Biology and Biological Control

of Yellow Starthistle

Carol Bell Randall, Rachel L. Winston, Cynthia A. Jette, 

Michael J. Pitcairn, and Joseph M. DiTomaso

United States Department of Agriculture

Technology

Transfer Biological 

Control

Forest Health Technology

Enterprise Team

FHTET-2016-08

4th Ed., June 2017
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/

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use does not constitute an official endorsement or approval by the U.S. Department of Agriculture or the Forest Service 

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Printed on Recycled Paper

References to pesticides appear in this publication. These statements do not constitute 

endorsement or recommendation of them by the U.S. Department of Agriculture, nor do they 

imply that the uses discussed have been registered. Use of most pesticides is regulated 

by state and federal laws. Applicable regulations must be obtained from the appropriate 

regulatory agency prior to their use.

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

fish or other wildlife if they are not handled or applied properly. Use all pesticides selectively 

and carefully. Follow recommended practices given on the label for the use and disposal of 

pesticides and pesticide containers.

Cover Photos: (a) Yellow starthistle flower head (Peggy Grebb, USDA ARS, 

bugwood.org); (b) Bangasternus orientalis adult; (c) Eustenopus villosus

adult; (d) Larinus curtus adult; (e) Chaetorellia spp. adult; (f) Urophora 

sirunaseva adult (b-f Laura Parsons & Mark Schwarzländer, University of 

Idaho); (g) Puccinia jaceae var. solstitialis (Stephen Ausmus, USDA ARS, 

bugwood.org)

a

b

c

d

e

f

g
Biology and Biological Control

of Yellow Starthistle

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

d’Alene, ID; crandall@fs.fed.us

Rachel L. Winston, Environmental Consultant, MIA Consulting LLC, Sandpoint, ID; 

rachel@getmia.net

Cynthia A. Jette, Department of Plant, Soil and Entomological Sciences, University of 

Idaho, Moscow, ID

Michael J. Pitcairn, Biocontrol Program, Integrated Pest Management Branch, California 

Department of Food and Agriculture, Sacramento, CA; mike.pitcairn@cdfa.ca.gov

Joseph M. DiTomaso, Weed Science Program, Department of Plant Sciences, University of 

California, Davis, CA; jmditomaso@ucdavis.edu

FHTET-2016-08

4th Ed., June 2017
Acknowledgments

Joseph P. McCaffrey (University of Idaho), Leonard Lake (USDA Forest Service), Christina Allen (Nez 

Perce Biocontrol Center), John Connett (University of Idaho), and Linda M. Wilson (University of Idaho) 

contributed to the first, second, or third editions of this manual, and some of their contributions have been 

retained in this revision. We are very grateful to Cindy Roché for contributing many excellent photographs 

and drawings, and we appreciate all the other photographers who granted permission for the use of photos. 

We would like to thank all of the county weed coordinators and land managers that we have worked with 

through the years for encouraging us to develop our series of “Biology and Biological Control” manuals. 

Some of the material in this manual was adapted from past manuals in the series, and we wish to acknowledge 

the authors of the original material. We also extend our gratitude to Richard Reardon (Forest Service-Forest 

Health Technology Enterprise Team or FHTET) for producing this guide. The layout of this publication was 

designed by Rachel Winston.

How to cite this publication:

Randall, C.B., R.L. Winston, C. Jette, M.J. Pitcairn, and J.M. DiTomaso. 2017. Biology and Biological 

Control of Yellow starthistle. Fourth Edition. USDA Forest Service, Forest Health Technology 

Enterprise Team, Morgantown, West Virginia. FHTET-2016-08.

This publication is available online at: 

https://www.fs.fed.us/foresthealth/technology/pdfs/FHTET-2016-08_Biocontrol_Yellow_Starthistle.pdf

For additional copies of this publication, contact: 

Richard Reardon FHTET, USDA Forest Service, 180 Canfield Street, Morgantown, WV 26505

304.285.1566; rreardon@fs.fed.us

Carol Bell Randall USDA Forest Service, Forest Health Protection, 2502 E. Sherman Ave, Coeur 

d’Alene, ID 83814 208.769.3051; crandall@fs.fed.us
Biology and Biological Control of Yellow Starthistle

TABLE OF CONTENTS

CHAPTER 1: INTRODUCTION..........................................................................................................1

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

Responding to the Threat of Yellow Starthistle .................................................................................2

The Invasion Curve............................................................................................................................3

Management of Yellow Starthistle Infestations .................................................................................4

Classical Biological Control of Weeds...............................................................................................5

Code of Best Practices for Classical Biological Control of Weeds....................................................7

Biological Control of Yellow Starthistle.............................................................................................7

Is Biological Control of Yellow Starthistle Right for You?.................................................................8

About This Manual............................................................................................................................9

CHAPTER 2: GETTING TO KNOWYELLOW STARTHISTLE..................................................... 11

Taxonomy and Related Species.......................................................................................................11

Yellow Starthistle .............................................................................................................................12

Classification...........................................................................................................................................12

History....................................................................................................................................................12

Description.............................................................................................................................................12

Biology and Ecology...............................................................................................................................16

Habitat....................................................................................................................................................18

Distribution............................................................................................................................................18

Commonly Confused Species................................................................................................................19

CHAPTER 3: BIOLOGY OF YELLOW STARTHISTLE BIOCONTROL AGENTS...................... 25

Introduction.....................................................................................................................................25

Insects..............................................................................................................................................25

Beetles (Order Coleoptera).....................................................................................................................26

Flies (Order Diptera)..............................................................................................................................26

Differentiating Beetles and Flies............................................................................................................26

Fungi................................................................................................................................................27

Yellow Starthistle Weevils................................................................................................................28

Bangasternus orientalis ..........................................................................................................................30

Yellow starthistle bud weevil

Eustenopus villosus..................................................................................................................................32

Yellow starthistle hairy weevil

Larinus curtus.........................................................................................................................................35

Yellow starthistle flower weevil

Yellow Starthistle Flies.....................................................................................................................37

Chaetorellia australis & C. succinea ......................................................................................................39

Yellow starthistle peacock fly & False yellow starthistle peacock fly

Urophora sirunaseva ..............................................................................................................................43

Yellow starthistle gall fly

Puccinia jaceae var. solstitialis ........................................................................................................45

Yellow starthistle rust fungus

CHAPTER 4: ELEMENTS OF AYELLOW STARTHISTLE BIOCONTROL PROGRAM........... 47

Before You Begin..............................................................................................................................47

Determining the Scope of the Problem .................................................................................................48
Biology and Biological Control of Yellow Starthistle

Defining Goals and Objectives...............................................................................................................49

Understanding Yellow Starthistle Management Options......................................................................49

Developing, Implementing, and Managing a Yellow Starthistle Biological Control Program.....49

Selecting Biological Control Agent Release Sites...................................................................................50

Establish Goals for Your Release Site.....................................................................................50

Determine Site Characteristics ...............................................................................................50

Note Land Use and Disturbance Factors ................................................................................50

Survey for Presence of Biological Control Agents .................................................................51

Record Ownership and Access................................................................................................51

Choosing the Appropriate Biological Control Agents for Release........................................................52

Biocontrol Agent Efficacy.......................................................................................................52

Biocontrol Agent Availability .................................................................................................52

Release Site Characteristics ...................................................................................................54

Obtaining and Releasing Yellow Starthistle Biological Control Agents................................................54

Factors to Consider When Looking for Sources of Biological Control Agents......................54

Collecting Yellow Starthistle Biological Control Agents........................................................55

Collection methods ...........................................................................................................56

Sweep netting...........................................................................................................................56

Tapping ....................................................................................................................................57

Aspirating.................................................................................................................................57

Rearing out biocontrol agents..................................................................................................58

Suspending rust spores.............................................................................................................58

Transferring infested stems......................................................................................................58

Methods by species...........................................................................................................58

Bangasternus orientalis...........................................................................................................58

Chaetorellia australis ..............................................................................................................58

Eustenopus villosus..................................................................................................................59

Larinus curtus..........................................................................................................................59

Puccinia jaceae var. solstitialis................................................................................................59

Urophora sirunaseva ...............................................................................................................60

Release Containers for Yellow Starthistle Biological Control Agents....................................60

Transporting Yellow Starthistle Biological Control Agents....................................................61

Keep the containers cool at all times ...............................................................................61

Transporting short distances ...........................................................................................61

Shipping long distances....................................................................................................61

Other factors to consider..................................................................................................62

Avoiding common packaging mistakes.............................................................................63

Purchasing Yellow Starthistle Biological Control Agents.......................................................63

Releasing Yellow Starthistle Biological Control Agents.........................................................64

Establish permanent location marker ..............................................................................64

Record geographical coordinates at release point using GPS ........................................64

Prepare map .....................................................................................................................64

Complete relevant paperwork at site ...............................................................................65

Set up photo point .............................................................................................................65

Release as many biocontrol agents as possible ...............................................................65

Documenting, Monitoring, and Evaluating a Biological Control Program...................................... 67

The Need for Documentation .................................................................................................67

Information Databases.............................................................................................................68

Monitoring Methods................................................................................................................68

Assessing biological control agent populations ...............................................................68

Assessing the status of yellow starthistle and co-occurring plants ..................................70

Qualitative (descriptive) monitoring........................................................................................70

Quantitative monitoring...........................................................................................................71

Assessing impacts on non-target plants...................................................................................71
Biology and Biological Control of Yellow Starthistle

CHAPTER 5: AN INTEGRATEDYELLOW STARTHISTLE MANAGEMENT PROGRAM........ 73

Introduction.....................................................................................................................................73

Components of Integrated Pest Management Programs................................................................75

Education & Outreach...........................................................................................................................75

Inventory & Mapping............................................................................................................................76

Prevention...............................................................................................................................................77

EDRR ......................................................................................................................................78

Weed Control Activities..........................................................................................................................78

Biological Control...................................................................................................................78

Physical Treatment ..................................................................................................................78

Hand pulling .....................................................................................................................78

Mowing .............................................................................................................................79

Tillage ...............................................................................................................................80

Cultural Practices ....................................................................................................................80

Burning .............................................................................................................................80

Grazing .............................................................................................................................81

Seeding competitive species..............................................................................................82

Chemical Control ....................................................................................................................83

Broadleaf selective herbicides in CA................................................................................84

Non-selective herbicides in CA.........................................................................................85

Broadleaf selective herbicides outside of CA ...................................................................85

Non-selective herbicides outside of CA............................................................................86

GLOSSARY ........................................................................................................................................... 91

SELECTED REFERENCES................................................................................................................. 94

Chapter 1: Introduction ..................................................................................................................94

Chapter 2: Getting to Know Yellow Starthistle ...............................................................................95

Chapter 3: Biology of Yellow Starthistle Biological Control Agents...............................................96

Chapter 4: Elements of a Yellow Starthistle Biocontrol Program...................................................97

Chapter 5: An Integrated Yellow Starthistle Management Program...............................................97

APPENDIX........................................................................................................................................... 99

Appendix I: Troubleshooting Guide: When Things Go Wrong .......................................................99

Appendix II: Sample Biological Control Agent Release Form........................................................100

Appendix III: Standardized Impact Monitoring Protocol (SIMP).................................................102

Appendix IV: General Biological Control Agent Monitoring Form ..............................................107

Appendix V: Yellow Starthistle Qualitative Monitoring Form .......................................................108

Appendix IV: Yellow Starthistle Quantitative Monitoring Form....................................................109

Biology and Biological Control of Yellow Starthistle

CHAPTER 1: INTRODUCTION

Overview

Yellow starthistle (Centaurea solstitialis L., Figure 1-1a,b) is an invasive weed native to the Mediterranean 

region. It was introduced to North America in the 1800s. Though it is present throughout much of the 

United States (Figure 1-2a), yellow starthistle is most problematic in rangelands and grasslands in the 

western USA where it occupies more than 18 million acres (7.3 million ha, Figure 1-2b). 

a b

Figure 1-2. Yellow starthistle presence and abundance in 

North America (Credit: USDA PLANTS Database, 

EDDMapS; states with over 100 different reports 

of yellow starthistle have been marked as abundant) 

Please note, many plant distribution databases report 

yellow starthistle as present in Canada; however, while 

a few plants were previously found in Alberta, British 

Columbia, Ontario, and Saskatchewan, no populations 

have persisted to date.

Figure 1-1. Yellow starthistle, a. plant; b. infestation (Credits: a Mark Schwarzländer, University of Idaho; b Charles 

Turner, USDA ARS)

Chapter 1: Introduction 1
Biology and Biological Control of Yellow Starthistle

Throughout its native and introduced ranges, yellow starthistle is found in annual and perennial grasslands, 

pastures, shrub steppe, open woodlands, and disturbed habitats such as hayfields, orchards, vineyards, 

roadsides and abandoned areas. It is most commonly found in Mediterranean and steppe climates 

characterized by cool winters and hot, dry summers, and in relatively dry, fertile soils. 

Yellow starthistle is one of the most problematic exotic plant species currently threatening rangeland and 

conservation areas in the western USA. It reduces biodiversity by displacing native vegetation in grasslands 

and woodlands. The thorny spines that surround the flower heads of starthistle interfere with grazing by 

livestock, recreation, and wildlife management. It is toxic to horses, causing a chronic and potentially fatal 

neurological disorder known as “chewing disease”. Because of its excessive water usage, yellow starthistle 

threatens both human economic interests and native plant ecosystems.

Responding to the Threat of Yellow Starthistle

Yellow starthistle is an invasive species not native to North America whose introduction causes or is likely 

to cause economic or environmental harm. Yellow starthistle is responsible for millions of dollars of damage 

annually resulting from lowered quality of range forage, a decrease of land value, disruption of natural 

ecosystems, and a reduction of water available to recreation, agriculture, and native vegetation. 

A general management response to the threat of yellow starthistle and other invasive species is based on 

four key elements or intermediate outcomes: prevention and preparedness, eradication, containment, and 

asset-based protection. In order to ensure a timely and appropriate management response, land managers 

must continually monitor, evaluate, and report/map new yellow starthistle infestations and evaluate how 

yellow starthistle responds to each control effort. Research and development informed by the observations 

and needs of land managers will play a critical role in the eventual success or failure of yellow starthistle 

prevention and management activities in its invaded range.

Prevention and Preparedness

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

the threat they pose. Considerable resources and planning are required to maintain prevention of 

a large number of species. Preparedness encompasses all the activities and resources necessary to 

successfully manage new invasions.

Eradication

Eradication, getting rid of an invasive species completely, is generally only possible in the early stages 

of establishment when the distribution and abundance of the invasive species are low. This approach 

can be almost as cost-effective as prevention. 

Containment

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

preventing its further spread. Containment involves measures to eradicate outlying (satellite) 

infestations and prevent spread beyond the boundaries of core infestations (those that are too large 

and well established to eradicate). Obtaining a high degree of community support is a prerequisite for 

any long-term containment program.

2 Chapter 1: Introduction
Biology and Biological Control of Yellow Starthistle

Asset-Based Protection

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

so widespread that it would be inefficient to control the species everywhere it occurs and where 

containment would provide a low return on investment. The asset-based approach is used to manage 

the species only where reducing its adverse effects provides the greatest benefits by achieving 

protection and restoration outcomes for specific highly valued assets.

Monitoring, Evaluation, and Reporting

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

reporting are elements of adaptive management, whereby programs are continually reviewed and 

analyzed to ensure that their approaches are consistent with and supportive of any changes in 

environmental response, community expectation, or scientific knowledge.

Research and Development

The knowledge that comes from research and development is critical to implement evidence-based 

management approaches. In many cases, substantial advances in invasive species management will 

require development of new techniques and acquisition of greater and new knowledge. Investment in 

research needs to be sufficient to ensure future management is not seriously constrained by insufficient 

research and development support.

The Invasion Curve

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

becomes less likely and control costs increase as an invasive species spreads over time. Prevention is the 

most cost-effective solution, followed by eradication. If a species is not detected and removed early, intense 

and long-term control efforts will be unavoidable.

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

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

Chapter 1: Introduction 3
Biology and Biological Control of Yellow Starthistle

While yellow starthistle currently infests large acreages in some regions, there are entire states and provinces 

where yellow starthistle is absent or is present at very low levels. The diversity of yellow starthistle 

populations, from absent to widespread and abundant, throughout its potential range requires land managers 

to coordinate their management response to yellow starthistle across larger landscapes to prevent current 

infestations from spreading into uninfested areas. 

Identifying where yellow starthistle is on the invasion curve in a particular area is the first step to taking 

management action. Inventorying and mapping current yellow starthistle populations, coupled with research 

efforts to predict where yellow starthistle is most likely to inhabit, enables land managers to concentrate 

resources in areas where yellow starthistle is likely to invade, and then to treat individual plants and small 

populations before it is too late to remove them. 

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

generally not appropriate for areas on the left side of the invasion curve (species absent [prevention] - small 

number of localized populations [eradication]) because biological control alone will not result in yellow 

starthistle eradication. Biological control as a control method is best suited to yellow starthistle populations 

in the later phases of the invasion curve (rapid increase in distribution and abundance [containment] – 

widespread and abundant throughout potential range [asset based protection])

Management of Yellow Starthistle Infestations

Successful management of yellow starthistle is an intensive process which requires land managers to 

continuously inventory, map, and assess the extent and severity of yellow starthistle infestations. Land 

managers must also understand the benefits and shortcomings of each weed control method available, alone 

and in combination, when applied to yellow starthistle. Chemical control (using herbicides) may be used to 

successfully control small yellow starthistle infestations (left side of the invasion curve, Figure 1-3) where 

land managers are committed to annual monitoring and, when necessary, re-treatment; however, chemical 

control can be impractical, prohibitively expensive, and damaging to desired vegetation when treating large 

yellow starthistle infestations. Hand pulling small, individual yellow starthistle plants may again be feasible 

for small infestations on the left side of the invasion curve; however, new yellow starthistle plants may 

germinate throughout the growing season, and the extensive taproots on larger plants are difficult to remove. 

Repeated mowing can be utilized in the containment and asset-based portions of the invasion curve by 

reducing yellow starthistle vigor and seed production, but it may exacerbate the problem by triggering regrowth and spreading seeds. Burning may be used to decrease the yellow starthistle seed bank, but in the long 

term burning is a generally ineffective control method because it favors the germination of yellow starthistle 

and may have severe negative, long-term consequences for desired plants and wildlife. Grazing yellow 

starthistle can be an effective control method in certain circumstances (typically asset-based protection of 

the invasion curve), but grazing can be difficult and/or time-consuming, and may have severe negative, 

long-term consequences for plant communities. Because chemical, physical, and cultural control methods 

are not universally effective in managing yellow starthistle throughout its invaded range, a biological control 

program was initiated in the 1960s. Biological control as a control method is best suited to yellow starthistle 

populations in the later phases of the invasion curve. This manual discusses the biological control of yellow 

starthistle in North America, within the larger context of an integrated yellow starthistle management strategy. 

The most effective weed management strategies are based on regular inventory and monitoring of target 

weed populations, application of one or many weed control methods, evaluation of treatment efficacy, 

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

objectives in response to changing weed populations through time. Integrated Pest Management (IPM) 

4 Chapter 1: Introduction
Biology and Biological Control of Yellow Starthistle

incorporates additional activities that enable land managers to address the threat of yellow starthistle 

invasions in infested as well as uninfested areas across a landscape. IPM activities include education and 

outreach, inventory and mapping, prevention methods, and control methods (physical control [hand pulling 

or mowing], cultural control [revegetation, grazing, or fire], chemical control [herbicides], and biological 

control). IPM relies on the development of realistic pest management objectives, accurate pest identification 

and mapping, appropriate prevention and control methods, and post-treatment monitoring to ensure current 

pest-management activities are meeting yellow starthistle management goals.

Land managers choose control methods, either alone or in combination that enable them to achieve their 

yellow starthistle management goals or objectives in the most cost-effective manner. No single control 

method will enable managers to meet their yellow starthistle management goals in all environments or 

instances. Control method(s) employed will depend on the size and location of the infested area and specific 

management goals (e.g., eradication vs. weed density reduction). Small patches of yellow starthistle may 

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

require the use of additional control methods. A combination of control methods consistently applied, 

evaluated, and adjusted through time is usually necessary to attain and maintain weed management goals 

for yellow starthistle.

Classical Biological Control of Weeds

Most invasive plants (weeds) in the United States are not native to North America; they arrived with 

immigrants, through commerce, or by accident from different parts of the world. These non-native plants 

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

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

weeds when introduced to areas outside of their native range. 

Biological control (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 in its introduced 

range. Natural enemies used in classical biological control of weeds include different organisms, such as 

insects, mites, nematodes, and pathogens. In North America, most weed biological control agents are plantfeeding insects, of which beetles, flies, and moths are among the most commonly used. 

Biological control agents may attack a weed’s flowers, seeds, roots, foliage, and/or stems. Effective biological 

control agents seldom kill weeds outright, but work with other stressors such as moisture or nutrient 

shortages to reduce vigor and reproductive capability, or facilitate secondary infection from pathogens—all 

of which compromise the weed’s ability to compete with other plant species. Once established, root- and 

crown-feeding biocontrol agents are usually more effective on perennial plants that primarily spread by root 

buds. Flower- and seed-feeding biocontrol agents are typically more effective on annual or biennial plants 

that spread only by seed. Regardless of the plant part attacked by biocontrol agents, the aim is always to 

reduce populations and vigor of the target weed. 

Although weed biological control is an effective and important weed management tool, it does not work in all 

cases and should not be expected to eradicate the target weed. Even in the most successful cases, biocontrol 

often requires multiple years before impacts become noticeable. When classical biological control alone 

does not result in an acceptable level of weed control, other weed control methods (e.g., physical, cultural, 

or chemical control) may be incorporated to achieve desired results. For a more in-depth description of 

weed control methods in the context of yellow starthistle management, please refer to Chapter 5.

Chapter 1: Introduction 5
Biology and Biological Control of Yellow Starthistle

There are advantages and disadvantages to biological control of weeds as a management tool. These are 

listed in Table 1-1. 

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

Advantages Disadvantages

Target specificity Will not work on every weed in every setting 

Continuous action Permanent; cannot be undone

Long-term cost-effective; can provide sustained control at the 

landscape scale

Funding and testing candidate biocontrol agents is expensive; 

measurable impact may take years or even decades to materialize

Integrates well with other control methods Approved biocontrol agents are not available for all exotic weeds

Generally environmentally benign Like all weed control methods, non-target effects are possible, 

but pre-release testing reduces the risks

Self-dispersing, even into rough or difficult to access terrain Unpredictable level of control; does not eliminate weed

6 Chapter 1: Introduction

To be approved for release in North America, weed biocontrol agents must be host-specific, meaning they 

must develop only on the target weed. Rigorous testing is required to confirm that biocontrol agents are 

host specific and effective. Potential biocontrol agents often undergo five or more years of testing to ensure 

that rigid host specificity requirements are met, and results are vetted at a number of stages in the approval 

process. 

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

and Quarantine (USDA-APHIS-PPQ) is the federal regulatory agency responsible for providing testing 

guidelines and authorizing the importation of biocontrol agents into the USA. Though not currently found in 

Canada, if yellow starthistle were to become established in Canada, The Canadian Food Inspection Agency 

(CFIA) serves the same regulatory role in Canada. Federal laws and regulations are in place to identify and 

avoid potential risks to native and economically valuable plants and animals that could result from exotic 

organisms introduced to manage weeds. The Technical Advisory Group (TAG) for Biological Control Agents 

of Weeds is an expert committee with representatives from USA federal regulatory, resource management, 

and environmental protection agencies, and regulatory counterparts from Canada and Mexico. TAG members 

review all petitions to import new biocontrol agents into the USA, and make recommendations to USDAAPHIS-PPQ regarding the safety and potential impact of prospective biocontrol agents. Weed biocontrol 

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

of potential weed biocontrol agents and programs. In addition, some states in the USA have their own 

approval process to permit field release of weed biocontrol agents. In Canada, the Biological Control 

Review Committee (BCRC) draws upon the expertise and perspectives of Canadian-based researchers 

(e.g., entomologists, botanists, ecologists, weed biological control scientists) from academic, government, 

and private sectors for scientific review of petitions submitted to the CFIA. The BCRC reviews submissions 

for compliance with the North American Plant Protection Organization’s (NAPPO) Regional Standards 

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

conclusions factor into the final TAG recommendation to APHIS on whether to support the release of the 

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

and Canada, APHIS and the CFIA attempt to coordinate decisions based on the assessed safety of each 

country’s plant resources. 
Biology and Biological Control of Yellow Starthistle

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 in 1999 by delegates and participants in the Tenth International 

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

impacts from, weed biological control. In following the Code, practitioners reduce the potential for causing 

environmental damage through the use of weed biological control by voluntarily restricting biocontrol 

activities to those most likely to result in success and least likely to cause harm.

International Code of Best Practices for Classical Biological Control of Weeds1

1.	 Ensure that the target weed’s potential impact justifies release of non-endemic biocontrol agents

2.	 Obtain multi-agency approval for biocontrol target

3.	 Select biocontrol agents with potential to control target

4.	 Release safe and approved biocontrol agents

5.	 Ensure that only the intended biocontrol agent is released

6.	 Use appropriate protocols for release and documentation

7.	 Monitor impact on the target

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

9.	 Monitor impacts on potential non-targets

10.	 Encourage assessment of changes in plant and animal communities

11.	 Monitor interaction among biocontrol agents

12.	 Communicate results to public

1

Ratified July 9, 1999, by the delegates to the X International Symposium on Biological Control of Weeds, Bozeman, MT

Chapter 1: Introduction 7

There are several resources that provide additional information about general weed biocontrol practices 

and specific weed-biocontrol systems, which can be found in the Chapter 3 references under Andreas et al. 

2017, Coombs et al. 2004, and Winston et al. 2014b. 

Biological Control of Yellow Starthistle

In 1969, what was first identified as the banded yellow 

starthistle gall fly, Urophora sirunaseva Hering, became 

the first biocontrol agent approved and released in North 

America on yellow starthistle (Figure 1-4). By 2003, six 

additional species had been approved and released in the 

United States, including a rust fungus, two flies, and three 

beetles. None of these biological control agents have been 

released in Canada because the weed does not occur in 

Canada. Though a few plants have been reported from 

Alberta, British Columbia, Ontario, and Saskatchewan in 

the past, no Canadian yellow starthistle populations have 

persisted to date. 

Figure 1-4. Adult Urophora sirunaseva, the 

banded yellow starthistle gall fly (Credit: Mark 

Schwarzländer, University of Idaho)
Biology and Biological Control of Yellow Starthistle

Is Biological Control of Yellow Starthistle Right for You?

When biological control is successful, biocontrol agents increase in abundance until they suppress (or 

contribute to the suppression of) the target weed. As local target weed populations are reduced, their biological 

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

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

followed by increases of its biocontrol agent, until the target weed/biocontrol agent populations stabilize at 

a much lower abundance. 

