adult stage using a sweep net and aspirator 

when host plants are flowering. 

Comments

Trichosirocalus horridus has been observed feeding (at least in small numbers) on native 

thistle species. Although it is still approved for redistribution, care should be taken when 

releasing this agent in the proximity of native thistles.

Populations of this insect build up slowly, taking three to five years. However, once it is 

well established, this species disperses quite successfully. 

Recent taxonomic work with this insect indicates that T. horridus is a complex of many 

species. Weevils released in North America are likely T. mortadelo. This weevil is also known 

as Ceuthorhynchidius horridus in the scientific literature.

Because of concerns over host specificity, some states do not recommend releasing this agent. 

Please check with your state department of agriculture before introducing this agent.
58 Chapter 3: Biology of Biological Control Agents

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Cheilosia corydon

Thistle stem hover fly

Description

Larvae are tan in color and grub-like, completing three instars and growing up to 19 mm 

(3⁄4 inch) long. Adults are fuzzy with orange-tan hairs and have large black eyes. Adults can 

be up to 15 mm (5/8 inch) long, including their wings, and somewhat resemble bees. 

Life Cycle

Adults emerge in very early spring and deposit eggs on young leaves as host plants bolt. 

Larvae soon emerge and mine into shoots and stems. As the season progresses, they mine 

into roots and continue feeding. Pupae overwinter in roots or in soil litter. There is one 

generation per year.

Egg            

Larva

Pupa

Adult

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Figure 3-14. Life cycle of C. corydon. Bars indicate the approximate length of activity for 

each of the life stages. Black bars represent the overwintering period. 

Figure 3-13. Cheilosia corydon adult. 

Photo: Eric Coombs (Oregon 

Department of Agriculture).

Order Diptera

Family Syrphidae

Native distribution Europe

Original sources Italy

First U.S. release 1990: Maryland

Species attacked bull, Italian, marsh, 

musk, plumeless, and 

slenderflower thistles

Nontarget effects Other species in the tribe 

Cardueae might be 

marginal hosts

Establishment OR
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Chapter 3: Biology of Biological Control Agents 59

Impact

Larval mining interferes with plant function and ultimately results in a decrease of seed 

production, sometimes even death. This insect attacks bull, Italian, marsh, musk, plumeless, and slenderflower thistles.

Figure 3-15. Chelosia corydon larval mining 

damage. Photo: Eric Coombs 

(Oregon Department of Agriculture).

Habitat Preference

This insect survives a wide array of climatic conditions throughout the range of its host 

thistles. It tends to do better in areas where host plants flower early.

Availability

This species is available for redistribution in the western states, but is not widespread. 

Adults can be collected with a sweep net in spring when host plants are beginning to bolt. 

Alternatively, pupae can be collected and transferred by digging roots in late summer and 

early fall as the host plants die back with lower temperatures.

Comments

Because emerging adults may need early flowering plants from which to collect nectar, this 

species may be limited in distribution. This has not been confirmed. 
60 Chapter 3: Biology of Biological Control Agents

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Urophora cardui

Canada thistle stem gall fly

Description

Larvae are barrel-shaped, white, and have dark brown anal plates. They complete three 

instars and can be up to 5 mm (1/5 inch) long at maturity. Adults have dark bodies and 

dark bands on their white wings that form a ‘W’. Adults can be up to 8 mm (1

/3) long.

Life Cycle

Larvae overwintering inside galls pupate in early spring. Adults emerge in late spring and 

early summer when new plants are flowering. Eggs are deposited on plant shoots in axillary 

buds throughout the summer. Hatching larvae burrow into stems and cause galls to form. 

There are often multiple larvae in one gall. Larvae overwinter in the third instar, with pupation occurring in early spring as plants start bolting. There is one generation per year.

Egg            

Larva

Pupa

Adult

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Figure 3-17. Life cycle of U. cardui. Bars indicate the approximate length of activity for 

each of the life stages. Black bars represent the overwintering period. 

Figure 3-16. Urophora cardui adult. 

Photo: Laura Parsons, 

(University of Idaho).

Order Diptera

Family Tephritidae

Native distribution Europe

Original sources Central Europe

First U.S. release 1980 Oregon

Species attacked Canada thistle

Nontarget effects None reported

Establishment CA, MD, MT, NV, OR, VA, 

WA, WY
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Chapter 3: Biology of Biological Control Agents 61

Impact

Larval feeding causes galls to be formed that act as metabolic sinks, diverting resources 

away from normal plant development. Attacked plants produce fewer seeds, are less competitive, and may be more susceptible to pathogens and other insects.

Figure 3-19. Urophora cardui pupa in multichambered gall. Photo: Norman Rees 

Figure 3-18. Urophora cardui (USDA-ARS, Bugwood.org). gall. Photo: 

Jennifer Andreas (Washington 

State University Extension).

Habitat Preferences

This fly does well in moist, open and partially shaded areas where Canada thistle is scattered. Areas subject to other means of control (grazing, mowing, chemicaltreatment , etc.) 

are not suitable for this fly’s survival.

Availability 

This fly is available for collection, particularly in western Idaho, Oregon, and Washington. 

Urophora cardui is best distributed by collecting galls in fall after the first major frost or 

from dead stems throughout the winter and very early spring. Galls are then kept cool 

throughout winter. In spring when host plants begin to grow, galls are transferred to the 

new site. Alternatively, adults can be reared out in screened cages in early spring or collected 

with a sweep net and aspirator throughout the spring as Canada thistle is bolting. Rearing 

insects out of galls has the advantage of preventing the spread of parasitic wasps.

Comments

Galls and/or the larvae within them are often preyed upon by rodents, birds, and an unidentified mite.
62 Chapter 3: Biology of Biological Control Agents

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Urophora solstitialis

Musk thistle seed head fly

Description

Larvae are white and barrel-shaped with a dark brown anal plate. Larvae can be up to 4 

mm (1/6 inch) long at maturity and complete three instars. Adults are brownish black 

with a yellow head and yellow legs. Adults have two black, ‘VII’-shaped markings on their 

wings and can be up to 5 mm (1/5 inch) long.

Life Cycle

Larvae overwintering in galls pupate in early spring. Adults emerge in later spring when 

plants have bolted. Eggs are deposited in developing seed heads. Hatching larvae feed 

on seeds and receptacle tissue; hardened galls form around them as they feed. There may 

be multiple larvae per seed head. Most larvae overwinter in the third instar, but early 

maturing larvae may pupate in early summer as a second generation. There are up to two 

generations per year.

Egg            

Larva

Pupa

Adult

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Figure 3-21. Life cycle of U. solstitialis. Bars indicate the approximate length of activity for 

each of the life stages. Black bars represent the overwintering period. 

Figure 3-20. Urophora solstitialis

adult. Photo: Eric Coombs 

(Oregon Department of 

Agriculture).

Order Diptera

Family Tephritidae

Native distribution Europe to Central Asia

Original sources Italy

First U.S. release 1993: Maryland

Species attacked Musk and plumeless thistle

Nontarget effects None reported

Establishment Not established
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Chapter 3: Biology of Biological Control Agents 63

Impact

Larval feeding reduces seed production in musk and plumeless thistles. Galls from larval 

feeding act as metabolic sinks, diverting resources away from normal plant development. 

Figure 3-22. Urophora solstitialis larvae in a flower head. Photo: Peter 

Harris (Agriculture and Agri-Food Canada, Bugwood.org).

Habitat Preferences

Site preferences and habitat requirements are unknown.

Availability

This insect’s establishment within the United States has not been confirmed, so it is unavailable for collection.

Comments

This fly may be outcompeted by Rhinocyllus conicus, an introduced biological control 

agent against exotic thistles that is no longer approved for re-distribution due to nontarget 

effects on native thistles.
64 Chapter 3: Biology of Biological Control Agents

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Urophora stylata

Bull thistle seed head gall fly

Description

Larvae are barrel-shaped, off-white in color, and have dark anal plates. Larvae complete 

three instars and can reach lengths of 5 mm (1/5 inch) at maturity. Adults are brownish 

gray with a yellow head and brownish legs. Wings are clear and with a gray-brown ‘IV’ 

marking, the ‘V’ being near the tip of the wing. Adult males can be up to 5 mm (1/5 inch) 

long while females can be 7 mm (1/4 inch) long.

Life Cycle

Overwintering larvae pupate in galls in spring when new plants are bolting. Adults emerge 

throughout the early summer and deposit eggs on maturing buds. Hatching larvae burrow 

into seed heads and feed on seed producing tissue, inducing the formation of galls. There 

may be multiple larvae per seed head. Third instar larvae overwinter within galls. There 

is one generation per year. 

Egg            

Larva

Pupa

Adult

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Figure 3-24. Life cycle of U. stylata. Bars indicate the approximate length of activity for each 

of the life stages. Black bars represent the overwintering period. 

Figure 3-23. Urophora stylata adult. 

Photo: Peter Harris 

(Agriculture and Agri-Food 

Canada).

Order Diptera

Family Tephritidae

Native distribution Western Europe

Original sources Germany and Switzerland

First U.S. release 1983: Washington

Species attacked Bull thistle

Nontarget effects None reported

Establishment CO, MD, OR, WA
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Chapter 3: Biology of Biological Control Agents 65

Impact

Larval feeding reduces seed production in bull thistle. Galls from larval feeding act as 

metabolic sinks, diverting resources away from normal plant development. Seeds in infested 

seed heads are physically stuck to gall tissue, thereby reducing dispersal. 

Figure 3-25. Urophora stylata larvae in a flower head. Photo: Peter 

Harris (Agriculture and Agri-Food Canada, Bugwood.org).

Habitat Preferences

This insect does best in open meadows with scattered plants. It does not do as well in dense 

stands of bull thistle. It does not favor sites with flooding and high winds. 

Availability

Available for redistributrion in the western states, particularly in Oregon and Washington. 

Urophora stylata galls can be collected from fall through early spring so that adults might 

be reared out in the spring within screened cages held at room temperature. Urophora 

stylata-infested galls could also be transferred to uninfested sites in spring. Alternatively, 

adults can be collected with a sweep net and aspirator in spring and early summer when 

host plants are in bud and flowering.

Comments

Because bull thistle is short-lived, it is difficult to maintain large populations of U. stylata.
66 Chapter 3: Biology of Biological Control Agents

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Native Natural Enemies of Exotic Thistles

There are many native insect species that attack exotic true thistles. Because it is likely 

that you will occasionally encounter them while monitoring your exotic thistle biological 

controls, we describe two species here (see Table 3-2). These insects are generalists and 

should not be collected for redistribution. 

Table 3-2. Selected insects native to North America that attack exotic thistles.

Type of Insect Scientific Name

Thistle Species Attacked

Bull Canada Marsh Scotch

Moth Platyptilia carduidactyla X X X

Butterfly Vanessa cardui X X X

Figure 3-26. Platyptilia carduidactyla adult. 

Photo: Jack Kelly Clark (Courtesy 

University of California Statewide 

IPM Program).

Platyptilia carduidactyla

Artichoke plume moth

Description

Larvae are orange with black heads and black marks on the last abdominal segments during 

early instars but turn light yellowish white in later instars. Larvae complete five instars and 

can reach lengths of 15 mm (5/8 inch) at maturity. Adults are buff to brownish in color, 

with a wingspan of 32 mm (11⁄4 inches). Both front and hind wings are divided into lobes, 

and the hind wings are fringed.

Life Cycle

Adults emerge from overwintering pupal cases and fly to suitable host species prior to plant 

bolting. Pupation occurs in cocoons on leaves; adults are not long-lived. There are three to 

four overlapping generations per year; larvae of all generations feed on foliage and stems 

of thistles. Overwintering is done in the pupal stage within cocoons. 

Order Lepidoptera

Family Pterophoridae

Native distribution North America

Exotic thsitles 

attacked

Bull, Canada, and 

marsh thistles
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 3: Biology of Biological Control Agents 67

Egg            

Larva

Pupa

Adult

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Figure 3-27. Life cycle of P. carduidactyla. Bars indicate the approximate length of activity 

for each of the life stages. Black bars represent the overwintering period. 

Impact

This native species can (with sufficient larval feeding) severely impact and prevent attacked 

plants of bull, Canada, and marsh thistle from flowering. Because the species is native, 

insect presence and density can vary tremendously. Platyptilia carduidactyla is also an 

economic pest of artichokes.

Figure 3-28. Platyptilia carduidactyla larva. Photo: Jack Kelly 

Clark (Courtesy University of California Statewide 

IPM Program).

Habitat Preferences

The artichoke plume moth has a wide distribution across North America and can tolerate 

a variety of climatic conditions.

Availability

Natural populations of this moth should not be augmented.
68 Chapter 3: Biology of Biological Control Agents

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Vanessa cardui

Painted lady butterfly

Figure 3-29. Vanessa cardui adult. Photo: Whitney 

Cranshaw (Colorado State University, 

Bugwood.org).

Description

Larvae are spiny, green to mottled brown, and have a light back stripe and side stripes. 

They grow through five instars and up to 38 mm (11⁄2 inches) long. The upper wings of 

adults have an irregular pattern of gold, orange and black marks and white spots near the 

tips of its forewings. The undersides of the hind wings have a large rosy area and four bluish eye-spots. The wingspan can reach 75 mm (3 inches).

Life Cycle

Adults live year-round in the Southwest of North America. They fly to the Northwest in 

spring and early summer when plants bolt and flower. Adults lay eggs on the upper leaf 

surface of host plants. Larvae feed on plant foliage and pupate within a cocoon attached 

to host leaves. There are two generations per year. Second generation adults migrate back 

to the southwestern U.S. in fall. 

Figure 3-30. Vanessa cardui larva 

and pupa. Photo: 

Eric Coombs (Oregon 

Department of 

Order Lepidoptera Agriculture).

Family Nymphalidae

Native distribution North America

Exotic thistles 

attacked

Bull, Canada, and 

Scotch thistles
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 3: Biology of Biological Control Agents 69

Egg            

Larva

Pupa

Adult

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Figure 3-31. Life cycle of V. cardui. Bars indicate the approximate length of activity for each of 

the life stages. This species does not overwinter in the Northwest. 

Impact

Swarms of this native species can rapidly defoliate patches of bull, Canada, and Scotch 

thistles, though plants are often able to recover. Because the species is native, insect presence and density can vary tremendously. Larvae also feed on sunflowers and other crop and 

native plants. At irregular intervals, mass migrations of adult painted lady butterflies from 

southern to northern states can occur in late spring or early summer. When mass migrations 

occur, subsequent larval populations on thistles and other plants can be quite high. 

Figure 3-32. Vanessa cardui larva and damage. Photo: 

Whitney Cranshaw (Colorado State University, 

Bugwood.org).

Habitat Preferences

The painted lady butterfly has a wide distribution across North America and can tolerate 

a variety of climatic conditions.

Availability

Natural populations of this native insect should not be artificially supplemented.
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Unapproved and Adventive Natural Enemies of Exotic Thistles

Numerous species attack exotic thistles throughout North America in addition to the ones 

already described. Unfortunately, some of these additional species were either unintentionally introduced or were intentional releases that attacked native thistles and so were not 

approved for interstate shipment. Most of these species are well established throughout 

the range of their host plants. It is important to be able to recognize these species to avoid 

collecting and redistributing them along with approved biological control agents. 

Table 3-3. Thistle biological control agents and adventive natural enemies present in North America but not 

approved. PL= plumeless thistle, SF= slenderflower thistle.

Type of

Agent

Scientific Name

Thistle Species Attacked

Bull Canada Italian Marsh Milk Musk PL Scotch SF

Beetle

Cassida rubiginosa X X

Cleonis pigra X X X X X X X X X

Larinus planus X X X X

Rhinocyllus conicus X X X X X X X X

Fly Terellia ruficauda X

Mite Aceria anthocoptes X

Rust

Puccinia carduorum X

Puccinia 

punctiformis

X
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Chapter 3: Biology of Biological Control Agents 71

Aceria anthocoptes

Canada thistle rust mite

Order Acari

Family Eriophyidae

Figure 3-33. Aceria anthocoptes. Photo: Eric Erbe 

(USDA-ARS).

Description and Biology

Mites appear on Canada thistle foliage in the spring when rosettes begin to bolt and are 

present until plants die back in the fall. Nymphs and adults are white, tan, pink, or yellow, 

depending on the developmental stage. They are extremely small (0.15-0.20 mm long) and 

are visible only under a microscope. Females exist in reproductive (summer) and overwintering forms. Canada thistle rust mites feed by sucking out the contents of leaf cells. These 

mites have multiple generations per year and likely overwinter on roots or root buds. 

While never approved as biological control agents, mite populations have recently been 

found throughout the U.S., suggesting one or more accidental introductions. Significant 

damage to Canada thistle is rarely if ever seen in the field. Eriophyid mites have also been 

collected from native Cirsium thistles, but it is not yet known if these are A. anthocoptes

or another, unknown mite species.
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Cassida rubiginosa

Thistle tortoise beetle

Figure 3-34. Cassida rubiginosa adult. Photo: Laura 

Parsons (University of Idaho).

Description and Biology

Adults are green, oval, and have black undersides. They can be up to 7.5 mm (1/3 inch) 

long and have a hard covering over their bodies resembling the shell of a tortoise. Overwintering adults emerge in early spring and feed on new foliage.