As stated in Table 1-1, biological control is not effective in every weed system or at every infestation. We 

recommend that you develop an integrated weed management program in which biological control is one of 

several control methods considered. Here are some questions you should ask before you begin a biological 

control program:

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

Biological control does not eradicate target weeds, so it is not a good fit with an eradication goal; 

however, depending on the target weed, which biological control agent is used, and land use, biological 

control can be effective at reducing the abundance and vigor of a target weed to an acceptable level.

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

ten years?

Biological control requires time and patience to work. Generally, it can take one to three years after 

release to confirm that biological control agents are established at a site, and even longer for biocontrol 

agents to cause significant impacts to the target weed. For some weed infestations, 5-30 years may be 

needed for biological control to reach its weed management potential.

What resources can I devote to my weed problem?

If you have only a small yellow starthistle problem (< 1 acre [0.4 ha] or much smaller), weed control 

methods such as hand pulling and/or herbicides, followed by regular monitoring for re-growth and retreatment when necessary, may be most effective. These intensive control methods may allow you to 

achieve rapid control and prevent the weed from spreading and infesting additional areas, especially 

when infestations occur in high-priority treatment areas such as travel corridors where the weed is 

more likely to readily disperse. If yellow starthistle is well established over a large area (>1 acre, 0.4 

ha), and resources are limited, biological control may be the 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 or continue to be 

disturbed. Without restoration of a desirable, resilient plant community, and especially if disturbance 

continues, biological control is unlikely to solve your weed problems.

8 Chapter 1: Introduction
Biology and Biological Control of Yellow Starthistle

The ideal biological control program:

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

management objectives

2. Is part of a broader integrated weed management program

3. Has considered all weed control methods and determined that biological control is the best option 

based on available resources and weed management objectives

4. Has realistic weed management goals and timetables

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

and populations of biological control agents

About This Manual

This manual provides information on the biology and ecology of yellow starthistle and each of its biological 

control agents. It also presents guidelines to establish and manage approved biological control agents as 

part of an integrated yellow starthistle management program. Throughout this manual, English units are 

given first for descriptions of plants and areas, followed by their metric system equivalents in parentheses. 

Metric units are the preferred and traditional reference for insects and are used throughout Chapter 3 for 

describing the yellow starthistle biological control agents. Table 1-2 provides English/metric conversions 

and abbreviations.

Table 1-2. English/metric conversion table

English System Metric System

1/16 inch (in) 3.2 millimeters (mm)

1 inch (in) 2.54 centimeters (cm)

1 foot (ft) 30 centimeters (cm)

1 yard (yd) 0.9 meters (m)

1 mile (mi) 1.6 kilometers (km)

1 acre (ac) 0.4 hectares (ha)

Chapter 1: Introduction 9

Chapter 1: Introduction provides introductory information on yellow starthistle (including its distribution, 

habitat, and economic impact) and classical biological control.

Chapter 2: Getting to Know Yellow Starthistle provides detailed descriptions of the taxonomy, growth 

characteristics and features, invaded habitats, and occurrence of yellow starthistle in North America. It also 

describes how to differentiate yellow starthistle from look-alike species.

Chapter 3: Biology of Yellow Starthistle Biological Control Agents describes biological control agents 

of yellow starthistle, including details on each biocontrol agent’s native range, original source of releases 
Biology and Biological Control of Yellow Starthistle

in North America, parts of yellow starthistle plants attacked, life cycle, description, 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 Yellow Starthistle Biological Control Program includes detailed information 

and guidelines on how to plan, implement, monitor, and evaluate an effective yellow starthistle biological 

control program. Included are guidelines and methods for:

• Selecting and preparing biological control agent release sites

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

• Monitoring biological control agents and vegetation

Chapter 5: An Integrated Yellow Starthistle Management Program discusses the role of biological 

control in the context of an integrated yellow starthistle management program.

The Glossary defines technical terms frequently used by those involved in yellow starthistle biological 

control and found throughout this manual.

References lists selected publications and resources utilized to compile this manual.

Appendices:

I. Troubleshooting Guide: When Things Go Wrong

II. Sample Biological Control Agent Release Form

III. Yellow Starthistle Standardized Impact Monitoring Protocol

IV. General Biological Control Agent Monitoring Form

V. Yellow Starthistle Qualitative Monitoring Form

VI. Yellow Starthistle Quantitative Monitoring Form

10 Chapter 1: Introduction
Biology and Biological Control of Yellow Starthistle

Chapter 2: Getting to Know Yellow Starthistle 11

CHAPTER 2: GETTING TO KNOW YELLOW STARTHISTLE

Taxonomy and Related Species

Yellow starthistle belongs to the sunflower family (Asteraceae). Members of the sunflower family produce 

flower heads, or capitula, that are an aggregation of many individual flowers (Figure 2-1a). These flowers, 

called florets, are clustered together and attached to a receptacle. There are two types of florets: disc and 

ray (Figure 2-1b, 2-1c). Some species produce both types of florets, while others (like yellow starthistle) 

produce only disc florets. The receptacle and florets are enclosed by modified leaves called involucral 

bracts. Each floret produces one seed (achene) from early to late summer. Seeds often have a tuft of whitish 

hairs (pappus) on one end, similar to those on dandelion seeds (Figure 2-1d).

Yellow starthistle is in the genus Centaurea. There are approximately 500 species in this genus worldwide, 

all of which are annual, biennial or perennial forbs. This genus can be found on all continents of the Northern 

Hemisphere, but it is particularly species-rich in the Middle East. No species of Centaurea are native to 

North America. Two species native to 

the southern USA and northern Mexico 

(Plectocephalus americanus [Nutt.] 

D. Don and P. rothrockii [Greenman] 

D.J.N. Hind) were once assigned 

to the genus Centaurea, but have 

recently been recognized as belonging 

to Plectocephalus. Taxonomic 

classification within the Centaurea 

is still uncertain; this variable 

group includes the knapweeds and 

cornflowers as well as the starthistles. 

Representatives from all three groups 

are among the 20 Centaurea species 

and two additional hybrids currently 

listed as established outside of 

cultivation in North America. Five 

of these species and one hybrid are 

starthistles.

a b c d

Figure 2-1. Sunflower family, a. capitulum; b. disc floret; c. ray floret; d. 

seed with pappus (Credits: a-d Prof. Dr. Otto Wilhelm Thomé Flora 

von Deutschland, Österreich und der Schweiz 1885, Gera, Germany, 

www.biolib.de; © expired)
Biology and Biological Control of Yellow Starthistle

12 Chapter 2: Getting to Know Yellow Starthistle

Unlike cornflowers, starthistles typically have grayish-green and often rough or hairy leaves sometimes 

deeply divided into irregular lobes. Some knapweed species are similar to starthistles in that bracts 

subtending flower heads can end in stiff spines; however, unlike starthistles, the spines of knapweeds are 

never longer than the bracts. Only yellow starthistle is discussed in this manual. 

Yellow Starthistle

Scientific Name

Centaurea solstitialis L.

Common Names

Yellow starthistle, yellow star-thistle, St. Barnaby’s thistle, golden thistle, cotton-tip thistle

Classification

Ranking Scientific Name Common Name

Kingdom Plantae Plants

Subkingdom Tracheobionta Vascular plants

Superdivision Spermatophyta Seed plants

Division Magnoliophyta Flowering plants

Class Magnoliopsida Dicotyledons

Subclass Asteridae

Order Asterales

Family Asteraceae Sunflower family

Genus Centaurea Knapweed, starthistle

Species Centaurea solstitialis L. Yellow starthistle

History

Yellow starthistle is a native of Eurasia and the Mediterranean. Yellow starthistle seeds were found in 

adobe bricks in California beginning in the early 1800s. It was likely introduced to North America in 

the mid-1800s via contaminated alfalfa seed and other crops. 

Description

At a Glance 

Yellow starthistle is an herbaceous winter annual typically growing 1-3 feet (0.3-0.9 m) tall from a 

deep taproot. Rosettes have deeply lobed leaves up to 6 inches (15 cm) long. Plants produce one or 

more rigid, upright stems with multiple spreading and ascending branches (Figure 2-2, 2-6). Stem 

leaves are typically narrow, unlobed, and attach directly to plant stems by wings that run down the 

sides of the stems. All stems, leaves, and wings are densely covered with cobwebby hairs, resulting 

in a characteristic gray-green color. Flower heads are up to 0.6 inches (1.4 cm) across and consist 

of 10-80 bright yellow florets. Flower heads are produced at tips of branches, and sometimes at leaf 

or branch axils. The bracts are spiny and up to 1 inch (2.5 cm) long. Seeds are oblong and up to 1/8 
Biology and Biological Control of Yellow Starthistle

Chapter 2: Getting to Know Yellow Starthistle 13

inch (3 mm) long. Plumed seeds are tan to brown, located 

at the center of the seed head, and have a ring of pappus 

consisting of fine, white bristles. Plumeless seeds are 

dark brown to black, fewer in number, occur around the 

periphery of the seed head, and lack pappus. 

Roots

Yellow starthistle develops taproots that are slender and 

deep, growing more than 3.3 feet (1 m) long. Roots have 

fine, thread-like extensions from the main stem. 

Stems

Plant height varies depending on site conditions. While most 

rangeland plants are 1-3 feet (0.3-0.9 m) tall, individuals 

growing in wet locations can reach up to 6 feet (1.8 m) 

tall (Figure 2-3a), and those growing in hot, dry areas can 

be as short as 6 inches (15 cm, Figure 2-3b). Plants have 

one or more flowering stems with multiple spreading and 

ascending branches. Stems are upright, rigid, and winged 

(Figure 2-3c). Stems and wings are covered with dense hair, 

resulting in a characteristic gray-green color.

Figure 2-2. Yellow starthistle flowering plant 

(Credit: Laura Parsons, University of Idaho)

a b c

Figure 2-3. Yellow starthistle, a. plant growing over 6 feet (1.8 m) tall after a moist winter and spring; b. plants 

growing less than 6 inches (15 cm) tall at a dry site; c. winged stem (Credits: a,c Rachel Winston, MIA Consulting; 

b Eric Coombs, Oregon Department of Agriculture, bugwood.org)

Leaves

Rosettes consist of numerous leaves. At open sites these lie close to the ground (Figure 2-4a); where 

numerous yellow starthistle plants are crowded together, rosette leaves grow more upright (Figure 

2-4b). Each basal leaf is 2-6 inches (5-15 cm) long with lobed margins. The lobes are irregular, they 

may be opposite or alternate from each other, and the terminal lobe is larger than the side lobes. Stem 

leaves attach directly to plant stems by wings that run down the sides of the stems (Figure 2-4c). 

Stem leaves may be lobed but are more often narrow with smooth margins. Stem leaves are typically 

smaller than rosette leaves; they may be 0.4-4 inches (1-10 cm) long. All leaves and wings are densely 

covered with cobwebby hairs, resulting in a characteristic gray-green color.
Biology and Biological Control of Yellow Starthistle

14 Chapter 2: Getting to Know Yellow Starthistle

a b c

Figure 2-4. Yellow starthistle, a. rosette leaves lying close to the ground; b. rosette leaves growing more upright in a 

dense infestation; c. stem leaf and wing (Credits: a,c Steve Dewey, Utah State University, bugwood.org; b John 

D. Byrd, Mississippi State University, bugwood.org)

Flowers

Solitary flower heads are produced at the tips of branches, though vigorous plants may also produce 

solitary flower heads at leaf and branch axils. Each flower head consists of 10-80 disc florets (Figure 

2-5a). Each floret consists of five bright yellow fused petals. The involucre (base of the flower head) 

is 0.16-0.6 inches (0.4-1.4 cm) wide. Bracts occur in shingle-like layers around the involucre and are 

densely to sparsely covered with hairs. Each bract has one long central spine 0.4-1 inch (1-2.5 cm) 

long and two or more pairs of short side spines.

Fruits and Seeds

Fruits are oblong achenes (hereafter referred to as seeds) up to 1/8 inch (3 mm) long. Two types of 

seeds are produced: plumed and plumeless in an average ratio of 3:1, respectively. Plumed seeds are 

tan to brown, located at the center of the seed head, and have a ring of pappus consisting of fine, white 

bristles (Figure 2-5b top). Plumeless seeds are dark brown to black, fewer in number, occur around 

the periphery of the seed head, and lack pappus (Figure 2-5b bottom). Seed production varies greatly 

depending on soil moisture, nutrients, and plant competition. A mature plant produces 1-200 capitula, 

and seed heads yield between 1 and 80 seeds each (an average of 30). Consequently, a single mature 

plant can produce 1-6,000 seeds. 

a b

Figure 2-5. Yellow starthistle, a. 

flower head and spines; b. seed 

with pappus (above) and without 

pappus (below) (Credits: a Peggy 

Grebb, USDA ARS, bugwood.

org; b C. Roché)
Biology and Biological Control of Yellow Starthistle

Chapter 2: Getting to Know Yellow Starthistle 15

Figure 2-6. Artist’s rendition of yellow starthistle key traits (Credit: Reichenbach, H. G. (1852-3) Icones florae Germanicae 

et Helveticae. 15. Band. Ambrosius Abel, Leipzig. Tafel 64/795; © expired)
Biology and Biological Control of Yellow Starthistle

16 Chapter 2: Getting to Know Yellow Starthistle

Biology and Ecology

Yellow starthistle spreads only by seed. It is mostly self-incompatible, relying on outcrossing through 

pollination for successful seed production. Most seeds fall within 3.3 feet (1 m) of the parent plant. 

Short to medium dispersal of plumed seeds is facilitated by animals and humans; the seed pappus 

readily tangles in fur, feathers, and clothing. Long distance dispersal typically occurs with the 

movement of seed-contaminated livestock, vehicles, equipment, and hay and crop seed. Birds also 

contribute to long distance dispersal when some seeds pass through their digestive tracts still viable.

Germination typically begins in the fall and winter following autumn rains in regions with a 

Mediterranean climate (mild wet winters and dry summers), though early spring germination also 

occurs. Most seeds germinate at low temperatures and days with short amounts of daylight, provided 

sufficient soil moisture is present. During periods of higher temperatures and longer amounts of daylight, 

the seeds often enter an induced dormancy. Later, if openings form in the plant canopy, subsequent 

yellow starthistle germination will often occur to fill in these open patches. Plumed and plumeless 

seeds respond differently to varying temperature, light, and moisture regimes. These differences are 

believed to aid yellow starthistle in exploiting variable conditions in semiarid environments. While 

the majority of yellow starthistle seeds germinate within three years, the soil seed bank can at times 

yield seedlings up to 10 years after seed drop.

After germination, two oblong cotyledons or seed leaves appear on seedlings, followed by 5 or more 

elliptic leaves (Figure 2-7a). Rosettes develop from fall through spring, consisting of 7-20 lobed basal 

leaves (Figure 2-7b). In wet years, the roots of overwintering rosettes grow rapidly, often extending 

3.3 feet (1 m) before the plants bolt in spring. In dry years with reduced soil moisture recharge, root 

growth is limited to shallow depths of less than 2 feet (<0.60 m). Starthistle roots typically grow 

faster and to greater depths than do potentially competitive species, both weedy and native, allowing 

yellow starthistle to utilize limited resources. A deep root system also prolongs the period of resource 

availability for yellow starthistle, supporting the water-dependent production of seeds well into the 

dry months of summer when many competing species have already senesced. 

Stems bolt from spring through the summer, sending up rigid, winged flower stalks (Figure 2-7c). 

Flower heads then develop through four distinct bud stages:

BU-1 (Figure 2-7d) Small buds with yellow-green spines begin to be visible at the top. 

BU-2 (Figure 2-7e) Spines protrude more than half of the bud length.

BU-3 (Figure 2-7f) Spines are equal to or greater than 45° angle from the stem.

BU-4 (Figure 2-7g) Spines are straw-colored and equal to or greater than 90° angle from the stem.

Flowering occurs throughout summer, and each flower head produces 10-80 bright yellow florets 

(Figure 2-7h). During the seed formation stage, there is a progressive loss of color throughout the 

plant, though the involucre still retains some green (Figure 2-7i). By late summer as plants mature, 

the leaves wither and dry, the bright yellow flowers fade, and the plants take on a straw-colored 

appearance (Figure 2-7j). The size of the plant at maturity will depend, in part, on soil type, nutrients, 

water availability, the genetic potential of the plant, and plant density. Growth is most typically limited 

by low moisture and nitrogen.

During the seed dissemination stage from late summer to early fall, the flower head dries to a tan color, and 

the bracts dry and release the plumed seeds for dispersal (Figure 2-7k). Senescence (Figure 2-7l) begins 

in autumn and continues through the following spring as the plants wither and lose their leaves, becoming
Biology and Biological Control of Yellow Starthistle

Chapter 2: Getting to Know Yellow Starthistle 17

silver-gray skeletons with decaying capitula that resemble white, cottony tufts by December or January. 

The flower heads have lost most of their spines and the plumeless seeds by this time. Eventually the head 

disintegrates and plumed seeds are shed.

a b c

d e f g

h i j

k l

Figure 2-7. Yellow starthistle life cycle, a. seedling; b. rosette; c. bolting; d. BU-1; e. BU-2; f. BU-3; g. BU-4; h. flowering; 

i. seed formation; j. mature; k. dissemination; l. senescence (Credits: a-l C. Roché)
Biology and Biological Control of Yellow Starthistle

18 Chapter 2: Getting to Know Yellow Starthistle

Habitat

Yellow starthistle capitalizes on soil disturbance, but can spread into undisturbed sites as well. It is 

a serious weed of roadsides, rangeland, pastures, shrub steppe, abandoned cropland, and disturbed 

places (Figure 2-8a-c). Yellow starthistle occurs in a variety of soils and conditions but is most 

common below 4000 feet (1200 m) in elevation on south-facing slopes in areas with more than 6 

inches (15 cm) of rain annually.

a b c

Figure 2-8. Yellow starthistle infestations, a. open rangeland; b. fenced pastures; c. along a roadway (Credits: a,b 

Steve Dewey, Utah State University, bugwood.org; c John M. Randall, The Nature Conservancy, bugwood.org)

Distribution

As of 2016, yellow starthistle is considered established in 38 states but no Canadian provinces 

(Figure 1-2, repeated here in Figure 2-9a). While a few plants were previously found in Alberta, 

British Columbia, Ontario, and Saskatchewan Canada, no populations have persisted to date. Yellow 

starthistle has been declared noxious in eleven of the states in which it is currently found in the USA, 

as well as three Canadian provinces where it has not yet established (Figure 2-9b). 

a b

Figure 2-9. States and provinces where yellow starthistle is a. present and abundant in North America (Credit: 

USDA PLANTS Database, EDDMapS; states with over 100 different reports of yellow starthistle have been 

marked as abundant. Please note, many plant distribution databases report yellow starthistle as present in Canada; 

however, while a few plants were previously found in Alberta, British Columbia, Ontario, and Saskatchewan, no 

populations have persisted to date.); b. listed as noxious.
Biology and Biological Control of Yellow Starthistle

Chapter 2: Getting to Know Yellow Starthistle 19

Comments

Yellow starthistle exhibits morphological and genetic variation throughout its range in the USA, 

and this variation is present among individuals at most infestations. This suggests that multiple 

introductions may have occurred, but that there is only one highly mixed population in the USA. High 

genetic diversity enables yellow starthistle to easily adapt to climate and soil differences between 

invaded sites, and it has likely contributed to yellow starthistle’s invasion success in North America. 

Commonly Confused Species

Numerous species present in North America have an appearance similar to yellow starthistle, especially 

those in the same family and genus. The species most closely resembling yellow starthistle are listed 

in Tables 2-1 and 2-2, along with key characteristics that can be used to differentiate the look-alikes. 
Biology and Biological Control of Yellow Starthistle

20 Chapter 2: Getting to Know Yellow Starthistle

Table 2-1. Non-native/invasive Centaurea species present in North America that are similar in appearance to yellow 

starthistle (YST), along with key traits for differentiation.

Species Similarities to YST Differences from YST 

Yellow starthistle

Centaurea solstitialis

Exotic annual forb

Iberian and Purple 

Starthistle

Centaurea iberica and C. 

calcitrapa

Exotic annuals to shortlived perennial forbs

Rosette leaves are lobed.

Foliage is gray-green and covered with 

hairs.

Involucral bracts end in long spines.

Similar height under typical conditions.

Stems are not winged.

Florets are pink or purple.

All seeds of Iberian starthistle have pappus.

All seeds of purple starthistle lack pappus.

Maltese starthistle

Centaurea melitensis

Exotic annual or 

biennial forb

Rosette leaves are lobed.

Foliage is gray-green and covered with 

hairs.

Stems are winged.

Involucral bracts end in long spines.

Florets are yellow.

Spiny bracts are smaller and purple-colored at their base.

All seeds have pappus.

Grows relatively shorter under typical conditions.

Flowers earlier in the year.

Sicilian starthistle

Centaurea sulphurea

Exotic annual forb

Stems are winged.

Florets are yellow.

Involucral bracts end in long spines.

Similar height under typical conditions.

Florets are yellow.

Leaves are not lobed.

Foliage is only sparsely covered with hairs; not gray-green.

Flower heads are larger (up to 1 in or 2.5 cm)

Seeds are twice as large and all have dark pappus.

Spotted knapweed

Centaurea stoebe

Exotic biennial or 

perennial forb

Rosette leaves are lobed.

Foliage is gray-green and covered with 

hairs.

Involucral bracts are in shingle-like layers 

and may be slightly spiny.

Similar height under typical conditions.

Stems are not winged.

Involucral bracts have black tips and lack long terminal 

spines.

Florets are pink or purple.

All seeds have pappus.

Credits: Yellow starthistle: 1 (Rachel Winston, MIA Consulting), 2 (Steve Dewey, Utah State University, bugwood.

org), 3 (Peggy Greb, USDA ARS, bugwood.org), 4 (Cindy Roche, bugwood.org); Iberian and Purple Starthistle: 

1 (Xemenendura), 2,4a top (©2001 CDFA), 3 (Joan Simon), 4b bottom (D. Walters and C. Southwick, USDA, 

bugwood.org); Maltese starthistle: 1 (David Thornburg, swbiodiversity.org), 2 (Max Licher, swbiodiversity.org), 

3 (Franco Folini), 4 (CDFA Seed Laboratory); Sicilian starthistle: 1,2 (Joseph DiTomaso, University of CaliforniaDavis, bugwood.org), 3,4 (Daniel Montesinos); Spotted knapweed: 1 (Rob Routledge, Sault College, bugwood.org), 2 

(James Miller, USDA Forest Service, bugwood.org), 3 (Michael Shephard, USDA Forest Service, bugwood.org), 4 (Ken 

Chamberlain, Ohio State University, bugwood.org)
Biology and Biological Control of Yellow Starthistle

Chapter 2: Getting to Know Yellow Starthistle 21

Table 2-1. Continued

Look Alike Appearance

Leaves Plant Flower Head Seeds
Biology and Biological Control of Yellow Starthistle

22 Chapter 2: Getting to Know Yellow Starthistle

Table 2-2. Other species present in North America that are less related to yellow starthistle (YST) but that are in the same 

family (Asteraceae) and similar in appearance, along with key traits for differentiation.

Credits: Yellow starthistle: 1 (Rachel Winston, MIA Consulting), 2 (Steve Dewey, Utah State University, bugwood.org), 

3 (Peggy Greb, USDA ARS, bugwood.org), 4 (Cindy Roche, bugwood.org); Dandelion: 1 (Bruce Ackley, Ohio State 

University, bugwood.org), 2 (Robert Vidéki, Doronicum Kft, bugwood.org), 3 (Chris Evans, Illinois Wildlife Action 

Plan, bugwood.org), 4 (Joseph Berger, bugwood.org); Prickly lettuce: 1 (Ohio State University Weed Lab Archive, 

bugwood.org), 2,3 (Mary Ellen (Mel) Harte, bugwood.org), 4 (Steve Dewey, Utah State University, bugwood.org); 

Rush skeletonweed: 1 (Richard Old, XID Services, Inc, www.xidservices.com, bugwood.org), 2 (Joseph Milan, BLM), 

3 (Rachel Winston, MIA Consulting), 4 (Steve Dewey, Utah State University, bugwood.org)

Species Similarities to YST Differences from YST 

Yellow starthistle

Centaurea solstitialis

Exotic annual forb

Dandelion

Taraxacum officinale

Exotic and native 

perennial forb

Rosette leaves are lobed.

Florets are yellow.

Foliage is not gray-green.

Stems are hollow, leafless, unbranched, and non-winged.

Flower heads are much larger and lack spines.

All parts of the plant exude a milky latex when torn.

Prickly lettuce

Lactuca serriola

Exotic annual or 

biennial forb

Rosette leaves are lobed.

Florets are yellow.

The flowering stem is often branched.

Stiff hairs extend all along the stem and leaf undersides.

Flower heads are much smaller and lack spines.

All parts of the plant exude a milky latex when torn.

Stems do not have wings.

Rush skeletonweed

Chondrilla juncea

Exotic perennial forb

Rosette leaves are lobed. 

Florets are yellow.

Foliage is not gray-green.

Stems lack wings and have stiff, downward-pointing hairs.

Flower heads are much smaller and lack spines. 

All parts of the plant exude a milky latex when torn.
Biology and Biological Control of Yellow Starthistle

Chapter 2: Getting to Know Yellow Starthistle 23

Table 2-2. Continued

Look Alike Appearance

Leaves Plant Flower Head Seeds

Biology and Biological Control of Yellow Starthistle

CHAPTER 3: BIOLOGY OF YELLOW STARTHISTLE 

BIOLOGICAL CONTROL AGENTS

Introduction

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

biocontrol agents are invertebrates in the kingdom Animalia and the phylum Arthropoda. More specifically, 

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

and moths), and Diptera (true flies). In addition to insects, there are also mites (arthropods in the class 

Arachnida), nematodes (kingdom Animalia and phylum Nematoda), and fungi (kingdom Fungi) biocontrol 

agents. Yellow starthistle biocontrol agents currently approved for use in North America include five species 

of insects (three beetles and two flies) and a rust fungus. One additional fly species was inadvertently 

introduced along with an approved species, and it is now widespread throughout the western USA. The 

taxonomic groups of yellow starthistle biocontrol agents are described in greater detail in the following 

sections.

Insects

Insects are the largest and most diverse class of animals. Basic knowledge of insect anatomy and life cycles 

will help in understanding insects and recognizing them in the field. Most insects used in weed biocontrol 

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

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

body divided into three regions (head, thorax, and abdomen, Figure 3-1b). Adult insects have three pairs 

of segmented legs attached to the thorax, and a head with one pair each of compound eyes and antennae.

Because insects have an external skeleton, they must shed their skeleton in order to grow. This process of 

shedding the exoskeleton is called molting. Larval stages between molts are called “instars.” Larvae of 

insects with complete metamorphosis generally complete three to five instars before they molt into pupae. 