Eggs are deposited in spring and summer. Larvae are green, grow to 6 mm (1/4 inch) 

long, and complete five instars. They have spines on their margins and a forked tail spine 

on which they accumulate molted skins and waste to serve as a protective parasol. Larvae 

pupate in late summer; pupae are brown, oval-shaped, and have black spikes along their 

margins. Young larvae feed on the undersides of new leaves, leaving a thin layer of leaf 

tissue (“window feeding”), while older larvae make irregular feeding holes from the top 

sides of leaves. Adults emerge and feed on young foliage until late fall and then overwinter 

in soil litter. There is one generation per year. 

This species can significantly damage exotic thistle populations. However, it was an accidental introduction and is not a permitted biological control agent. It feeds on a wide 

array of plant species within the Cardueae, many of which are native or of economic importance in North America. 

Order Coleoptera

Family Chrysomelidae
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Chapter 3: Biology of Biological Control Agents 73

Cleonis pigra

Stem-base weevil

Order Coleoptera

Family Curculionidae

Description and Biology

Adults are oval-shaped and a mottled brown color. Their thorax and front part of the elytra 

have many shiny black lumps. Their wide snouts have two ridges on either side of a central 

groove. These weevils can be up to 7 mm (1/4 inch) long. Overwintering adults emerge in 

spring and feed on young leaves. 

Eggs are deposited about one month after emergence, with larvae hatching throughout 

summer. Larvae bore to the stem base of their target plant, where they feed internally. 

They are white with a brown head and are grub-like. Larvae complete four instars and 

are up to 3 mm (1/8 inch) long at maturity. Pupation occurs in soil litter near the base of 

host plants during late summer. Adults emerge throughout fall and then overwinter in soil 

litter. There is one generation per year. 

This species was an accidental introduction. It feeds on a wide array of plant species within 

the Cardueae, many of which are native or of economic importance in North America. It 

does little damage to exotic thistles.

Figure 3-35. Cleonis pigra adult. Photo: Ivo 

Tosevski (CAB International).
74 Chapter 3: Biology of Biological Control Agents

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Larinus planus

Canada thistle stem weevil

Order Coleoptera

Family Curculionidae

Description and Biology

Adults are elongate and have black bodies with mottled tan or yellow hairs and long snouts. 

Adults can be up to 8 mm (1/3

 inch) long. Overwintering adults emerge in early spring 

and feed on young foliage near shoot tips as plants begin to bolt. Feeding damage results 

in holes within leaves and deformed bracts and seed heads. 

Eggs are deposited inside developing flower buds in spring. Larvae feed on developing 

seeds and receptacle tissue throughout the summer. Larvae are white and grub-like, complete three instars, and can be up to 5 mm (1

/5 inch) long. Pupation occurs within the 

seed head; adults emerge in late summer and early fall and overwinter in soil litter. There 

is one generation per year. 

This species was an accidental introduction. It feeds on many native thistle species.

Figure 3-36. Larinus planus adult. Photo: Alec 

McClay (McClay Ecosciences, 

Bugwood.org).
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Chapter 3: Biology of Biological Control Agents 75

Puccinia carduorum

Musk thistle rust

Order Uredinales

Family Pucciniaceae

Figure 3-37. Puccinia carduorum. Photo: USDAARS, Bugwood.org.

Description and Biology

This fungus has five stages in its life cycle and produces four types of spores. Teliospores are 

the overwintering stage of the musk thistle rust and can withstand freezing temperatures. 

These germinate in spring and produce basidiospores, which infect musk thistle plants 

in the early rosette and bolting stages. Aeciospores and urediospores are then produced. 

Urediospores are the most characteristic spore: they are gold brown, flat, round, and tiny 

(25 μ in diameter). The spore surface is covered with short spines. Spores occur in mass 

clusters (pustules) on infected leaves; pustules are reddish brown and powdery. Urediospores are easily blown to uninfected plants to repeat the cycle, a process that takes two 

weeks to complete. When host plants die back in the fall, the fungus produces teliospores 

in preparation for winter. 

This species was purposely introduced throughout North America although it is not currently approved for general release. It is now widespread. Infected musk thistle plants 

experience stunted growth, a reduced seed production, and a higher susceptibility to 

interspecific competition and attack from additional biological control agents. It is easily 

spread by some thistle biological control insects.
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Puccinia punctiformis

Canada thistle rust

Order Uredinales

Family Pucciniaceae

Figure 3-38. Puccinia punctiformis. Photo: Malcolm 

Storey (www.bioimages.org.uk).

Description and Biology

This rust occurs in four types of spores: overwintering teliospores, basidiospores. aeciospores, and urediospores. In the spring, Canada thistle root volatiles stimulate the teliospores 

to germinate and produce basidiospores. Aeociospores are then produced in a scented, 

sweet, sticky nectar that attracts flies to disperse spores of opposite mating types. Resulting 

urediospores form densely packed brown pustules on upper leaf surfaces that are easily 

blown to uninfected plants to repeat the cycle. There are many generations per year. When 

host plants die back in the fall, the fungus produces teliospores in preparation for winter.

This species was accidentally introduced to North America and is now widespread throughout the U.S. and Canada. It is not approved for redistribution. This rust can stunt the 

growth of young Canada thistle but does not provide significant control. It is easily spread 

by some thistle biological control insects.
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Chapter 3: Biology of Biological Control Agents 77

Rhinocyllus conicus

Thistle seed head weevil

Description and Biology

Adults are dark brown or black with yellowish tufts of hair giving them a mottled appearance in spring. Overwintering adults emerge in early spring and deposit eggs on bracts of 

thistle flowers in late spring. Adult feeding leaves pits on stems and holes in leaves. Oviposition leaves bracts brown with bumps of chewed plant tissue. As they age, adults lose 

some of these hairs and turn brownish black. They can be up to 6 mm (1/4 inch) long and 

have shorter snouts than Larinus planus. 

Larvae hatch in spring and early summer. They burrow into seed heads and feed on receptacle tissue and developing seeds. Larvae are white with brown head capsules, complete 

four instars, and reach up to 4 mm (1/6 inch) long. Pupation occurs within the seed heads 

in late summer as seeds mature. Adults emerge for a brief time before overwintering in 

sheltered areas. There is one generation per year.

This weevil was originally approved for release in North America against exotic thistles, 

aiding the control of Italian and musk thistles. It is by far the most common insect found 

on Italian and musk thistles, often with several larvae attacking (and severely damaging) a 

single capitulum. However, after establishment, it was soon discovered feeding on a large 

number of native thistles even in the presence of its exotic hosts. Consequently, its interstate 

shipment permits were revoked. 

Figure 3-39. Rhinocyllus conicus adult. Photo: Laura 

Parsons (University of Idaho).

Order Coleoptera

Family Curculionidae
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Terellia ruficauda

Canada thistle seed head fly

Order Diptera

Family Tephritidae

Figure 3-40. Terellia ruficauda adult. Photo: Peter 

Harris (Agriculture and Agri-Food 

Canada).

Description and Biology

Adults are yellow-orange with a black mark on the thorax and four large black marks that 

make the abdomen appear mostly black. Their wings have three black marks along the 

leading margins and fainter mark near the center of the hind margins. Adult flies are about 

5 mm (1/5 inch) long. Adults emerge from overwintering pupae in early spring as plants 

begin to bolt. 

Eggs are laid in immature female seed heads; larvae feed on developing seeds and receptacle tissue. They are white and grub-like, complete three instars, and are up to 6 mm 

(1/4 inch) long at maturity. Just before seeds begin to disseminate, larvae form cocoons 

of tangled pappus hairs in which they overwinter. Pupation occurs in spring, and there is 

one generation per year.

This species was an accidental introduction. Its host range in North America has not been 

tested, but in Europe it feeds on many thistle species in the genus Cirsium. It does little 

or no damage to exotic thistle species in North America.
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 4: Elements of a Biological Control Program 79

Chapter 4: Elements of an Exotic True Thistle Biological Control Program

Rich Hansen and Rachel Winston

Exotic thistles are difficult to control with any method, including biological control. When 

biological control agents are available against a particular exotic thistle, the level of control 

achieved may vary greatly from site to site due to varying conditions. Land managers need 

to develop biological control programs that address management conditions and objectives 

unique to their area. The following steps can help in this endeavor. 

Defining Your Goals and Objectives

Defining your weed management goals and objectives is the first and most important step 

in developing a 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. 

The first thing to do is define as precisely as possible what will constitute a successful exotic 

thistle management program. For example, the goal of “a noticeable reduction in exotic 

thistle density over the next 10 years” might be achievable, but “a 50 percent reduction 

in exotic thistle stems over the next three years” is a more precise and measurable goal.

Biological control might be an appropriate exotic thistle weed management tool if your 

goal is to reduce the abundance of exotic thistles. However, by itself, biological control will 

not eradicate exotic thistles from the landscape. If your goal is to eradicate exotic thistles, 

then you should plan to employ other weed control techniques in addition to biological 

control (see Chapter 5 for more details). 

Taking Stock: Your Infestation and Your Options

Before embarking on exotic thistle management activities, you should understand the scope 

of your exotic thistle problem, identify areas of special concern, and review and understand 

all weed management tools at your disposal.
80 Chapter 4: Elements of a Biological Control Program

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Your first step should be to develop a distribution map of each species of exotic thistle at a 

scale that will allow you to address the exotic thistle problem in a manner consistent with 

your land-management objectives and your weed management resources. For example, in 

large management areas with significant exotic thistle infestations and limited resources, 

aerial mapping of large patches of exotic thistles may be sufficient to identify priority areas 

for additional survey and weed management activities (Figure 4-1a). In other management 

areas with small, discrete exotic thistle patches or where an exotic thistle infestation affects 

your ability to meet management objectives, intensive mapping and characterization of 

exotic thistle infestations (e.g., species of exotic thistle, location, size, density, and cover) 

may be necessary to develop an appropriate weed management strategy (Figure 4-1b). 

 Figure 4-1. Hypothetical weed infestation maps for a) Idaho Forest Service land and b) Snake River Birds of 

Prey National Conservation Area.

Review the exotic thistle management tools available (Chapter 5)—including use of herbicides, mechanical treatments, cultural practices, and biological control—and determine 

the conditions (when, where, if, etc.) under which it might be appropriate to use each 

tool or combination of tools. Consult your agency or university biological control expert, 

cooperative weed management area, or county weed coordinator/supervisor to learn about 

other exotic thistle management activities, underway or planned, in your area and the level 

of control that might be achieved by each.

Identify the resources that will be available for weed management activities, and determine 

if these resources will be consistently available until you meet your weed management program objectives. If resources are not available, identify what will happen at the treatment 

site if the activities are not implemented.
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 4: Elements of a Biological Control Program 81

With a map of each species of exotic thistle in your management area, an understanding of 

your land management objectives, and a list of the weed management tools available and 

the level of control you can realistically expect from each, you can identify the sites where 

biological control would be a good fit.

Developing, Implementing, and Managing a Biological Control Program

Important factors to consider when planning your exotic thistle biological control program 

include selecting appropriate release sites, obtaining and releasing insects, and monitoring 

the success of the program. These items are discussed below. If problems are encountered 

following the initiation of a biological control program, refer to the troubleshooting guide 

in Appendix I for potential responses. 

Before You Begin

There is a fair amount of preliminary work to do before you can implement a biological 

control release program. Some factors to address include:

•	 Many biological control programs do not result in visible weed reduction for a 

number of years (typically, at least three to five years). Make sure that you can make 

a long-term commitment to the program.

•	 Decide how long you can let biological control activities continue if weed control 

goals are not being met.

•	 Discuss your biological control program plans with neighboring landowners and 

land managers. Ask local weed managers about their experiences with biological 

control. Determine which agents they have used, alone or in combination with 

other weed control tools, and what level of control they achieved. Would their level 

of control be acceptable for your management area? Talk to neighboring managers about any activities, such as herbicide use, grazing, or mowing programs they 

have planned on their land. These measures could have a direct impact on your 

proposed biological control activities.

•	 Set short- and long-term goals. For example, a short-term goal might be to release 

and determine establishment of biological control agents; a long-term goal might 

be to reduce exotic thistle density by 50 percent within 10 years.

•	 Determine what resources will be consistently available for 5 to 10 years for 

implementing, monitoring, and assessing your biological control program. These 

include:

- Commiting resources for field equipment and supplies,

- Recruiting and train personnel, and

- Identifying sources of biological control agents.
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Selecting Biological Control Agent Release Sites

Establish goals for your release site. The overall management goals for a given site 

must be considered when evaluating its suitability for the release of biological control 

agents. Additional factors must be evaluated even after deciding that biological control 

is appropriate for a given site. Suitability factors will differ, depending on whether the 

release is to be:

•	 A general release, where agents are simply released for exotic thistle management,

•	 A field insectary (nursery) release, primarily employed for production of biological 

control agents for distribution to other sites, or

•	 A research release, used to document biological control agent biology and/or the 

agent’s impact on the target weed and nontarget plant community.

A site chose 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 release).

Identify site characteristics. Consider accessibility, slope, and cover. Generally, all 

exotic thistle biological control agents do best on warm, sunny sites. Areas with a flat 

topography or a south-facing exposure are preferable to cooler, north-facing sites. Areas 

with an abundance of shade should be avoided; eventually, some agents might colonize 

shaded sites, but usually only after becoming established in adjacent, unshaded areas. 

Favorable biological control agent sites are generally not overly moist. Loamy soils 

are preferable to high clay or sand soils, especially for root-feeding insects: invasive 

thistles tend to prefer loamy soils as well, though many can grow under a variety of 

conditions. 

For practical purposes, an exotic thistle infestation cannot be too large for biological 

control releases; however, it might be too small. Small, isolated patches may not allow 

biological control agent populations to build up and persist and may be better suited 

for other weed control tactics, such as herbicide applications. An area with at least 1 

acre (0.40 hectares) of exotic thistles might be considered as a minimum release site 

size, but a larger area of infestation is more desirable, especially for field insectaries 

(Figure 4-2). Infestations should be contiguous; relatively uniform weed populations 

are preferable to scattered patches over a given area. Most thistle biological control 

agents do best in a moderately dense area of infestation. 

Note land use and disturbance factors. Preferred release sites are those that experience little to no human (or other) disturbances. Fallow sites and natural areas are 

good choices for biological control agent releases. If a site must be disturbed (e.g., 

mowed or grazed), the activities should not take place during the spring and summer 

months when most biological control agents are active above ground. Sites where 

insecticide use is routine should not be used for agent releases. Such sites include 

those near wetlands that are subject to mosquito control efforts, where grasshopper 
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 4: Elements of a Biological Control Program 83

outbreaks routinely require chemical control, or near agricultural fields that receive 

regular insecticide sprays. Avoid sites prone to seasonal flooding. Do not use sites where 

significant conversion will take place, such as road construction, cultivation, building 

construction, and mineral or petroleum extraction. Do not use sites where burning 

practices occur regularly.

Survey for presence of biological control agents. Examine your prospective release 

sites to determine if one or more exotic thistle biological control agent is present. If 

an agent you are planning to release is already established at a site, you can release it 

at that site to augment the existing population, but it may be better to release it at 

another site. You should re-evaluate the release of the planned species if a different 

species of biological control agent is present. 

Record ownership and access. In general, release sites on public land are preferable to 

sites on private land. If you must 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 exotic thistle populations require lessfrequent and short-term access; you may need to visit such a site only once or twice 

after initial release. If you are releasing insects on private land, it may be a good idea 

to obtain the following:

Figure 4-2. Canada thistle infestation suitable for a biological control 

program. Thistles are in the foreground and middle ground.

Photo: Mark Schwarzländer (University of Idaho).
84 Chapter 4: Elements of a Biological Control Program

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•	 Written permission from the landowner or land manager allowing use of the area 

as a release site,

•	 Written agreement by 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), and

•	 Permission to put a permanent location marker at the site.

You may wish to restrict access to release locations, especially field insectaries and 

research sites, and allow only authorized project partners to visit the sites and collect 

insects or plants. The simplest way to accomplish this would be to select only locations that are not visible to, or accessible by, the general public. Being 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 public land manager. These formalities 

will involve such things as posting trespassing restrictions, installing locks on gates, 

etc.

Another consideration is the 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 selecting biological control for release at a site, 

including agent efficacy, availability, and site preferences (Table 4-1). 

Agent efficacy. Efficacy means the ability of the agent to directly or indirectly reduce 

the population of the target weed below acceptable damage thresholds or cause weed 

mortality resulting in control. Most of the available data on efficacy is anecdotal, observational, or based on limited experimental data. The potential efficacy of several 

agents that are not established in the U.S. is not known.

It is preferable to release only the most effective biological control agents rather than 

releasing all agents that might be available against a target weed. Consult with local 

weed biological control experts, neighboring land managers, and landowners to identify 

the agent(s) that appear more effective given local site characteristics and management 

scenarios.

Agent availability. Several exotic thistle biological control agents do not have established, 

collectable populations in the U.S. and are not available for distribution. Among established agents, readily available insects likely have several collectable populations in 

all states with exotic thistle infestations. These insects should be easy to obtain from 

intrastate and local sources. Exotic thistle agents with limited availability are likely to 

have collectable populations only in some states, and depending on the exact locations, 

might not be easily accessible. Federal agencies and commercial biological control suppliers may be able to assist you in acquiring exotic thistle agents that are not available 
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 4: Elements of a Biological Control Program 85

A

gent

Characteristics

Release Site Characteristics

Scientific Name Plants Attacked Efficacy Availability Favorable Conditions Unfavorable Conditions

Ceutrorhynchus litura Canada thistle Low alone Readily 

available

Moist, disturbed sites with 

dense thistle stands

Dry areas and where 

host thistle is stressed by drought, grazing, and other control methods

Psylliodes chalcomera Italian, musk, 

plumeless, and slenderflower thistles

Moderate Not established 

in North America

Little known; can survive 

both hot and cold climates

Unknown

Trichosirocalus horridus Italian, musk, and 

plumeless thistles

Low alone Readily 

available

Open sites on perimeter 

where thistle plants are not as tall

Marshy conditions or high 

elevations

Cheilosia corydon bull, Italian, musk, 

plumeless, and slenderflower thistles

Moderate Available in the 

West but not widespread

Does well under a variety 

of circumstances; best at early-flowering sites

Late-blooming thistle 

patches

Urophora cardui Canada thistle Moderate Readily 

available

Moist, partially shaded sites 

with scattered thistle plants

Where host thistle is stressed 

by drought, grazing, and other control methods; sites where R. conicus is established

Urophora soltitialis musk and plumeless 

thistles

Low alone Not established 

in North America

Unknown Unknown

Urophora stylata bull thistle Moderate Readily 

available

Open meadows with 

scattered plants

Dense stands; sites with 

flooding or high winds

Table 4-1. Summary of general characteristics and site preferences of exotic thistle biological control agents released in the United States 

(through 2007).
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in your state (see Obtaining and Releasing Exotic Thistle Biological Control Agents, 

below). County weed managers, extension agents, or federal and university weed or 

biological control specialists should be able to recommend in-state sources for various 

exotic thistle biological control agents.

Release site characteristics. General physical and biological site preferences for each 

agent have been developed from anecdotal observations and experimental data. As 

noted above, most exotic thistle biological control agents seem to prefer exotic thistle 

stands of moderate density. Such sites also have well-drained, loamy soils, a medium 

to dry moisture regime, and level ground or south-facing slopes (Figure 4-3). 

Figure 4-3. Typical musk thistle release site. Photo: Mark Schwarzländer 

(University of Idaho).

Obtaining and Releasing Exotic Thistle Biological Control Agents

You can obtain exotic thistle biological control agents either by collecting them yourself, 

having someone collect them for you, or by purchasing them from a commercial supplier. 

Typically, the last two methods will require packaging and shipping form the collection 

site to your release location (see Collecting Exotic Thistle Biological Control Agents, page 

94). Be sure to only release approved biological control agents.

Here are factors to consider when looking for sources of biological control:

•	 You do not need to take a “lottery approach” and release all types of biological 

control agents at a site and hope that one of them will work. In fact, some biological 

control agents will not be available even if you want them, and some have shown 

to have little or no effectiveness even after released. The best strategy is to release 

the best agent! Ask the county, state, or federal biological control experts in your 

state for recommendations of agents for your particular region. 
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Chapter 4: Elements of a Biological Control Program 87

•	 If available, biological control agents from local sources are best. Using local 

sources increases the likelihood that agents are adapted to the abiotic and biotic 

environmental conditions present and are available at appropriate times for release 

at your site. Local sources may include neighboring properties or other locations 

in your county and adjacent counties. Remember: Interstate transport of biological 

control agents requires a USDA-APHIS-PPQ permit (see Regulations Pertaining 

to Exotic Thistles on page 99). Get your permits early to avoid delays.

•	 Some states, counties, and universities have “field days” at productive insectary 

sites. On these days, land managers and landowners are invited to collect or receive 

freshly collected exotic thistle biological control agents for quick release at their 

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 can often see first-hand the impacts of various agents on exotic thistles and 

plant communities. 

Figure 4-4. Participants in a field day in New Mexico, receiving information about 

musk thistle control and distribution of biological control agents. Photo: 

Mark Schwarzländer (University of Idaho).

Typically, field days are conducted at several sites in a state and on several dates 

during the summer. Although designed primarily for intrastate collection and 

distribution, out-of-state participants may be welcome to participate. (Remember 

that USDA 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 state and/or adjacent states.
88 Chapter 4: Elements of a Biological Control Program

Biology and Biological Control of Exotic True Thistles _________________________________________

Collecting Exotic Thistle Biological Control Agents

Planning and timing of collection is critical. The species of biological control agent, the 

species of exotic thistle, and weather characteristics at your collection 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, especially 

given the large number of unapproved biological control agents established in the U.S. 

General guidelines for collecting exotic thistle biological control agents are listed in Table 

4-2. For all species, collect only on a day with good weather; insects are usually not active 

in rainy and very windy conditions. 

Beetles. Beetles are best collected in the adult stage. Adults of Ceutorhynchus litura can 

be collected with a sweep net and aspirator during spring when host plants are bolting. 

Adult Trichosirocalus horridus are best collected after emerging from overwintering: 

this is done by hand-picking these weevils from among the rosette leaves of their host 

thistle plant. 

Flies. Sweeping adult seed head flies is possible, though this is not always the best stage 

for collection. Adult flies are fragile and can be damaged during collection. Gathering heads and stem galls infested with larvae or pupae is the least-damaging way to 

collect and redistribute flies. If adults are to be collected, all exotic thistle fly species 

can be collected with a net and aspirator. For example, Cheilosia corydon in spring as 

host plants bolt and Urophora cardui and Urophora stylata in summer as host plants 

flower. Alternatively, U. cardui stem galls and U. stylata seed head galls may be collected in late fall or early spring and held in a screened cage or container and kept at 

room temperature until adult flies emerge in the spring. Freshly-emerged adults can 

then be collected and distributed for field release. 

Sweep net. A sweep net is made of cotton or muslin on a hoop 10 to 15 inches in diameter 

(25 to 38 cm) attached to a handle 3 feet (0.9 m) long (Figure 

4-5). As its name implies, it is used to “sweep” insects off the 

exotic thistles. A sweep is made by swinging the net through 

the plant canopy. It is relatively easy and efficient, though one 

should avoid breaking thistle seed heads in the process. It is 

best to alternate between sweeping insects off the thistle and 

aspirating them out of the net. Sweep no more than 25 times 

before aspirating. This reduces the potential harm that could 

result from knocking biological control agents around with 

debris or from other insects inside the net. 

Flies are very delicate, and collecting them with sweep nets can 

be damaging or fatal to them. For this reason, sweep-netting is 

not recommended for collecting flies, and alternative methods, 

such as aspiration (see below) should be used. However, 

this method is ideal for collecting adult beetles. The best 

time for sweeping exotic thistle insects is during the warmest part of the day (between 1 and 6 p.m.) as this is when 

they are most active. Sweep nets can be purchased from entomological, forestry, and 

biological supply companies, or you can construct them yourself.

Figure 4-5. Canvas sweep net. 

Photo: Laura Parsons 

(University of Idaho).
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Chapter 4: Elements of a Biological Control Program 89

Type Scientific

Name Life Stage Plant

Gro

wth

Stage Timing

Method

Beetle

Ceutrorhynchus litura Adult Bolting Spring (April-June) Net and aspirator

Psylliodes chalcomera Not established in the U.S.Trichosirocalus horridus Adult Rosette Early spring (April-June) Hand collected from rosettes

Fly

Cheilosia corydon Adults Bolting Spring (April-June) Net and aspirator

Pupae Flowering Late summer/early fall 

(August-November)

Dig roots to harvest pupae

Urophora cardui Larvae or pupae 

in galls

Bolting Collect in fall (OctoberNovember); transfer in 

spring (April-June)

Transfer galls, or rear adults 

from galls in laboratory to limit spread of parasitoids

Adults Flowering Summer (JuneSeptember) Net and aspirator

Urophora soltitialis Not established in the U.S.Urophora stylata Larvae or pupae 

in galls

Senescence to bolting Fall, Winter, Spring 

(September-May)

Transfer galls, or rear adults 

from galls in laboratory to limit spread of parasitoids

Adults Flowering Summer (JuneSeptember) Net and aspirator

Table 4-2. Recommended timetable and methods for collecting approved exotic thistle biological control agents in the U.S.
90 Chapter 4: Elements of a Biological Control Program

Biology and Biological Control of Exotic True Thistles _________________________________________

Aspirator. Use an aspirator (Figure 4-6) to suck 

the insects from the exotic thistles or sweep 

net. This provides selective sorting (no unwanted or unknown material is inadvertently 

collected). Aspirating can be done in the field 

or indoors. When aspirating indoors, cool the 

insects to make them less active and easier to 

collect. A variety of aspirators can be purchased 

from entomological, forestry, and biological 

supply companies, or you can construct them 

yourself. For the latter, make sure that tubing 

reaching your mouth is covered by fine-mesh 

screening, so that insects and small particles are 

not inhaled.

Hand-picking. Simply pick the insects from the exotic thistle plants by hand or tap them 

into a net or white plastic tray using a tool such as a badminton racquet. Forceps or 

tweezers may be helpful. Hand-picking works best for stationary or slow-moving insects, such as Trichosirocalus horridus upon its emergence in early spring. Use clippers 

if collecting galls. 

Containers For Exotic Thistle Biological Control Agents

How biological control agents are handled after collection and transported to the release 

site can affect whether they will survive to multiply at the new site. It is best to redistribute 

the agents the same day they are collected to reduce mortality or injury. 

Following collection, insects need to be 

transferred to containers intended to 

protect the biological control agents and 

prevent them from escaping. Containers 

should be rigid enough to resist crushing 

and ventilated to provide adequate air 

flow and prevent condensation. Unwaxed 

paperboard cartons (Figure 4-7) are ideal 

for all exotic thistle biological control 

agents. They are rigid yet permeable to air 

and water vapor and are available in many 

sizes. Unfortunately, most manufacturers 

have stopped producing them, and they are 

becoming increasingly difficult to find. 

As an alternative, you can use either light-colored, lined or waxed-paper containers or 

plastic containers, providing they are ventilated. Simply cut holes in the container or its 

lid, and cover the holes with a fine mesh screen. Untreated paper bags (lunch bags) work 

well for transporting agents across short distances. However, they are fragile and offer little 

physical protection for the agents within, and you must take care to seal them tightly to 

Figure 4-6. Aspirator. Photo: Laura 

Parsons (University of 

Idaho).

Figure 4-7. Cardboard containers. Photo: 

Martin Moses (University of Idaho, 

Bugwood.org).
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 4: Elements of a Biological Control Program 91

prevent the agents from escaping. Do not use glass or metal containers; they are breakable 

and make it difficult to regulate temperature, air flow, and humidity.

Fill the containers two-thirds full with paper towels to provide a substrate for insects to 

rest and hide and to help regulate humidity. Include fresh sprigs of the proper host thistle 

(as food) before adding the agents. Thistle sprigs should be free of seeds, flowers, and any 

other insects. Do not place thistle sprigs in water-filled containers; if the water leaks, it will 

likely drown your agents. Seal the container lids either with masking tape or label tape. 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 collection 

date and site, and the name of the person(s) who did the collecting.

Transporting Exotic Thistle Biological Control Agents 

Keep the containers cool at all times. If you sort and package the agents while 

in the field, place the containers in large coolers with forzen 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 

containers. Place extra packing material in the coolers to prevent the ice packs from 

shifting and damaging the containers. Always keep coolers out of the direct sun, and 

only open them again when you are ready to remove the containers to place them in 

a refrigerator for overnight storage or to release the agents. If you sort and package 

your agents indoors, keep them in a refrigerator (no lower than 40°F or 4.4°C) until 

you transport or ship them.

Transporting short distances. If you can transport your biological control agents 

directly to their release sites within 3 hours after collecting them and release them the 

same day or early the next, you need not take any measures other than those already 

described.

Shipping long distances. You might need to use a bonded carrier service with overnight 

delivery (e.g., USPS, FedEx, UPS, or DHL) if your release sites are far from your collection sites or you have to deliver your biological control agents to several sites. In 

such cases, the containers should be placed in insulated shipping containers with one 

or more ice packs, depending on the size of the packs. Some specially designed foam 

shippers have pre-cut slots to hold agent containers and ice packs (Figure 4-8). This 

construction allows cool air to circulate but prevents direct contact between the ice 

and the 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.

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 insect containers during shipping. Empty spaces in the shipper should be loosely filled with crumbled or shredded 

paper, bubble wrap, packing “peanuts,” or other soft, insulating material. Use enough 
92 Chapter 4: Elements of a Biological Control Program

Biology and Biological Control of Exotic True Thistles _________________________________________

insulation to prevent agent containers and ice packs from shifting during shipment but 

not so much that air movement is restricted. Tape the container lids shut. Enclose all 

paperwork accompanying the agents before sealing the shipping container. For additional security and protection, you may place the sealed shipping containers or coolers 

inside cardboard boxes. 

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, 

Saturday, or Sunday, delaying release of the agents for several days.

•	 Clearly label the contents of your containers and specify that they are living insects.

Common Packaging Mistakes

Excess heat. Do not expose biological control agents to direct 

sunlight or temperatures above 80°F.

Excess moisture. Remove spilled or excess water in the 

container.

Lack of air. Provide adequate ventilation; use only air-permeable 

containers.

Figure 4-8. Commercially made shipping container. Photo: University 

of Idaho, Bugwood.org.
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Chapter 4: Elements of a Biological Control Program 93

•	 Check with a prospective courier to make sure that they can accept this type of 

cargo and will not X-ray or otherwise 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, verify that someone will be there to accept the shipment, and 

instruct them not to open or X-ray the container. 

Regulations Pertaining to Exotic Thistle Biological Control Agents

U.S., intrastate. Generally, there are few if any restrictions governing collection and 

shipment of biological control 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.

U.S., interstate. The interstate transportation of biological control agents is regulated 

by the U.S. Department of Agriculture (USDA), and an approved permit is required 

to transport living biological control agents across state lines. You should apply for 

a Plant Protection Quarantine (PPQ) permit from the Animal and Plant Health 

Inspection Service (APHIS) as early as possible—ideally, 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 and obtain the permit application form, PPQ Form 526 (Appendix II), at 

the USDA-APHIS-PPQ website at http://www.aphis.usda.gov/ppq/permits. A 

recently initiated ePermit process can be accessed at this website; this allows the 

complete online processing of biological control agent permit requests

Canada. Canada requires an import permit for any new or previously released biological 

control agent. Permits are issued by the Plant Health Division of the Canadian Food 

Inspection Agency. Redistribution of exotic thistle biological control agents within 

a province is generally not an issue; however, you should consult with provincial 

authorities and specialists prior to moving biological control agents across provincial 

boundaries. More information on thistle biological control agents and their biology 

is available online at http://res2.agr.ca/lethbridge/weedbio/index_e.htm#toc.
94 Chapter 4: Elements of a Biological Control Program

Biology and Biological Control of Exotic True Thistles _________________________________________

Purchasing Exotic Thistle Biological Control Agents

A number of commercial suppliers provide exotic thistle biological control agents. County 

weed managers, extension agents, or university weed or biological control specialists maybe 

able to recommend one or more suppliers. Make sure that a prospective supplier can provide 

the species you want and can deliver it to your area at a time appropriate for field release. 

(You may want to know where and when the agents were collected.) Interstate shipments 

of exotic thistle biological control agents by commercial suppliers also require a USDA 

permit (see the box on the previous page). Determine in advance whether you or the 

shipper is responsible for obtaining the permit. Do not purchase or release unapproved, 

non-permitted biological control organisms (see Chapter 3, Table 3-2). 

Releasing Exotic Thistle Biological Control Agents

Establish a permanent location marker. Place a steel fence post or plastic or fiberglass 

pole at least 4 feet (1.2 m) tall as a marker at the release point (Figure 4-9). 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. If tall, conspicuous posts are not 

practical or suitable at your release site because of too much human or large animal 

traffic or a high risk of vandalism, etc., mark your release sites with short, colorful 

plastic tent or surveyor’s stakes or steel plates that can be etched or tagged with release 

information and located later with a metal detector and GPS.

Figure 4-9. Permanent marker for biological control agent release site. 

Photo: Rachel Winston (MIA Consulting).
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Chapter 4: Elements of a Biological Control Program 95

Record the geographical coordinates at the release point from a GPS unit. 

This should be done as a complement to, rather than a replacement for, a physical 

marker and will help locate release points if markers are damaged or removed. With the 

coordinates, be sure to record what coordinate system you are using (e.g., latitude/

longitude or UTM).

Prepare a map that describes access to your release site, including roads, trails, 

and relevant landmarks. The map should be a complement to, not a replacement 

for, a physical marker and latitude and longitude or UTM coordinates. It will be especially useful for a long-lived project 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 that are difficult to access. 

Complete relevant paperwork at the release site before or just after releasing 

biological control agents. Your agency may have release forms for you to fill out. 

Typically, the information you would provide would include a description of the site’s 

physical location, including GPS-derived latitude, longitude, and elevation coordinates; 

a summary of its biological and physical characteristics and use; the names of the biological control agent(s) released; date and time of the release; weather conditions during 

the release; and the names of the person(s) who released the agents. (See Biological 

Control Agent Release Form in Appendix III). The best time to record this information is while you are at the field site; don’t wait until you are back in the office to do 

it. Once back in the office, submit the information to your county extension agent, 

university, or state department of agriculture. Keep a copy for your own records. 

Set up a photo point. A photo point is used to visually document changes in exotic 

thistle infestations and the plant community over time following release of biological 

control agents at a site. Use a permanent feature in the background as a reference point 

(e.g., a mountain, large rocks, trees, or a permanent structure). Pre- and post-release 

photographs should be taken from roughly the same place and at the same time of 

year. Make sure each photograph includes your release point marker.