During the pupal stage, insects change from larvae to adults. Insects do not feed or molt during the pupal 

stage. Adult insects emerge from the pupal stage and do not grow or molt.

Chapter 3: Biology of Yellow Starthistle Biological Control Agents 25
Biology and Biological Control of Yellow Starthistle

Beetles (Order Coleoptera)

Adult beetles are hard-bodied insects with tough exoskeletons. Adult beetles possess two pairs of 

wings. The two front wings, called elytra, are thickened and meet in a straight line down the abdomen, 

forming a hard, shell-like, protective covering (Figure 3-1b). The two hind wings are membranous 

and used for flight; these are larger and are folded under the elytra when not in use. Beetle larvae are 

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

white with a brown or black head capsule, though some may be quite colorful and change markedly 

in appearance as they grow. All beetles have chewing mouth parts.

Flies (Order Diptera)

Many insects have the word “fly” in their 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 name (e.g., butterfly in the order Lepidoptera and mayfly in the 

order Ephemeroptera). Adult true flies are easily distinguished from other orders of insects by their 

single pair of membranous wings and typically soft bodies (Figure 3-1a,b). Larvae of most true flies, 

called maggots, are legless and worm-like. 

Differentiating Beetles and Flies

An important part of any successful biocontrol program is to be able to identify the biocontrol agents 

in the field. As adults, the insects are relatively easy to identify, with their variable size, form, color, 

and habits. The larvae are more challenging to identify than the adults, and yet are probably more 

important to recognize as this is the stage that 1) does the damage, 2) is monitored in the field, and 3) 

provides the best evidence that the insects are established in the field.

As illustrated in Figure 3-2a, the simplest method to determine if a larva found in yellow starthistle is 

a beetle or a fly is to look for a head capsule. Beetle larvae are as variable as adult beetles are, but all 

have an obvious head capsule (Figure 3-2b). Fly larvae have no obvious head capsule. Fly larvae are 

sometimes confused with other larvae because they appear to have a broad, dark head (Figure 3-2c); 

however, this is actually a dark, hardened anal plate that is used to anchor the larva to its host.

Figure 3-3a illustrates the differences between beetles and flies in the pupal stage. Beetle pupae have 

a b

Figure 3-1. Line drawings of a. fly life cycle showing complete metamorphosis; b. fly and beetle anatomy: A. head, B. 

antenna, C. thorax, D. abdomen, E. wing (Credits: a L. Wilson, University of Idaho; b adapted from Biological Control 

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

26 Chapter 3: Biology of Yellow Starthistle Biological Control Agents
Biology and Biological Control of Yellow Starthistle

a b c

Figure 3-2. Differentiating beetle and fly larvae, a. line drawing key; b. beetle larva with an obvious head capsule; 

c. fly larva without an obvious head capsule (Credits: a L. Wilson, University of Idaho; b Charles Turner, USDA 

ARS, bugwood.org; c © Malcolm Storey, www.bioimages.org.uk)

Chapter 3: Biology of Yellow Starthistle Biological Control Agents 27

a b c

Figure 3-3. Differentiating beetle and fly pupae, a. line drawing key; b. beetle pupa with appendages; c. fly pupae 

contained in puparia (Credits: a L. Wilson, University of Idaho; b USDA ARS European Biological Control 

Laboratory, bugwood.org; c © Malcolm Storey, www.bioimages.org.uk)

well-developed appendages that are obviously not fused to the body (Figure 3-3b). Fly pupae are 

contained inside a puparium (Figure 3-3c).

Fungi

Fungi belong to their own kingdom (Fungi). The fungus described in this manual is a rust, which is in the 

phylum Basidiomycota. Rust fungi are obligate parasites; they require a living host to complete their life 

cycle. Rusts typically attack leaves and stems of the host plant. Rust infections usually appear as numerous 

rusty, orange, yellow, or even white colored spots (pustules) that rupture the leaf surface and release spores 

that resemble colored powder (typically yellow, orange, or brown). Most rust infections are local spots 

but some may spread internally through the plant. Rusts spread from plant to plant mostly by windblown 

spores, although insects, rain, and animals may aide in the transmission and infection process.

The life cycle of rust fungi can be very complicated. Rust fungi can produce up to five distinctive spore 

types which have different functions from infesting a new host plant, re-infecting the same host plant, 

producing pustules on infected plant leaves and stems, and overwintering.
Biology and Biological Control of Yellow Starthistle

Yellow Starthistle Weevils

All three yellow starthistle beetles are weevils, which are plant-feeding beetles with long snouts bearing 

chewing mouth parts at the tip. They use the snout to chew and feed on plant tissues, or to notch out holes 

in which to lay their eggs. Antennae are elbowed and attached to the snout approximately halfway along 

its length. 

Starthistle weevils complete one generation per year. The adults overwinter in protected areas on the ground 

and become active the following spring. Thus, the first weevils seen in spring are from the overwintering 

generation, and the new weevils that emerge in late summer are from the new generation.

As adults, weevils generally cause little damage to the plant. They may do some feeding of the foliage, 

but the amount is usually negligible. The exception is the hairy weevil (Eustenopus villosus) which feeds 

on starthistle buds, often killing the buds. For the other starthistle weevils, however, it is primarily the 

internally-feeding larval stage that causes damage. Larvae develop through three or four instars inside 

starthistle seed heads. Weevil larvae are distinctly white, C-shaped, and have a brown head capsule.

Table 3-1 provides details on the three weevils available for starthistle biocontrol as of 2016. Table 3-2 

compares life cycles of each of the weevils.

Table 3-1. Comparison of adult yellow starthistle weevils

Bangasternus orientalis Eustenopus villosus Larinus curtus

Emerges 1st (bolting) Emerges 2nd (BU-I) Emerges 3rd (BU-1)

Cylindrical body shape Cylindrical-oblong body shape Oblong body shape

4-6 mm 4-6 mm 5-6 mm

Brown with yellow/white hairs Brown with gray/white hairs Brown

Mottled appearance Striped Yellow-spotted

Non-hairy looking Hairy-looking Pollen-covered

Short snout Long, slender snout Medium-sized snout

Release at BU-1 to BU-3 stages Release at BU-2 to BU-4 stages Release at flowering stage

28 Chapter 3: Biology of Yellow Starthistle Biological Control Agents

Credits: Weevil line drawings (J. Johnson, University of Idaho); Photos (Laura Parsons & Mark Schwarzländer, University 

of Idaho)
Biology and Biological Control of Yellow Starthistle

Table 3-2. Comparison of weevil life cycles by yellow starthistle growth stages. BU 1-4 represent the four different bud 

stages (refer to Figure 2-7 to see pictures of each bud stage).

YST Stage Bangasternus orientalis Eustenopus villosus Larinus curtus

Seedling

Adults overwinter in ground litter

Rosette Adults overwinter in ground litter Adults overwinter in ground litter

Bolting Adults emerge; visible on plants

BU-1 Females oviposit; black egg cases 

visible

Adults emerge, feed on buds; 

visible on plant BU-2 Adults emerge; visible on plant Larvae develop; black egg cases 

visible

BU-3

BU-4

Larvae develop; find in seed head

Females oviposit. Feeding damage 

visible

Females oviposit

Flowering

Larvae develop; find in seed head

Seed formation Larvae develop; find in seed head

Mature Larvae pupate Larvae pupate Larvae pupate

Dissemination Adults emerge; visible on plant Adults emerge; visible on plant Adults emerge; visible on plant

Senescence Adults overwinter in ground litter Adults overwinter in ground litter Adults overwinter in ground litter

Chapter 3: Biology of Yellow Starthistle Biological Control Agents 29
Biology and Biological Control of Yellow Starthistle

Bangasternus orientalis (Capiomont)

Yellow starthistle bud weevil

Quick Facts

Order Coleoptera

Family Curculionidae

Native Distribution Eurasia, Mediterranean

Original Source Greece

First Release 1985

Non-target Effects None reported

Description

Eggs are tiny yellow spheres covered with dark egg caps (Figure 3-4a). Larvae are up to 6 mm long 

with white, C-shaped bodies and brown head capsules (Figure 3-4b). Adults are 4-6 mm long with 

somewhat flattened, cylinder-shaped bodies and short snouts (Figure 3-4c). Adult bodies are brown 

with yellow-white hairs that give them a mottled appearance.

a b

30 Chapter 3: Biology of Yellow Starthistle Biological Control Agents

c

Figure 3-4. Bangasternus orientalis, a. egg on plant stem (red arrow); b. larva in seed head; c. adult (Credits: a 

University of Idaho Archives; b California Department of Agriculture; c Laura Parsons & Mark Schwarzländer, 

University of Idaho)

Life Cycle

Overwintering adults resume activity in spring when yellow starthistle is bolting. Females lay eggs 

singly just below buds. They may lay up to 470 eggs in a lifetime. Eggs are covered with a dark, 

protective substance. Larvae hatch in late spring and tunnel through the stem to reach the developing 

flower head where they feed on bracts, receptacle tissue, and developing seeds. Larvae grow through 

four instars; the first instar mines the stem and bracts of the flower head while instars 2-4 occur inside 

the head and feed on the developing seeds. Pupation occurs within seed heads in chambers made of 

damaged seed and receptacle tissue. Adults emerge in late summer and overwinter in soil litter. There 

is one generation per year (Figure 3-5).

Habitat Preference

This weevil initially established throughout most yellow starthistle infestations in the western United 

States, indicating it is well-adapted to a variety of conditions. It appears to do better at mid-elevation 

sites where the weevil Eustenopus villosus is not already established or abundant than at hotter and 

drier locations.
Biology and Biological Control of Yellow Starthistle

Figure 3-5. Schematic life cycles of Bangasternus orientalis and yellow starthistle in North America. Bars indicate 

the approximate length of activity for each life stage; dates will vary depending on local conditions. Black bars 

represent the inactive overwintering period.

Damage

Though adults may feed on yellow starthistle stem wings and leaves, the feeding is superficial and 

does not significantly damage the plant. Larvae feed on seed head tissue and developing seeds. Seed 

consumption does not kill existing plants, but reduces seed production which may help decrease the 

rate of spread of yellow starthistle populations and may reduce infestations in some habitats.

Current Status and Availability

In the western United States (Figure 3-6), larval feeding typically destroys 60% of seeds within 

attacked seed heads. It was initially the most widespread of established biocontrol agents. However, 

densities of the weevil have been declining since their peak a few years after their original release. The 

current attack rate is only 1% of available capitula. Predation, parasitism, and displacement by other 

established biocontrol agents limit populations in many areas.

Chapter 3: Biology of Yellow Starthistle Biological Control Agents 31

a b

Figure 3-6. North American establishment of a. yellow starthistle; b. Bangasternus orientalis (Credits: a USDA 

PLANTS Database, EDDMapS; b Winston et al. 2014a)
Biology and Biological Control of Yellow Starthistle

Eustenopus villosus (Boheman)

Yellow starthistle hairy weevil

Quick Facts

Order Coleoptera

Family Curculionidae

Native Distribution Europe, Mediterranean

Original Source Greece

First Release 1990

Non-target Effects None reported

Description

Eggs are translucent, oval-shaped, and tiny (Figure 3-7a). Larvae are up to 6 mm long with white, 

C-shaped bodies and brown head capsules (Figure 3-7b). Adults are 4-6 mm long with oblong, 

cylindrical bodies and long, slender snouts (Figure 3-7d). Adult bodies are brown with lighter-colored 

longitudinal stripes and are covered with long hairs.

a b

32 Chapter 3: Biology of Yellow Starthistle Biological Control Agents

c d

Figure 3-7. Eustenopus villosus, a. egg between starthistle bracts (red arrow); b. larva and seed head damage; c. pupa; 

d. adult (Credits: a John Connett, University of Idaho; b,c J. Johnson, University of Idaho; d Laura Parsons & 

Mark Schwarzländer, University of Idaho)

Life Cycle

Overwintering adults emerge in spring when yellow starthistle is entering the spiny bud stage. They 

feed heavily on early immature starthistle buds. Females chew holes in the sides of mature, closed buds 

(pre-flower) in early to midsummer, lay eggs inside, and cap holes with a dark substance (Figure 3-8a). 

Larvae hatch throughout summer and feed on developing seeds through three instars. Pupation occurs 

in seed heads in chambers made of damaged seed and receptacle tissue (Figure 3-7c, 3-8c). Adults 

emerge in late summer and overwinter in soil litter. There is one generation per year (Figure 3-9). 

Habitat Preference

This weevil does well throughout the majority of conditions yellow starthistle has invaded in the USA. 

In California, it is most common in the foothills and less common in the Great Central Valley. 

Damage

Larvae feed on receptacle tissue and developing seeds. Adult feeding can lead to a high percentage of 

mortality of young flower buds (Figure 3-8b). Both forms of feeding do not kill existing plants, but 
Biology and Biological Control of Yellow Starthistle

a b c

Figure 3-8. Eustenopus villosus damage, a. oviposition hole; b. buds aborted from adult feeding; c. pupal chamber 

and chewed up florets, seeds, and receptacle tissue (Credits: a John Connett, University of Idaho; b Mark 

Schwarzländer, University of Idaho; c Charles Turner, USDA ARS)

Chapter 3: Biology of Yellow Starthistle Biological Control Agents 33

Figure 3-9. Schematic life cycles of Eustenopus villosus and yellow starthistle in North America. Bars indicate the 

approximate length of activity for each life stage; dates will vary depending on local conditions. Black bars 

represent the inactive overwintering period. 

reduce seed production which may help decrease the rate of spread of yellow starthistle populations 

and may reduce infestations in some habitats.

Current Status and Availability

Eustenopus villosus is abundant and widespread in the western USA (Figure 3-10) where larval feeding 

destroys up to 100% of seeds within attacked seed heads. Adult feeding also delays production of 

mature flowers by 2-3 weeks and often causes abortion of attacked young flower buds. The combined 

effect of larval and adult feeding has the largest total impact on yellow starthistle’s reproductive 

output compared with the other biological control insects. However, delayed flower production and 

mortality of the early flower buds reduces the plant’s synchronicity with and attractiveness to other 

ovipositing seed predators.

Eustenopus villosus consumes a slightly higher proportion of seeds in yellow starthistle seed heads not 

infected with the rust Puccinia jacea var. solstitialis. However, this difference is not significant when 

comparing overall seed production per plant; seed production for plants attacked by seed biocontrol 

agents is similar whether or not plants are infected with the rust. 
Biology and Biological Control of Yellow Starthistle

In conjunction with the fly, Chaetorellia succinea, E. villosus can reduce seed production by >70% 

overall. This is believed to have contributed to significant declines of yellow starthistle populations at 

some sites that have no disturbance and have competition from grasses and other forbs. No declines 

in yellow starthistle have been observed at several other areas, especially those with high disturbance 

such as along roadsides. At many sites, yellow starthistle plants compensate for any decreases in 

density by growing larger and producing more seed heads. Parasitism and predation may negate the 

impact of this weevil at some sites. 

a b

Figure 3-10. North American establishment of a. yellow starthistle; b. Eustenopus villosus (Credits: a USDA PLANTS 

Database, EDDMapS; b Winston et al. 2014a)

34 Chapter 3: Biology of Yellow Starthistle Biological Control Agents
Biology and Biological Control of Yellow Starthistle

Larinus curtus Hochhut

Yellow starthistle flower weevil

Quick Facts

Order Coleoptera

Family Curculionidae

Native Distribution Eurasia, Mediterranean

Original Source Greece

First Release 1992

Non-target Effects None reported

Description

Eggs are tiny, yellow, and elongate. Larvae are up to 6 mm long with white, C-shaped bodies and brown 

head capsules (Figure 3-11a). Adults are 5-6 mm long with oval-shaped bodies and medium-length 

snouts (Figure 3-11b). Adult bodies are brown with white hairs that give them a mottled appearance. 

The body hairs are often yellowish from yellow starthistle pollen.

a b

Figure 3-11. Larinus curtus, a. larvae; b. adult (Credits: a Charles Turner, USDA ARS, bugwood.org; b Laura 

Parsons & Mark Schwarzländer, University of Idaho)

Chapter 3: Biology of Yellow Starthistle Biological Control Agents 35

Life Cycle

Overwintering adults emerge in summer when yellow starthistle is in bud and flowering and feed on florets and 

pollen. Females lay eggs singly in open flower heads at the base of florets. Larvae hatch in late summer and feed 

on developing seeds through three instars. Pupation occurs in seed heads in chambers made of damaged seed 

and receptacle tissue. Adults emerge in late summer and overwinter in the soil litter. There is one generation per 

year (Figure 3-12). 

Habitat Preference

This weevil does well throughout the majority of conditions yellow starthistle has invaded in the USA.

Damage

Adults feed on pollen and florets, though the impact is typically minor. Larvae feed on flower head tissue and 

developing seeds. Seed consumption does not kill existing plants, but reduces seed production which may help 

decrease the rate of spread of yellow starthistle populations and may reduce infestations in some habitats.
Biology and Biological Control of Yellow Starthistle

Figure 3-12. Schematic life cycles of Larinus curtus and yellow starthistle in North America. Bars indicate the 

approximate length of activity for each life stage; dates will vary depending on local conditions. Black bars 

represent the inactive overwintering period. 

Current Status and Availability

In the western United States (Figure 3-13), larval feeding destroys up to 100% of seeds within attacked 

seed heads; however, attack rates are typically low. Abundance of this weevil plateaued within a 

few years of its introduction, and its current abundance varies from high in portions of Oregon, to 

moderate in Washington, to becoming more limited in Idaho and California. It is less abundant than the 

other seed-feeding biocontrol agents which have been unable to impact yellow starthistle population 

trajectories at many sites. Impact is highest at sites with significant competition from other plant 

species. Some L. curtus populations have been found to be infected with microsporidia (Nosema sp.), 

which are internal parasites of the digestive tract. It has been suggested that microsporidia may reduce 

survivorship and fecundity of infected adults; however, population-level impacts on L. curtus field 

populations are unknown. 

a b

Figure 3-13. North American establishment of a. yellow starthistle; b. Larinus curtus (Credits: a USDA PLANTS 

Database, EDDMapS; b Winston et al. 2014a)

36 Chapter 3: Biology of Yellow Starthistle Biological Control Agents

Non-target Effects

Adult L. curtus were found feeding on open flower heads of cultivated safflower (Carthamus tinctorius), 

but no egg laying or development occurred. Consequently, there has been no impact to safflower production.
Biology and Biological Control of Yellow Starthistle

Yellow Starthistle Flies

Flies used for yellow starthistle biological control are fruit flies; larvae of the flies eat the developing 

yellow starthistle seeds (fruit). Adult flies are small with short antennae and patterned wings. They obtain 

nectar from yellow starthistle and other flowers, but adults do not damage yellow starthistle plants. It is the 

internally-feeding larval stage that causes damage. Larvae develop through three instars inside yellow 

starthistle seed heads. Fly larvae are white, turning yellowish with maturity. They are legless, without 

an obvious head capsule, and slightly thicker at one end. 

All three yellow starthistle flies have two generations per year, although the Chaetorellia spp. may have 

three at sites with long growing seasons. Overwintering larvae pupate in old seed heads in the spring, and 

adults emerge to feed, mate, and lay eggs in newly-formed seed heads. The summer generation adults 

emerge during early to mid-summer to feed, mate, and oviposit eggs. Larvae from this generation typically 

overwinter in seed heads and finish development the following spring.

Table 3-3 provides a comparison of the three flies used for yellow starthistle biological control. Table 3-4 

compares life cycles of each of the flies.

Table 3-3. Comparison of adult yellow starthistle flies.

Chapter 3: Biology of Yellow Starthistle Biological Control Agents 37

Chaetorellia australis Chaetorellia succinea Urophora sirunaseva

4 black spots on thorax (2 per side) 6 spots on thorax (2 per side) Yellow spot on thorax

Straw-colored body Straw-colored body Black body

Straw-colored wing bands Straw-colored wing bands Black wing bands

2-3 generations per year 2-3 generations per year 2 generations per year

Female oviposits inner side lateral bracts Female oviposits inner side lateral bracts Female oviposits on top of buds

Release at BU-3 stage Accidental, not approved for release Release at BU-2 to BU-3

Credits: Fly line drawings (J. Johnson, University of Idaho); Photos (C. australis Charles Turner USDA ARS; C. succinea

and U. sirunaseva Laura Parsons & Mark Schwarzländer, University of Idaho)
Biology and Biological Control of Yellow Starthistle

Table 3-4. Comparison of fly life cycles by yellow starthistle growth stages. BU 1-4 represent the four different bud stages 

(refer to Figure 2-7 to see pictures of each bud stage).

YST Stage Chaetorellia australis Chaetorellia succinea Urophora sirunaseva

Seedling

Larvae overwinter in old seed 

heads

Larvae overwinter in old seed 

heads

Larvae overwinter in woody galls 

in seed heads Rosette

Bolting

Adults emerge and are visible 

on plants; sometimes utilizing 

Centaurea cyanus

Adults emerge and are visible 

on plants; sometimes utilizing 

Centaurea melitensis and C. cyanus

BU-1 Females oviposit between bracts of 

closed buds

Adults continue to emerge; females 

oviposit between bracts of closed 

buds

Adults emerge; visible on plant

BU-2 Larvae develop, feed on seeds; 

pupation begins

Larvae develop, feed on seeds; 

pupation begins Females oviposit in closed buds

BU-3 New generation adults emerge; 

oviposit in buds

New generation adults emerge; 

oviposit in buds

Larvae tunnel in and feed within 

galls; pupation begins in galls

BU-4 Adults still present on plants; new 

larvae develop, feed on seeds

Adults still present on plants; new 

larvae develop, feed on seeds

New generation adults emerge; 

oviposit on buds

Flowering Larvae continue to develop; feed 

on seeds

Larvae continue to develop; feed 

on seeds

Adults still present on plants; new 

larvae develop, feed within galls

Seed formation

Mature larvae found in seed heads; 

overwinter in seed heads

Mature larvae found in seed heads; 

overwinter in seed heads

Larvae continue to develop and 

feed within galls

Mature

Mature larvae overwinter in woody 

galls in seed heads Dissemination

Senescence

38 Chapter 3: Biology of Yellow Starthistle Biological Control Agents
Biology and Biological Control of Yellow Starthistle

Chaetorellia australis (Treitschke) & C. succinea (Costa)

Yellow starthistle peacock fly & False yellow starthistle peacock fly

Chaetorellia australis

Order Diptera

Family Tephritidae

Native Distribution Europe, Mediterranean

Original Source Greece

First Release 1988

Non-target Effects None reported

Chaetorellia succinea

Order Diptera

Family Tephritidae

Native Distribution Europe, Mediterranean

Original Source Greece

First Release 1991, Accidental

Non-target Effects Carthamus tinctorius

Description

Both species are very similar in appearance, with the exception of the spots on the thorax of adults. 

Eggs are elongated and white or pale yellow. Larvae are somewhat barrel-shaped (though slightly 

thicker at one end) and up to 4 mm long. They are white, turning yellowish with maturity, and 

lack an obvious head capsule (Figure 3-14a,d). Pupae are concealed in a barrel-shaped puparium up 

to 3.5 mm long and pale yellow with dark ends. Adults have tan bodies with black spots on the thorax; 

there are 8 spots on the thorax of C. australis while C. succinea has 10 (Figure 3-14b,e). Their eyes are 

multi-colored and metallic, and their wings are clear with thick brown bands (Figure 3-15c,f). Males 

are 3-4 mm long; females are 4-6 mm, including ovipositors.

a b

Chapter 3: Biology of Yellow Starthistle Biological Control Agents 39

c

d e f

Figure 3-14. Chaetorellia australis, a. larva, b. line drawing, c. adult; C. succinea d. larva, e. line drawing with extra 

spot on thorax (red arrow), f. adult with extra spot on thorax (red arrow) (Credits; a Gary Piper, Washington 

State University; b,e J. Johnson, University of Idaho; c Charles Turner, USDA ARS; d Rachel Winston, MIA 

Consulting; f Laura Parsons & Mark Schwarzländer, University of Idaho)

Life Cycle

Adults of both species emerge in spring before yellow starthistle has begun to bolt. Adults often 

feed on nectar of vetch (Vicia spp.) or other plants with early-developing flowers while waiting for 
Biology and Biological Control of Yellow Starthistle

their target plants to develop to the appropriate stage for oviposition. Adult females lay eggs singly 

beneath bracts of closed buds. After hatching, larvae tunnel into capitula and feed on receptacle tissue 

and developing seeds through three instars. Pupation occurs in puparia within flower heads inside 

chambers made of pappus and chewed seeds. Adults emerge in early to mid-summer, mate, and lay 

eggs in more starthistle buds. Larvae of the summer generation overwinter as mature larvae within 

starthistle seed heads. There are usually two generations, though three generations are possible where 

the growing season is sufficiently long (Figure 3-15, 3-16). 

Both C. australis and C. succinea adults often emerge too early to utilize yellow starthistle. Chaetorellia 

australis will frequently oviposit in buds of the earlier-flowering invasive weed bachelor’s button 

(Centaurea cyanus); adults emerging as part of the next generation will then utilize yellow starthistle. 

Likewise, C. succinea females will often oviposit in seed heads of C. cyanus and the invasive Maltese 

starthistle (C. melitensis) in spring, then move to yellow starthistle in the subsequent generation.

Figure 3-15. Schematic life cycles of Chaetorellia australis and yellow starthistle in North America. Bars indicate 

the approximate length of activity for each life stage; dates will vary depending on local conditions. Black bars 

represent the inactive overwintering period. There are typically two generations per year, sometimes three where 

growing seasons are long.

Figure 3-16. Schematic life cycles of Chaetorellia succinea and yellow starthistle in North America. Bars indicate 

the approximate length of activity for each life stage; dates will vary depending on local conditions. Black bars 

represent the inactive overwintering period. There are typically two generations per year, sometimes three where 

growing seasons are long.

40 Chapter 3: Biology of Yellow Starthistle Biological Control Agents
Biology and Biological Control of Yellow Starthistle

Habitat Preference

In the western United States, C. succinea occurs wherever yellow starthistle grows (Figure 3-17c). 

Chaetorellia australis (Figure 3-17b) does best where bachelor’s button (Centaurea cyanus) co-occurs 

with yellow starthistle. Both fly species prefer warm sites. Chaetorellia australis appears to do best at 

low-elevation sites, while elevation does not limit C. succinea populations.

a b

c

Figure 3-17. North American establishment of a. yellow 

starthistle; b. Chaetorellia australis; c. C. succinea

(Credits: a USDA PLANTS Database, EDDMapS; 

b,c Winston et al. 2014a)

Chapter 3: Biology of Yellow Starthistle Biological Control Agents 41

Damage

Larvae feed on developing seeds. Seed consumption does not kill existing plants, but reduces seed 

production which may help decrease the rate of spread of yellow starthistle populations and may 

reduce infestations in some habitats.