Release as many agents as possible. For practical purposes, there is probably no maximum number of biological control agents that could be released: in other words, you 

can never release too may insects! As a general rule of thumb, it is better to release as 

many individuals of an agent as you can at one site than to spread those individuals 

thinly over two or more sites. Concentrating the release 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 agents, but releases of 200 individuals or more are encouraged.
96 Chapter 4: Elements of a Biological Control Program

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Adults of exotic thistle biological control agents should be released in a group at the 

marked release point from mid-morning to late afternoon on warm, sunny days. This 

is preferred to scattering released biological control agents throughout the thistle infestation. Avoid making releases on cold or rainy days. If you encounter an extended 

period of poor weather, however, it is better to release the insects than wait three or 

more days for conditions to improve as the agents’ vitality may decline with extended 

storage.

When larvae or pupae are collected in galls and stems of infested thistle tissue, it is 

best to broadcast the infested plant material into the litter from previous years’ thistle 

growth. Emerging biological control agents can utilize nearby soil and litter for pupation or shelter or crawl up new host thistle plants to complete their life cycles. If there 

are a number of ant mounds or ground dwelling animals in the area, a better option 

may be to tie a bouquet of infested material to a nearby fencepost. 

Documenting, Monitoring, and Evaluating a Biological Control Program

Weed biological control success is measured by how well the biological control agents reduce 

the targeted weed densities near or below a pre-determined threshold. Measurement of 

the status of the weed population in relation to this threshold will determine if you efforts 

have been successful. The effects of biological control agents usually take much longer to 

appear than those of herbicide and mechanical management strategies and at least several 

years to have full impact on the targeted weed. 

The Need for Documentation

Documenting outcomes (both successes and failures) will help generate a more complete 

picture of biological control impacts, guide future management strategies, and serve education and public relations functions. Documenting initial conditions, coupled with data 

from periodic evaluations of the biological control agent’s establishment and impact, can 

indicate whether or not the biological control program is working as desired or if additional releases of the same or different biological control agents are needed. Similarly, it 

can provide critical information for other land managers and help them predict where and 

when biological control might be successful.

The value of monitoring and evaluation efforts will be greatly enhanced if the information 

you collect is recorded and accessible by other land managers and researchers. Institutional 

memory is short if based on personal recollection, and documentation of initial conditions, 

release locations, successes, and failures will provide critical information to those who will 

follow you. Documenting successes and failures can help prioritize future research and 

collection efforts. At the very least, it should help others avoid releasing biological control 

agents that do not work and concentrate on those that do. Publicly accessible information on release locations, sizes, and outcomes can be extraordinarily useful information 

for biological control researchers and policy makers. Finally, other land managers need 

to know the location of your releases so that they can avoid engaging in activities—such 

as cultivating, mowing, and applying herbicides or insecticides—that would harm your 

biological control agent populations.
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 4: Elements of a Biological Control Program 97

Information Databases

Many federal and state agencies have electronic databases for archiving information from 

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 III).

At the federal level, the USDA Animal and Plant Health Inspection Service (APHIS) 

maintains the Cooperative Agricultural Pest Survey (CAPS) database, which is part of 

the National Agricultural Pest Information System (NAPIS) (http://ceris.purdue.edu/

napis/). Biological control release information is entered into CAPS by a number of state 

and federal agency personnel who serve on the state’s CAPS survey committee. Contact 

your local APHIS officials or state department of agriculture for more information on 

participation.

The USDA Forest Service maintains a database that can store information on biological 

control agent releases on federal and non-federal lands. As of the writing of this document, biological control releases made on Forest Service lands should be entered into the 

FACTs database, and the extent of the targetted weed infestation should be entered into 

the TERRA database. Other agencies may maintain their own databases for this information. Many of the databases maintained by state and federal agencies have some safeguards 

in place to prevent undesirable uses of the information they contain.

Information Collection

For any weed biological control program, pre- and 

post-release monitoring is critical to determine if 

management goals have been achieved (Figure 4-10). 

Information on both biological control agent populations and the status of exotic thistles are collected 

during monitoring.

Populations of biological control agents

•	 Are biological control agents established at 

the release site?

•	 Are biological control agent populations increasing in size?

•	 How far beyond the initial release point(s) 

at a given site have biological control agents 

spread?

•	 Are surplus biological control agents collectable at the site?

Figure 4-10. Establishment of a Canada thistle biological 

control monitoring site. Photo: Mark 

Schwarzländer (University of Idaho).
98 Chapter 4: Elements of a Biological Control Program

Biology and Biological Control of Exotic True Thistles _________________________________________

Status of exotic thistles and other plants

•	 What is the distribution and density of the target exotic thistle plants?

•	 Are the biological control agents causing damage to the target exotic thistle plants 

and/or nontarget vegetation? What percentage of the plants or seed heads are 

attacked? 

•	 Has there been a change in the exotic thistle population and distribution since 

introducing the biological control agents?

•	 Has there been a change in desirable vegetation at the release site? 

•	 Is there a change in undesirable plants, such as other noxious invasive exotic weeds, 

at the release site?

To address these questions, monitoring activities must be focused on biological control 

agents, their impacts (damage) on individual exotic thistle plants, the exotic thistle population, and the rest of the plant community in the vicinity of the release. 

Assessing the Status of Exotic Thistles and Other Plants

The ultimate goal of a biological control program is to reduce the abundance of the target 

weed and enable the recovery of more desirable vegetation on the site. To determine the 

efficacy of biological control efforts, there must be monitoring of plant community attributes, such as target weed distribution and density. Monitoring occurs before biological 

control efforts are started (pre-release) and at regular intervals after release. 

The methods used in pre-release vegetation monitoring should enable land managers to 

determine later if they are achieving the objectives of the weed biological control program. 

Often, land managers use reductions in exotic thistle patch size or density to gauge the 

success of weed management efforts. Pre-release estimates of exotic thistle stem density, 

flowering and vegetative stems, patch size, and patch perimeter at the release sites are 

frequently measured to enable pre- and post-treatment comparisons. 

Land managers may have a goal of changing the structure and composition of the plant 

community through biological control. Pre-release sampling techniques, which allow 

managers to describe pre-treatment vegetation, are integral to assessing progress towards 

this goal. 

Pre-release monitoring should include the establishment of control plots where no insects 

will be released. These plots should be as similar as possible in habitat type (the same soil 

type, aspect, and exposure) to the release plots. Control sites should be far enough away 

from release sites so that it is unlikely they will be colonized by biological control agents 

at least during the monitoring period of the program. For consistency, the same data collection protocols should be used at control and release sites. 

In order to measure biological control agent impact accurately, methods for assessing plant 

densities after biological control agents are released must be the same as the pre-release 

methods. Post-release assessments should be planned for at least three to five years after 

the initial agent release.
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Chapter 4: Elements of a Biological Control Program 99

There are many ways to qualitatively (descriptively) or quantitatively (numerically) assess 

exotic thistle populations and other plant community attributes at release sites.

Qualitative (descriptive) vegetation monitoring. Qualitative monitoring uses descriptive elements about exotic thistles at the management site. Examples of qualitative 

monitoring include listing plant species occurring at the site, estimates of density, age 

and distribution classes, visual infestation mapping, and location of the photo points. 

Qualitative monitoring provides insight into the status or change of exotic thistle 

populations. However, its descriptive nature does not generally allow for detailed statistical analyses. Data obtained in qualitative monitoring may trigger more intensive 

monitoring later on. 

Quantitative vegetation monitoring. The purpose of quantitative monitoring is to 

measure changes in the exotic thistle population before and after release of biological 

control agents. Quantitative monitoring may be as simple as counting the number of 

flowering exotic thistle plants in an area or as complex as measuring plant height, seed 

production, rosette diameter, biomass, or plant species diversity. If designed properly, 

quantitative data can be statistically analyzed and give precise information on population or community changes. 

The simplified protocol used by the state of Idaho for exotic thistle vegetation monitoring is described in Appendix IV. It is a combination of qualitative and quantitative 

elements and can be easily modified to meet your personal or agency needs. For additional vegetation monitoring protocols, see Appendices VI and VII. 

Assessing Exotic Thistle Biological Control Agent Populations

All biological control agents go through a population cycle of gradual increase, peak, and 

decline during the season. It is easier to assess insect establishment when populations are 

peaking, so we recommend you make multiple visits to a site throughout the season and 

sample when populations appear highest. Populations of some biological control agents 

take two to three years to reach detectable levels. Thus, if no agents are detected a year 

after release, it does not mean that the insects failed to establish. Revisit the site for three 

years. If no evidence of insects is seen, 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 their opinion.

General biological control agent surveys. If you wish to determine whether or not 

an exotic thistle biological control agent has established after initial release, you may 

simply need to find the biological control agents themselves and/or evidence (that is, 

plant damage) of their presence. The easiest way to confirm biological control agent 

establishment in the years following release is to find one or more of the insect’s life 

stages at the release site (Table 4-3). Begin looking for biological control agents where 

they were first released. If you do not find any, continue to explore the area around 

the release site. Sometimes, biological control agents do not like the area where they 

were released and move to patches of exotic thistles nearby. Damage characteristic of 

individual biological control agents can also indicate successful establishment. Typical 

damage traits are listed in Table 4-3. 
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Table 4-3. Life stages/damage to look for to determine establishment of exotic thistle biological control agents.

Scientific Name Life Stage Where to Look When to Look Damage

Ceutorhynchus litura, 

Trichosirocalus 

horridus, and Cheilosia 

corydon

Adults Foliage Spring (April-June) Adult feeding by any 

of these species is 

minimal

Larvae In stems and in/around 

root crown

Summer 

(June-September)

Plants appear 

stunted and wilty; 

dissection will 

reveal mined tissue 

and dark frass

Psylliodes chalcomera Not established in the U.S.

Urophora cardui Adults Foliage and stems Late spring 

(May-July)

Adults do not do 

any appreciable 

damage to host 

plants

Larvae In stem galls Summer, Fall, Winter 

(July-March)

Form stem galls 

that stunt growth 

of host plants; 

give malformed 

appearance

Urophora soltitialis Not established in the U.S.

Urophora stylata Adults On foliage and seed 

heads

Summer 

(June-September)

Adults do not do 

any appreciable 

damage to host 

plants

Larvae In seed head galls Summer, Fall, Winter 

(July-March)

Form seed head 

galls that prevent 

developing seeds 

from dispersing; 

give seed heads a 

malformed, swollen 

appearance

Additional monitoring methods. To determine the density of insects at the release 

site or how far the biological control agents have spread from the release point, a more 

systematic monitoring method is needed. Numerous approaches can be taken in setting 

up a monitoring program; these will vary depending on the information you wish to 

obtain from monitoring efforts. 

A simple method for monitoring the abundance of two common species of exotic 

thistle biological control agents (Ceutorhynchus litura and Urophora cardui) used 

by the state of Idaho is described in Appendix IV. This approach was designed to be 

simple, efficient, and sufficiently versatile to allow for the collection of information 

from the same sites over multiple years. You can easily modify these protocols to meet 

your personal or agency needs. Alternative general biological control agent monitoring 

forms can be found in Appendix V.
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 4: Elements of a Biological Control Program 101

Cheilosia corydon. To assess the density of the less widespread Cheilosia corydon, an even 

simpler method would suffice. This specie is not common and is difficult to collect in 

the adult stage. Consequently, the larval stage is easiest to observe. 

•	 Randomly select 20 host (Italian, musk, plumeless, or slenderflower) thistle plants 

scattered throughout the target weed infestation.

•	 Harvest the plants during the flowering stage (throughout summer) by digging 

down to include collection of the roots.

•	 Dissect the stems and roots, looking for evidence of C. corydon larvae. Be sure that 

you properly distinguish between C. corydon larvae and T. horridus larvae, which 

may also be present in the same portions of the plant (Figure 4-11).

Figure 4-11. Cheilosia corydon larval mining damage. Photo: Eric Coombs (Oregon 

Department of Agriculture).

•	 Record the number of larvae found per plant, and repeat the process annually.

Assessing Impacts on Non-Target Plants

Sampling of vegetation other than exotic thistles should be included in a vegetation monitoring program in order to assess potential non-target impacts of the biological control 

program. Specific methods will depend on the species targeted for sampling in particular 

areas.

Changes in abundance of other desirable or undesirable vegetation. Biological 

control agent releases, among other land management strategies, can affect the presence and relative abundance of many other plant species even though the agents do not 

directly utilize them. If biological control works to remove exotic thistles at a site, it 

will create an empty niche to be filled by alternative—hopefully desirable—vegetation. 

These indirect effects are the result of changes in exotic thistle abundance. As exotic 

thistles become less abundant, the utilization of site resources is altered; some plants 

become more abundant, while others become less so. Within the overall management 

plan for your site, it may be important to document the changes in other vegetation 

after you release your biological control agents.
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Depending on your program goals, you may need to document quantitative and/

or qualitative changes for groups of plants, such as native forbs or exotic perennial 

grasses, or on individual species, such as a rare plant or a food plant for a native butterfly. Plant species may be considered beneficial (e.g., native and introduced forage 

plants) or deleterious (e.g., other invasive weeds). One important management goal 

should be to avoid invasion of a site by another exotic weed after successful biological 

control of exotic thistles; in other words, you do not want to replace one invasive weed 

with another. For this reason, we strongly recommend that you monitor populations 

of other exotic weeds that are known to be problematic in your area.

You will need to clearly define site management goals and become familiar with the 

plant communities at your release location and nearby sites. You can easily modify the 

vegetation sampling procedures described above to monitor changes in density and/or 

cover for exotic thistles as well as other plant species, both before and after you release 

your biological control agents.

Direct impacts of biological control 

on nontarget plants. The host 

ranges of all currently approved exotic 

thistle biological control agents are 

restricted to species of exotic thistles 

(see Chapter 3). Most other exotic 

thistle species in the western U.S. and 

Canada are considered weeds, so nontarget feeding on these plants should 

not be a cause for concern. Many native thistle species are found throughout the western U.S. (e.g., the wavyleaved thistle in Figure 4-12) and are 

likely to occur within or relatively near 

exotic thistle infestations and releases 

of biological control agents. Some 

native thistle species may be at risk 

of non-target utilization by released 

agents, so we recommend monitoring native thistles that occur in exotic 

thistle infestations being treated with 

biological control agents.

The first step in addressing possible 

non-target attacks on native thistles 

is to become familiar with the plant 

communities present at and around 

your release sites. A visual, pre-release survey may locate native thistles that are present. 

You may have to consult with a local botanist, if available, for advice on areas where 

these plants might be growing, what specific habitats they typically utilize, and how 

you can identify them. Some of the more common native thistles are presented in 

Figure 4-12. Native Cirsium undulatum. Photo: 

Rachel Winston (MIA Consulting).
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 4: Elements of a Biological Control Program 103

Chapter 2. Herbarium records at a university or other research institution may provide 

guidance about the local or statewide distribution of native thistle species.

If you do find one or more native thistles at a potential biological control release site, 

you should not immediately cancel plans to release biological control agents; generally, native thistles are not attacked or may experience limited exploratory feeding. The 

vegetation sampling procedures described above can be easily modified to monitor 

changes in density and/or cover of specific, known native thistle species, before and 

after biological control agents are released. Concurrently, you may wish to collect additional data, such as the number of agents observed on non-target thistles, the amount 

of foliar feeding observed, or the presence of galls.

If you observe approved biological control agents feeding on and/or developing on 

native thistles, collect samples and take them to a biological control specialist in your 

area. Alternatively, you may send the specialist the site data so he or she can survey the 

site for nontarget impacts. Be sure not to ascribe native thistle damage to any specific 

insect and thus bias its identification.

__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 5: An Integrated Exotic Thistle Management Program 105

Chapter 5: An Integrated Exotic Thistle Management Program

Rodney G. Lym, Rachel Winston, and Mark Schwarzländer

Introduction

Integrated weed management (IWM) is a systems approach to management of undesirable 

plants. IWM is described in the Federal Noxious Weed Act as a system for the planning and 

implementation of a program using an interdisciplinary approach to incorporate multiple 

methods for containing or controlling an undesirable plant species or group of species. 

Components include but are not limited to:

•	 Education and Outreach

•	 Prevention

•	 Monitoring

•	 Early Detection and Rapid Response (EDRR)

•	 Biological controls (insects, mites, or pathogens)

•	 Physical or mechanical treatments (tilling, mowing, etc.)

•	 Cultural practices (grazing, reseeding, etc.) 

•	 Chemical treatment

•	 General long-term land management practices 

An integrated, coordinated approach to weed management has two interdependent 

goals:

•	 The development of a long-term plan to manage all land in a designated area, with 

landowners and land managers working together towards effective management. 

•	 The implementation of the most effective weed control methods for the target 

weed, regularly assessed and adjusted as needed. 

A program that integrates multiple control methods—such as biological agents, chemical 

controls, and cultural practices—is far more likely to achieve long-term success against 

exotic thistles than any single control method used alone.
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Biology and Biological Control of Exotic True Thistles _________________________________________

Integrating Biological Control Methods

Classical biological control has been applied to many invasive weed species, and there are 

several examples in which both single- and multiple-agent introductions have successfully 

controlled the target weeds, though this has not been commonly documented for exotic 

thistles. The use of biological control agents alone to control weeds can be effective with 

some invasive plants, but may take three to five years or more to reduce weed populations 

to manageable levels. The success rate for classical biological control may increase when 

multiple species of biological control agent are used so long as the different species attack 

different plant parts (leaves, roots, stems, etc.) of the target weed at different times during 

the growing season or are released over a larger range of infestation

Some agents have helped reduced exotic thistle densities (Figure 5-1), but biological control 

agents have not established in all areas where exotic thistles occur. Even when established, 

biological control agents do not eradicate the target weed. Where ideally suited, biological 

control can maintain some species of exotic thistle densities below economically significant 

levels. Biological control agents are not going to work against exotic thistles every time at 

every site; integration with other management tools or simply resorting to other tools may 

be required and is often encouraged in order to attain thistle management objectives.