Current Status and Availability

Chaetorellia australis initially established only at sites where bachelor’s button, Centaurea cyanus, 

was present. At one successful release site where bachelor’s button was not established, it was 

determined the release inadvertently contained a mix of C. australis and the closely related C. succinea. 

Chaetorellia succinea subsequently spread rapidly throughout western USA. 

Chaetorellia australis is currently established on both bachelor’s button and yellow starthistle. Its 

abundance varies, often in relation to bachelor’s button presence. Attack rates are often higher on 
Biology and Biological Control of Yellow Starthistle

bachelor’s button, on which it can reduce seed production by up to 70% in Washington. It can destroy 

up to 90% of seeds in attacked yellow starthistle capitula; however, attack rates have typically not 

exceeded 10% of available capitula, likely due to poor synchrony in spring. 

Of the two species, C. succinea is considered the more effective biocontrol agent for yellow starthistle 

because it is widespread and often builds up high populations locally. Larval feeding destroys up 

to 80% of seeds within attacked yellow starthistle seed heads and decreases pollinator visitation. 

Chaetorellia succinea consumes a slightly higher proportion of seeds in yellow starthistle seed heads 

not infected with the rust Puccinia jacea var. solstitialis. However, this difference is not significant when 

comparing overall seed production per plant; seed production for plants attacked by seed biocontrol 

agents is similar whether or not plants are infected with the rust. Eustenopus villosus populations 

are associated with lower populations of C. succinea (and the other seed head insects), due to E. 

villosus causing delayed flower production and mortality of early flower buds, and also possibly due 

to E. villosus larvae killing larvae of other insects co-habiting the seed head. Further studies of these 

interactions and their outcomes are warranted. Despite this, C. succinea and E. villosus in combination 

can reduce yellow starthistle seed production by >70% overall. Whether this level of seed destruction 

results in a decline of yellow starthistle abundance depends on many factors. At many infestations, 

yellow starthistle plants compensate for any decreases in density by growing larger and producing 

more seed heads. Impact is highest at sites with significant competition from other plant species.

Non-target Effects

Chaetorellia succinea larvae were found feeding in the heads of commercial safflower (Carthamus 

tinctorius) in California, though damage was minimal. Field and laboratory trials documented C. 

succinea ovipositing and developing in the flower heads of Plectocephalus americana, a related 

genus indigenous to the central and southwestern United States. An application and permit (USDAAPHIS-PPQ 526 permit) to move C. succinea has not been submitted, and this species remains 

unauthorized for release or redistribution. It is likely that this fly will attack P. americana and its 

relative P. rothrockii in the field if the fly should reach their geographic range (Arizona to Missouri) 

through its own natural dispersal. 

Field and laboratory trials also found C. succinea to oviposit and develop on the weedy and exotic 

Maltese starthistle (Centaurea melitensis) and Sicilian starthistle (C. sulphurea). 

42 Chapter 3: Biology of Yellow Starthistle Biological Control Agents
Biology and Biological Control of Yellow Starthistle

Urophora sirunaseva (Héring)

Yellow starthistle gall fly

Quick Facts

Order Diptera

Family Tephritidae

Native Distribution Europe, Mediterranean

Original Source Greece

First Release 1984

Non-target Effects None reported

Description

Eggs are elongated and white or pale yellow. Larvae are somewhat barrel-shaped (though slightly 

thicker at one end) and up to 4 mm long. They are white, turning yellowish with maturity, and 

lack an obvious head capsule but have a dark anal plate (Figure 3-18a). Pupae are concealed in 

a barrel-shaped puparium up to 3.5 mm long and pale yellow with dark ends. Puparia are contained 

in woody galls (Figure 3-18a,b). Adults have black bodies with a yellow spot on the rear part of the 

thorax. Their eyes are multi-colored and metallic, and their wings are clear with dark crossbands 

(Figure 3-18c). Males are 3-4 mm long; females are 4-6 mm, including ovipositors.

a b

Chapter 3: Biology of Yellow Starthistle Biological Control Agents 43

c

Figure 3-18. Urophora sirunaseva, a. larva in gall; b. multiple woody galls in capitulum; c. adult (Credits: a,b Charles 

Turner, USDA ARS; c Laura Parsons & Mark Schwarzländer, University of Idaho)

Life Cycle

Adults emerge in spring as yellow starthistle is bolting and starting to form buds. Females lay eggs in 

immature, closed buds amongst the developing florets. After hatching, the larva follows a floret tube 

down to the ovule at the base of the floret. The larva enters the ovule (young achene) and the plant 

responds by beginning formation of a gall. Multiple galls will occur in one flower head. There are 

three larval instars, prior to pupation within galls. Adults emerge in early to mid-summer, mate, and 

lay eggs in more yellow starthistle buds. Larvae of the summer generation overwinter within galls 

inside the seed heads. There are two generations per year (Figure 3-19).

Habitat Preference

Specific habitat requirements are unknown for Urophora sirunaseva, but this fly does not seem to do 

well at overly windy locations. In California, U. sirunaseva is more common in the coastal mountains 

and less common in the interior of the state.
Biology and Biological Control of Yellow Starthistle

Figure 3-19. Schematic life cycles of Urophora sirunaseva and yellow starthistle in North America. Bars indicate 

the approximate length of activity for each life stage; dates will vary depending on local conditions. Black bars 

represent the inactive overwintering period. There are typically two generations per year, sometimes three where 

growing seasons are long.

44 Chapter 3: Biology of Yellow Starthistle Biological Control Agents

Damage

Floret feeding and gall formation reduce seed production. Galls are believed to act as nutrient sinks, 

diverting plant resources away from regular plant function and seed formation.

Current Status and Availability

The first introductions occurred on yellow starthistle in California beginning in 1969, utilizing flies 

collected in Italy. After multiple releases failed to establish, it was determined the species introduced 

was in fact Urophora jaculata, which is specific to yellow starthistle biotypes growing only in Italy. 

The true U. sirunaseva was subsequently released in the USA beginning in 1984. 

In the western United States (Figure 3-20), gall formation decreases yellow starthistle seed production, 

though multiple galls are required per seed head before seed reduction is significant, and high gall 

density per capitulum is not common. While this species is widely distributed, abundance is low. Attack 

rates peaked at approximately 50% a few years following their successful establishment. More recently, 

attack rates are closer to 10% of yellow starthistle seed heads. Consequently, its overall impact is now 

limited. Populations at some sites are hindered by competition with other seed head agents. 

a b

Figure 3-20. North American establishment of a. yellow starthistle; b. Urophora sirunaseva (Credits: a USDA 

PLANTS Database, EDDMapS; b Winston et al. 2014a)
Biology and Biological Control of Yellow Starthistle

Puccinia jaceae var. solstitialis (Savile)

Yellow starthistle rust fungus

Quick Facts

Class Pucciniomycetes

Order Pucciniales

Native Distribution Eurasia, Mediterranean

Original Source Turkey

First Release 2003

Non-target Effects None reported

Description and Life Cycle

The fungus produces up to five spore stages throughout the growing season. In the spring, overwintering 

spores germinate and infest yellow starthistle rosette leaves, forming yellowish chlorotic lesions with 

raised centers. These turn into orangish-brown pustules that produce large amounts of dry, powdery, 

round, spores that are rusty or dark brown in coloration (Figure 3-21). These spread rapidly from plant 

to plant; they are easily dispersed by both wind and rain. The entire process can take two weeks to 

complete, and multiple cycles may be produced throughout the year.

a b

Chapter 3: Biology of Yellow Starthistle Biological Control Agents 45

c

Figure 3-21. Puccinia jaceae var. solstitialis infecting yellow starthistle (Credits: a,b Eric Coombs, Oregon Department 

of Agriculture, bugwood.org; c Stephen Ausmus, USDA ARS, bugwood.org)

Habitat Preference

The yellow starthistle rust fungus requires climates with dew periods that result in moisture forming 

or collecting on starthistle foliage.

Damage

Infected plants can experience stunted growth, reduced seed production, and a higher susceptibility 

both to competition from other plants and attack from additional biocontrol agents.

Current Status and Availability

The yellow starthistle rust fungus is established only in California and Oregon (Figure 3-22). Under 
Biology and Biological Control of Yellow Starthistle

optimal conditions (moist, mild temperatures) the rust can reduce yellow starthistle biomass and the 

number of capitula, especially in conjunction with high plant competition. At drier sites, impact is 

decreased and likely to be of only minor biological significance. Across much of yellow starthistle’s 

range, suboptimal climatic conditions prevent the rust’s persistence and/or significant impact.

a b

Figure 3-22. North American establishment of a. yellow starthistle; b. Puccinia jaceae var. solstitialis (Credits: a 

USDA PLANTS Database, EDDMapS; b Winston et al. 2014a)

46 Chapter 3: Biology of Yellow Starthistle Biological Control Agents
Biology and Biological Control of Yellow Starthistle

CHAPTER 4: ELEMENTS OF A YELLOW STARTHISTLE 

BIOLOGICAL CONTROL PROGRAM

Before You Begin

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

is not appropriate for areas where yellow starthistle is not present or where a small number of localized 

populations occur. Biological control as a control method is best suited to yellow starthistle populations in 

the later phases of the invasion curve, where yellow starthistle populations are experiencing a rapid increase 

in distribution and abundance (containment), or where yellow starthistle is widespread and abundant 

throughout its potential range (asset based protection, Figure 1-3 repeated here in Figure 4-1).

Figure 4-1. Generalized invasion curve showing actions appropriate to each stage (Credit: © State of Victoria, 

Department of Economic Development, Jobs, Transport and Resources, Reproduced with permission)

Chapter 4: Elements of a Yellow Starthistle Biological Control Program 47
Biology and Biological Control of Yellow Starthistle

The results of using biological control to treat yellow starthistle may vary greatly from site to site for 

a variety of reasons. Land managers should develop treatment programs that complement management 

activities and objectives unique to the area. This is accomplished by first understanding the scope of the 

yellow starthistle problem, defining overall goals for the yellow starthistle management program, and 

understanding the control methods available for accomplishing the goals. 

Determining the Scope of the Problem

The first step should be to develop a distribution map of yellow starthistle at a scale that will allow 

you to address the problem in a manner consistent with your overall land-management objectives and 

available weed management resources. The most appropriate scale may encompass a large landscape 

with a variety of site characteristics and land uses managed by many different land owners/managers– all 

of whom contribute to mapping efforts (Figure 4-2a). In large management areas with significant yellow 

starthistle infestations and limited resources, aerial mapping of large patches of yellow starthistle may 

be sufficient to identify priority areas for additional survey, mapping, and weed management activities. 

In other management areas with smaller, more discrete yellow starthistle infestations, or where an 

infestation’s characteristics affect your ability to meet management objectives, your weed management 

strategy might have to include more extensive mapping and analysis of the scope of the infestations (e.g., 

size, density, cover, or location in relation to roads and waterways over time) (Figure 4-2b).

a b

Figure 4-2. Yellow starthistle data for a. USDA Forest Service land with yellow starthistle infestations in the state of 

Oregon; b. infestations in Oregon’s Whitman National Forest (Credits: a,b Becca VanKampen, MIA Consulting, 

EDDMapS)

48 Chapter 4: Elements of a Yellow Starthistle Biological Control Program

In many cases, it may prove useful to check for existing yellow starthistle distribution data before 

collecting your own. Several agencies and organizations maintain weed distribution databases, 

including state agricultural departments, provincial ministries (e.g., British Columbia IAPP 

Application), invasive plant/species councils, USDA PLANTS database, EDDMapS, and many 

others. EDDMapS can be particularly useful for land mangers interested in creating yellow starthistle 

distribution maps for their area. By visiting www.eddmaps.org and creating a free account, users can 

view existing distribution maps for yellow starthistle or other weeds at the state, county, or point 

level. By selecting the GIS view option, users can view yellow starthistle data on various backgrounds 

and zoomed into different scales, add hand drawn labels, boundaries, points and other shapes to the 

map, perform measurements such as perimeter estimates or distance between points, add new yellow 

starthistle data from user shapefiles, edit the management status of various infestations, and print 

finished maps (see page 68 for more information on EDDMapS).
Biology and Biological Control of Yellow Starthistle

Defining Goals and Objectives

Goals broadly define the “what” or desired outcome of management; objectives define the “how” or 

specific activities through which desired outcomes can be achieved. To be effective, objectives must 

be SMART: specific, measurable, achievable, realistic, and timely. Defining your weed management 

goals and objectives is the crucial first step in developing a successful biological control program. By 

defining what you want to achieve, you will be able to determine if, when, and where you should use 

biological control.

As precisely as possible, you must define what will constitute a successful yellow starthistle 

management program. For example, the objective of “. . . a noticeable reduction in yellow starthistle 

density over the next ten years . . . ” might be achievable, but it uses a subjective measurement of 

success that is open to observer bias. Alternatively, the objective of “. . . a 50% reduction in yellow 

starthistle stems over the next three years . . .” is objectively measurable (and therefore SMART). If 

your goal is to reduce the abundance of yellow starthistle or slow its rate of spread, then biological 

control might be an appropriate weed management tool; however, by itself biological control will not 

completely and permanently remove yellow starthistle from the landscape. If your goal is to eradicate 

yellow starthistle, then you should plan to employ other weed control techniques instead of, or in 

addition to, biological control (see Chapter 5 for more details).

Understanding Yellow Starthistle Management Options

Once you determine the scope of your yellow starthistle infestations and define your overall program 

goals, review all the weed control methods available (biological control, physical treatments, cultural 

practices, and herbicides), and determine the conditions (when, where, if, etc.) under which it might 

be appropriate to use each method or combination of methods (see Chapter 5). Consult commercial, 

agency, or university biological control experts, cooperative weed management area partners, or 

county weed coordinator/supervisors to learn about other yellow starthistle management activities 

(herbicide use, grazing, etc.) underway or planned for your area, and the level and persistence of 

control that might be achieved by each.

Identify the resources that will be available for weed management activities, and determine if they will 

be consistently available until you meet your weed management program objectives. If resources are 

not currently available, or will not be available consistently, identify what will happen at the treatment 

site if planned management activities are not implemented. This information will help you determine 

the best management activities to use as you initiate and continue your integrated yellow starthistle 

management program. 

With a map of yellow starthistle infestations in your management area, an understanding of your land 

management goals, well defined weed management objectives, and a list of the weed control methods 

available with the level of control you can realistically expect from each, you can identify sites where 

biological control would be a good fit, alone or in combination with other control methods.

Developing, Implementing, and Managing a Yellow Starthistle 

Biological Control Program

When biological control is deemed suitable for treating your yellow starthistle infestations, there are several 

important factors to consider. These include selecting appropriate release sites, obtaining and releasing 

biocontrol agents, and monitoring the success of the program. Familiarity with all aspects of a biocontrol 

Chapter 4: Elements of a Yellow Starthistle Biological Control Program 49
Biology and Biological Control of Yellow Starthistle

program before beginning will greatly facilitate its implementation and increase its chances of success. 

These items are discussed in their own sections below. If problems are encountered following the initiation 

of a biological control program, refer to the troubleshooting guide in Appendix I for potential solutions.

Selecting Biological Control Agent Release Sites

Establish Goals for Your Release Site

You must consider your overall management goals for a given site when you evaluate its suitability 

for the release of biological control agents. Suitability factors will differ depending on whether the 

release is to be a 

1. general release, where biological control agents are simply released for yellow starthistle 

management;

2. field insectary (nursery) release, used primarily to mass produce biological control agents for 

redistribution to other sites; or 

3. research release, used to investigate biological control agent biology and/or the biocontrol 

agent’s impact on the target weed and non-target plant community.

A site chosen to serve one of the roles listed above may also serve additional functions over time (e.g., 

biocontrol agents might eventually be collected for redistribution from a research or general release).

Determine Site Characteristics

For practical purposes, no yellow starthistle infestation is too large for biocontrol releases; however, 

it might not be large enough (Figure 4-3a). Very small, isolated patches of yellow starthistle may 

not be adequate for biological control agent populations to build up and persist and are often better 

treated with other weed control methods, such as physical control or herbicides. An area with at 

least 1 acre (0.4 ha) of yellow starthistle is the minimum size to better ensure a successful biological 

control agent release site, but larger infestations are more desirable (Figure 4-3b), especially if the 

land manager hopes to someday use the release site as a field insectary. However, smaller infestations 

may be acceptable release sites in some cases, such as critical habitat zones where disturbance from 

physical control would be detrimental or sites where herbicides are prohibited. If the yellow starthistle 

populations are extensive within a region but the individual population is below an acre, biocontrol 

agents can be released to establish populations and encourage spread throughout the region. In 

addition, control of yellow starthistle may be considered a low priority in some regions and is often 

overlooked for intensive management. In these cases, land managers may wish to use biocontrol as a 

way to reduce further weed spread. Nevertheless, biocontrol agents disperse more easily in contiguous 

yellow starthistle infestations than in infestations with only a few scattered plants and distant patches. If 

your biological control program goals involve evaluating the program’s efficacy, establish permanent 

monitoring sites before you release any biocontrol agents. The monitoring sites will require regular 

inspections, so consider the site’s ease of accessibility, terrain, and slope.

Note Land Use and Disturbance Factors

Release sites should experience little to no regular disturbance. Abandoned fields/pastures, vacant 

lots, and natural areas are good choices for biological control agent releases. Sites where insecticides 

are used should not be utilized for biocontrol agent releases. Such sites include those near wetlands 

that are subject to mosquito abatement, rangelands that are subjected to grasshopper control, or 

50 Chapter 4: Elements of a Yellow Starthistle Biological Control Program
Biology and Biological Control of Yellow Starthistle

a b

Figure 4-3. Yellow starthistle infestations, a. too small for biological control (single plant); b. appropriate for 

biological control (Credits: a John M. Randall, The Nature Conservancy, bugwood.org; b Charles Turner, USDA 

ARS, bugwood.org)

Chapter 4: Elements of a Yellow Starthistle Biological Control Program 51

infestations near agricultural fields or orchards where pesticide applications occur regularly. Roadside 

infestations along dirt or gravel roads with heavy traffic should also be avoided; extensive dust makes 

yellow starthistle plants less attractive to biocontrol agents. Do not use sites where significant land 

use changes will take place, such as road construction, cultivation, building construction, and mineral 

or petroleum extraction. If supply of biocontrol agents is limited, prioritize release sites that are not 

regularly mowed, burned, or treated with herbicides.

Survey for Presence of Biological Control Agents

Always examine your prospective release sites to determine if yellow starthistle biological control 

agents are already present. If a biocontrol agent you are planning to release is already established 

at a site, you may want to consider making the release at another site where the biocontrol agent is 

not yet present. If observed biocontrol agent populations are low at a site, you can release additional 

biocontrol agents at that site to augment the existing population. 

Record Ownership and Access

If you release biological control agents on private land, it is a good idea to select sites on land likely 

to have long-standing, stable ownership and management. Stable ownership will help you establish 

long-term agreements with a landowner, permitting access to the sites to sample or harvest biological 

control agents and collect biocontrol agent 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 biocontrol agent harvesting or data collection. General releases 

of biological control agents to control yellow starthistle populations require less-frequent and shortterm access; you may need to visit such a site only once or twice after initial release. When releasing 

biocontrol agents on private land, it may be a good idea to obtain the following:

• written permission from the landowner allowing use of the area as a release site

• written agreement with the landowner allowing access to the site for monitoring and collection for a 

period of at least six years (three years for establishment and buildup and three years for collection)

• permission to put a permanent marker at the site

• written agreement with the landowner that land management practices at the release site will not 

interfere with biological control agent activity
Biology and Biological Control of Yellow Starthistle

The above list can also be helpful for releases made on 

public land where the goal is to establish an insectary. 

In particular, an agreement should be reached that land 

management practices will not interfere with biological 

control agent activity (e.g., chemically spraying or 

physically destroying the weed infestation). It is often 

useful to visit the landowner or land manager at the 

release site annually to ensure they are reminded of the 

biological control endeavors and agreement. Always 

re-check with the landowner prior to inspecting release 

sites; in some cases the ownership may have changed.

You may wish to restrict access to release locations, 

especially research sites and insectaries, and allow only 

authorized project partners to visit the sites and collect 

biocontrol agents. The simplest approach is to select 

locations that are not visible to or accessible by the 

general public. To be practical, most if not all of your 

sites will be readily accessible, so in order to restrict 

access you should formalize arrangements with the 

landowner or manager. This will require you to post notrespassing signs, install locks on gates, etc. (Figure 4-4).

Figure 4-4. “No disturbance” sign (Credit: 

Alan Martinson, Latah County Weed 

Control & Paul Brusven, Nez Perce 

BioControl Center)

52 Chapter 4: Elements of a Yellow Starthistle Biological Control Program

Another consideration is physical access to a release site. You will need to drive to or near the release 

locations, so determine if travel on access roads might be interrupted by periodic flooding or inclement 

weather. You might have to accommodate occasional road closures by private landowners and public 

land managers for other reasons, such as wildlife protection.

Choosing the Appropriate Biological Control Agents for Release

You should consider several factors when considering which biological control agent to release at a 

site, including biocontrol agent efficacy, availability, and site preferences (Table 4-1).

Biocontrol Agent Efficacy

Efficacy refers to the ability of the biological control agent to directly or indirectly reduce the 

population of the target weed below acceptable damage thresholds or cause weed mortality resulting 

in control. It is preferable to release only the most effective biocontrol agents rather than releasing all 

biocontrol agents that might be available for a target weed. Consult with local weed biological control 

experts, neighboring land managers, and landowners to identify the biocontrol agent(s) that appear(s) 

more effective given local site characteristics and management scenarios.

Biocontrol Agent Availability 

All six of the USA-approved biological control agents described in this manual are established in 

the continental USA; however, availability varies greatly between species and sites (Table 4-1). The 

unapproved Chaetorellia succinea is the most widespread of all yellow starthistle biocontrol agents, 

and care should be taken when collecting any other species to ensure C. succinea is not collected along 

with approved biocontrol agents. The hairy weevil Eustenopus villosus is also very widespread and 

effective on yellow starthistle in the USA and is available for collection in Arizona, California, Idaho, 
Biology and Biological Control of Yellow Starthistle

Biocontrol Agent Characteristics Site Characteristics

Species

Part

Attacked

Efficacy Availability

Favorable

Conditions

Unfavorable

Conditions

Bangasternus 

orientalis

Yellow starthistle 

bud weevil

Receptacle tissue 

and seeds

Destroys 60% seeds in attacked 

seed heads, but attacks only 1% of 

seed heads; hindered by predation, 

parasitism, and competition with 

other established biocontrol agents

Initially 

widespread, but 

now limited in 

abundance in 

CA, ID, OR, 

WA and UT

Mesic climates; 

sites where 

Eustenopus villosus

is absent or 

limited

Cold or hot/dry 

climates; sites with 

high predator, 

parasite, and/or 

Eustenopus villosus

populations

Chaetorellia 

australis

Yellow starthistle 

peacock fly

Receptacle tissue 

and seeds

Destroys up to 90% seeds in 

attacked seed heads, but usually 

attacks ≤10% of seed heads; limited 

by poor synchrony with yellow 

starthistle in spring

Abundance in 

CA, ID, OR 

and WA varies 

in relation 

to bachelor’s 

button presence

Sites with 

populations of 

bachelor’s button; 

warm, lowelevation sites

Sites with no 

bachelor’s button 

available in 

spring; cold, highelevation sites

Chaetorellia 

succinea

False yellow 

starthistle 

peacock fly

Receptacle tissue 

and seeds

Destroys up to 80% seeds in 

attacked seed heads and decreases 

pollinator visits; in conjunction 

with E. villosus, reduces seed 

produced by >70% overall; 

most effective at sites with high 

interspecific plant competition

Abundant in 

CA, ID, NV, 

OR and WA. 

Not approved 

for release or 

redistribution

Does well under 

conditions 

found at most 

yellow starthistle 

infestations in the 

USA

Sites with no 

alternative 

Centaurea species 

or other nectarproducing species 

available in spring; 

cold sites

Eustenopus villosus

Yellow starthistle 

hairy weevil

Receptacle tissue 

and seeds

Destroys up to 100% seeds in 

attacked seed heads and causes bud 

abortion; in conjunction with C. 

succinea, reduces seed produced 

by >70% overall; most effective at 

sites with high interspecific plant 

competition

Abundant in 

AZ, CA, ID, 

OR, WA and 

UT

Does well under 

conditions 

found at most 

yellow starthistle 

infestations in the 

USA

Sites with high 

predator and/

or parasite 

populations

Larinus curtus

Yellow starthistle 

flower weevil

Receptacle tissue 

and seeds

Destroys up to 100% seeds in 

attacked seed heads, but seed 

head attack rates are typically low; 

hindered by parasitism

Abundant in 

portions of 

OR, moderate 

in WA, 

becoming more 

limited in ID 

and CA

Does well under 

conditions 

found at most 

yellow starthistle 

infestations in the 

USA

Sites with Nosema 

sp. microsporidia 

present

Puccinia jaceae var. 

solstitialis

Yellow starthistle 

rust

All aboveground growth

Reduces biomass and number of 

seed heads, but only under moist, 

mild conditions. At dry sites, 

impact is limited. Most effective at 

sites with high interspecific plant 

competition. 

Abundance 

limited in CA 

and OR

Moist, mild 

temperatures

Hot, dry sites 

with limited dew 

periods

Urophora sirunaseva

Yellow starthistle 

gall fly

Receptacle tissue 

and seeds

Gall formation decreases seed 

production; multiple galls per 

seed head are needed before seed 

reduction is significant; attacks 

typically only 10% of seed heads

Though widely 

distributed 

in CA, ID, 

OR and WA, 

abundance is 

low

Sites where other 

biocontrol agents 

are limited or 

absent

Windy locations; 

sites with high 

populations of 

other biocontrol 

agents

Chapter 4: Elements of a Yellow Starthistle Biological Control Program 53

Table 4-1. Summary of general characteristics and site preferences of yellow starthistle biological control agents released 

in North America
Biology and Biological Control of Yellow Starthistle

Oregon, Washington, and Utah. The weevils Bangasternus orientalis and Larinus curtus and the flies 

Chaetorellia australis and Urophora sirunaseva are all established in California, Idaho, Oregon, and 

Washington, but they vary in their abundance and impact. Bangasternus orientalis (also established 

in Utah) and U. sirunaseva were initially more widespread but are both now limited in abundance. 

Chaetorellia australis is most abundant at sites where bachelor’s button (Centaurea cyanus) is also 

established. Larinus curtus abundance is high in portions of Oregon, moderate in Washington, and 

becoming more limited in Idaho and California. The rust Puccinia jaceae var. solstitialis is the least 

abundant biocontrol agent; it is sparsely established on yellow starthistle only in California and Oregon.