Figure 5-1. A pasture a) before and b) five years after the release of Rhinocyllus conicus, a 

biological control agent (no longer approved for release) released for must thistle 

control. Photos: Loke T. Kok (Virginia Polytechnic Institute and State University).

Land managers have learned successful, long-term exotic thistle control programs must 

be cost-effective. Because exotic thistles occur in a wide variety of environments across 

western North America, no single control method can be successful in all infestations 

(Figure 5-2). Land managers have also recognized that they must operate under social 

constraints that will limit the weed management tools they can use in sensitive areas, such 

as wilderness, near waterways, and on public lands. A wide variety of successful exotic 

thistle management methods—including herbicide mixtures, selective grazing practices, 

reseeding, and biological control agent release—have long been used. Each method was 

initially used alone, but long-term management is greatly improved when various control 

methods are used in combination according to infested habitat type, land use, ownership, 

and available resources. 
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 5: An Integrated Exotic Thistle Management Program 107

Weed Control Methods Used to Manage Exotic Thistles

There are seven commonly cited activities for control of invasive plants: education; prevention; and biological, physical or mechanical, cultural, and chemical controls and practices. 

Associated activities include monitoring, early detection and rapid response (EDRR), and 

long-term management practices.

Education and Outreach

Education and outreach activities should aim to increase public awareness of noxious weeds 

and provide information on ways to manage the problem. Education efforts should be a 

significant component of any IWM strategy and program, regardless of the other weed 

control methods employed. Education and outreach should inform both the public and 

land managers of all available control methods.

Prevention and Exclusion

Prevention and exclusion activities are aimed at areas not currently infested by exotic thistles 

and intended to keep uninfested areas weed-free. Exotic thistles are spread by the movement 

of seed-contaminated hay, wind, motorized equipment, wildlife, or water dispersion into 

Figure 5-2. Various habitats where exotic thistles can thrive: a. arid bunchgrass community 

(Rachel Winston, MIA Consulting); b. farmland (Ohio State Weed Lab Archive, 

Ohio State University); c. wet meadow (Malcolm Storey, www.bioimages.org.uk); 

d. sagebrush steppe (Mark Schwarzländer, University of Idaho); e. forest understory 

(Marianna Szucs, University of Idaho); f. rangeland (Mark Schwartzländer, 

University of Idaho).
108 Chapter 5: An Integrated Exotic Thistle Management Program

Biology and Biological Control of Exotic True Thistles _________________________________________

uninfested areas. Where these factors can be controlled, preventing the spread of thistle 

requires cooperation among all local landowners. 

In areas where exotic invasive thistles 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. Prevention and exclusion activities are typically paired with education efforts 

(Figure 5-3). Examples of exclusion efforts include weed-free forage programs, state seed 

laws, and mandatory equipment cleaning before entering uninfested sites.

Figure 5-3. Weed education sign erected by Sublette County, Wyoming, 

on USDA Forest Service land. Photo: Rick VanBebber (USDA 

Forest Service, Region 4).

Monitoring

Monitoring supports prevention by using frequent surveys in weed-free areas to ensure 

that exotic thistles have not invaded. Similarly, mapping of known infestations allows land 

managers to document the direction and rate of spread.

Early Detection and Rapid Response

An early-detection and rapid-response (EDRR) program is a specific protocol for tracking 

and responding to infestation spread. It relies heavily on education and outreach activities to be effective. EDRR programs target areas into which a weed may spread and take 

a two-prong approach by: 1) educating the public to aid in detection of the weed and 2) 

initiation of rapid response eradication efforts at all verified locations of the weed. 

Biological Control 

Biological control involves the use of living organisms, usually insects, mites, or pathogens, 

to control a weed infestation and recreate the balance of plant species with their natural 

predators and pathogens. Classical biological control focuses on the introduction of natural 

enemies from the origin of the invasive weed.
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 5: An Integrated Exotic Thistle Management Program 109

Physical or Mechanical Treatment

Physical treatment uses hand pulling and hoeing to disrupt and remove weeds and is the 

oldest method of weed control. Mulching can be used to control regrowth. Physical methods are very effective in controlling biennial thistles, but are labor-intensive. Mechanical 

treatments include tilling and mowing: these methods are equipment-intensive.

Tilling. Tilling will control most species of exotic thistles if done on a timely basis and if 

roots are cut below the soil surface. Using farming equipment to cut annual or biennial 

thistles off below the crown area is an effective way to kill individual plants (Figure 

5-4). Control is enhanced when an herbicide treatment is used in conjunction with 

tilling. Tilling is generally not practical or desirable in wildlands and nature preserves 

and is not recommended for Canada thistle, as this technique can result in the vegetative spread of the weed from root fragments.

Figure 5-4. Mechanical treatment. Photo: John Byrd (Mississippii State University, 

Bugwood.org).

Tilling is not always compatible with biological control efforts. Tilling and disking 

frequently disrupt and destroy biological control agents overwintering in exotic thistle 

roots and galls or in soil litter.

Mowing. Cutting back the above-ground portion of a plant will remove topgrowth and 

can reduce exotic thistle seed production, especially if the plants are mowed frequently 

and during flowering. Mowing should be done as close to the ground surface as possible when terminal flower heads start blooming (10% bloom). If the mowing height 

is too high, the plant may recover or produce flowers below the cutting height. Likewise, mowing conducted after plants have started producing seed may help distribute 

seed. 

A single mowing treatment does not injure the root system, and long-term control is 

not possible on perennial species unless done often enough so that root carbohydrates 

are depleted and roots die. Mowing has been shown to be effective on annual and biennial species. Frequent mowing, however, might be too costly or infeasible for larger 

infestations. Mowing can be used to reduce non-target plant cover and litter prior to 
110 Chapter 5: An Integrated Exotic Thistle Management Program

Biology and Biological Control of Exotic True Thistles _________________________________________

herbicide application or the release of biological control agents, increasing the success 

of both techniques. Mowing in the summer prior to a fall herbicide application will 

improve coverage of the chemical on fall thistle rosettes. Mowing, if applied infrequently 

or if not well-timed, however, can increase the spread of Canada thistle (in particular) 

by encouraging new growth from underground rhizomes.

Prior to mowing, it is important to consider the life cycles of the biological control 

agents and when and where they will be on the plant. For example, mowing excess 

plant litter during late winter and very early spring when C. litura, T. horridus, and 

C. corydon are overwintering either in the soil litter or roots is compatible with, and 

perhaps even conducive to, biological control. However, mowing during this same time 

period would destroy larvae of U. cardui and U. stylata overwintering in galls within 

exotic thistle seed heads and stems if host plants are erect. Mowing during spring when 

all species are active in some stage in the above-ground portion of exotic thistles could 

kill large numbers of insects.

Cultural Practices

Cultural methods of weed control, including seeding with competitive grass species, 

burning, and grazing, can enhance the growth of desired vegetation, which may slow 

the invasion of exotic thistles onto a site. Regardless of which method is used, all cultural 

control methods are more successful when combined with other control methods, such 

as biological controls and chemical treatments. 

Seeding competitive grasses. Seeding can be used to help establish competitive native 

species, such as grasses and forbs, in an exotic thistle infestation. Once grasses and forbs 

have established (usually one to two years after seeding), biological control agents can 

be released to help control surviving exotic thistles. 

Exotic thistle control programs that include establishment of introduced and native 

perennial grasses have been more successful at increasing forage production and longterm weed control than any single method. Some perennial grasses can compete effectively with exotic thistles and provide a long-term reduction in density. The most 

competitive grasses include wheatgrass, wild rye, and smooth brome; however, the best 

type of grass to plant in competition with exotic thistles varies by region. Consult your 

local county extension agent or Natural Resource Conservation Services representative 

for best alternatives. Control of exotic thistles prior to seeding grasses is important: 

herbicides such as aminopyralid, clopyralid, MCPA, and 2,4-D can be applied once or 

twice (check the label) during the summer to reduce exotic thistle vigor prior to a fall 

or early spring seeding of grasses.

Incorporating biological control agents with re-seeding has been 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. In 

order to establish a suitable site for re-seeding, either an area must be tilled to provide 

an acceptable seed bed and/or herbicides such as glyphosate must be applied to reduce 

competition from exotic thistles. Tilling can disrupt and destroy biological control 
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 5: An Integrated Exotic Thistle Management Program 111

agents overwintering in soil litter and plant roots and heavy herbicide use will reduce 

the exotic thistle shoots on which biological control agents feed and may hinder suitable establishment of biological control agent populations.

Seeding of competitive species using a no-till seeder would be less disruptive of an 

established exotic thistle biological control population than conventional seeding 

techniques. Unfortunately, no-till seeding has been successful only when the site was 

mowed or burned prior to seeding and an herbicide was applied to control broadleaf 

and grass weeds. The thick thatch of dead thistle stems often found in old stands can 

reduce grass seedling establishment and ultimately may result in undesirable species 

replacing the seeded grasses. Intensive management techniques often establish competitive species first, using biological control agents only after the seeded species have 

become established and the weed has begun to re-grow. 

Prescribed fire. This method of cultural control is used against many domestic and 

exotic plants (Figure 5-5). The success of prescribed fire against exotic thistles varies 

considerably depending on season of burn, burn severity, site conditions, and plant 

community composition and phenology before and after the fire. Prescribed burns 

can reduce thistle density if used in conjunction with other control methods such as 

reseeding or herbicide treatments. Fire may also increase competitive plant species (i.e., 

some perennial grasses), allowing them to compete with the exotic thistles. 

Figure 5-5. Prescribed fire. Photo: Robert Masters (USDA Agricultural Research Service, 

Bugwood.org).

Biological control agents must be able to survive controlled burns that aid in returning 

native vegetation to exotic thistle-infested areas. Often, the timing of the controlled 

burn will determine if an agent survives. Generally, soil-inhabiting agents are able to 

survive fast-moving, low- or moderate-intensity fires. If a fire event occurs while the 

biological control agents are in the adult stage, they can often escape the fire by flying 

off, readily re-establishing on recovering thistles not killed during the fire event.

Grazing. Domestic livestock such as cattle, sheep, and goats have been observed grazing 

on seedlings, flower heads, and seeds of some exotic thistle species (Figure 5-6). It 
112 Chapter 5: An Integrated Exotic Thistle Management Program

Biology and Biological Control of Exotic True Thistles _________________________________________

has not been determined what effects such feeding has on the population dynamics of 

exotic thistles. Domestic livestock are discouraged from grazing the foliage and stems 

of exotic thistles due to their spiny nature. In most instances, domestic livestock actively 

graze species other than exotic thistles, thus decreasing the competition against exotic 

thistle species. Additional control methods should be used in conjunction with or in 

replacement of grazing by domestic livestock. 

Figure 5-6. Cattle and sheep grazing. Photo: Scott Bauer (USDA Agricultural Research 

Service, Bugwood.org).

While grazing by domestic livestock would not likely control exotic thistle infestations 

alone, proper grazing of desirable range and pasture species can increase plant productivity to competitive levels. Precision grazing using cattle can successfully reduce 

Canada thistle stands in rangeland and pasture habitats. Unlike season-long grazing, 

so-called high-intensity, low-frequency (HILF) grazing reduces Canada thistle stem 

density and biomass while increasing range yield. A HILF grazing system uses higher 

stocking rates of cattle (high-intensity) to graze heavily on a pasture for a short period 

of time followed by an eight-week recovery time (no grazing) of the pasture or paddock (low-frequency) before the grazing treatment is repeated. The relatively long 

recovery time benefits the re-growth of grasses, increasing the competitive ability of 

the desired vegetation. It should be noted, however, that improper or over-grazing can 

result in conditions conducive to exotic thistle invasion (bare ground and an outright 

removal of desirable forage species) and may have led to the original infestation. 

Integration of grazing and biological control agents for exotic thistles may be complicated. As stated above, thistle grazing is most successful when plants are heavily defoliated under high intensity grazing regimes. Under these conditions, a good proportion 

of the thistles get trampled. Both trampling and consumption of thistle shoots will kill 

juvenile biological control insects developing in galls or feeding in the stems of exotic 

thistles and adult insects hiding or overwintering in leaf litter. Integration of biological 

control and precision grazing can be more successful than each control method alone 

when grazing events are carefully timed and synchronized with the activity period of 

biological control agents.
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 5: An Integrated Exotic Thistle Management Program 113

Chemical Treatment

Herbicides are important tools for controlling noxious 

weeds and are available for exotic thistle control in a 

variety of environments (Figure 5-7). Herbicide timing 

and application rates and grazing and haying restrictions 

vary by state and region. Please consult your local weed 

officer or county agricultural extension agent to learn 

which herbicides work best and when to apply them in 

your situation. (See the following pesticide sse text box.) 

The most widely used products include the following.

•	 Aminopyralid can be applied in the spring or 

early summer on rosettes or bolting plants or, 

alternately, in the fall on rosettes. Aminopyralid 

is less detrimental to desirable broadleaf species 

than most other auxin herbicides. Aminopyralid 

can also be applied near many tree species where 

dicamba and picloram cannot be used. However, 

legume species are especially susceptible to aminopyralid.

•	 Clopyralid can be used on young, actively growing thistles prior to the bud stage and on rosettes 

in the fall. Clopyralid will not injure established 

grasses. Clopyralid is more expensive than most 

other herbicides used for thistle control but is very effective. 

•	 Use dicamba after thistles emerge. Annuals can be treated when small and actively 

growing. Biennial thistles are best treated prior to flowering and in the fall rosette 

growth stage. Dicamba should be applied in the fall for optimum Canada thistle 

control. A commercial mixture of dicamba plus diflufenzopyr (Overdrive®) is also 

available for thistle control in non-crop lands and wildlands.

•	 MCPA can be used when thistles are small and actively growing. MCPA will not 

control perennial thistles. MCPA is often used prior to and following reseeding of 

desirable grasses for annual weed control. 

•	 Use picloram after thistles emerge, throughout active growth stages, or in late 

summer or fall. Picloram requires a higher use rates than aminopyralid and has a 

longer soil residual period, which will reduce regrowth from roots or seedlings.

•	 2,4-D can be used when annual and biennial thistles are young and actively growing. 2,4-D will not reduce Canada thistle, but can be used to prevent flowering 

and allowing haying without weed spread.

•	 Metsulfuron should be applied post-emergence to actively growing thistles. It is 

best to use a nonionic or organosilicone surfactant with metsulfuron. Grazing restrictions apply to its use, as do restrictions for use on some grasses. Metsulfuron is 

often mixed with 2,4-D and/or dicamba to increase control of perennial thistles.

Figure 5-7. Applying herbicides. 

Photo: James Miller 

(USDA Forest Service, 

Bugwood.org).
114 Chapter 5: An Integrated Exotic Thistle Management Program

Biology and Biological Control of Exotic True Thistles _________________________________________

Application timing is very important to ensure the most effective use of herbicides. It is good 

to treat biennial and winter annual species with fall applications when plants are seedlings 

or rosettes as these stages are more vulnerable and are often easiest to control. Growth 

stage, stand density, and environmental conditions (e.g., drought or cold temperatures) 

all determine the best choice of product and application rate. 

Depending on the species and the location, herbicides and biological control agents can 

be combined or used separately. For example, the Canada thistle stem gall fly, Urophora 

cardui, can be used to reduce seed production in forested areas where herbicides generally 

cannot be used; in turn, herbicides can be applied away from trees to reduce exotic thistle 

spread. In a separate example, recent research has shown that Ceutorhynchus litura can be 

well integrated with herbicide treatments for the control of Canada thistle if the herbicide 

is applied after the mature larvae have left the stems for pupation in the soil. The herbicide 

treatment is thought to reduce the weed’s ability to compensate for the biological control 

feeding damage. In greenhouse trials, clopyralid was the most effective herbicide used in 

conjunction with Ceutorhynchus litura. 

Herbicide application should occur at a time least disruptive to the biological control agent 

even if the timing is not ideal for the chemical. In order to ensure that the insects maintain 

viable populations as the exotic thistle infestation is reduced, 25 percent of the area should 

remain untreated to serve as “refuges” for biological control agents. 

Use Pesticides Safely!

•	 Read the pesticide label, even if you have used the pesticide before. Follow the all

instructions on the label.

•	 Wear protective clothing and safety devices as rec¬ommended on the label.

•	 Bathe or shower after each pesticide application.

•	 Be cautious when you apply pesticides. Know your legal responsibility as a pesticide

applicator. You may be liable for injury or damage resulting from pesticide use. 

 Peter M. Rice

General Long-Term Land Management Practices

Exotic thistles have persistent growth characteristics, and seed can remain viable in the 

soil for years. Therefore, you should implement long-term weed management programs. 

Long-term weed management includes re-treatment with herbicides or continued cultural, 

mechanical, or biological control practices to maintain low exotic thistle populations. Range 

improvements—such as grazing systems, cross-fencing, and water development—will help 

retard the invasion of many weed species, including exotic thistles. Sites with no desirable 

species should be reseeded with a competitive plant species as part of the total management program (Table 5-1).
__________________________________________Biology and Biological Control of Exotic True Thistles

Chapter 5: An Integrated Exotic Thistle Management Program 115

 Table 5-1. Comparison of exotic thistle management options.