Federal and state departments or commercial biological control suppliers may be able to assist you 

in acquiring biocontrol agents not yet available but permitted for use in your area (see Obtaining and 

Releasing Yellow Starthistle Biological Control Agents, below). In the USA, state departments of 

agriculture, county weed managers, extension educators, or federal and university weed biological 

control specialists should be able to recommend in-state collection sites where appropriate. Remember 

that in the USA, interstate transport of biological control agents requires a USDA-APHIS-PPQ 526 

Permit (see Regulations for the Transfer of Yellow Starthistle Biological Control Agents, page 66). 

Get your permits early to avoid delays.

Release Site Characteristics 

General physical site and biological preferences for each biocontrol agent have been developed from 

anecdotal observations and experimental data. These are listed in Table 4-1 to help land managers 

ensure that biocontrol agents are released at sites with suitable conditions. 

Obtaining and Releasing Yellow Starthistle Biological Control Agents

You can obtain yellow starthistle biological control agents by collecting or rearing them yourself, 

having someone collect them for you, or by purchasing them from a commercial supplier. This section 

provides information on collecting and purchasing yellow starthistle biocontrol agents.

Factors to Consider When Looking for Sources of Biological Control Agents

You do not need to take a “lottery approach” and release all approved biological control agents at 

a site in the hopes that one of them will work. Some biological control agents will not be available 

even if you want them, and some are already widespread and/or have been shown to have little or no 

effectiveness in certain areas. The best strategy is to release the best biocontrol agent. Ask the county, 

state, or federal biological control experts in your area for recommendations of biocontrol agents for 

your particular project.

If available, biological control agents from local sources are best. Using local sources increases 

the likelihood that biocontrol agents are adapted to the climate and site conditions present and are 

available at appropriate times for release at your target infestation. Using locally sourced biological 

control agents also reduces the possibility of accidentally introducing biocontrol agent pathogens 

or natural enemies to your area. Local sources may include neighboring properties or locations in 

adjacent counties/districts. Remember that in the USA, interstate transport of biological control agents 

requires a USDA-APHIS-PPQ 526 Permit (see Regulations Pertaining to the Transfer of Yellow 

starthistle Biological Control Agents, page 66). Get your permits early to avoid delays.

Some USA states, counties, and universities have “field days” at productive insectary sites (Figure 

4-5). On these days, land managers and landowners are invited to collect or receive locally collected 

yellow starthistle biological control agents for quick release at other sites. These sessions are an easy 

54 Chapter 4: Elements of a Yellow Starthistle Biological Control Program
Biology and Biological Control of Yellow Starthistle

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 any impacts the various biocontrol agents 

might be having on yellow starthistle plant communities. 

a b c

Figure 4-5. Yellow starthistle field day including, a. indoor instruction; b. field tour; c. biocontrol agent collection 

(Credits: a-c Paul Brusven, Nez Perce BioControl Center)

Chapter 4: Elements of a Yellow Starthistle Biological Control Program 55

Typically, field days are conducted at several sites in a state and on several dates. Although designed 

for intrastate collection and redistribution, out-of-state participants may be welcome to participate 

(remember that USDA PPQ 526 Permits are required for interstate movement and release of biological 

control agents). Contact county weed supervisors, university weed or biological control specialists, or 

federal weed managers for information about field days in your region.

Collecting Yellow Starthistle Biological Control Agents

Planning and timing of collection is critical. For all species, it is usually most efficient to scout the 

potential collection site well in advance to ensure your desired species is present at suitable densities. 

The species of biological control agent and weather characteristics at your collection and release site 

will determine the best time in the season to collect. Ensure that all necessary collection supplies are 

on hand. Also, accurate identification of the biological control agents is essential. General guidelines 

for collecting yellow starthistle biological control agents are listed in the following sections and in 

Table 4-2. 

For all yellow starthistle insects, collect only on a day with good weather. Do not collect in the rain; 

flying insects will not be active during rain, and crawling insects will hide and become difficult to 

find in rainy weather. In addition, excess moisture causes adverse effects, and biocontrol agents may 

drown in wet collection containers. The only exception to this rule is the rust fungus Puccinia jaceae 

var. solstitialis, for which overcast and rainy days are optimal for collection.
Biology and Biological Control of Yellow Starthistle

Table 4-2. Recommended timetable and methods for collecting yellow starthistle biological control agents in North 

America. Methods are listed in the order of ease of collection and efficacy. Refer to Chapter 2 for detailed descriptions 

of yellow starthistle stages. Plant and biocontrol agent stages will vary by location.

BIOCONTROL

AGENT

BIOCONTROL

AGENT STAGE PLANT STAGE TIMING METHOD

Bangasternus orientalis

Yellow starthistle bud 

weevil

Adult BU-1 to BU-3 Late May to late 

June

Sweep or tap adults from foliage 

in afternoon and aspirate into 

collection container

Chaetorellia australis

Yellow starthistle 

peacock fly

Larva; or Adult

Flowering to 

senescence; or 

bolting to BU-1 or 

BU-3 to BU-4

August to March; 

or sweep adults in 

April/May or June/

July

Rear indoors; or sweep adults from 

foliage and aspirate into collection 

container

Chaetorellia succinea

False yellow starthistle 

peacock fly

N/A N/A N/A

This species is not approved for 

release or redistribution in the 

USA

Eustenopus villosus

Yellow starthistle 

hairy weevil

Adult BU-2 to BU-4 June to July

Sweep or tap adults from foliage 

in afternoon and aspirate into 

collection container

Larinus curtus

Yellow starthistle 

flower weevil

Larva, Pupa BU-4 to mature Late June to 

September

Rear indoors; only Nosema-free 

populations (lab-reared) are 

permitted for release in the USA

Puccinia jaceae var. 

solstitialis

Yellow starthistle rust

All stages Seedling to mature

Whenever green 

yellow starthistle 

plant material is 

present

Vacuum spores, suspend in water, 

and spray on uninfected leaves prior 

to dew period; or move infected 

plant stems to uninfected sites

Urophora sirunaseva

Yellow starthistle 

gall fly

Larva; or Adult

Seed formation 

to senescence; or 

bolting to BU-2 or 

BU-4 to flowering

August to March; 

or sweep adults in 

May or June/July

Rear indoors; or sweep adults from 

foliage and aspirate into collection 

container

56 Chapter 4: Elements of a Yellow Starthistle Biological Control Program

Collection methods

Sweep netting: Using a sweep net is the best method for collecting the weevils Bangasternus 

orientalis and Eustenopus villosus. Though sweeping can be damaging to adult flies, it is also an 

alternative method for collecting Chaetorellia australis and Urophora sirunaseva. Extreme caution 

should be used when sweeping for adult C. australis, however, to ensure that the very similar but 

unapproved fly C. succinea is not collected and redistributed. 

A sweep net consists of a conical canvas or muslin bag held open on one end by a sturdy wire hoop 

10-15 inches (25-38 cm) in diameter attached to a handle 3 feet (0.9 m) long (Figure 4-6a,b). They 

can be purchased from entomological, forestry, and biological supply companies, or you can construct 

them yourself. As their name implies, these are heavy duty nets used to “sweep” insects off yellow 

starthistle. A sweep is made by swinging the net through the plant canopy and collecting insects off 

the foliage. It is best to use no more than 25 sweeps (10 sweeps for delicate biocontrol agents such as 

adult flies) before removing the biocontrol agents from the net. Removing insects at regular intervals 
Biology and Biological Control of Yellow Starthistle

reduces the potential harm that could result from knocking biocontrol agents around with debris, and 

reduces the opportunity for predator insects and spiders swept up with the biocontrol agents from 

incapacitating or devouring the biocontrol agents.

a b

Figure 4-6. Sweep net, a. closeup; b. being used to collect yellow starthistle biocontrol agents (Credits: a Laura 

Parsons, University of Idaho; b Rachel Winston, MIA Consulting)

Chapter 4: Elements of a Yellow Starthistle Biological Control Program 57

Tapping: If a sturdy sweep net is not available (see above), adult Bangasternus orientalis and 

Eustenopus villosus can be collected using a tool such as a racket to tap the biocontrol agents off of 

their host plant foliage and into a tray placed strategically beneath the stem being tapped. Biocontrol 

agents thus tapped off the foliage can then be gathered directly using an aspirator (see below). Avoid 

disturbing the yellow starthistle before tapping because this will often cause beetles to drop to the 

ground or fly away. This method is effective for yellow starthistle beetles but is not appropriate for 

collecting the yellow starthistle flies.

Aspirating: An aspirator is a device used to suck insects from a surface into a collection vial (Figure 

4-7a). An aspirator is used to collect insects out of a sweep net (described above), though it can also be 

used to remove adult Bangasternus orientalis and Eustenopus villosus directly from yellow starthistle 

plants. A variety of aspirators can be purchased from entomological, forestry, and biological supply 

companies, or you can construct them yourself. For the latter, inline filters (e.g., HEPA filters) are 

commercially available to prevent unintentional inhalation or swallowing of particles or debris during 

mouth aspiration. At the very least, make sure that tubing reaching your mouth is covered by finemesh screening, so that insects and small particles are not inhaled (Figure 4-7b).

a b

Figure 4-7. Aspirator, a. components; b. diagram (Credits: a wiki.bugwood.org; b Karen Loeffelman, University of 

Idaho, bugwood.org)
Biology and Biological Control of Yellow Starthistle

Rearing out biocontrol agents: The preferred collection method for delicate insects such as moths 

or flies is to rear adults out indoors. This method is also useful for biocontrol agents plagued by 

parasitoids or microorganisms/diseases, such as the Nosema-affected weevil Larinus curtus. Yellow 

starthistle seed heads infested with larvae or pupae of L. curtus, C. australis, and U. sirunaseva can be 

collected in the fall or winter and stored at 39-46°F (4-8°C). Two to three weeks prior to their normal 

emergence time, bring them to room temperature in rearing cages or breathable, clear containers. 

Any parasitoids that emerge should be separated and destroyed. Emerging biocontrol agents can be 

transferred to new yellow starthistle patches during the appropriate plant stage. Care should be taken 

to ensure that all emerging Chaetorellia flies are indeed C. australis and not the very similar but 

unapproved C. succinea.

Suspending rust spores: Rust fungus spores can 

be vacuumed from infected yellow starthistle leaves 

throughout the growing season (Figure 4-8), suspended 

in a carrier (typically distilled water and a surfactant and 

sprayed on new, uninfected yellow starthistle foliage prior 

to a dew period. Because the methods for spore collection, 

suspension, and application are varied and more consistent 

with a bioherbicide, we do not attempt to describe them in 

depth in this manual. For more information, contact your 

local biocontrol specialist.

Transferring infected stems: An alternative method for 

collecting the rust fungus is to transfer infected stems to 

uninfected sites. Infected stems can be cut, bundled in 

groups of 20-50, and moved to new sites where the rust 

is not yet established. See the section “Release as many 

biocontrol agents as possible” on page 65 for detailed 

instructions on the proper way to utilize bundled plants 

at new yellow starthistle sites. Care should be taken not 

to spread yellow starthistle seeds to new sites as this may 

exacerbate the yellow starthistle problem. Care should also 

be taken to avoid spreading other plant or insect species to 

new sites as this may inadvertently create future problems.

Methods by species

Bangasternus orientalis: Collect adult beetles by sweeping yellow starthistle foliage or tapping stems 

over trays with tools such as rackets to dislodge the insects. Adult B. orientalis should then be aspirated 

from the net or tray. Collect the bud weevils during the heat of the day. The optimal time to collect is 

in spring when yellow starthistle is in the bud stage (BU-1 to BU-3) and when adult B. orientalis are 

currently or have recently finished mating. Depending on location, the collecting period is generally 

from late May to late June. Because this beetle has been decreasing in abundance and efficacy in the 

USA, it is not a high priority for redistribution. 

Chaetorellia australis: The preferred method for obtaining C. australis is to rear the fly out indoors. 

Yellow starthistle seed heads infested with larvae can be collected in the fall or winter and stored at 

39-46°F (4-8°C). Two to three weeks prior to their normal emergence time (April/May), bring them to 

room temperature in rearing cages or breathable, clear containers. Any parasitoids that emerge should 

be separated and destroyed. Emerging C. australis adults can be transferred to new yellow starthistle 

Figure 4-8. Vacuuming yellow starthistle spores 

(Credit: Stephen Ausmus, USDA ARS)

58 Chapter 4: Elements of a Yellow Starthistle Biological Control Program
Biology and Biological Control of Yellow Starthistle

patches during the bolting to BUI-1 stages. Care should be taken to ensure that all emerging flies 

are indeed C. australis and not the very similar but unapproved C. succinea. Refer to Chapter 3 

for tips on differentiating the two species. 

Alternatively, C. australis adults can be collected from yellow starthistle (they are present during the 

late bolting or late bud stages of yellow starthistle). Depending on location, the two adult collection 

periods are generally April/May or June/July. Sweeping can be very damaging to the delicate adult 

flies, so this method is less desirable than rearing adults out indoors. In addition, differentiating 

between C. australis and the unapproved C. succinea can be very difficult in the field, making this 

collection method even less desirable. Any adult Chaetorellia collected in the field should be properly 

identified in a laboratory setting before redistributions occur. Because C. australis has an overall 

lower efficacy and is often limited to sites where bachelor’s button (Centaurea cyanus) is also present, 

it is a low priority for redistribution. 

Eustenopus villosus: Collect adult beetles by sweeping yellow starthistle foliage or tapping stems 

over trays with tools such as rackets to dislodge the insects. Adult E. villosus should then be aspirated 

from the net or tray. Collect the hairy weevils during the heat of the day. The optimal time to collect is 

in spring and early summer when yellow starthistle is in the bud stage (BU-2 to BU-4) and when adult 

E. villosus are currently or have recently finished mating. Depending on location, the collecting period 

is generally from June to July. This weevil is already widespread throughout much of western North 

America, but it should be redistributed to yellow starthistle sites where it is not already established.

Larinus curtus: Many populations of this species are plagued by Nosema microsporidia that likely 

reduce its reproductive output and life span. Only Nosema-free (lab reared) individuals are approved 

for redistribution. In most cases, it will be necessary to obtain the beetles from research or professional 

rearing operations. 

Rearing healthy beetles out indoors is also possible with assistance from biocontrol experts that have 

experience with Nosema. Yellow starthistle seed heads infested with larvae can be collected in the fall 

or winter and stored at 39-46°F (4-8°C). Two to three weeks prior to their normal emergence time 

(June/July), bring them to room temperature in rearing cages or breathable, clear containers. Any 

parasitoids that emerge should be separated and destroyed. Emerging L. curtus adults can be transferred 

to new yellow starthistle patches during the bud to early flowering stages. Because this beetle has been 

decreasing in abundance and efficacy in the USA, it is not a high priority for redistribution.

Puccinia jaceae var. solstitialis: Spores of this rust fungus can be vacuumed from infected yellow 

starthistle leaves throughout the growing season, suspended in a carrier (typically distilled water and 

a surfactant) and sprayed on new, uninfected yellow starthistle foliage prior to a dew period. Because 

the methods for spore collection, suspension, and application are varied and more consistent with a 

bioherbicide, we do not attempt to describe them in this manual. For more information, contact your 

local biocontrol specialist.

Alternatively, the rust fungus can be transferred to new sites via infected stems. Infected stems can 

be cut, bundled in groups of 20-50, and moved to new sites where the rust is not yet established. See 

the section “Release as many biocontrol agents as possible” on page 65 for detailed instructions on 

the proper way to utilize bundled plants at new yellow starthistle sites. Care should be taken not to 

spread yellow starthistle seeds to new sites as this may exacerbate the yellow starthistle problem. 

Care should also be taken to avoid spreading other plant or insect species to new sites as this may 

inadvertently create future problems. Because establishment and efficacy of this rust are limited by 

Chapter 4: Elements of a Yellow Starthistle Biological Control Program 59
Biology and Biological Control of Yellow Starthistle

climatic conditions, it is only a high priority for redistribution at sites with moist, mild climates with 

significant periods of dew and/or fog.

Urophora sirunaseva: The preferred method for obtaining U. sirunaseva is to rear the fly out indoors. 

Yellow starthistle seed heads infested with larvae can be collected in the fall or winter and stored at 

39-46°F (4-8°C). Two to three weeks prior to their normal emergence time (May), bring them to room 

temperature in rearing cages or breathable, clear containers. Any parasitoids that emerge should be 

separated and destroyed. Emerging U. sirunaseva adults can be transferred to new yellow starthistle 

patches during the BUI-1 to BU-2 stages. 

Alternatively, U. sirunaseva adults can be collected from yellow starthistle (they are present during 

bolting to early bud stage and the late bud/early flowering stages of yellow starthistle). Depending 

on location, the two adult collection periods are generally May and June/July. Sweeping can be very 

damaging to the delicate adult flies, so this method is less desirable than rearing adults out indoors. 

Because this fly has been decreasing in abundance and efficacy in the USA, it is not a high priority 

for redistribution.

Release Containers for Yellow Starthistle Biological Control Agents

The manner in which biological control agents are handled during transportation to the release site 

will affect whether they will survive and multiply at the new site. To reduce mortality or injury, it is 

best to redistribute the biocontrol agents the same day they are collected.

Following collection, biocontrol agents should be transferred to release containers intended to 

protect them (and to prevent them from escaping en route). When large sections of infected stems are 

transferred between sites to redistribute Puccinia jaceae var. solstitialis, the stems should be stored in 

large paper bags (minus all flowers and seeds). Paper bags provide sufficient ventilation while plastic 

bags may cause moist plant material to rot. Though paper bags are appropriate for rust-infected stems, 

they are less ideal for transferring yellow starthistle weevils and flies because they offer little physical 

protection for the material within, must be sealed tightly to prevent biocontrol agents from escaping, 

and some biocontrol agents are capable of chewing through them.

When transferring lab-reared or field-collected adult B. orientalis, E. villosus, L. curtus, C. australis 

or U. sirunaseva, release containers should 

be rigid enough to resist crushing but also 

ventilated to provide adequate airflow and 

reduce condensation. Un-waxed paperboard 

cartons are ideal; they are rigid, permeable to 

air and water vapor, and are available in many 

sizes. As an alternative, you can use release 

containers made of either light-colored lined 

or waxed paper (e.g., ice cream cartons are 

particularly suitable, see Figure 4-9) or plastic, 

providing they are ventilated; simply poke 

numerous holes in the container or its lid with 

an ordinary push pin or thumb tack, and cover 

the holes with a fine mesh screen. Do not 

use glass or metal release containers; they 

are breakable and make it difficult to regulate 

temperature, airflow, and humidity.

Figure 4-9. Cardboard release containers for transporting 

yellow starthistle biocontrol agents (Credit: Martin 

Moses, University of Idaho, bugwood.org)

60 Chapter 4: Elements of a Yellow Starthistle Biological Control Program
Biology and Biological Control of Yellow Starthistle

Fill release containers half full with crumpled paper towels or tissue paper to provide a substrate for 

biocontrol agents to rest on and hide in, and to help regulate humidity. Fill the other half with yellow 

starthistle sprigs. All sprigs should be free of seeds, flowers, dirt, spiders, and other insects and should 

not be placed in water in the release container. Seal the release container lids with masking or label 

tape or with tightly fitting rubber bands. If you are using paper bags, fold over the tops several times 

and staple them shut. Be sure to label each container with (at least) the biological control agent(s) 

name, the number of biological control agents in the container, the collection date and site, and the 

name of the person(s) who did the collecting. 

Transporting Yellow Starthistle Biological Control Agents

Keep the containers cool at all times 

Once you collect and package the biocontrol agents, maintain them at temperatures between 50 and 

65°F (10-18°C). If possible, place the release containers in large coolers equipped with frozen ice 

packs. Do not use ice cubes unless they are contained in a separate, closed, leak-proof container. 

Wrap the ice packs in crumpled newspaper or bubble wrap to prevent direct contact with release 

containers and to absorb any condensation that forms. Place extra packing material in coolers to 

prevent ice packs from shifting and damaging biocontrol agent containers. As an alternative to coolers 

with ice packs, electric car-charged coolers may be utilized, provided the cycle is set to cool and not 

warm. Always keep coolers out of direct sun, and only open them when you are ready to release the 

biocontrol agents. If you cannot release them immediately, place them in a refrigerator for short-term 

storage (no lower than 40°F or 4.4°C) until you transport or ship them (which should occur as soon as 

possible and preferably not longer than 48 hours).

Transporting short distances 

If you can transport biocontrol agents to their release sites within three hours of collection, and release 

them the same day or early the next, you need not take any measures other than those already described.

Shipping long distances

If you will be shipping your biocontrol agents to their final destination, use a bonded carrier service with 

guaranteed overnight delivery (e.g., USPS, FedEx, UPS, or DHL) and send the recipient the tracking 

number. In such cases, the release containers should be placed in insulated shipping containers with 

one or more ice packs. Some specially 

designed foam shippers have pre-cut slots 

to hold small biocontrol agent containers 

and ice packs (Figure 4-10). This 

construction allows cool air to circulate 

but prevents direct contact between the 

ice and the release containers. Laboratory 

and medical suppliers sell foam 

“bioshippers” that are used to transport 

medical specimens or frozen foods. If 

neither foam product is available, you 

can use a heavy-duty plastic cooler, 

which also may be better suited to large 

yellow starthistle stems infected with the 

rust. Please note that for safety reasons, 

dry ice cannot be used for transporting 

biocontrol agents.

Figure 4-10. Commercially made shipping container suitable 

for biocontrol agent transport (Credit: University of Idaho, 

bugwood.org)

Chapter 4: Elements of a Yellow Starthistle Biological Control Program 61
Biology and Biological Control of Yellow Starthistle

Careful packaging is very important regardless of the shipping container you use. Ice packs need to be 

wrapped in crumpled newspaper, wrapping paper, or bubble wrap, and should be firmly taped to the 

inside walls of the shipping container to prevent them from bumping against and possibly crushing 

the release containers during shipping. Empty spaces in the shipping container should be loosely filled 

with crumbled or shredded paper, bubble wrap, packing “peanuts,” or other soft, insulating material. 

Use enough insulation to prevent release containers and ice packs from shifting during shipment, 

but not so much that air movement is restricted. Enclose all paperwork accompanying the biocontrol 

agents (including copies of permits and release forms) before sealing the shipping container. For 

additional security and protection, you may place the sealed shipping containers or coolers inside 

cardboard boxes.

Other factors to consider

• Make your overnight shipping arrangements well before you collect your biological control agents, 

and make sure the carrier you select can guarantee overnight delivery.

• Plan collection and packaging schedules so that overnight shipments can be made early in the 

week. Avoid late-week shipments that may result in delivery on Friday through Sunday, potentially 

delaying release of the biocontrol agents for several days.

• Clearly label the contents of containers and specify that they contain perishable material.

• Check with a prospective courier to make sure that they can accept this type of cargo and will 

not treat the packages in ways that could harm the biological control agents. If the courier cannot 

guarantee that such treatments will not occur, choose a different carrier.

• Provide the receiver with a tracking number and verify someone will be there to accept the shipment.

• Releases should be made immediately upon receipt. If that is not possible, biocontrol agents should 

be checked for food depletion, excess moisture, and overcrowding and then be refrigerated.

• Have the receiver provide feedback to the shipper on the overall condition of the shipment. This can 

provide important guidance on packing/shipping methods.

62 Chapter 4: Elements of a Yellow Starthistle Biological Control Program
Biology and Biological Control of Yellow Starthistle

Avoiding Common Packaging Mistakes

Crushing- Secure all material included in the shipping container so that blue ice, bundles of 

plant material, etc. do not become loose and move around in transit thereby crushing, tearing 

or popping open release containers and killing or scattering the biocontrol agents inside.

Escape- Seal release containers securely with rubber bands or easily removable/resealable 

tape (e.g., masking tape) to prevent mobile biocontrol agents from escaping into the shipping 

container.

Excess heat- Do not expose release containers to direct sunlight or temperatures above 65°F 

(18°C). Avoid shipping delays that can expose biocontrol agents to high temperatures.

Excess moisture- Remove spilled or excess water in release and shipping containers. Do 

not ship weed sprigs with any type of water source (e.g., floral foam or tubes) inside release 

containers. Add crumpled paper towels to release containers to absorb incidental moisture or 

condensation.

Lack of ventilation- Provide adequate ventilation; use air-permeable release containers or 

make air holes in plastic containers with push pins or other small diameter tools, covering the 

holes with a fine mesh screen to prevent the escape of mobile biocontrol agents. 

Starvation- Provide sufficient food, and do not store release containers with biological control 

agents more than 48 hours. 

Stress- Provide root-, flower- and seed-free sprigs of the target weed (free also of other weed 

species’ seeds, flowers, dirt, spiders, or other insects) and crumpled paper towels where 

biocontrol agents can shelter; avoid over-crowding.

Chapter 4: Elements of a Yellow Starthistle Biological Control Program 63

Purchasing Yellow Starthistle Biological Control Agents

A number of commercial suppliers provide yellow starthistle biological control agents. In the USA, 

county weed managers, extension educators, or university weed or biological control specialists may 

be able to recommend one or more suppliers. Make sure that a prospective supplier is reputable, can 

provide healthy individuals of the species you want (parasite- and pathogen-free), and can deliver 

them to your area at a time appropriate for field release (you will want to know where and when the 

biocontrol agents were collected). Avoid purchasing biocontrol agents from a supplier who collects 

biocontrol agents from an environment significantly different from your planned release location. 

Interstate shipments of yellow starthistle biological control agents by commercial suppliers also 

require a USDA PPQ 526 Permit (see page 66), a copy of which should be enclosed in the shipping 

box. It is the responsibility of the person receiving and releasing biocontrol agents to secure the 

required permits, though some vendors will help buyers with this process. Confirm in advance that 

there is a permit in place for the species you are acquiring as well as the region in which the release 

will occur. DO NOT purchase or release unapproved or non-permitted biological control organisms. 

Note that before any biocontrol agents can be taken across national borders, whether collected or 

purchased, an importation permit from the regulatory agency of the receiving country is required 

(USDA-APHIS in the USA and CFIA in Canada).
Biology and Biological Control of Yellow Starthistle

Releasing Yellow Starthistle Biological Control Agents

Establish permanent location marker 

Place a steel fence post or plastic/fiberglass pole as a marker at the release point (Figure 4-11a). Avoid 

wooden posts; they are vulnerable to weather and decay. Markers should be colorful and conspicuous. 

White, bright orange, pink, and red are preferred over yellow and green, which may blend into 

surrounding vegetation. In addition, white posts will not fade over time. Where conspicuous posts 

may encourage vandalism, mark your release sites with short, colorful plastic tent/surveyor’s stakes 

or steel plates that can be tagged with release information and located later with a metal detector and 

GPS. Depending on the land ownership or management status at the release site, it may be necessary 

to attach a sign to the post or pole indicating a biological control release has occurred there and that the 

site should not be sprayed with chemicals or be mechanically disturbed (see Figure 4-4 on page 52). 

Where a sign is appropriate, the landowner/land manager and the local weed management authority 

(county, state, federal, and/or provincial) should be notified and given a map of the release location.

Record geographical coordinates at release point using GPS 

Map coordinates of the site marker should be determined using a global positioning system device 

(GPS) or a GPS-capable tablet/smartphone. There are numerous free apps available for recording GPS 

coordinates on a tablet/smartphone (Figure 4-11b). Coordinates should complement but not replace 

a physical marker. Accurate coordinates will help re-locate release points if markers are damaged or 

removed. Along with the coordinates, be sure to record what coordinate system and datum you are 

using, e.g., latitude/longitude in WGS 84 or UTM in NAD83.

Prepare map 

The map should be detailed 

and describe access to 

the release site, including 

roads, trails, and unique 

landmarks/terrain features 

that are not likely to change 

through time (e.g., large 

rocks or rocky outcrops, 

creeks, valleys, etc.). Avoid 

using ephemeral landmarks 

such as “red bush”, “grazing 

cows”, etc. and descriptors 

which may not be obvious 

to everyone, such as “the 

Miller place”, or “where 

the old barn used to be”, etc.. Use your vehicle’s trip odometer to measure and record mileage between 

specified locations on your map, e.g., when you turn on to a new road, at cattle guards along the 

route, and where you park. The map should complement but not replace a physical marker and GPS 

coordinates. Maps are especially useful for long-term biological control programs in which more than 

one person will be involved or participants are likely to change. Maps are often necessary to locate 

release sites in remote locations or places physically difficult or confusing to access.

Complete relevant paperwork at site 

Your local land management agency/authority may have standard biocontrol agent release forms for 

a b

Figure 4-11. Biocontrol agent release site tools, a. permanent marker; b. 

smartphone with free weed and biocontrol agent mapping app iBioControl 

(Credits: a,b Rachel Winston, MIA Consulting)

64 Chapter 4: Elements of a Yellow Starthistle Biological Control Program
Biology and Biological Control of Yellow Starthistle

you to complete. Typically, the information you provide includes a description of the site’s physical 

location, including GPS-derived latitude, longitude, and elevation; a summary of its biological and 

physical characteristics and land use; the name(s) of the target weed and biocontrol agent(s) released; 

the number and life cycle stage of the agent(s) released; date and time of the release; weather conditions 

during the release; and the name(s) of the person(s) who released the biocontrol agents (see Sample 

Biological Control Agent Release Form in Appendix II). The best time to record this information is 

while you are at the field site. Consider using a smartphone and reporting app such as iBioControl. 

This free application uses EDDMapS (see page 68 for more information) to help county, state, and 

federal agencies track releases and occurrences of biological control agents of noxious weeds. Once 

back in the office, submit the information to your local weed control office, land management agency, 

or other relevant authority/database. Always keep a copy for your own records.

Set up photo point

A photo point is used to visually document changes in yellow starthistle infestations and other 

components of the plant community over time following the release of biocontrol agents. Use a 

permanent feature in the background as a reference point (e.g., a mountain, large rocks, trees, or a 

permanent structure) and make sure each photo includes your release point marker. Pre- and postrelease photographs should be taken from roughly the same place and at the same time of year. 

Label all photos with the year and location; many smartphone and tablet apps such as GrassSnap or 

Theodolite do this automatically or with minimal input.

Release as many biocontrol agents as possible 

As a general rule of thumb, it is better to release many individuals of a biocontrol agent species at 

one yellow starthistle infestation than it is to spread those individuals too thinly over multiple yellow 

starthistle infestations. Releasing all the biocontrol agents within a release container in one spot will 

help ensure that adequate numbers of males and females are present for reproduction and reduce the 

risks of inbreeding and other genetic problems. Guidelines for a minimum release size are uncertain 

for most biocontrol agents, but releases of 200 adult yellow starthistle biocontrol insects (or more) are 

encouraged.

Often, a single release will be sufficient to establish a biocontrol agent population, especially if a large 

number of individuals are released. The only way to determine if biocontrol agents have established is 

to inspect release sites annually for up to 5 years (or more) after releases are made. Additional releases 

may be necessary if initial releases fail to establish. For species or locations where establishment is 

likely to be slow (e.g., due to high levels of overwintering mortality), planning to make releases on 

the same site for two or three consecutive years may increase successful establishment and reduce the 

time until biocontrol agent impact on target weed populations is visible. If more than one release of a 

biocontrol agent is available in a given year, be sure to put some distance between releases; 2/3 mile 

(1 km) is ideal. If possible, make more than one release per drainage or in adjoining drainages; if one 

of your releases is wiped out by flooding, fire, herbicide application or other catastrophic disturbance, 

then biocontrol agents from adjoining releases can repopulate it. 

In general, you can release biocontrol agents either in open releases or cages. For open releases, get 

to the desired release location and open the release container. When releasing adult yellow starthistle 

weevils and flies, gently shake out all biocontrol agents in one small area, taking care to dislodge 

any insects hiding in or clinging to the paper towels in the release containers (Figure 4-12a). Do not 

scatter biocontrol agents throughout the infestation. Do not walk back over the area where you 

just made a release.

Chapter 4: Elements of a Yellow Starthistle Biological Control Program 65
Biology and Biological Control of Yellow Starthistle

When Puccinia jaceae var. solstitialis spores have been vacuum-collected from yellow starthistle 

foliage, a suspension can be made by combining spores with a carrier (typically distilled water and a 

surfactant) and sprayed on new (uninfected) yellow starthistle foliage prior to a dew period (Figure 

4-12b). Temporary “tents” can be used to maintain humidity after the inoculation, aiding in the 

establishment of the rust (Figure 4-12c).

When using the alternative method of transferring large yellow starthistle stems infected with the rust,

first ensure collected stems have no flowers or seeds or other insects or plant species. Take bundles of 

20-50 stems and remove the ties on one end of each bundle so that stems can be fanned out at the loose 

a b c

Figure 4-12. Releasing yellow starthistle biocontrol agents, a. releasing adult weevils; b. inoculating yellow starthistle 

plants with a suspension of the rust fungus; c. tent used to increase humidity following yellow starthistle rust 

inoculation (Credits: a Rachel Winston, MIA Consulting; b,c Eric Coombs, Oregon Department of Agriculture, 

bugwood.org)

66 Chapter 4: Elements of a Yellow Starthistle Biological Control Program

Regulations for the Transfer of Yellow Starthistle 

Biological Control Agents

USA, intrastate Generally, there are few if any restrictions governing the collection and shipment of approved 

biological control agents within the same state; however, you should check with your state’s department of 

agriculture or agriculture extension service about regulations governing the release and intrastate transport 

of your specific biological control agent. The state of California regulates release permits at the county 

level. It is illegal to redistribute unapproved species in the USA.

USA, interstate The interstate transportation of biological control agents is regulated by the U.S. Department 

of Agriculture (USDA), and a valid permit is required to transport living biological control agents across 

state lines. You should apply for a Plant Protection and Quarantine (PPQ) permit from the Animal and Plant 

Health Inspection Service (APHIS) as early as possible—but at least six months before actual delivery 

date of your biological control agent. You can check the current status of regulations governing intrastate 

shipment of weed biological control agents, PPQ Form 526 at the USDA-APHIS-PPQ website. The 

ePermit process can be accessed by doing an internet search for “USDA APHIS 526 permit application”. 

This allows the complete online processing of biological control agent permit requests. It is illegal to 

redistribute unapproved species across state lines in the USA.

Canada Canada requires an import permit for any new biological control agent or shipment of previouslyreleased biocontrol agents entering the country. These permit requests are reviewed and issued by the Plant 

Health Division of the Canadian Food Inspection Agency. Redistribution within a province (or within 

Canada) of weed biological control agents that have been officially approved for release in Canada is not 

prohibited; however, you should consult with federal and provincial authorities and specialists prior to 

moving any weed biological control agent, especially across ecozones (e.g., from the prairies to the interior 

or coast of British Columbia). Similarly, you should consult with appropriate experts when considering the 

movement of adventive biocontrol agents that have become established in a region, or native organisms 

that may feed on a weed targeted for control.
Biology and Biological Control of Yellow Starthistle

end, providing a supportive base. Place the fanned bundles upright within dense stands of uninfected 

yellow starthistle. In less dense yellow starthistle infestations or at windy locations, tying the fanned 

bundle against uninfected yellow starthistle plants may aid in successful establishment. Four to five 

rust-infected bundles should be used per site, though more or fewer may be required, depending on 

the yellow starthistle infestation size. The transfer should take place in the evening, and uninfected 

starthistle plants should be sprayed with water to increase inoculation success.

Caged releases confine biocontrol agents for a period of time so they adjust to the site and easily find 

one another for mating. Cages may help increase establishment success at new locations, but they 

require you to put up and take down equipment. For caged releases, place a mesh bag over a yellow 

starthistle plant or a small area containing multiple plants. Release the adult weevils or flies inside 

the cage, and secure the bottom of the cage to either the yellow starthistle stem or the ground. Cages 

should be removed within a few days (for plants) or weeks (for areas).

Releases of adult weevils or flies should be made under moderate weather conditions (mornings 

or evenings of hot summer days, mid-day for cold season releases). Making releases under these 

conditions reduces the immediate dispersal of stressed insects when they are dumped out of release 

containers; appropriately timed releases can significantly enhance the probability of establishment. 

Milder temperatures are also more conducive to successful rust establishment. Avoid making releases/

transfers on rainy days, unless dealing with the rust, which is aided by moist conditions. If you 

encounter an extended period of poor weather, it is better to release the biological control agents than 

wait three or more days for conditions to improve as the biocontrol agents’ vitality may decline with 

extended storage. Avoid transferring biocontrol agents to areas with obvious ant mounds or ground 

dwelling animals that may prey upon some species of biocontrol agents.

Documenting, Monitoring, and Evaluating a Biological Control Program

The Need for Documentation 

The purpose of monitoring is to evaluate the success of your yellow starthistle biological control 

program and to determine if you are meeting your weed management goals. Documenting outcomes 

(both successes and failures) of biocontrol release programs will help generate a more complete 

picture of biocontrol impacts, guide future management strategies, and serve education and public 

relations functions. Monitoring can provide critical information for other land managers by helping 

them predict where and when biological control might be successful, helping them avoid releasing 

ineffective biocontrol agents or the same biocontrol agent in an area where they were previously 

released, and/or helping them avoid land management activities that would harm local biocontrol 

agent populations or worsen the yellow starthistle problem. (See the Code of Best Practices for 

Classical Biological Control of Weeds on page 7).

Monitoring activities utilize standardized procedures over time to assess changes in populations of 

the biocontrol agents, yellow starthistle, other plants in the community, and other components of the 

community. Monitoring can help determine:

• if the biological control agents have become established at the release site

• if biological control agent populations are increasing or decreasing and how far they have spread 

from the initial release point

• if the biological control agents are having an impact on yellow starthistle

• if/how the plant community or site factors have changed over time

Chapter 4: Elements of a Yellow Starthistle Biological Control Program 67
Biology and Biological Control of Yellow Starthistle

Monitoring methods can be simple or complex. A single year of monitoring may demonstrate whether 

the biocontrol agents established, while multiple years of monitoring may allow you to identify trends 

in the population of the biocontrol agents, changes in the target weed population and plant community, 

and changes in other factors such as climate or soil.

Information Databases

Many federal and state/provincial departments have electronic databases for archiving information 

about weed biological control releases. We have included a standardized biological control agent 

release form that, when completed, should provide sufficient information for inclusion in any number 

of databases (see Appendix II).

The USDA Forest Service (in conjunction with the University of Georgia, MIA Consulting, University 

of Idaho, CAB International, and the Queensland Government) also maintains a worldwide database 

for the Biological Control of Weeds: A World Catalogue of Agents and their Target Weeds. The 

database includes entries for all weed biocontrol agents released to date, including the year of first 

release within each country, the biocontrol agents’ current overall abundance and impact in each 

country, and more. This database can be accessed at www.ibiocontrol.org/catalog/.

EDDMapS (Early Detection & Distribution Mapping System) is a web-based mapping system 

increasingly being used for documenting invasive species as well as biocontrol agent distribution in 

North America. EDDMapS combines data from existing sources (e.g., databases and organizations) 

while soliciting and verifying volunteer observations, creating an inclusive invasive species geodatabase 

that is shared with educators, land managers, conservation biologists, and beyond. Information can 

be added in online forms through home computers and/or apps created for smartphones. For more 

information on how to utilize or contribute to these tools, visit www.eddmaps.org/about/ and apps.

bugwood.org/.

In addition, some states/provinces have county/district weed departments or employ weed biocontrol 

specialists, often affiliated with state/province departments of agriculture, county extension offices, or 

Animal and Plant Health Inspection Service Plant Protection and Quarantine (APHIS-PPQ) offices. 

Contact local entities for more information.

Monitoring Methods 

There are three main components to measure in a yellow starthistle monitoring program: biological 

control agent populations, yellow starthistle populations, and the rest of the plant community (including 

non-target plants). More detailed monitoring might also examine effects on other biotic community 

components (such as other insects, birds, mammals, etc.) or abiotic factors (such as erosion, soil 

chemistry, etc.). Only the three main monitoring components are discussed in this manual. 

Assessing biological control agent populations

If you wish to determine whether yellow starthistle biocontrol agents have established after initial 

release, you simply need to find the biocontrol agents in one or more of their life stages, or evidence 

of their presence (Table 4-3). Begin looking for biocontrol agents where they were first released, and 

then expand to the area around the release site. 

68 Chapter 4: Elements of a Yellow Starthistle Biological Control Program
Biology and Biological Control of Yellow Starthistle

Table 4-3. Life stages/damage to look for to determine establishment of yellow starthistle biological control agents. a. Bangasternus 

orientalis egg on plant stem; b. B. orientalis larva in seed head; c. Chaetorellia australis larva in seed head; d. C. australis adult; e. C. succinea larva; f. C. succinea adult; g. Eustenopus villosus pupal chamber and larval-chewed florets, seeds, and receptacle 

tissue; h. seed heads aborted from E. villosus feeding; i. Larinus curtus larvae in a seed head; j. L. curtus adult face down in 

starthistle florets; k. leaf infected by Puccinia jaceae var. solstitialis; l. Urophora sirunaseva larva in gall; m. U. sirunaseva adult

Credits from top to bottom: University of Idaho Archives; California Department of Agriculture; Gary Piper, Washington 

State University; Charles Turner, USDA ARS; Rachel Winston, MIA Consulting; Laura Parsons & Mark Schwarzländer, 

University of Idaho; Charles Turner, USDA ARS; Laura Parsons & Mark Schwarzländer, University of Idaho; Charles 

Turner, USDA ARS; Rachel Winston, MIA Consulting; Stephen Ausmus, USDA ARS; Charles Turner, USDA ARS; 

Laura Parsons & Mark Schwarzländer, University of Idaho

Biocontrol

Agent

Life

Stage

Where to Look

When to

Look

Most Frequently Observed Damage Appearance

Bangasternus 

orientalis

Yellow starthistle 

bud weevil

Larva In seed heads June to August

Larva feeding in seed head within chamber made 

of plant tissue and frass; C-shaped body with 

brown head capsule, indistinguishable from other 

seedhead weevil biocontrol agent larvae

Adult

Mating or 

ovipositing females 

around buds

Late May to late 

June

Adults do not cause any direct damage to yellow 

starthistle; females oviposit black-covered eggs 

beneath seed heads

Chaetorellia 

australis

Yellow starthistle 

peacock fly

Larva In seed heads

May to midJune; August to 

April

Larva feeding in seed head within chamber made 

of plant tissue and frass; barrel-shaped body with 

no true head capsule, indistinguishable from larva 

of C. succinea

Adult

Mating or 

ovipositing females 

around buds

Mid-April to 

mid-May; June 

to mid-July

Adults do not cause any direct damage to yellow 

starthistle; females oviposit beneath the bracts of 

closed buds

Chaetorellia 

succinea

False yellow 

starthistle 

peacock fly

Larva In seed heads

May to midJune; August to 

April

Larva feeding in seed head within chamber made 

of plant tissue and frass; barrel-shaped body with 

no true head capsule, indistinguishable from larva 

of C. australia

Adult

Mating or 

ovipositing females 

around buds

May; Mid-June 

to July

Adults do not cause any direct damage to yellow 

starthistle; females oviposit beneath the bracts of 

closed buds

Eustenopus 

villosus

Yellow starthistle 

hairy weevil

Larva In seed heads Late June to 

early September

Larva feeding in seed head within chamber made 

of plant tissue and frass; C-shaped body with 

brown head capsule, indistinguishable from other 

seedhead weevil biocontrol agent larvae

Adult

Mating, feeding, or 

ovipositing females 

on buds

June to July

Adult feeding causes buds to abort (some aborted 

buds tip sideways); oviposition and feeding holes 

in seed head bracts are dark-colored

Larinus curtus

Yellow starthistle 

flower weevil

Larva In seed heads July to 

September

Larva feeding in seed head within chamber made 

of plant tissue and frass; C-shaped body with 

brown head capsule, indistinguishable from other 

seedhead weevil biocontrol agent larvae

Adult

Mating, feeding, or 

ovipositing females 

among florets

Late June to July Adults feeding on florets (often face down); 

females oviposit among florets

Puccinia jaceae 

var. solstitialis

Yellow starthistle 

rust

All All above-ground

growth

Year-round 

(there can be multiple 

cycles per year; 

symptoms and 

stages will vary)

Foliage covered with yellowish chlorotic lesions 

with raised centers, becoming orangish-brown 

pustules that produce powdery, round, and dark 

brown/rust-colored spores; infected plants stunted 

Urophora 

sirunaseva

Yellow starthistle 

gall fly

Larva In woody galls in 

seed heads

Mid-May to 

mid-June; 

August to April

Larva feeding in seed head within woody galls; 

barrel-shaped body with no true head capsule but 

with a dark anal plate

Adult

Mating or 

ovipositing females 

around buds

May; June to 

July

Adults do not cause any direct damage to yellow 

starthistle; females oviposit on top of closed buds

Chapter 4: Elements of a Yellow Starthistle Biological Control Program 69
Biology and Biological Control of Yellow Starthistle

Populations of some biocontrol agents take two or more years to reach detectable levels. Thus if no 

biocontrol agents are detected a year after release, it does not mean they failed to establish. Revisit the 

site at least once annually for three years. If no evidence of biocontrol agents is found, either select 

another site for release or make additional releases at the monitored site. Consult with your county 

extension educator or local biological control of weeds expert for assistance.

A systematic monitoring approach is required to determine the changing densities of biocontrol 

agent populations. The Standardized Impact Monitoring Protocol (SIMP) is one such approach to 

monitoring biocontrol agent populations, weed populations, and the surrounding plant community 

over time (Appendix III). This protocol was developed through cooperation among the Bureau of 

Land Management, the University of Idaho, U.S. Forest Service Forest Health Protection, the Nez 

Perce Biocontrol Center, and the Idaho State Department of Agriculture. SIMP was designed to be 

simple, efficient, and sufficiently versatile to allow for the collection of information from the same 

sites over multiple years. The yellow starthistle SIMP system is designed to monitor yellow starthistle 

seed-feeding insects. An alternative general biological control agent monitoring form can be found 

in Appendix IV. In addition to measuring seed-feeding insect populations, Appendix IV is useful for 

estimating the presence and/or extent of yellow starthistle rust infection. Existing data sheets may be 

modified to meet the needs of each land manager by adding extra columns, descriptive classes, etc..

Assessing the status of yellow starthistle and co-occurring plants

The ultimate goal of a yellow starthistle biological control program is to permanently reduce the 

abundance and vigor of yellow starthistle and enable the recovery of more desirable vegetation on 

the site. To determine the efficacy of biocontrol efforts, there must be monitoring of plant community 

attributes, such as target weed distribution and density. Ideally, monitoring begins before biological 

control efforts are started (pre-release) and occurs at regular intervals after release. There are many 

ways to qualitatively (descriptively) or quantitatively (numerically) assess weed populations and other 

plant community attributes at release sites.

Qualitative (descriptive) vegetation monitoring: Qualitative monitoring uses subjective measurements 

to describe the yellow starthistle and the rest of the plant community at the management site. Examples 

include listing plant species occurring at the site, estimates of density, age and distribution classes, 

visual infestation mapping (as opposed to mapping with a GPS unit), and maintaining a series of photos 

from designated photo points over time (Figure 4-13a-c). 

Qualitative monitoring provides insight into the status or change of yellow starthistle populations; 

however, its descriptive nature does not generally allow for detailed statistical analyses. Data obtained 

a b c

Figure 4-13. Yellow starthistle biocontrol release site in a. 1997; b. 2009; c. 2012 (Credits: a Leonard L. Lake, 

USDA Forest Service; b,c Rachel Winston, MIA Consulting)

70 Chapter 4: Elements of a Yellow Starthistle Biological Control Program
Biology and Biological Control of Yellow Starthistle

in qualitative monitoring may trigger more quantitative monitoring later. See Appendix V for a sample 

data form useful for recording qualitative yellow starthistle monitoring data along with information 

on associated vegetation.

Quantitative vegetation monitoring: Quantitative 

monitoring measures changes in the yellow 

starthistle population as well as the vegetative 

community as a whole before and after a biocontrol 

agent release using numbers and statistics. It may 

be as simple as counting the number of yellow 

starthistle plants in a small sample area, or as 

complex as measuring yellow starthistle plant 

height, flower and seed production, biomass, 

species diversity, and species cover (Figure 4-14). 

Quantitative sampling data can be more readily 

analyzed using statistical methods to demonstrate 

significant plant community changes. Pre- and postrelease monitoring should follow the same protocol 

and be employed at the same time of year. Postrelease assessments should be planned annually for 

at least three to five years after the initial biocontrol agent release (and ideally longer than that).

See Appendix VI for a sample data form useful for recording quantitative yellow starthistle monitoring 

data along with information on associated vegetation. The SIMP approach described earlier and 

found in Appendix III is a combination of qualitative and quantitative elements as well as counts for 

biological control agents. 

Assessing impacts on non-target plants: To address possible non-target attacks on species related to 

or just growing adjacent to yellow starthistle, you must become familiar with the plant communities 

present at and around your release sites and be aware of species related to yellow starthistle. Start by 

compiling a list of other species in the Asteraceae family and the genus Centaurea that are present at 

the site (see Chapter 2 for more information). You may need to consult with local, state, or regional 

botanical experts, or review local herbarium records for guidance on areas where related non-target 

plants might be growing and additional information on how you can identify them. Care should be 

taken in the management of your yellow starthistle biocontrol program to ensure that all closelyrelated native or desirable species are identified and monitored along with yellow starthistle. 

Please be aware that there are many “look-alike” native insects that feed on related native plants. 

Correct identification by insect specialists is needed to confirm such records. If you observe approved 

biological control agents feeding on and/or developing on native species, collect samples and take 

them to a biocontrol specialist in your area. Alternatively, you may send the specialist the site data 

and/or pictures so he or she can survey the site for non-target impacts. Be sure not to ascribe any 

damage you observe on native species to any specific species and thus bias the confirmation of attack 

and the identification of the species causing the attack.

If you observe approved biocontrol agents feeding on and/or developing on non-target plant species, 

the vegetation sampling procedures described above can be easily modified to monitor changes in 

density and/or cover of the non-target species. Concurrently, you may wish to collect additional 

data, such as the number of biocontrol agents observed on non-target plants, the amount of foliar 

Figure 4-14. Quantitatively monitoring new yellow 

starthistle rosettes in a 1m2 quadrat (Credit: Mark 

Schwarzländer, University of Idaho)

Chapter 4: Elements of a Yellow Starthistle Biological Control Program 71
Biology and Biological Control of Yellow Starthistle

attack observed, or the presence of characteristic biocontrol agent damage. Collecting this data over 

subsequent years can help determine if there is a population level impact or if the non-target feeding 

is temporary or of minor consequence to the non-target species. 

72 Chapter 4: Elements of a Yellow Starthistle Biological Control Program
Biology and Biological Control of Yellow Starthistle

CHAPTER 5: AN INTEGRATED YELLOW STARTHISTLE 

MANAGEMENT PROGRAM

Introduction

The invasion curve (Figure 1-3, repeated here in Figure 5-1) shows that eradication of an invasive species 

such as yellow starthistle becomes less likely and control costs increase as an invasive species spreads 

over time. Prevention is the most cost-effective solution, followed by eradication. If a species is not 

detected and removed early, intense and long-term control efforts will be unavoidable. Identifying where 

yellow starthistle is on the invasion curve in a particular area is the first step to taking management action. 

Inventorying and mapping current yellow starthistle populations coupled with research efforts to predict 

where yellow starthistle is most likely to move enables land managers to concentrate resources in areas 

which are likely to be invaded, and then to treat individual plants and small populations of yellow starthistle 

before it is too late to remove them.

Classical biological control has 

been applied to many invasive 

plant species, but biological 

control is generally not 

appropriate for areas on the left 

side (species absent [prevention] 

- small number of localized 

populations [eradication]) of 

the invasion curve. Biological 

control as a control method is 

best suited to yellow starthistle 

populations in the later phases 

of the invasion curve (rapid 

increase in distribution and 

abundance [containment] 

- widespread and abundant 

throughout its potential range 

[asset based protection]). 

Figure 5-1. Generalized invasion curve showing actions appropriate to each stage 

(Credit: © State of Victoria, Department of Economic Development, Jobs, 

Transport and Resources, Reproduced with permission)

Chapter 5: An Integrated Yellow Starthistle Management Program 73
Biology and Biological Control of Yellow Starthistle

There are several examples in which both single- and multiple-biocontrol agent introductions have 

successfully controlled the targeted weeds. Where ideally suited, biological control can help maintain 

yellow starthistle densities below economically or ecologically significant levels, enabling land managers 

to live with the weed; however, it may take three to five years or more for biological control to help reduce 

weed populations to such manageable levels. Furthermore, yellow starthistle occurs across a wide range 

of conditions. Some habitats are unsuitable to biocontrol agents, so biological control is not going to work 

on yellow starthistle every time at every site. Depending on the infestation, integration with other weed 

control methods or resorting to other control measures entirely may be required to attain yellow starthistle 

management objectives. 

A wide variety of successful weed control methods have been developed and may be useful for helping meet 

management goals for yellow starthistle. The most successful long-term yellow starthistle management 

efforts have a number of common features, including:

• Education and Outreach

• Inventory and Monitoring

• Prevention 

• Weed Control Activities: A variety of yellow starthistle control activities which are selected based 

on characteristics of the target infestation and planned in advance to use the most appropriate 

method or combination of methods at each site, including:

o Biological control

o Physical treatment

o Cultural practices

o Chemical treatment

Integrated Pest Management (IPM) incorporates all efforts noted above, and addresses several aspects of 

land management, not just how to get rid of weed populations. Land managers or landowners engaged in 

IPM take the time to educate themselves and others about the threat invasive species pose to the land and 

how management may facilitate invasion. IPM requires land managers to regularly inventory and map the 

land they manage, identifying areas where the vegetation is not meeting their management objectives and 

identifying reasons why. When a weed infestation is found, IPM dictates that land managers map it and 

make plans to address it utilizing control methods most appropriate for their particular infestation and land 

use. After initiating control activities, IPM encourages land managers to monitor the site to determine if the 

control activity was successful in subsequent years. If re-treatment or additional treatments are necessary, 

these are applied in a timely manner with appropriate post-treatment monitoring to ensure that management 

objectives are being met. 