Management

Technique Advantages Disadvantages C

omments

Herbicides

Fast acting. Expensive for large areas. Best used on small patches when exotic 

thistles first appear or on the edges of a large infestation to keep it from spreading while other methods, such as biological control, have time to establish.

High success rate for 

reducing exotic thistle densities.

May harm desirable vegetation, especially 

broadleaf species.

Rapidly enhances grass 

production.

Many natural areas are inaccessible to 

spray equipment.

Public resistance to chemical controls.Regulations or policies may prohibit use in 

some areas.

Biological Control

Can be very selective. 

Agents generally do not have to be reintroduced once established.

Some risk of undesirable 

effects on native plants. Biological control is not successful in all situations.

Most economical option for large 

infestations.

Public acceptance is 

generally higher than with other weed control methods.

Permanent: cannot be undone. Will control exotic thistles in a variety 

of environments in which the weed occurs, especially if multiple agents are introduced.

Measurable changes in weed densities 

may take several to many years.

Grazing

Allows use of the land even 

with heavy exotic thistle infestations.

Cannot be used in many natural areas 

such as national parks and wilderness areas.

Will remove top-growth only, but does not 

reduce the root mass.

Can be used in combination 

with biological or chemical control methods.

Expensive. 

Non-selective.

Can exacerbate the problem.

The same areas must be grazed annually 

or exotic thistles will rapidly reestablish.

Mechanical/Cultural 

Treatments

Very effective. Not appropriate for natural areas and 

wildlands.

Not compatible with biological control 

agents.

Can be used to reseed native 

species

Expensive for larger infestations. Best used when an areas is being 

“reclaimed.”

__________________________________________Biology and Biological Control of Exotic True Thistles

Glossary 117

Glossary

abdomen The last of the three insect body regions; usually containing the 

digestive and reproductive organs

achene A small, one-seeded fruit that does not split at maturity

aeciospore A binucleate spore of a rust fungus, formed in a chainlike series 

in an aecium

allelopathy One plant harms another by exuding specific biomolecules

anal plate Darkened segment at the tip of the abdomen

annual A plant that flowers and dies within a period of one year from 

germination

antenna (pl. antennae) In arthropods, an appendages on the head, normally many-jointed, 

and of sensory function

aspirator An apparatus used to suck insects into a container. Can be as 

simple (as in-mouth aspirator) or mechanical (as in a gasolineor battery-powered vacuum aspirator)

basal Located at the base of a plant or plant part

basidiospore A sexually produced fungal spore borne on a basidium

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

bract A small, leaf-like structure below a flower
118 Glossary

Biology and Biological Control of Exotic True Thistles _________________________________________

capitulum (pl. capitula) Seed head of a plant in the sunflower family

complete metamorphosis An insect life cycle with four distinct stages (egg, larvae, pupae, 

adult)

compound eyes Paired eyes consisting of many facets, or ommatidia, in most adult 

Arthropoda

coordinates Any of a set of numbers used to specify a location on a line or in 

a plot

density Number of individuals per unit area

dioecious The condition in which male and female reproductive organs are 

on separate plants

dissemination Dispersal. Can be applied to seeds or insects

elytron (pl. elytra) Hardened front wing of a beetle

emergence Act of adult insect leaving the pupal exoskeleton, or leaving winter 

or summer dormancy

exoskeleton Hard, external skeleton of the body of an insect

exotic Not native 

floret One of the small, closely clustered flowers forming the head of 

a composite flower in the sunflower family

flower head A special type of inflorescence consisting of numerous florets 

that actually look like one flower

gall An abnormal growth on a plant, usually induced by an insect that 

lives within the gall

genera A taxonomic category ranking below a family and above a species and generally consisting of a group of species exhibiting 

similar characteristics. In taxonomic nomenclature the genus 

name is used, either alone or followed by a Latin adjective or 

epithet, to form the name of a species.

glandular Having glands (a group of specialized cells often in the form of 

sticky hairs that produce and secrete a specific substance)

grub A soft, thick-bodied, ‘C’-shaped beetle larva
__________________________________________Biology and Biological Control of Exotic True Thistles

Glossary 119

head (insect) The segment of an insect that consists of mouthparts, antennae, 

and eyes

head capsule Hardened covering of the head of an immature insect

herbivory Feeding on plants

hermaphroditic Having both male and female reproductive parts

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.

inflorescence The flowering part of a plant

instar The phase of an insect’s development between molts

integrated weed management A system for the planning and implementation of a program, 

using an interdisciplinary approach, to select a method for 

containing or controlling an undesirable plant species or group 

of species using all available methods

involucre A circle of bracts under an inflorescence

larva (pl. larvae) Immature insect stage between the egg and pupa (examples include 

grubs, caterpillars, and maggots)

lobed A leaf with shallow or deep, rounded segments, as in a thistle 

rosette leaf

marbled Of variegated or mottled color

membranous Thin and transparent

metabolic sink Site of a plant that receives photosynthate (food) produced by 

the plant, diverting the resource away from the plant’s normal 

use

molting Process of insect development that involves shedding its exoskeleton and producing an exoskeleton for the next instar
120 Glossary

Biology and Biological Control of Exotic True Thistles _________________________________________

nontarget effect When biological control agents feed upon plant species other than 

the hosts they were introduced to control 

oviposit To lay or deposit eggs

pappus A tuft of hairs, scales, or bristles at the base of an achene in flowers of the sunflower family

perennial A plant that lives more than two years

proleg A fleshy, unsegmented, abdominal walking appendage of some 

insect larvae, common among caterpillars

pupa (pl. pupae) (v. pupate) Non-feeding, inactive stage between larvae and adult in insects

qualitative Measurement of descriptive elements (e.g., age class, distribution)

quantitative Measurement of quantity; the number or amount (e.g., seeds per 

capitulum)

ramet An individual part of a clone

receptacle Part of the stem to which the flower is attached

rhizomatous A rootlike subterranean stem, commonly horizontal in position, 

that produces true roots and sends up shoots progressively to 

the soil surface

rosette A compact, circular, and normally basal cluster of leaves

seed head Synonym for capitulum of a plant in the sunflower family. The 

seed head consists of a receptacle and florets.

senescence Final stage in a plant’s lifecycle

snout ‘Nose’ of a weevil. The elongate head of a weevil with mouth 

parts at the tip (apex)

species A fundamental category of taxonomic classification, ranking below a genus or subgenus and consisting of related organisms 

capable of interbreeding

spine A stiff, pointed plant part
__________________________________________Biology and Biological Control of Exotic True Thistles

Glossary 121

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

teliospores A spore of certain rust fungi that carries the fungus through the 

winter and that, on germination, produces the basidium

thorax Body region of an insect behind the head and abdomen, bearing 

the legs and wings

transect A straight line of varying length along which plants are periodically sampled individually or in quadrants

urediospore The spore of the rust fungi that appears between the aeciospore 

and the teliospore: commonly, the summer spore

__________________________________________Biology and Biological Control of Exotic True Thistels

Selected References 123

Selected References

Chapter 1: Introduction

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Press, Corvallis. 467 p.

Harley, K.L.S., and I.W. Forno. 1992. Biological Control of Weeds: A Handbook 

for Practitioners and Students. Inkata Press, Melbourne, Australia. 74 p.

McFadyen, R.E.C. 1998. Biological control of weeds. Annual Review of Entomology

43:369-393.

Wilson, L.M., and J.P. McCaffrey. 1999. Biological Control of Noxious Rangeland 

Weeds. Pp. 97-115 in R. Sheley and J. Petroff (eds.), Biology and Management of 

Noxious Rangeland Weeds. Oregon State University Press, Corvallis. 

Chapter 2: Getting to Know Exotic Thistles

Abrams, L., and R.S. Ferris. 1960. Illustrated Flora of the Pacific States. Vol. IV. Stanford University Press. Stanford, CA. 540 p.

Austin, M.P., R.H. Groves, L.M.F. Fresco, and P.E. Kaye. 1985. Relative growth of 

six thistle species along a nutrient gradient with multispecies competition. Journal 

of Ecology 73:667-684.

Bendall, G.M. 1973. The control of slender thistle, Carduus pycnocephalus L. and 

C. tenuiflorus Curt. (Compositae), in pasture by grazing management. Australian 

Journal of Agricultural Research 24:327-332.

Bremer, K. 1994. Asteraceae: Cladistics and Classification. Timber Press, Portland, 

Oregon.
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Del Rio-Celestino, M., R. Font, R. Moreno-Rojas, and A. De Haro-Bailon. Uptake 

of lead and zinc by wild plants growing on contaminated soils. Industrial Crops & 

Products 24(3):230-237.

Dennis, LaRea J. 1980. Gilkey’s Weeds of the Pacific Northwest. Oregon State University Press, Corvallis. Pp.346-347.

Desrochers, A.M., J.F. Bain, and S.I. Warwick. 1988a. The biology of Canadian 

weeds. 89. Carduus nutans L. and Carduus acanthoides L. Canadian Journal of 

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Desrochers, A.M., J.F. Bain, and S.I. Warwick. 1988b. A biosystematic study of the 

Carduus nutans complex in Canada. Canadian Journal of Botany 66:1621-1631.

Dewey, S.A. 1991. Weed thistles of the western United States. Pp. 247-253 in James, 

L.F., J.O. Evans, M.H. Ralphs, and R.D. Child (eds.), Noxious Range Weeds. 

Westview Press, Boulder, Colorado.

Donald, W.W. 1994. The biology of Canada thistle (Cirsium arvense). Reviews of 

Weed Science 6: 77-101.

Doucet, C., and P.B. Cavers. 1996. A persistent seed bank of the bull thistle Cirsium 

vulgare. Canadian Journal of Botany 74:1386-1391

Dunn, P.H. 1976. Distribution of Carduus nutans, C. acanthoides, C. pycnocephalus

and C. crispus in the United States. Weed Science 24(5):518-524.

Erickson, L.C. 1983. A review of early introductions of field (Canada) thistle (Cirsium arvense (L.) Scop.) to North America and its present distribution. Proceedings, Western Society of Weed Science 36:200-204

Evans, J.E. 1984. Canada thistle (Cirsium arvense): a literature review of management 

practices. Natural Areas Journal 4:11-21

Evans, R.A., J.A. Young, and R. Hawkes. 1979. Germination characteristics of Italian 

thistle and slenderflower thistle. Weed Science 27(3):327-332.

Feldman, S.R., J.L. Vesprini, and J.P. Lewis. 1994. Survival and establishment of Carduus acanthoides L. Weed Research 34:265-273

Gabay, R., U. Plitmann, and A. Danin. 1994. Factors affecting the dominane of Silybum marianum L. (Asteraceae) in its specific habitats. Flora (1994)189:201-206.

Gleason, H.A. 1952. The New Britton and Brown Illustrated Flora of the Northeastern United States and Adjacent Canada. The New York Botanical Garden. New 

York. Vol. 3. 595 p.
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Gleason, H.A., and A. Cronquist. 1991. Manual of vascular plants of northeastern 

United States and adjacent Canada. 2nd ed. New York Botanical Garden, New 

York. 910 p.

Groves, R.H., and P.E. Kaye. 1989. Germination and phenology of seven introduced 

thistle species in southern Australia. Australian Jornal of Botany 37(1):351-359.

Haggar, R.J., A.K. Oswald, and W.G. Richardson. 1986. A review of the impact 

and control of creeping thistle (Cirsium arvense L.) in grassland. Crop Protection

5:73-76

Hitchcock, C.L., and A. Cronquist. Flora of the Pacific Northwest. University of 

Washington Press. Seattle and London. P. 549.

Holm, L.G., J.V. Pancho, J.P. Herberger, and D.L. Plucknett. 1979. A Geographical 

Atlas of World Weeds. John Wiley and Sons, New York. 

Holmgren, N.H. 1998. Illustrated Companion to Gleason and Cronquist’s Manual. 

New York Botanical Garden, Bronx, New York. 

Huenneke, L.F. 1996. Case study: Cirsium vinaceum, a threatened thistle endemic 

to the Sacramento Mountains of New Mexico. New Mexico Journal of Science

36:141-151

Hull, A.C., Jr., and J.O. Evans. 1973. Musk thistle (Carduus nutans) an undesirable 

range plant. Journal of Range Management 26(5):383-385.

Joley, D.B., D.M. Woods, and M.J. Pitcairn. 1998. Field studies to examine growth 

habit and population resurgence of Scotch thistle in northern California. CDFA 

Biological Control Program: 1998 Annual Report. California Department of Food 

and Agriculture, http://www.cdfa.ca.gov/.

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dynamics of the invasive thistle Carduus nutans in its native range? Journal of Applied Ecology 43:877-886

Jongejans, E., O. Skarpaas, P.W. Tipping, and K. Shea. 2007. Establishment and 

spread of founding populations of an invasive thistle: the role of competition and 

seed limitation. Biological Invasions 9:317-325

Kelch, D.G., and B.G. Baldwin. 2003. Phylogeny and ecological radiation of New 

World thistles (Cirsium, Cardueae – Compositae) based on ITS and ETS rDNA 

sequence data. Molecular Ecology 12:141-151

Kingsbury, J.M. 1964. Poisonous Plants of the United States and Canada. PrenticeHall, Inc. Englewood Cliffs, NJ. Pp. 435-6.

Lloyd, D. G., and A. J. Myall. 1976. Sexual dimorphism in Cirsium arvense (L.) 

Scop. Annals of Botany 40:115-123.
126 Selected References

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Moore, R. J. 1975. The biology of Canadian weeds. 13. Cirsium arvense (L.) Scop. 

Canadian Journal of Plant Science 55:1033-1048.

Lym, R.G., and K.M. Christianson. 1996. The Thistles of North Dakota. North Dakota State Univ. Extension Service, Fargo, ND Publication W-1120. 26 p.

Moore, R.J., and C. Frankton. 1974. The Thistles of Canada. Canada Department of 

Agriculture Monograph No. 10. Ottawa. 111 p. 

Munz, Philip A., and David D. Keck. 1963. A California Flora. University of California Press. Berkley, CA. Pp. 1280-1281.

Nadeau, L. B., and W. H. Vanden Born. 1989. The root system of Canada thistle. 

Canadian Journal of Plant Science 69:1199-1206.

Olivieri, I. 1985. Comparative electrophoretic studies of Carduus pycnocephalus L., C. tenuiflorus Curt. (Asteraceae), and their hybrids. American of Botany

72:715-718.

Omtvedt, I.T. (Dir.). 1984. A Descriptive Guide for Major Nebraska Thistles. Agriculture Experimental Station University of Nebraska - Lincoln. SB493. Pp. 23-24. 

Piper, G. 1984. Scotch thistle–a continuing menace in the Pacific Northwest. Pacific 

Northwest Weed Topics 84:1-2.

Pook, E.W. 1983. The effect of shade on the growth of variegated thistle (Silybum 

marianum L.) and cotton thistle (Onopordum sp.). Weed Research 23:11-17.

Qaderi, M.M., and P.B. Cavers. 2000. Variation in germination response among local 

populations of Scotch thistle, Onopordum acanthium L. Seed Science and Technology 28(3):881-886. 

Randall, J.M., and M. Rejmánek. 1993. Interference of bull thistle (Cirsium vulgare) 

with growth of ponderosa pine (Pinus ponderosa) seedlings in a forest plantation. 

Canadian Journal of Forest Research 23:1507-1513

Robbins. W.W., M.K. Bellue, and W.S. Ball. Weeds of California. Pp. 433.

Roberts, H.A., and R.J. Chancellor. 1979. Periodicity of seedling emergence and 

achene survival in some species of Carduus, Cirsium, and Onopordum. Journal of 

Applied Ecology 16:641-647.

Roché, C. 1991. Milk thistle (Silybum marianum (L.) Gaertn.) Pacific Northwest 

Extension Publication. Washington State University, Oregon State University and 

University of Idaho Cooperative Extension. PNW382.

Sindel, B.M. 1997. The persistence and management of thistles in Australian pastures. 

Proceedings of the 50th New Zealand Plant Protection Conference. Pp. 453-456.
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Sindel, B.M. 1991. A review of the ecology and control of thistles in Australia. Weed 

Research 31:189-201.

Smith, L. M., and L. T. Kok. 1984. Dispersal of musk thistle (Carduus nutans) seeds. 

Weed Science 32:120-125.

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54:1080-1085.

Susanna, A., N. Garcia Jacas, D.E. Soltis, and P.S. Soltis. 1995. Phylogenetic relationships in tribe Cardueae (Asteraceae) based on ITS sequences. American Journal of 

Botany 82:1056-1068.

Tiemann, L., and T.R. Seastedt. 2006. Study shows introduced thistle may facilitate 

growth of some native grasses (Colorado). Ecological Restoration 24:58-59.

Tutin, T.G., V.H. Heywood, N.A. Burges, and D.H. Valentine. 1976. Flora Europeae. 

4:143-144. Cambridge University Press, Cambridge, U.K.

University of Illinois Agriculture Experiment Station. 1954. Weeds of North Central 

States. North Central Regional Publication No. 36, Circular 718. University of 

Illinois Agriculture Experiment Station, Urbana. 239 p.

Voss, E.G. 1996. Michigan Flora Part III. Dicots (Pyrolaceae-Compositae). Bulletin 

of the Cranbrook Institute of Science 61 & Univ. of Michigan Herbarium. 622 p.

Warwick, S.I, B.K. Thompson, and L.D. Black. 1990. Comparative growth response 

in Carduus nutans, C. acanthoides and their F1 hybrids. Canadian Journal of 

Botany 68:1675-1679. 