Integrated Pest Management programs undertaken on a landscape level over many years can, at times, 

prove logistically difficult, expensive, and time-consuming. The concept of Cooperative Weed Management 

Areas (CWMA) was created in the western USA in order to erase jurisdictional boundaries as an 

impediment to weed control and make a landscape IPM approach to weed management more feasible 

and successful. CWMAs consist of federal, state and local land managers, as well as concerned private 

landowners, within a designated zone who combine and coordinate efforts against exotic plants, pooling 

and stretching limited resources and labor for managing invasive species and protecting/restoring habitat. 

Cooperation between neighboring CWMAs helps transfer knowledge and experience between heavily 

treated regions and places not yet as impacted by yellow starthistle. Sharing successes and failures in 

yellow starthistle management saves time and funding and reduces the incidence of both failure and 

74 Chapter 5: An Integrated Yellow Starthistle Management Program
Biology and Biological Control of Yellow Starthistle

negative impacts from management efforts, such as destruction of wildlife habitat and damage to non-target 

species. Numerous CWMAs exist throughout the western states of the USA and are excellent sources of 

information, experience, and resources for treating yellow starthistle infestations using an IPM approach. 

Components of Successful Integrated Pest Management Programs to 

Manage Yellow Starthistle

Though each component of IPM is an important tool for managing yellow starthistle, it is important to note 

that these components work best when used in a combined approach. Rather than applying only one tool 

per site (e.g., applying herbicides at one infestation, mowing at another, and using biological control at still 

another), the most effective IPM strategy is to employ as many tools as necessary at a single site in order to 

maximize the efficacy of each tool and ultimately reduce yellow starthistle infestations, while at the same 

time achieving land use objectives. 

Education, inventorying/mapping, and prevention are important and applicable across all landscapes, whether 

or not yellow starthistle is already present. When yellow starthistle is established and control methods are 

warranted, long-term management success is greatly improved when control methods are identified according 

to infested habitat type, land use, ownership, and available resources and then integrated where appropriate. 

As described above, biological control is most appropriately used on large infestations where multiple years 

may be required before impacts are realized. During this time, chemical and physical control methods are best 

applied to smaller new or satellite populations where immediate eradication is warranted, and to the edges of 

large infestations to contain the existing infestation 

while preventing further spread. Cultural control 

methods work to enhance the growth of more 

desirable vegetation and are best applied as 

complements to all other control methods.

The components of a yellow starthistle IPM program 

are described individually below. Because the focus 

of this manual is the biological control of yellow 

starthistle, the potential to integrate biocontrol with 

other weed control methods is described at the end 

of each control method’s section.

Education & Outreach

Education and outreach activities increase 

public awareness of noxious weeds, the 

problems they cause, their distribution, and 

ways to manage them (Figure 5-2). Ideally, 

education and outreach activities also foster 

cooperation and collaboration across land 

ownership boundaries to facilitate the 

development of a landscape-level weed 

management response. Education efforts 

should be an important component of any 

weed management plan, regardless of 

the target weed or weed control method 

employed. Figure 5-2. Yellow starthistle education poster (Credit: 

University of Nevada Cooperative Extension)

Chapter 5: An Integrated Yellow Starthistle Management Program 75
Biology and Biological Control of Yellow Starthistle

Yellow starthistle education and outreach should focus on conveying to the public:

• the threat yellow starthistle poses

• how to identify yellow starthistle in different life stages (seedling – senescence)

• ways in which they can help in yellow starthistle management

By educating land managers, landowners, recreationalists and the public about the threat of yellow 

starthistle, enabling them to identify infestations, and enlisting them in mapping and management 

efforts, it becomes possible to cooperatively develop successful weed management responses at the 

landscape level. 

Inventory & Mapping

Inventory and mapping are key elements of a successful pest management program. It is imperative to 

accurately characterize the size and extent of weed infestations before control activities are identified, 

prioritized, and implemented because the best treatment methods are often determined by the size and 

location of the infestation. Education and outreach activities that foster collaboration between adjacent 

landowners are particularly useful when developing landscape-level maps of weed infestations. Once 

land managers and landowners fully understand the threat yellow starthistle poses to their land, they 

are often more willing to participate to ensure that their land is inventoried and accurate maps of 

yellow starthistle are developed so the best control activities can be implemented. 

Yellow starthistle infestations are often mapped by foot, vehicle, horse, or airplane using a global 

positioning system (GPS) unit and a geographical information system (GIS), though hard copy 

maps made by hand are suitable for some locations. Different infestations are best monitored by 

different means. Small infestations can be very difficult to spot, given the morphology of starthistle 

stems which make the plant difficult to distinguish from neighboring vegetation (Figure 5-3a). These 

infestations are often best spotted with small-scale search operations such as those done by foot 

or on horseback. Larger infestations are often more easily spotted by their gray-green appearance, 

monotypic tendencies and (during mid- to late summer) the presence of numerous scattered yellow 

flower heads. Many large infestations can be seen from hovering aircraft and helicopters; however, 

surrounding vegetation still determines the ease with which yellow starthistle can be distinguished. 

Some large infestations can be especially cryptic during early growth stages (Figure 5-3b) or as plants 

senesce and all vegetation begins to look similar (5-3c). For very large infestations, GoogleEarth can 

be a useful tool for providing birds-eye-view imagery that allows for visual delineation of yellow 

starthistle boundaries; however, image quality and timing can make it difficult to make comparisons.

An increasing number of free smartphone and tablet apps help make accurate, detailed, and versatile 

weed mapping available to anyone (e.g., the apps available from EDDMapS, see page 68 for 

more information). Inventory efforts should document the following for each infestation: location 

coordinates, boundaries, estimated density (number of stems of target weed per area, e.g., square meter 

or square yard), land usage, treatment history, disturbance history (e.g., fire, flooding), habitat type 

(desert, upland, shrubland, grassland), and date. Photos of the infestation and a list of co-occurring 

species are also very useful. Documenting inventory and mapping efforts enables land managers to 

determine if all known yellow starthistle infestations have been treated, and facilitates post-treatment 

monitoring. In turn, this allows land managers to judge the effectiveness of various treatment methods. 

See Chapter 4 for suggested techniques to monitor infestations. 

76 Chapter 5: An Integrated Yellow Starthistle Management Program
Biology and Biological Control of Yellow Starthistle

a b c

Figure 5-3. Yellow starthistle infestations difficult to see, a. scattered starthistle plants in middle of image, infestations 

in foreground and background are different species; b. dense infestation of yellow starthistle rosettes in foreground 

obscured by previous-year stems and two other yellow-flowering species that are not yellow starthistle; c. yellow 

starthistle infestation interspersed with few other species though all look similar in the senescent stage (Credits: 

a-c Rachel Winston, MIA Consulting)

Chapter 5: An Integrated Yellow Starthistle Management Program 77

Prevention

Prevention activities focus on areas not currently infested by yellow starthistle with the goal of keeping 

these areas weed-free. Though yellow starthistle is already present throughout much of western North 

America, there are many sites where it is absent or remains at low densities, and entire counties/states/

provinces where yellow starthistle has not yet invaded. Inventory and mapping efforts help identify 

the precise borders of existing yellow starthistle infestations as well as identify weed-free areas. 

Preventing introduction and spread of yellow starthistle to uninfested areas is more environmentally 

desirable and cost-effective than treating large-scale infestations.

Yellow starthistle is spread by the movement of seed in/on hay, motorized equipment, livestock, 

wildlife, or humans. Preventing the spread of yellow starthistle requires cooperation among all 

landowners and land managers. In areas where yellow starthistle is not yet present, it is important to 

ensure that possible invasion avenues are identified and management actions taken to reduce the risk 

of spread. This includes minimizing soil disturbances and regularly monitoring uninfested sites to 

confirm that they have remained uninfested. 

Cultivation, soil erosion (especially following flooding events and prescribed burns or wildfire), road 

grading, recreational activities (e.g., riding dirt bikes or four wheelers), and overgrazing all weaken 

existing plant communities, decrease plant cover, and cause disturbance. These conditions favor yellow 

starthistle establishment and persistence 

(Figure 5-4). Because such activities 

are also potential ways of spreading 

yellow starthistle seeds, they should 

either be avoided or closely monitored 

in starthistle-prone areas. Where 

grazing does occur, proper livestock 

management (such as strategic timing 

and stocking rates) will allow grazed 

vegetation to recover and competitive 

plants to increase, which, in turn, will 

help prevent the establishment of yellow 

starthistle. If possible, livestock should 

be kept off weed-infested land when 

they are most likely to spread viable 

Figure 5-4. Overgrazing and erosion (Credit: Paul Bolstad, 

University of Minnesota, bugwood.org)
Biology and Biological Control of Yellow Starthistle

seeds (e.g., after seed formation). If it is not possible to avoid driving vehicles and machinery (e.g., 

logging, construction, or rangeland fire-fighting equipment) through yellow starthistle infestations, it 

is crucial that a thorough cleaning take place before equipment leaves the contaminated area.

Prevention and exclusion activities are typically paired with education efforts. Examples of exclusion 

efforts include weed-free forage programs, state and provincial seed laws, and mandatory equipment 

cleaning when leaving infested sites and before entering uninfested sites. 

EDRR

An early detection and rapid response (EDRR) program is a specific protocol for tracking and 

responding to new infestations. It relies heavily on education and outreach activities to be effective. An 

EDRR program targets areas where yellow starthistle may spread. It consists of three complementary 

activities: 1) educating land managers and the public on weed identification and mapping techniques, 

2) enlisting their aid in immediate and thorough detection of the weed, and 3) initiating rapid response 

eradication efforts at all verified locations of the weed.

The most cost-effective strategies for dealing with yellow starthistle are prevention and EDRR. 

Because yellow starthistle is difficult to control once established, it is imperative that every effort be 

made to inventory regularly and immediately eliminate all early invaders. 

Weed Control Activities

Biological Control

Biological control involves the use of living organisms, usually insects, mites, or pathogens, to control 

a weed infestation and regain the balance among coexisting plant species. Classical biological control 

focuses on the introduction of host specific natural enemies from the invasive weed’s native range. This 

method of yellow starthistle management is the most economical and suitable for larger infestations 

(tens to thousands of acres). For small patches (less than 4 acres or 1.6 ha) of new satellite infestations 

(those growing outside of well-established yellow starthistle populations), more aggressive control 

methods should be utilized (e.g., physical control or herbicides). Refer to Chapter 3 for detailed 

descriptions of the biological control agents currently approved for use on yellow starthistle and 

Chapter 4 for how to implement a yellow starthistle biological control program in your area.

Physical Treatment

Physical treatment utilizes hand pulling, hoes or other hand-held equipment, mowing, or tillage to 

remove or disrupt the growth of weeds and is the oldest method of weed control. Physical methods 

have shown some success in controlling yellow starthistle when properly timed and applied repeatedly, 

but they are labor-intensive and often not suitable for the more rugged and inaccessible sites where 

starthistle has invaded. Regardless of the physical method employed, it is imperative that all equipment 

be thoroughly cleaned following use to prevent the spread of yellow starthistle seeds.

Hand pulling

Hand pulling can be useful in steep or uneven terrain where other forms of mechanical control (e.g., 

mowing, tillage) are not feasible. It is most appropriate in the EDRR stage of a yellow starthistle 

infestation or on satellite populations occurring outside larger containment areas. While hand pulling 

is most often used to control small yellow starthistle infestations (under 1 acre or 0.4 ha), larger 

populations (up to 40 acres or 16 ha) can also be controlled by starting at the outward edge of the 

population and moving inwards. Successful use of hand pulling requires repeated sessions to account 

for new yellow starthistle individuals sprouting throughout the growing season. 

78 Chapter 5: An Integrated Yellow Starthistle Management Program
Biology and Biological Control of Yellow Starthistle

Hand pulling is especially effective on young plants; as plants age, or for plants growing in compacted 

soils, well-developed root systems anchor plants in the soil and can lead to stem breakage during 

pulling. Leaving even two inches (5 cm) of rooted stems with leaves attached can result in plant 

recovery. It is important to remove as much of the yellow starthistle root system as possible, while 

minimizing soil disturbance. The most appropriate and easiest time to pull plants is before they go to 

seed and before they produce the spiny flower heads. This is often the time when the soil is moist and 

plants are easy to pull. Because plants are not in flower, they can be left at the site with no threat of 

recovery as long as the roots are not contacting the soil. When yellow starthistle plants are in flower 

or seed, cut off and bag all flower stalks prior to pulling. Otherwise, the jarring action of pulling may 

dislodge and distribute seeds at the site. All stems, flowers, and seeds should be securely bagged 

and taken to the trash or a transfer site to prevent possible yellow starthistle seed dispersal from 

pulled material (Figure 5-5a). Re-seeding the open space resulting from yellow starthistle removal 

with seeds of desirable vegetation can provide competition to decrease yellow starthistle seedling 

germination and persistence.

Because hand pulling removes the biocontrol agent’s host from the site, this control method is not 

compatible with biological control. Hand pulling is most appropriate for small infestations where 

immediate eradication is feasible, while biological control is more appropriate for much larger, 

established infestations where the management goal is containment or asset-based protection. One 

way to successfully combine these two methods is to release biological control agents in a large, main 

infestation while employing hand pulling to remove individual plants and to control small, satellite 

patches arising outside of the main yellow starthistle infestation.

Mowing

Mowing yellow starthistle has proven effective for infestations along roadsides and rights-of-way 

(Figure 5-5b) when timed correctly and repeated. The most appropriate time to mow is when yellow 

starthistle plants are in the spiny bud (BU-3 to BU-4) to very early flowering stages. Mowing postflowering can exacerbate the problem by spreading viable starthistle seeds. Mowing too early, when 

plants are at the bolting stage, can stimulate starthistle re-growth (and subsequent flowering) and 

suppress competing vegetation. Repeated mowing is often necessary. The lower the mowing blade 

is set, the more effective the treatment; plants should be cut below the height of the lowest branches. 

However, low mowing is typically not feasible in rough, rocky, or remote terrain where yellow 

starthistle is so problematic in the western USA. Alternatively, mowing can be used to reduce nontarget plant cover and litter prior to fall herbicide applications, as this will improve coverage of the 

chemical on fall yellow starthistle rosettes. 

a b c

Figure 5-5. Physical weed treatments, a. hand pulling yellow starthistle; b. roadside mowing; c. tillage (Credits: a 

Michael G. Common, National Park Service; b Joost J. Bakker, Ijmuiden; c Howard F. Schwartz, Colorado State 

University, bugwood.org)

Chapter 5: An Integrated Yellow Starthistle Management Program 79
Biology and Biological Control of Yellow Starthistle

Because the most effective time to mow is during the spiny bud and early flowering stages when 

seed-feeding biocontrol agents are active in the seed heads, mowing is not compatible with yellow 

starthistle biocontrol insects. Though it hasn’t been explicitly studied because of the rust’s limited 

current distribution, mowing may actually help distribute the yellow starthistle rust (Puccinia jaceae 

var. solstitialis), and the rust could theoretically re-establish on yellow starthistle plants recovering 

from mowing efforts. 

Tillage

Cultivation (Figure 5-5c) can be used successfully for the control of yellow starthistle in an agricultural 

setting, if the timing and frequency are accurately applied. Tillage in early summer (when starthistle 

plants are in the bolting and bud stages) will control yellow starthistle plants provided the roots 

are removed from the shoots. Cultivation during the flowering and seed set stages will only help 

facilitate seed dispersal. Infestations should be tilled multiple times in a season if additional starthistle 

germination occurs between tillage events. Alternatively, a single tillage can be used if it is conducted 

during the early flowering stage of starthistle before the plant produces viable seed. At this timing, 

there is little chance of newly germinated seedlings surviving the dry summer months. In the extensive 

rangeland system yellow starthistle has invaded in western North America, frequent cultivation is not 

feasible.

Because cultivation destroys yellow starthistle plants, this form of weed control is destructive to 

all six approved yellow starthistle biocontrol agents. Repeated tillage is most applicable in a crop 

setting, where chronic disturbance and the need to attain immediate control make biological control 

fundamentally incompatible.

Cultural Practices

Cultural methods of weed control (including burning, grazing, and seeding with competitive species) 

are used to manipulate the environment by non-mechanical means and can enhance the growth of 

desired vegetation, which may slow the invasion of noxious weeds onto a site. Regardless of which 

method is used, all cultural control techniques are more successful when combined with other control 

methods, such as hand pulling prior to re-seeding or burning prior to applying herbicides. 

Burning

Burning has yielded mixed results for controlling yellow starthistle. The most effective prescribed 

burns (Figure 5-6a) are applied during the early flowering stage of starthistle before the plant produces 

viable seed. This timing typically corresponds with other desirable species having already dispersed 

their seeds and senesced, contributing more fuel. When prescribed burns were applied during this 

stage at one study site and repeated for three years, the fires significantly reduced yellow starthistle 

by depleting its seed bank, while simultaneously increasing desirable perennial grasses. At the same 

study site four years after prescribed burns were no longer applied, the plant community transitioned 

back to a starthistle-dominated grassland. The general consensus is that burning is most effective 

when used as just one tool in an integrated approach over several years. 

Fire can be used intentionally to burn off plant litter in order to stimulate germination and make new 

starthistle growth easier to see when applying herbicides. The bare soil following a fire is ideal for 

yellow starthistle germination. If new flushes of starthistle growth are treated rapidly with physical 

or chemical means, the starthistle seed bank can be depleted quickly. When prescribed fire kills off 

competing vegetation, it will only increase the starthistle problem, unless it is followed by a subsequent 

herbicide application. Revegetation with desired vegetation is recommended wherever fire is utilized 

80 Chapter 5: An Integrated Yellow Starthistle Management Program
Biology and Biological Control of Yellow Starthistle

a b

Figure 5-6. Yellow starthistle cultural management techniques, a. prescribed fire; b. grazing sheep (Credits: a David 

Cappaert, Michigan State University, bugwood.org; b Flying Mule Farm)

Chapter 5: An Integrated Yellow Starthistle Management Program 81

to aid in yellow starthistle herbicide control. Fire has deleterious effects on insects, small animals, and 

some native plant species, and it can increase soil erosion; caution should be applied when using this 

tool as part of an integrative pest management system.

Because the most effective time to burn is during the early flowering stages when seed-feeding 

biocontrol agents are active in the seed heads, burning is not compatible with yellow starthistle 

biocontrol insects. Burning also destroys the yellow starthistle rust. 

Grazing

Most domestic livestock (including goats, sheep, and cattle) and some species of wildlife will graze 

yellow starthistle growing in the rosette and bolting stages prior to the production of the spiny seed 

heads. Yellow starthistle provides palatable, high-protein forage for cattle, sheep, and goats prior to 

the formation of the spiny bracts, though infested fields have less forage value than uninfested fields. 

Goats and sheep have also been observed feeding on the spiny seed heads (Figure 5-6b). Horses 

should not be exposed to yellow starthistle as this species contains compounds that are toxic to horses, 

leading to the fatal neurological disorder of nigropallidal encephalomalacia or “chewing disease”.

Spring grazing reduces the growth, survivability, and reproductive ability of yellow starthistle. The 

best results have been found with continuous grazing rather than rotational grazing. When livestock 

are moved as part of rotational grazing, yellow starthistle quickly recovers and sets seeds. Continuous 

grazing keeps the plant from bolting when other green feed is scarce; however, this heavy feeding is 

considered by many to be overgrazing, which has been shown to exacerbate the starthistle problem 

by reducing competing species and promoting the germination of starthistle seeds. Although grazing 

can be effective for suppressing yellow starthistle under the right management circumstances, the 

difficulties and costs associated with proper livestock management often limit the implementation of 

this control method. Where it is feasible to utilize livestock to manage yellow starthistle, it is important 

the animals do not graze during seed set, as this can assist in the distribution of starthistle seeds. 

The impact of grazing on the biological control of yellow starthistle is largely unknown. Feeding on 

yellow starthistle seed heads infested with the seed-feeding biocontrol agents would destroy those 

insects; however, most grazing animals prefer to feed on yellow starthistle prior to this stage, making 

grazing compatible with seed-feeding biological control agents.
Biology and Biological Control of Yellow Starthistle

Seeding competitive species

Where yellow starthistle is established and then suppressed by one or more control methods, reinvasion 

by yellow starthistle or other undesirable species is likely if the ecological niche they occupied remains 

unfilled. Successful long-term management requires the establishment and maintenance of desirable 

competitive species to avoid reinvasions. 

Yellow starthistle is sensitive to competition for light and resources during early growth stages. When 

cool season plants emerge before yellow starthistle, their dense shade restricts the growth of starthistle 

seedlings and rosettes. This can reduce yellow starthistle populations by as much as 90%. In addition to 

competition for light, certain species hinder the growth of yellow starthistle through other mechanisms. 

Deep-rooted perennials such as alfalfa (Medicago sativa, Figure 5-7a) or perennial grasses compete 

with yellow starthistle for much-needed soil moisture over the summer months. Alfalfa and other 

legumes such as sub-clover (Trifolium subterraneum, Figure 5-7b) also improve the soil nitrogen 

status by fixing their own nitrogen. This increases growth and competition from additional desirable 

pastoral species previously limited by nitrogen availability. The level of management required to 

maintain dense stands of shading and/or nitrogen-fixing species is often difficult to achieve, especially 

in the vast natural and rangeland habitat yellow starthistle has invaded in western North America.

a b

Figure 5-7. Legume species that compete well with yellow starthistle in a pastoral setting, but are best used in 

combination with other weed control methods, a. alfalfa (Medicago sativa); b. sub-clover (Trifolium subterraneum) 

(Credits: a Olivier Pichard; b AnRo0002)

82 Chapter 5: An Integrated Yellow Starthistle Management Program

In more natural settings, the most suitable plant species to use for competition with yellow starthistle 

depends on habitat, site conditions, climate, management goals, and future land use. Ideally, planted 

seeds should contain a mix of species, some of which should be quick to germinate and others to 

provide more long-term competition to yellow starthistle seedlings. Utilizing ecologically equivalent 

species (those with root and growth patterns similar to yellow starthistle) may provide the best 

competition. Inventorying nearby sites that are uninvaded by yellow starthistle may provide insight 

into the best replacement species. Consult your local county extension educator or Natural Resource 

Conservation Service representative for additional help in determining the best alternatives in your 

area. Further suggestions for ecoregions throughout the United States may be found on the Native 

Seed Network website (see Chapter 5 References for the URL). Likewise, the “links” section of the 

USDA PLANTS website offers numerous revegetation guideline manuals specific to different regions 

of both the United States and Canada. This site also provides access to a program and fact sheets that 

utilize soil, plant, and climate data to select plant species that are site-specifically adapted, suitable for 

the selected practice, and appropriate for the goals and objectives of the revegetation project.
Biology and Biological Control of Yellow Starthistle

Control of yellow starthistle prior to seeding more desirable species is important because established 

starthistle plants are highly competitive. Seeding of competitors should take place immediately 

following exposure of soil to maximize their competitive advantage. For example, seeding should 

occur in bare soil following burning or after young starthistle plants have been hand pulled or killed 

with herbicides. Because high populations of rodents can reduce the success of re-seeding, erecting 

a raptor perch/pole may discourage rodent habitation and help ensure seeded species successfully 

germinate and establish. Drill seeding (rather than broadcast seeding) may also reduce predation by 

rodents. Besides exposing seeds to predation, broadcasted seeds are also more susceptible to decay 

and to desiccation following germination.

Incorporating biocontrol agents with re-seeding can be difficult, primarily because the methods 

used to establish a productive stand of competitive species are not always compatible with the 

establishment and survival of biological control agents. Any method used to initially reduce yellow 

starthistle stems and leaves to promote the growth of competitive species hinders the survival of 

all established starthistle biocontrol agents. Consequently, many successful revegetation programs 

establish competitive species first, using biological control agents after the seeded species have 

become established and yellow starthistle begins to reappear. Alternatively, revegetation projects can 

target only a small portion of the infestation annually, leaving a reservoir of yellow starthistle plants 

to support biocontrol agent populations. In some settings, it may be the biological control agents that 

open up the competing plant canopy, allowing for subsequent re-seeding to occur.

Chemical Control

Many herbicides are registered for use on yellow starthistle growing in a variety of locations. Herbicides 

are most effective when applied to small infestations, including newly established populations and 

recently established satellite patches arising from nearby older, larger yellow starthistle infestations. 

If utilized appropriately, herbicides are also useful on the leading edge of large, advancing yellow 

starthistle infestations. 

Herbicides may be too costly to be of practical use in treating extensive infestations of yellow 

starthistle and, similar to physical and cultural control methods, are also impractical in hard-to-access 

and environmentally sensitive areas. Repeated herbicide applications are often required over time as 

yellow starthistle plants repeatedly germinate from the seed bank. It generally takes about three years 

of herbicide treatment to reduce the population to fairly low levels. Potential non-target damage to 

associated vegetation must also be considered when using herbicides. For these reasons, herbicides 

are best used as part of a larger, integrated pest management program that employs regular (annual) 

inventory and mapping, and incorporates other weed control methods in areas where herbicides are 

less likely to be cost effective or the most appropriate control choice. 

Herbicides are generally applied in one of two ways: spot or broadcast applications. Spot treatments 

are used for individual yellow starthistle plants or small patches (Figure 5-8a). In spot applications, an 

appropriate herbicide is applied to the foliage of target plants only, thus reducing non-target effects. 

Broadcast treatments are when herbicides are applied to an entire weed infestation rather than to 

single plants (Figure 5-8b). Broadcast treatments should be used with caution as many herbicides may 

also impact plants that land managers may want to retain. If a broadcast treatment should kill all plants 

in a treated area, the resulting bare soil may allow yellow starthistle to reinvade from the seedbank, 

creating a denser yellow starthistle infestation than was there originally. 

Selective herbicides are those that target selected species (e.g., broad-leafed forbs vs. grasses) while 

leaving other species virtually unharmed. Utilizing selective herbicides and/or spot treatments helps 

Chapter 5: An Integrated Yellow Starthistle Management Program 83
Biology and Biological Control of Yellow Starthistle

a b

Figure 5-8. Yellow starthistle herbicide applications, a. spot-treating individual plants; b. broadcast treating 

infestations in rough terrain with a helicopter (Credits: a Mark Schwarzländer, University of Idaho; b Leon 

Slichter, Idaho County Weed Control)

84 Chapter 5: An Integrated Yellow Starthistle Management Program

reduce the non-target impacts of herbicide applications, and is the recommended approach for treating 

yellow starthistle infestations with chemical control. The herbicide label should always be referenced 

to help evaluate the risk of non-target species damage.