Whitson, T.D.(ed.), L.C. Burrill, S.A. Dewey, D.W. Cudney, B.E. Nelson, R.D. Lee, 

and R. Parker. 1996. Plumeless thistle in Weeds of the West. Western Society of 

Weed Science, in cooperation with the Western United States Land Grant Universities Cooperative Extension Services, Newark CA. 74 p.

Wilson, R.G., Jr. 1979. Germination and seedling development of Canada thistle 

(Cirsium arvense). Weed Science 27:146-151.

Wisconsin Department of Natural Resources. 1998. Musk or Nodding Thistle (Carduus nutans), Plumeless or Bristly Thistle (Carduus acanthoides), and Bull Thistle 

(Cirsium vulgare). Available: http://www.dnr.state.wi.us/org/land/er/invasive/

factsheets/thistles.htm.

Young, J.A, R.A. Evans, and R.B. Hawkes. 1978. Milk thistle (Silybum marianum) 

seed germination. Weed Science 26:395-398.

Young, J.A., and R.A. Evans. 1972. Germination and persistence of achenes of Scotch 

thistle. Weed Science 20:98-101.
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Chapter 3: Biology of Exotic Thistle Biological Control Agents

Alonso-Zarazaga, M.A. and M. Sánchez-Ruiz. 2002. Revision of the Trichosirocalus 

horridus (Panzer) species complex, with description of two new species infesting thistles (Coleoptera: Curculionidae, Ceutorhynchinae). Australian Journal of 

Entomology 41:199-208.

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. Pp. 365–368.

Goeden, R.D. and D.W. Ricker. 1987. Phytophagous insect faunas of two introduced 

Cirsium thistles, C. ochrocentrum and C. vulgare, in southern California. Annals 

of the Entomological Society of America 79:945-952.

Kok, L.T. 2001. Classical biological control of nodding and plumeless thistles. Biological Control 21:206-213.

McDonald, R.C., K.A. Kidd, and N.S. Robbins. 1994. Establishment of the rosette 

weevil, Trichosirocalus horridus (Panzer) (Coleoptera: Curculionidae) in North 

Carolina. Journal of Entomological Science 29:302-304.

Peschken, D.P., and P. Harris. 1975. Host specificity and biology of Urophora cardui

(Diptera: Tephritidae), a biocontrol agent for Canada thistle (Cirsium arvense). 

Canadian Entomologist 107:1101-1110.

Peschken, D.P., and J.L. Derby. 1992. Effect of Urophora cardui (L.) (Diptera: Tephritidae) and Ceutorhynchus litura (F.) (Coleoptera: Curculionidae) on the weed 

Canada thistle, Cirsium arvense. Canadian Entomologist 124:145-150.

Peschken, D.P. and A.T.S. Wilkinson. 1981. Biocontrol of Canada thistle (Cirsium 

arvense): releases and effectiveness of Ceutorhynchus litura (Coleoptera: Curculionidae) in Canada. Canadian Entomologist 113:777-785.

Rizza, A., G. Campobasso, P.H. Dunn, and M. Stazi. 1988. Cheilosia corydon (Diptera: Syrphidae), a candidate for the biological control of musk thistle in North 

America. Annals of the Entomological Society of America 81:225-232.

Sheppard, A.W., J.P. Aeschlimann, J.L. Sagliocco, and J. Vitou. 1995. Below-ground 

herbivory in Carduus nutans (Asteraceae) and the potential for biological control. 

Biocontrol Science and Technology 5:261-270.

Watts, J.D., and G.L. Piper. 2000. The phytophagous insect fauna of Scotch thistle, 

Onopordum acanthium L., in southeastern Washington and northwestern Idaho. 

Pp. 233-239 in Spencer, N.R. (ed.), Proceedings of the X International Symposium 

on Biological Control of Weeds. Bozeman, Montana. USDA-ARS, Sidney, MT. 

1029 p.
__________________________________________Biology and Biological Control of Exotic True Thistels

Selected References 129

Chapter 4: Elements of an Exotic Thistle Biological Control 

Program

Bendall, G.M. 1973. The control of slender thistle, Carduus pycnocephalus L. and 

C. tenuiflorus Curt. (Compositae), in pasture by grazing management. Australian 

Journal of Agricultural Research 24:327-332.

Briese, D.T., W.J. Pettit, A. Swirepik, and A. Walker. 2002. A strategy for the biological control of Onopordum spp. thistles in south-eastern Australia. Biocontrol 

Science and Technology 12:121-136.

Cartwright, B., and L.T. Kok. 1983. Evaluation of sampling techniques for three 

biological control agents of Carduus thistles. Environmental Entomology

12:1754-1759.

Hein, G.L., and R.G. Wilson. 2004. Impact of Ceutorhynchus litura feeding on root 

carbohydrate levels in Canada thistle (Cirsium arvense). Weed Science 52:628-633.

Feldman, S. R. 1997. Biological control of plumeless thistle (Carduus acanthoides L.) 

in Argentina. Weed Science 45:534-537.

Larson, G.A., T.A. Witting, K.F. Higgins, B. Turnipseed, and D.M. Gardner. 2005. 

Influence of biocontrol insects on Canada thistle: seed production, germinability, 

and viability. Prairie Naturalist 37:85-100.

Louda, S.M. and R.A. Masters. 1993. Biological control of weeds in Great Plains 

rangelands. Great Plains Research 3:215-247.

McAvoy, T.J., and L.T. Kok. 1987. Rearing of Trichosirocalus horridus (Coleoptera: 

Curculionidae) larvae in artificial diets. Journal of Entomological Science

22:330-335.

McFadyen, R.E.C. 2000. Successes in the biological control of weeds. Pp. 3-14 in

Spencer, N.R. (ed.). Proceedings of the X International Symposium on Biological 

Control of Weeds. Montana State Univ., Bozeman, MT.

Milbrath, L.R. and J.R. Nechols. 2004. Individual and combined effects of Trichosirocalus horridus and Rhinocyllus conicus (Coleoptera: Curculionidae) on musk 

thistle. Biological Control 30:418-429.

Rees, N.E. 1990. Establishment, dispersal, and influence of Ceutorhynchus litura on 

Canada thistle (Cirsium arvense) in the Gallatin Valley of Montana. Weed Science

38:198-200.

Stanforth, L.M., S.M. Louda, and R.L. Bevill. 1997. Insect herbivory on juveniles of 

a threatened plant, Cirsium pitcheri, in relation to plant size, density and distribution. Ecoscience 4:57-66.
130 Selected References

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Trumble, J.T., and L.T. Kok. 1978. Laboratory propagation of Ceuthorrhynchidius 

horridus (Coleoptera: Curculionidae), an introduced weevil for biocontrol of Carduus thistles. Canadian Entomologist 110:1091-1094.

Woodburn, T.L. 1997. Establishment in Australia of Trichosirocalus horridus, a biological control agent for Carduus nutans, and preliminary assessment of its impact 

on plant growth and reproductive potential. Biocontrol Science and Technology

7:645-656.

Chapter 5: An Integrated Exotic Thistle Management Program

Ang, B.N., L.T. Kok, G.I. Holtzman, and D.D. Wolf. 1994. Canada thistle (Cirsium 

arvense) response to simulated insect defoliation and plant competition. Weed Science 42:403-410.

Collier, T.R., S.F. Enloe, J.K. Sciegienka, and F.D. Menalled. 2007. Combined impacts 

of Ceutorhynchus litura and herbicide treatments for Canada thistle suppression. 

Biological Control 43:231-236.

Stoyer, T.L., and L.T. Kok. 1989. Oviposition by Trichosirocalus horridus (Coleoptera: Curculionidae), a biological control agent for Carduus thistles, on plants 

treated with low dosages of 2,4-dichlorophenoxyacetic acid. Environmental Entomology 18:715-718.

Trumble, J.T. and L.T. Kok. 1982. Integrated pest management techniques in thistle 

suppression in pastures of North America. Weed Research 22:345-359.

Zimdahl, R.L., and G. Foster. 1993. Canada thistle (Cirsium arvense) control with 

disking and herbicides. Weed Technology 7:146-149.
APPENDICES

Appendix I: Troubleshooting Guide; When Things Go Wrong

Appendix II: PPQ Form 526 Interstate Transport Permit 

Appendix III: Sample Biological Control Agent Release Form 

Appendix IV: Idaho Statewide Thistle Biological Control Agent 

2-Pagers and Monitoring Forms 

Appendix VI: Exotic Thistle Qualitative Monitoring Forms

Appendix VII: Exotic Thistle Biocontrol-Associated Vegetation 

Monitoring

Appendix I: Troubleshooting Guide; When Things Go Wrong

This guide is intended to assist those who encounter problems when establishing a biological control program. It 

identifies the probable cause of typical problems and offers solutions.

Problem Probable Cause Solution

Biological 

control 

agents 

unhealthy 

when 

received

Physical damage to 

agents in transport

Prevent containers from colliding; use crush-proof containers.

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 thistle foliage (no flowers, seeds, or roots) in containers.

Redistribution time Transport or ship agents immediately after collection.

Release agents at new site immediately upon arrival or receipt of agent.

Parasitism and/or 

disease

Check source agents. Ensure the insect population is disease-free when 

collecting or receiving shipment.

Number of 

eggs low

Agents past 

reproductive stage

Collect at peak activity (i.e., when insects are mating).

Sex ratio: not enough 

males or females

Observe mating among biocontrol agents before collecting; males often 

emerge earlier than females.

Synchrony Agents not synchronized with the exotic thistle growth stage; biological 

control agents require thistles to be at specific growth stage for optimal 

oviposition.

Few biological 

control 

agents 

collected

Wrong method used Refer to Table 4-2 for recommended collection time and technique.

Collection done at 

wrong time

Refer to Table 4-2 for recommended collection time and technique.

Collection technique Biological control agents can be killed/damaged during sweeping or 

aspirating, so sweep lightly.

Use vacuum aspirator if aspirating by mouth is not working.

Practice sweeping to avoid debris.

Conditions at time of 

collection wrong

Refer to Chapter 4 “Collecting Exotic Thistle Biocontrol Agents” for 

guidelines on desirable weather conditions.

Agents not 

found after 

release

Site is unsuitable Refer to Chapter 4 “Selecting Biocontrol Agent Release Sites.”

Site too small Select a larger site with a dense, uniform stand of exotic thistle.

Pesticide used in area Select pesticide-free site.

Released on wrong 

thistle species

Ensure the correct species of thistle and biological control agent are 

used.

Cannot locate 

release site

Permanent location 

marker not obvious

Use bright-colored wooden, metal, or plastic stake.

Map poorly or 

incorrectly drawn

Check map; redraw with more detail or add landmarks; GPS.
APPENDIX II: PPQ FORM 526 INTERSTATE TRANSPORT PERMIT

Please see http://www.aphis.usda.gov/ppq/permits to download permits. 
APPENDIX III: SAMPLE BIOLOGICAL CONTROL AGENT RELEASE FORM
APPENDIX IV:  IDAHO STATEWIDE THISTLE BIOLOGICAL CONTROL AGENT 2‐PAGERS AND

MONITORING FORM

Monitoring Guidelines for Ceutorhynchus litura on Canada Thistle

Overview

A  critical  part  of  successful  weed  biological  control  programs  is  a 

monitoring  process  to  measure  populations  of  the  biological  control 

agents  and  the  impact  that  they  are  having  on  the  target  weed. 

Monitoring  should  be  conducted  on  an  annual  basis  for  a  number  of 

years.  The  Idaho State Department of Agriculture, in conjunction with 

the University  of  Idaho, Nez  Perce  Biocontrol  Center,  and  federal land 

management  agencies  has  developed  the  monitoring  protocol  below

which enables land managers  to  take a more active role in monitoring 

populations  and  the  weed  control  ability  of  the  Canada  thistle  stem 

weevil, Ceutorhynchus litura (CELI) in efforts  to control Canada  thistle, 

Cirsium arvense.    This  monitoring  protocol  was  designed  to  be 

implemented by land managers in a timely manner while providing data 

which will enable researchers  to better quantify  the impact of CELI on 

Canada thistle throughout the state. 

Canada thistle (Cirsium arvense)

Canada thistle is an aggressive, colony‐forming perennial weed that reproduces 

by  seed  and  deep,  extensive  horizontal  roots.    Flowering  occurs  from  June 

through August.  The flowers are urn‐shaped and purple (sometimes white), and 

male and female flowers occur on separate plants with heads ranging from 1⁄2 to 

3⁄4‐inch in diameter.  Fruits are about 1/8 inch long and brownish with a tuft of 

hairs  at  the  top.    Stems  are  typically  1  to  4  feet  tall  with  alternate,  oblong  or 

lance‐shaped leaves divided into spiny‐tipped irregular lobes.  Canada thistle is 

native to Eurasia and was introduced to Canada as a contaminant of crop seed in 

the 18th century.   It can commonly be  found in gardens,  flower beds, pastures, 

cultivated  fields,  rangelands,  forests,  and  along  river  banks,  ditches  and 

roadsides.    Canada  thistle  can  tolerate  a  wide  range  of  environmental 

conditions,  but  requires  good  light  intensity  for  optimal  growth.    It  is  highly 

competitive  with  crops  and,  in  heavy  concentrations,  effectively  prevents 

razing.    A  number  of  accidentally  introduced  insects  attack  Canada  thistle.  

owever,  only  two  insects  are  approved  for  release;  Urophora cardui  and 

g

H

Ceutorhynchus litura. 

Canada thistle stem weevil (Ceutorhynchus litura)

CELI is a biological control agent that attacks Canada thistle stems and rosettes. Adults feed on rosette leaf 

foliage in spring, and larvae consume tissues while mining within shoots. At low densities, larval and adult 

feeding does not  significantly impact populations  of Canada  thistle directly.  Feeding does cause  secondary 

damage,  however, as  pathogens and  other  organisms enter  the  stems  of  Canada  thistle  via  holes made  by 

exiting larvae. At high densities, feeding by CELI will reduce the vigor of both rosettes and flowering stems to 

the point of deterioration. Overwintering weevils emerge from soil litter and feed on leaf and stem tissue in 

early spring.  Eggs are laid in spring within Canada thistle’s bolting shoots, with hatching larvae mining in the 

stems and root crowns of Canada thistle throughout spring and summer. Multiple larvae (up  to 20) can be 

found in individual stems. Mature larvae tunnel out of the stems, drop to the soil surface, and pupate in the 

soil. Adults of the new generation emerge in August and overwinter in soil litter.  There is one generation per 

year. The weevil does best in open but moist areas with scattered Canada thistle plants.  
APPENDIX IV (CONT.):  IDAHO STATEWIDE THISTLE BIOLOGICAL CONTROL AGENT 2‐

P M F AGERS AND ONITORING ORM  

Monitoring

This  biological  control  monitoring  protocol  is  based  upon  a  permanent  20‐meter  vegetation  sampling 

transect randomly placed in a suitable (at least 1 acre) infestation of Canada thistle and timed counts of CELI 

adults.    Annual  vegetation  sampling  will  allow  researchers    to  characterize  the  plant  community  and  the 

abundance  and  vigor  of  Canada  thistle.    Visual  counts  of  CELI  adults  will  provide  researchers  with  an 

20 m

2 

4 

6 

8 

10 

12 

14 

16 

18 

20 

estimate of CELI population levels. 

rior to monitoring activities and to filling out the form included below, it is necessary to first set out your 

erman

P

p ent monitoring frames. This will require:  

1)  25  x  50  cm  (10  x  20 inches) Daubenmire 

frame made from PVC (preferred) or rebar  

2) 20 m (66 ft) tape measure for the transect 

Permanent 

marker 

locations and plant height 

3) 10 permanent markers (road whiskers and 

16 penny nails) 

4)  A  post  or  rebar  to  permanently  mark  the 

site  (see  diagrams  for  examples  of  field 

equipment) 

5)  1  hour  during  the  peak  phenological 

monitoring  time  of  the  target  exotic  thistle 

Post or 

rebar 

location 

species (first week of May) 

To  set  up  the  transect,  place  the  20  m  (66  ft)  tape 

randomly within  the infestation. Mark  the beginning 

of the transect with a post. Place permanent markers 

every  2  m  or  6.5  feet  (for  a  total  of  10  markers) 

beginning at  the  2 m mark and ending with  the  20 m mark  on  the tape measure  (see  diagram).  Place  the 

Daubenmire frame with the long (50‐cm) side parallel to the tape and the permanent marker at 2 m in the 

upper‐left corner(see diagrams). Conduct monitoring as described below, and then repeat the procedure at 

2‐m intervals along the tape for a total of 10 replicated measurement (at permanent markers). 

Transect breakdown (Joseph Milan, BLM)

Plot setup (Joseph Milan, BLM) 

25 cm

50 cm Permanent 

markers 

 4 m 

0 m
APPENDIX IV (CONT.):  IDAHO STATEWIDE THISTLE BIOLOGICAL CONTROL AGENT 2‐

PAGERS AND MONITORING FORM

Monitoring Guidelines for Urophora cardui on Canada Thistle

Overview:

A  critical  part  of  successful  weed  biological  control  programs  is  a 

monitoring  process  to  measure  populations  of  the  biological  control 

agents  and  the  impact  that  they  are  having  on  the  target  weed. 

Monitoring  should  be  conducted  on  an  annual  basis  for  a  number  of 

years.    The  Idaho  State  Department  of  Agriculture,  in  conjunction  with 

the  University  of  Idaho,  Nez  Perce  Biocontrol  Center,  and  federal  land 

management agencies, has developed a monitoring protocol that enables 

land managers to take a more active role in monitoring populations and 

the  weed  control  ability  of  the  Canada  thistle  stem  gall  fly,  Urophora

cardui (URCA) in efforts to control Canada thistle, Cirsium arvense.  This 

monitoring protocol was designed to be implemented by land managers 

in a timely manner while providing data which will enable researchers to 

better  quantify  the  impact  of  URCA  on  Canada  thistle  throughout  the 

state. 

Canada thistle (Cirsium arvense)

Canada  thistle  is  an  aggressive,  colony‐forming  perennial  weed  that 

reproduces by seed and deep, extensive horizontal  roots.  Flowering  occurs 

from  June  through August.  The  flowers are urn‐shaped, purple  (sometimes 

white),  and  male  and  female  flowers  occur  on  separate  plants  with  heads 

ranging  from 1⁄2  to 3⁄4 inch in  diameter.    Fruits  are  about  1/8 inch long  and 

brownish with a  tuft  of  hairs at  the  top.   Stems are  typically 1  to  4  feet  tall 

with  alternate,  oblong  or  lance‐shaped  leaves  divided  into  spiny‐tipped 

irregular  lobes.    Canada  thistle  is  native  to  Eurasia  and  was  introduced  to 

Canada as a contaminant of crop seed in the 18th century.  It can commonly be 

found in gardens, flower beds, pastures, cultivated fields, rangelands, forests, 

and along  river  banks,  ditches and  roadsides.    Canada  thistle  can  tolerate a 

wide range of environmental conditions, but requires good light intensity for 

optimal  growth.    It  is  highly  competitive  with  crops  and,  in  heavy 

concentrations,  effectively  prevents  grazing.    A  number  of  accidentally 

ntroduced  insects  attack  Canada  thistle.    However,  only  two  insects  are 

pproved for release; Urophora cardui and Ceutorhynchus  litura. 

i

a

Canada thistle stem gall fly (Urophora cardui)

URCA  is  a  biological  control  agent  that  attacks  Canada  thistle  stems  and 

forms a gall which acts as an energy sink.  It takes nutrients which would normally be available to the plant, 

thereby  reducing  plant  vigor  and  competitive  ability.   Adult  flies  emerge  from  galls in late  spring  to  early 

summer and mate, and  females deposit one  to 30 eggs in Canada  thistle’s vegetative shoots.  Larvae of  the 

gall fly initiate gall development by tunneling into the plant stem.  

Larvae grow slowly while the gall is developing and attain 98% of 

their  body weight as  the gall matures.   Multiple larvae  (3  to  10) 

can be found in individual large galls.  Pupation occurs within the 

gall in early spring.  Galls resemble a small, green crabapple and 

are typically found in the upper third of the main stem (see photo 

above). They can vary in size but are generally marble‐ or walnut‐

sized.    The  fly  does  best  in  semi‐shaded,  moist  areas  with 

scattered Canada thistle plants.  
APPENDIX IV (CONT.):  IDAHO STATEWIDE THISTLE BIOLOGICAL CONTROL AGENT 2‐

P M F AGERS AND ONITORING ORM

Monitoring:

This  biological  control  monitoring  protocol  is  based  upon  a  permanent  20‐meter  vegetation  sampling 

transect  randomly  placed  in  a  suitable  (at  least  1  acre)  infestation  of  Canada  thistle  and  timed  counts  of 

URCA galls.  Annual vegetation sampling will allow researchers  to characterize the plant community and the 

abundance and vigor of Canada thistle.  Visual counts of URCA galls will provide researchers with an estimate 

20 m

2 

4 

6 

8 

10 

12 

14 

16 

18 

20 

Post or 

rebar 

of URCA population levels. 

rior to monitoring activities and to filling out the form included below, it is necessary to first set out your 

erman

P

p ent monitoring frames. This will require:  

1)  25  x  50  cm  (10  x  20 inches) Daubenmire 

frame made from PVC (preferred) or rebar  

2) 20 m (66 ft) tape measure for the transect 

Permanent 

marker 

locations 

and plant height 

3) 10 permanent markers (road whiskers and 

16 penny nails) 

4)  A  post  or  rebar  to  permanently  mark  the 

site  (see  diagrams  for  examples  of  field 

equipment) 

5)  1  hour  during  the  peak  phenological 

monitoring  time  of  the  target  exotic  thistle 

location 

species (first week of August) 

To  set  up  the  transect,  place  the  20  m  (66  ft)  tape 

randomly within  the infestation. Mark  the beginning 

of the transect with a post. Place permanent markers 

every  2  m  or  6.5  feet  (for  a  total  of  10  markers) 

beginning at  the  2 m mark and ending with  the  20 m mark  on  the tape measure  (see  diagram).  Place  the 

Daubenmire frame with the long (50‐cm) side parallel to the tape and the permanent marker at 2 m in the 

upper‐left corner(see diagrams). Conduct monitoring as described below, and then repeat the procedure at 

2‐m intervals along the tape for a total of 10 replicated measurement (at permanent markers). 

Transect breakdown (Joseph Milan, BLM)

Plot setup (Joseph Milan, BLM) 

50 cm 

25 cm

Permanent 

markers 

 4 m 

0 m
APPENDIX IV (CONT.):    IDAHO STATEWIDE THISTLE BIOLOGICAL CONTROL AGENT 2‐

PAGERS AND MONITORING FORM

Timed (for use with Urophora cardui galls and Ceutorhynchus litura adults)

Monitoring biological control agents is an essential component of a successful biocontrol program that can be

used to accurately document impact and safety of this weed management practice. This monitoring form has been

endorsed by the Nez Perce Biocontrol Center, University of Idaho, Forest Health Protection, Bureau of Land

Management, and Idaho State Department of Agriculture. The monitoring information from this form will be used

to document vegetation cover, target weed density, and biological control agent abundance and the changes that

occur over time.   

General Information:

Observer(s): Date: Landowner:  

Permanent site?  Y   N Site name: Weed:

Biocontrol agent: Insect Stage:   Adult    Larvae    Pupae    Egg

Lat/Long: N       °                ‘ W       °                   ‘ UTM Datum:                 UTM E:

UTM Year : UTM N:

Weed Infestation:

Size in acres: Picture taken?     Yes         No If Y, picture direction:

Vegetation cover (all in %, rows add to 100%):

Frame Target

weed%

Other

weed% Forb/shrub% Grass% Bareground% Litter% Moss% Total%

1                        

2                        

3                        

4                        

5                        

6                        

7                        

8                        

9                        

10                        

Target weed size/density:                   Biological control agent:

Frame Number of stems Height of tallest stem

(cm)

Count

location

# insects (or galls)

per 3 min. count

1       1   

2       2   

3       3   

4       4   

5       5   

6       6   

7      

8      

9      

10      
INSTRUCTIONS FOR APPENDIX IV:  IDAHO STATEWIDE THISTLE BIOLOGICAL CONTROL

AGENT MONITORING FORM  

General Information:

• Observer(s) – Who are you?

• Date – Today’s date.

• Landowner – Who is the landowner/land manager?

• Permanent? – Is this a permanent monitoring site?

• Site name – Which site are you monitoring?  This could have a specific name if

it is a permanent site.

• Weed – Which target weed are you are monitoring?

• Biocontrol agent – Which biocontrol agent you are monitoring?

• Insect Stage – What is the growth stage of the agent are you monitoring?

• Lat/Long OR UTM – What are the coordinates of the site you are monitoring?  

If UTM (preferred), what datum and year are your coordinate system?

    Annual grass – note

stems which are

typically solitary or in a

few stemmed tufts.

Vegetation Cover (all in %, rows should add up to 100%) – All percentages are to be

estimated to the nearest 5%.  Put a “T” on the form for trace amounts less than 5%.

• Frame – Which frame number are you working on (1= 2m, 2= 4m, ...,10 = 20m

on transect)?

• Target weed % – What is % cover of the target weed to the nearest 5%?  

• Other weeds % – What is the % cover of any other weeds in the frame to the

nearest 5%?  Count undesirable annual grasses as weeds.

• Forb/Shrub % – What is the % cover of native forbs/shrubs in the frame to the

nearest 5%?

• Grass % – What is the % cover of grass to the nearest 5%?

• Bare Ground/Litter % – What is the % cover of bare ground/litter to the

nearest 5%?

Target Weed Size/Density

• Frame – Which frame number are you working on (1=2m,...,10=20m)? Perennial grass – note

the multiple stem base

with multiple year’s

growth.

• Number of stems – How many stems of the target weed are in the frame?

• Height of tallest stems (cm) – How tall is the tallest stem in the frame (in cm)?

Biological Control Agent Density Monitoring

Here, you collect data for Ceutorhynchus litura (counting adults) and Urophora cardui (counting galls) that helps to

get an unbiased assessment of the population size of the biological control agents. This is probably the most

important part of the data collection.    

• Count location – Do not count in the area where the transect is located. Instead, identify 6 similar

locations around or close‐by but at least 20 paces away from the transect.

• In 3 minutes, count as many URCA galls (search for stems with galls) or as many adult CELI (searching

for and inspecting rosette leaf petioles, and crevices in between, throughout the center of the rosette).

How many insects/galls can you find in the 3 minute period?  For weevil counts, carefully approach the

plants and be sure to count each insect only once.  Repeat the weevil/gall count 5 times (for a total of 6

3‐minute counts) in different areas. These are replications and provide the unbiased data to calculate

the population size of the biological control agents.
APPENDIX V: GENERAL BIOLOGICAL CONTROL AGENT MONITORING FORM  

SITE: _______________________      STATE: ________    COUNTY_______________________     DATE: _________ _________ _______

                               year            month         day

DATA COLLECTOR: ___________________________________________________________         TIME: ___________________________     

First and last name

UTM DATUM:________________  UTM YEAR:___________

LAT/LONG: N ______°__________'      W _____°_________'       UTM E: __________________ UTM N: _________________

                

ELEVATION: ______________   TEMPERATURE:  _________       WEATHER:  _______________________________________________  

INSECT COUNTS:

1 1‐10

2 11‐25

3 26‐100

4 100‐500

5 >500

Species Method # insects (use Chart A)

C. litura, T. horridus, C.

corydon

Randomly select 25 plants,

dissect root tissue and count

larval tunnels

U. cardui Randomly select 25 plants,

dissect galls and count larvae

U. stylata

Randomly select 25 plants,

dissect seed heads and count

larvae  

0 <1% 

1 1-5% 

2 6-25% 

3 26-50% 

4 51-75% 

5 76-95% 

6 >95% 

EXOTIC THISTLES:  

0 <1%

1 1‐5%

2 6‐25%

3 > 25%

Chart C: Cover Class 

Chart B: Damage Class 

Chart A: 

Insect abundance 

Exotic thistle Stems Quad

# % damage

(use Chart B)

% cover  

(use Chart C)

# mature

stems

# immature

stems

Height 4 tallest

stems (cm)

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20
APPENDIX V (CONT.): GENERAL BIOLOGICAL CONTROL AGENT MONITORING FORM

Exotic thistle Stems Quad

# % damage

(use Chart B)

% cover  

(use Chart C)

# mature

stems

# immature

stems

Height 4 tallest

stems (cm)

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

Notes:
INSTRUCTIONS FOR APPENDIX V: GENERAL BIOLOGICAL CONTROL AGENT

MONITORING FORM

Materials needed: 1 meter (1.1 yard) stick, 0.2 x 0.5 m (0.2 x 0.55 yard) quadrat frame, stopwatch, sweep net,

monitoring form, pencils, clipboard, camera, and GPS unit to relocate transects.   

General:  The purpose of this monitoring activity is to estimate the abundance of exotic thistle and its biocontrol

agents at the site and to record measurements of a sample of exotic thistle plants.  Conduct the monitoring when

the biocontrol agents are at their peak.  Monitoring is easier when conducted by two people: one to make the

observations and the other to record data.   

1) Site information: Fill out the site information at the top of the form.

2) Insect counting:   Use the chart for the method to count insects.  Carefully approach the site and avoid

disturbing the vegetation as adult insects often drop from the vegetation once you touch stems (or even as you

approach the quadrat).  Use Chart A to record the category of abundance (1‐5).

3) Locate the transect and position the quadrat:  After you have completed the insect counts, locate the transect

using the GPS coordinates and the permanent marker.   

4) Position the quadrat:  Position the quadrat along the transect, as close to the ground as possible, carefully

positioning the quadrat along that transect line. Be sure not to damage the plants.  The quadrat should be in the

same location as the previous year’s quadrat.  Move stems in or out of the frame area so that all stems originating

inside the quadrat are included.

5) Estimate feeding damage:  Examine the exotic thistle for any damage to the leaves, shoots, flower heads, etc.,

such as malformed flower heads due to agents feeding on capitula and seeds.  Standing over the frame, estimate

the percent of damage over the entire quadrat, using Chart B to determine the category of damage.  

6) Estimate percent cover:  Standing over the frame, estimate how much of the quadrat is covered by exotic

thistle.  Use cover estimates in Chart C to estimate percent cover class.   

7) Count stems:  Count the number of exotic thistle stems, beginning at one corner of the quadrat and working

systematically across the quadrat.  Count the number of mature (floral) and immature (vegetative) stems.   

8) Measure stems:  Select the four (4) tallest exotic thistle stems in each quadrat (if there are fewer than 4

stems/quadrat, measure all that are present).  Measure the stem height (to the closest cm)   

9) Other observations: Record any general observations or useful information; disturbances, grazing, fire, etc., for

the sample quadrat or the site in general.  
APPENDIX VI:  EXOTIC THISTLE QUALITATIVE MONITORING FORM

Name: _______________________________Date: _____________Time:____________ am/pm

Location:__________________________________________ Site #:__________________________________

Biocontrol species:__________________________________ Year of release: __________________________

Exotic thistle species________________________________________________________________________  

Cover class estimate by plant category

0% 1‐5% 6‐20% 21‐45% 46‐70% 71‐100%

Exotic thistle

Annual Grasses

Perennial Grasses

Forbs

Shrubs

Trees

Dominant plant species on site:

Other noxious weeds:

Estimate exotic thistle density class (9check one) Exotic thistle phenology class at time of

monitoring

Flowering

plants/meter sq

Exotic thistle distribution Exotic thistle stage

Estimated

percent

0 Isolated Seedling

1‐25 Scattered Rosette

26‐50 Scattered‐Patchy Bolting

50‐75 Patchy Flowering

>75 Continuous Senescent

Comments/Observations ____________________________________________________________________

__________________________________________________________________________

__________________________________________________________________________

__________________________________________________________________________

__________________________________________________________________________

__________________________________________________________________________
APPENDIX VII:  EXOTIC THISTLE BIOCONTROL ‐ ASSOCIATED VEGETATION MONITORING

SITE: _______________________      STATE: ________    COUNTY_______________________     DATE: _________ _________ _______

                               year            month         day

DATA COLLECTOR: ___________________________________________________________         TIME: ___________________________     

First and last name

UTM DATUM:________________  UTM YEAR:___________

LAT/LONG: N ______°__________'      W _____°_________'       UTM E: __________________ UTM N: _________________

                

ELEVATION: ______________   TEMPERATURE:  _________       WEATHER:  _______________________________________________  

0 <1%

1 1‐5%

2 6‐25%

3 26‐50%

4 51‐75%

5 76‐95%

6 >95%

Chart A: Cover Class 

Q1 Q2 Q3 Q4 Q5    Q6 Q7 Q8 Q9 Q10

Percent of Quadrat (Use Chart A; total for each column is 100%)

  Vegetated

  Soil, litter

  Rock

Vegetation Cover (Use Chart A; total for column may exceed 100% due to overlapping of vegetation)

   Exotic thistle

   All other vegetation:

        Forbs

        Grasses and Sedges

        Woody plants

Individual Species (names) (Check if present or use Chart A to indicate percent cover)
INSTRUCTIONS FOR APPENDIX VII:  EXOTIC THISTLE BIOCONTROL ‐ ASSOCIATED

VEGETATION MONITORING

Materials needed: 1 meter stick, 1.0 m2 quadrat frame, data sheets, pencils, clipboard, camera, and GPS

unit to relocate quadrats.   

General:  The purpose of this activity is to estimate the abundance of other vegetation in the community

and to record measurements of exotic thistle plant attributes.  Monitoring is easier with two people,

one to make the observations and the other to record data.   

1) Site information: Fill out the site information at the top of the form.

2) Position the quadrat:  Position the quadrat frame as close to the ground as possible, carefully

positioning the quadrat along that transect line.  Be sure not to damage the vegetation.  The quadrat

should be in the same location as the previous year’s quadrat.

3) Estimate amount of vegetation:  Standing over the frame, estimate how much of the quadrat is

vegetated and how much is not vegetated (bare ground, rock, etc).  Use cover estimates in Chart A to

estimate percent cover.  

4) Estimate percent cover of vegetation: Standing over the frame, estimate how much of the quadrat is

covered by exotic thistle and how much is covered by other forbs, grasses, or shrubs.  Use cover

estimates in Chart A to estimate percent cover.  Because vegetation can naturally overlap, it is possible

to have a combined total percent cover to exceed 100%.

5) Estimate percent cover of individual species: Standing over the frame, estimate how much of the

quadrat is covered by individual species other than exotic thistle.  Use this section to track specific

species: for example perennial grasses, native forbs, etc.   

6) Other observations: Record any general observations or useful information, such as disturbances,

grazing, fire, etc.