Both post-emergent (works on growing tissue) and pre-emergent (prevents seed germination) 

herbicides are registered for use against yellow starthistle. Some post-emergent herbicides are very 

effective on yellow starthistle seedlings and rosettes. Applying post-emergent herbicides later in the 

spring will control yellow starthistle plants which germinated in the fall, winter, or spring; however, 

yellow starthistle plants which germinated in the fall/winter will be larger plants that may require 

higher application rates to achieve desired control. Drought-stressed plants are also less susceptible to 

herbicides. Adding a surfactant when treating larger and/or drought-stressed plants may help increase 

post-emergent herbicide efficacy. 

Pre-emergent herbicides with long residual periods in the soil tend to be more effective against yellow 

starthistle because they remain effective throughout yellow starthistle’s long germination period. To 

achieve season-long control of yellow starthistle, the most successful programs utilize herbicides that 

have both post-emergence and pre-emergence activity.

Yellow starthistle is most prevalent in the state of California, and the California herbicide 

recommendations for yellow starthistle management include:

Broadleaf selective herbicides

• Aminopyralid is one of the most frequently used and effective herbicides for yellow starthistle. 

It can be applied from the seedling to bolting stages. Aminopyralid is very effective as a postemergent and pre-emergent herbicide. It has a fairly long soil residual period and broadcast 

applications may reduce re-growth from remaining starthistle seedlings for 3-4 months following 

application. Aminopyralid can damage other desirable forbs and trees, especially legumes, so 

its use should be carefully considered in relation to existing native species, or those that may 

be reintroduced to a site when used in conjunction with broadleaf revegetation efforts. This 

herbicide can be applied near some tree species where dicamba and picloram cannot be used, but 

is not registered for use in forestry, and care should still be taken to verify whether a given species 

is tolerant of having aminopyralid applied within its dripline. Additional information regarding 
Biology and Biological Control of Yellow Starthistle

the known tolerance of various tree species to aminopyralid is available on the manufacturer’s 

website. This herbicide does not kill grasses, sedges, cattails, or other monocots when applied 

post-emergence at broadcast label rates, but it demonstrates some pre-emergence control of 

medusahead and bromes dependent on application timing.

• Aminocyclopyrachlor is a highly effective herbicide on yellow starthistle and can be applied 

to seedlings and rosettes in uncultivated non-agricultural land, industrial sites, and natural areas. 

Like aminopyralid, it is active both post- and pre-emergent and has a similar length of soil residual 

activity. Low rates of aminocyclopyrachlor can kill non-target tree and shrub species, so do 

not apply within the dripline of trees or shrubs, to a distance equal to the height of the 

species of concern. It has a broader spectrum for control of broadleaf species compared to 

clopyralid or aminopyralid, but is still considered relatively selective for certain broadleaf 

plant families, particularly the sunflower and legume families. Its use should be carefully 

considered in relation to existing native species, or those that may be reintroduced to a site 

when used in conjunction with broadleaf revegetation efforts. 

• Clopyralid should be applied to yellow starthistle in the seedling to bolting stages. While it 

provides excellent control of starthistle foliar growth, its soil residual activity is about 2 to 

3 months, which is slightly less than either aminopyralid or aminocyclopyrachlor. It is more 

selective than aminopyralid and aminocyclopyrachlor, and is not as damaging to woody plants. 

Because it will kill desirable legume species and some other forbs, its use should be carefully 

considered in relation to existing native species, or those that may be reintroduced to a site 

when used in conjunction with broadleaf revegetation efforts.

• 2,4-D is only applied post-emergence to yellow starthistle and is often an option at later stages of 

growth, generally in the bolting or spiny stages. It is faster acting compared to the other products, 

so is a better choice when used late season. 2,4-D is a broadleaf herbicide and will not harm 

grasses, sedges, cattails, or other monocots, but will damage most other broadleaf species. Ester 

formulations are volatile and should not be applied when temperatures exceed 80°F (26.7°C). 

• Triclopyr works much like 2,4-D and is also considered a late season control option. While it 

provides good control of yellow starthistle foliar growth, it has little to no soil activity. Like 2,4-

D, it is a broadleaf herbicide that is largely safe on grasses, sedges, cattails, or other monocots, 

but it will damage most other broadleaf species.

Non-selective herbicides

• Glyphosate is active only as a post-emergence herbicide and is generally used for late season 

control of yellow starthistle when plants are at the bolting or spiny stage. It is a non-selective 

herbicide and should only be used in spot treatments and in situations where all desirable 

species have already senesced or where loss of non-target vegetation is acceptable. Glyphosate 

may temporarily result in bare ground if applied early in the season. 

There are also registered herbicides labeled for the control of yellow starthistle not used in California, 

including:

Broadleaf selective herbicides

• Picloram may be used for the control of yellow starthistle. It should be applied from the rosette 

to bud formation stages. Picloram has a long soil residual period, which will reduce re-growth 

from starthistle seedlings for 2-3 years following application. Picloram is mostly safe for 

Chapter 5: An Integrated Yellow Starthistle Management Program 85
Biology and Biological Control of Yellow Starthistle

use on grasses (young monocots may be affected, check the product label for additional 

information), but it will kill desirable legume species and other forbs so its use should be 

carefully considered in relation to existing native species, or those that may be reintroduced 

to a site when used in conjunction with broadleaf revegetation efforts. Picloram is less useful 

in hot, sunny conditions or in sandy soil because it is degraded by sunlight and can leach below 

the root zone in sandy soils. One yellow starthistle population has shown resistance to picloram. 

Resistant individuals are less fit than susceptible plants, and the population has not spread since 

its discovery.

• Dicamba should be applied to yellow starthistle rosettes or bolting plants, prior to bud formation. 

Dicamba is less effective alone and is often mixed with other herbicides to increase weed control 

results. When mixed with diflufenzopyr, dicamba is accumulated in the plant and is more effective 

against yellow starthistle. It has limited to no soil residual activity, so repeated applications are 

necessary to treat late-germinating plants. Dicamba will likely kill desirable broadleaf species, 

including legumes. Alone, it does not kill grasses, sedges, cattails or other monocots (though 

increased effects may be observed when it is used in combination with diflufenzopyr). Ester 

formulations are highly volatile and so should not be applied when temperatures exceed 80°F 

(26.7°C). 

• Chlorsulfuron is a pre-emergent herbicide that can be applied in fall (when used alone) or 

in spring (when in combination with other herbicides) to suppress the germination of yellow 

starthistle. It should not be applied to frozen ground. Chlorsulfuron has no post-emergent activity 

so must be combined with other herbicides to provide season-long control of yellow starthistle 

infestations. Chlorsulfuron is damaging to most forb species; while it is somewhat safe on 

grasses and other monocots, some grasses may be damaged. This herbicide in solution must be 

constantly shaken during application, and it can leach when in the presence of excess water.

Non-selective herbicides

• Imazapyr can be applied to yellow starthistle rosettes or bolting plants. It is soil-active with a 

long residual activity, so is effective in preventing seedling germination. Imazapyr’s soil residual 

activity varies with the rate applied, and may still provide weed control or harm new plantings 

anywhere from 3 months to 2 years post application. It is a non-selective herbicide and should 

only be used in spot treatments and in situations where loss of non-target vegetation is 

acceptable. Even when used as a spot treatment, imazapyr may harm other plants rooted in 

the general area or even downhill, depending on soil conditions and precipitation. It is not 

highly effective against yellow starthistle. Because of its low efficacy and high non-target effects, 

its use should be carefully considered.

When herbicides are used for the control of yellow starthistle, it is important that the applicator adhere 

to all label instructions to ensure the usage, surfactant requirement, application rate, application 

timing and location/site of herbicide application fall within label recommendations. Not all herbicides 

are registered for use on yellow starthistle in all settings (including on or near water), or for use in 

each state of the USA and in Canada. Some herbicides are restricted use and can only be applied by 

a certified and licensed applicator, and then only under specific conditions. Herbicide treatments can 

vary widely depending upon geographic location, climatic conditions and rate of application. Please 

consult your local weed control authority, county agricultural extension educator, or forest invasive 

coordinator to learn which herbicides work best for yellow starthistle control and when to apply them 

in your area. 

86 Chapter 5: An Integrated Yellow Starthistle Management Program
Biology and Biological Control of Yellow Starthistle

Use Herbicides Safely!

Read the herbicide label, even if you have used the herbicide before. Follow all instructions on the 

label.

Wear protective clothing and safety devices as recommended on the label.

Bathe or shower after each herbicide application.

Be cautious when you apply herbicides. Know your legal responsibility as an herbicide applicator. 

You may be liable for injury or damage resulting from herbicide use.

Follow all storage and disposal instructions on the herbicide label.

Chapter 5: An Integrated Yellow Starthistle Management Program 87

If land usage of treated areas includes grazing practices, consult the herbicide label for any grazing 

restrictions that might be applicable.

Heavy herbicide use will kill starthistle growth and is not compatible with biological control. In order 

to guarantee that some biological control agent populations remain viable as the yellow starthistle 

infestations are reduced, plants should either be sprayed late in the growing season when the seedfeeding weevils are overwintering in the soil, or some of the infested area should not be treated with 

herbicides to serve as “refuges” for biological control agents. 

The advantages and disadvantages of the most common yellow starthistle control methods are 

summarized in Table 5-1. 
Biology and Biological Control of Yellow Starthistle

Control Method Advantage Disadvantage 

Biological Control

Sustainable – biocontrol agents generally do not 

have to be reintroduced once established

Measurable changes in weed densities may take 

many years (eradication is not the goal)

Most economical option for large infestations Some risk of undesirable effects on non-target 

plants

Public acceptance is generally higher than with 

other weed control methods Permanent; cannot be undone

Selective Not successful in all situations

Physical Control

(Hand pulling)

Reduces seed production Expensive and time intensive 

Useful for small infestations that must be quickly 

eradicated

Must be repeated regularly due to constant threat of 

re-establishment from seed bank

Physical Control

(Mowing)

Repeated mowing may reduce seed production and 

populations if properly timed

May spread yellow starthistle if done during 

flowering or seeding; or lead to compensatory 

growth if done too early

Removes plant litter, increasing coverage of 

herbicide applications

Expensive and time intensive; requires proper 

timing and equipment

Impractical in rough or difficult to access areas; also, 

in rocky areas it can result in sparks that trigger 

wildfires

Physical Control

(Tillage)

Kills yellow starthistle plants when done frequently 

and when stems are completely removed from roots

May lead to erosion in sloped areas

Only applicable in limited crop settings; impractical 

in rocky soils

May increase yellow starthistle infestation if done 

too early or improperly

Expensive and time intensive; requires proper 

timing and equipment

Cultural Control 

(Burning)

Depletes yellow starthistle seed bank Can decrease desirable competing vegetation

Removes plant litter, increasing coverage of 

herbicide applications made on newly sprouting 

starthistle

Negative effects on animals, air, soil erosion, and 

water

Can stimulate desirable legumes and perennial 

grasses. Risk of fire escaping and damaging large areas

Cultural Control

(Grazing)

With the proper timing, allows use of the land even 

with heavy yellow starthistle infestations

Cannot be used in many natural areas such as 

national parks and wilderness areas

Non-selective; can exacerbate the problem

Can be used (under the right conditions) in 

combination with biological or chemical control 

methods

Can be expensive

Yellow starthistle can recover rapidly post-grazing

Cultural Control

(Re-seeding)

Can be used to restore native or more desirable 

competitive species Expensive for large areas 

Can be self-perpetuating May be ineffective if existing yellow starthistle seed 

bank is extensive

Table 5-1. Comparison of yellow starthistle management options

88 Chapter 5: An Integrated Yellow Starthistle Management Program
Biology and Biological Control of Yellow Starthistle

Compatibility with Biological Control

Eustenopus villosus oviposition and bud herbivory deter oviposition by the other seed-feeding biocontrol 

insects. Damage by rust infection can be additive to seed herbivory at some sites, but under other conditions 

the effects of the rust are negated because Chaetorellia succinea and E. villosus both consume a higher 

proportion of seeds when plants are not infected with the rust.

Applicable only to very small infestations where biocontrol is not recommended. Hand pulling is not directly 

compatible with any biocontrol agent; however, biocontrol can be applied to large, main infestations while 

hand pulling can be used on surrounding small, satellite populations. 

Not compatible with the seed-feeding species. Mowing may aid in the dispersal of Puccinia jaceae var. solstitialis

where conditions suit the rust.

Not compatible with all six approved biocontrol agents. Tillage is only suitable in certain crop settings, where 

biocontrol is not recommended in general.

Not compatible with all six approved biocontrol agents. Can only be compatible if burning is done only on 

some sections of the infestation annually, leaving yellow starthistle “refuges” for the biocontrol agents.

Grazing seed heads infested with insect larvae would destroy the biocontrol agents; however, most grazing 

occurs earlier, making grazing and seed-feeding biocontrol agent compatible.

Compatible if biocontrol agents are introduced after competitive species are established. Also compatible if 

re-seeding is done only on small sections of the infestation annually, leaving yellow starthistle “refuges” for the 

biocontrol agents. In some settings, it is biocontrol that may make re-seeding feasible. 

Chapter 5: An Integrated Yellow Starthistle Management Program 89

Biology and Biological Control of Yellow Starthistle

GLOSSARY

Term Definition

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

adventive A biocontrol species that arrived in the geographical area from elsewhere by any means, not 

through official biocontrol development processes

alternate Where leaves appear singly at stem nodes, on alternate sides of the stem

annual A plant that sprouts, flowers, and dies all in the same year 

antenna (pl. antennae) In arthropods, one of a pair of appendages on the head, normally many jointed and of sensory 

function

app (application) A self-contained program or piece of software designed to fulfill a particular purpose; an 

application, especially as downloaded by a user to a mobile device

arthropod An invertebrate animal having an exoskeleton, a segmented body, and jointed appendages. 

Arthropods form the phylum Arthropoda, which includes the insects, arachnids, myriapods, 

and crustaceans

basal Located at the base of a plant or plant part

biennial A plant that flowers and dies between its first and second years and does not flower in its first 

year

bioherbicide A biologically based control agent for weeds, often using a fungus, bacteria, virus, or other 

pathogen.

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

broadcast application Application of a pesticide to a broad area, as opposed to small localized spots

capitulum (capitula pl.) Seed head of a plant in the sunflower family

community A naturally-occurring group of different species of organisms that live together and interact as a 

more or less self-contained ‘unit’

complete metamorphosis An insect life cycle with four distinct stages (egg, larva, pupa, adult)

compound eyes Paired eyes consisting of many facets, or ommatidia, in most adult arthropods

coordinates A set of numbers used to specify a location 

density Number of individuals per unit area

dicot Plant with two seed leaves upon germination, including most common flowering species, 

excluding grasses, sedges, cattails, lilies and orchids

dissemination Dispersal. Can be applied to seeds or insects

dormancy Period in an organism's life cycle when growth, development, and physical activity are 

temporarily stopped. This minimizes metabolic activity and helps an organism conserve 

energy

dripline The outermost circumference of a tree’s canopy, from which water drips onto the ground

emergence (insect) Act of adult insect leaving the pupal exoskeleton, or leaving winter or summer dormancy

eradicate To get rid of something completely

erect Grows upright and vertical as opposed to prostrate (spreading on the ground)

Glossary 91
Biology and Biological Control of Yellow Starthistle

Term Definition

exoskeleton Hard, external skeleton of the body of arthropods, including insects and mites

exotic Originating in a distant foreign country; not native 

field insectary An area where host plants or animals are abundant and biological control agents are released 

and propagated with or without additional human manipulation

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

forb Herbaceous plant (does not have solid woody stems)

frass Plant fragments, usually mixed with excrement, deposited by feeding insects

gall A plant tumor; a localized proliferation of abnormal plant tissue that is induced by an insect, 

nematode, fungus or other organism and usually exhibits a characteristic shape and color; 

gall-making insects and mites usually live and feed within the gall

genus (pl. genera) A taxonomic category ranking below family and above species and consisting of a group of 

species exhibiting similar characteristics. The genus name is followed by a Latin adjective or 

epithet to form the name of a species

GPS Global Positioning System; a space-based navigational system providing location and time 

information by using four or more satellites

head Insect segment with the mouth parts, antennae, and eyes

herbivory Feeding on plants

host The plant or animal on which an organism feeds; the organism utilized by a parasitoid; a plant 

or animal susceptible to attack by a pathogen

host specificity The highly-evolved, often obligatory association between an insect and its host (i.e. weed). A 

highly host-specific insect feeds only on its host and on no other species

inflorescence The flowering part of a plant

instar The phase of an arthropod’s nymphal or larval development between molts

invasive Tending to spread prolifically and undesirably or harmfully

involucre A circle of bracts under an inflorescence

larva (pl. larvae) Immature stage of some animals, including insects and mites. In insects with complete 

metamorphosis, it is the stage between the egg and pupa (examples include grubs, caterpillars, 

and maggots)

litter Dead plant material, such as leaves, bark, needles, and twigs, that has fallen to the ground

margin (of leaf) The edge of a leaf. Margins typically fall within a handful of categories and are useful in plant 

identification 

membranous Thin and transparent

molting Process of arthropod development that involves shedding its exoskeleton and producing 

another as an arthropod grows

monocot Plant with only one seed leaf upon germination, including grasses, sedges, cattails, lilies, orchids

monotypic An area vegetated by a single plant species

NAD 83 North American Datum, the official datum used for the UTM geographic coordinate system in 

North America

native Of indigenous origin

node Part of the stem of a plant from which a leaf, branch, or root grows

non-target effect When control efforts affect a species other than the species they were enacted to control (can be 

positive or negative)

92 Glossary
Biology and Biological Control of Yellow Starthistle

Term Definition

Nosema Small microsporidian parasites that live in the digestive tract of arthropods

noxious weed A weed whose control is mandated, and whose movement is regulated by federal or state law

nymph Immature form of invertebrates, including mites and insects, that undergoes gradual 

metamorphosis. Resembles adults

outcrossing Introducing unrelated genetic material into a breeding line

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

parasitoid An insect (e.g., a wasp) whose larvae live as parasites, eventually killing their hosts (typically 

other insects)

perennial A plant that lives for more than two years

plant cover The portion of the vegetative canopy in a fixed area attributable to an individual or a single 

plant species

post-emergent herbicide Herbicide that kills plants at any point during the growth cycle

pre-emergent herbicide Herbicide that prevents the germination of seeds

pupa (pl. pupae; v. pupate) Non-feeding, inactive stage between larva and adult for an insect with complete metamorphosis

puparium A rigid outer shell formed from the last larval skin that covers the pupae of some insects, 

especially some fly species

qualitative Measurement of descriptive elements (e.g., age class, distribution)

quantitative Measurement of quantity; the number or amount (e.g., seeds per capitula)

receptacle Part of the stem to which the flower is attached

rosette A compact, circular, and normally basal cluster of leaves

self-incompatible Mechanism in flowering plants that prevents self-fertilization (inbreeding) and promotes 

outcrossing

senescence Final stage in a plant’s life cycle

seed head Mature flower head containing seeds

species A fundamental category of taxonomic classification, ranking below a genus or subgenus and 

consisting of related organisms capable of interbreeding

spot spray application Application of a pesticide to small localized spots or individuals, as opposed to a broad area

surfactant A compound often applied with an herbicide mix to help bring the herbicide into closer 

contact with the leaf surface in order to aid absorption

synchrony Occurring at the same time (e.g., plant flowering and insect oviposition)

taxonomy The classification of organisms in an ordered system that indicates natural relationships. The 

science, laws, or principles of classification; systematics

thorax Body region of an insect behind the head and abdomen, bearing the legs and wings

transect A straight line of varying length along which plants are periodically sampled individually or in 

quadrants

UTM Universal Transverse Mercator, a grid-based geographic coordinate system 

viability The proportion of propagules (e.g., seeds) that are alive and can germinate

weed A plant growing where it is not wanted

WGS 84 The World Geodetic System, a datum for latitude/longitude geographic coordinate systems

Glossary 93
Biology and Biological Control of Yellow Starthistle

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Harley, K.L.S. and I.W. Forno, eds. 1992. Biological control of weeds: a handbook for practitioners and students. 

Inkata Press, Melbourne and Sydney.

Hironaka, M. 1985. Frequency approaches to monitor rangeland vegetation. Proceedings 38th Society Range 

Management Conference at Salt Lake City, Utah, February 11-15: 84-86.

McFadyen, R.E.C. 1998. Biological control of weeds. Annual Review of Entomology 43:369- 393.

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H.C. Evans, P.E. Hatcher, H.L. Hinz, and B.G. Rector, Eds. Proceedings of the XII International Symposium 

on Biological Control of Weeds. 22-27 April 2007, La Grande Motte, France; CAB International. pp. 610-

616. 

Swope, S.M. and I.M. Parker. 2012. Complex interactions among biocontrol agents, pollinators, and an invasive 

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Forest Health Technology Enterprise Team, Morgantown, West Virginia. FHTET-2014-04. 838 pp.

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Chapter 5: An Integrated Yellow Starthistle Management Program

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growth and intensifies negative effects on neighbors. Biological Invasions. 8:1389-1397.

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98 References
Biology and Biological Control of Yellow Starthistle

APPENDIX

Appendix I: Troubleshooting Guide: When Things Go Wrong 

This guide is intended to assist those who encounter problems when establishing biological control agent 

populations. It identifies the probable cause of typical problems and offers solutions.

Problem Probable Cause Solution

Biological control 

agents unhealthy or 

dead when received

Physical damage to biocontrol 

agents in transport

Provide adequate packing material to minimize movement of 

containers and ice packs

Drowning

Do not put water in containers during transport; prevent 

accumulation of excess moisture; too much plant material causes 

condensation

Excess or prolonged heat or cold Keep containers cool at all times; use coolers and ice packs; avoid 

exposure to direct sunlight while in transit

Starvation Put yellow starthistle foliage (no flowers or seeds) in containers

Release delay

Transport or ship biocontrol agents immediately after collection

Release biocontrol agents at new site immediately upon arrival or 

receipt of biocontrol agent

Parasitism and/or disease Check source biocontrol agents. Ensure the insect population is 

disease-free when collecting or receiving shipment

Reproductive problems

Biocontrol agents past reproductive 

stage Collect at peak activity (i.e. insects are mating and ovipositing)

Sex ratio: not enough males or 

females

Collect at peak activity; observe mating among target biocontrol 

agents before collecting; males often emerge earlier than females

Biocontrol agents not synchronized 

with the yellow starthistle growth 

stage

Biological control agents require the weed to be at specific growth 

stage for optimal oviposition; collect biocontrol agents from sites 

with plants in similar stages

Few biological control 

agents collected

Collection at wrong time Refer to Table 4-2 for collection time and technique

Collection technique Biological control agents can be killed/damaged during sweeping or 

aspirating so sweep lightly; avoid debris

Conditions at time of collection 

wrong

Refer to the Chapter 4 section "Collecting Yellow Starthistle 

Biological Control Agents" for guidelines on desirable weather 

conditions

Population insufficient Only collect from well-established populations

Biocontrol agents not 

found after release

Site is unsuitable or too small Refer to the Chapter 4 section "Selecting Biological Control Agent 

Release Sites"

Not enough biocontrol agents 

released

Release as many biocontrol agents as is feasible to ensure survival 

and reproduction (preferably ≥ 200 individuals)

Pesticide use/mowing in area Select sites where land use and management practices do not 

interfere with biological control agent life cycles

Released on wrong species Ensure yellow starthistle is targeted and the correct biocontrol agent 

is used

Released at wrong time Release only during the correct plant stage and in the cool hours of 

the day. Refer to Table 4-2 for guidelines

Biocontrol agents not well adapted 

to conditions

Release field-collected biocontrol agents from local sources wherever 

possible rather than greenhouse-reared adults or insects collected 

from distant locations

Ants or other predators preyed 

upon biocontrol agents

Release only at sites with no obvious ant mounds or high insect 

predator populations (e.g., mice, voles)

Cannot locate release 

site

Location marker not obvious Use a bright-colored wooden, metal, or plastic stake

Site destroyed Communicate with all direct and neighboring land users

Map poorly/incorrectly drawn Check map; redraw with more detail or add landmarks; GPS

Appendix 99
Biology and Biological Control of Yellow Starthistle

Appendix II: Sample Biological Control Agent Release Form

Released By:_________________________ Release Date:___/___/___ State: _______ County:________ 

 (mm dd yy)

Biocontrol Agent:__________________________________________ # Released:________________

Target Weed:______________________________________________ Date Collected:____/____/____

 (mm dd yy) 

Source of Biocontrol Agents:_______________________________________________

Biocontrol Agent Life Stage (circle): Larvae Adults 

Land Ownership (circle): Private County State USFS BLM COE BOR BIA/Tribe TNC Other____________

Legal:T_____ R_____ Sec_____ Q_____ Lat: Deg____Min____Sec______ Long: Deg___Min_____Sec_____

UTM: UTM Datum Zone________ UTM Year________ UTM Easting: _____________ UTM Northing: _____________

ENVIRONMENT

Temperature (°F):________ Wind: Calm Light Moderate Strong Gusty Wind Direction: N S E W

Weather (circle): Clear Ptly Cloudy Cloudy Rain Snow Release Time:____________ AM/PM

Site Aspect (circle): N, NE, E, SE, S, SW, W, NW Elevation:_________________________

Site Slope: Flat (0-10%) ________ Gentle (10-30%) _________ Moderate (30-60%) ________ Steep (>60%) ________

Topographic Position (circle): Valley Bottom Terrace Lower Slope Mid/Upper Slope Crest 

Disturbance: (check all that apply, circle most prevalent) Cultivation ___ Fire ___ Flood ___ Grazing ___ 

 Logging___ Roads____ Mining ____ Recreation ____

SITE CHARACTERISTICS

Site Name:_________________________________ Size of Infestation (acres):_________ % Weed Cover:_________________

Est. Weed Height (cm):_______ Weed Density (# per m2):_______ Dominant Plant:________________________

Distribution of Weed: Isolated_____ Scattered_____ Sc-Patchy_____ Patchy_____ Continuous_____ Linear_____

Phenology: Seedling % ____ Rosette % ____ Bolt % ____ Bud % ____ Flowering % ____ Seed % ____ Dormant % ____

Vegetation Type (circle): Estimate % Cover:

Grassland Tree ____________

Pasture Shrub ____________

Dry Meadow Forb ____________

Moist Meadow Grass ____________

Shrubland Steppe Litter ____________

Conifer Forest Bare Ground ________

Deciduous Forest Rock ____________

Soil Texture: (check) Sand___ Silt___ Clay___ Gravel___ Loam___

100 Appendix
Biology and Biological Control of Yellow Starthistle

Appendix II (cont.): Sample Biological Control Agent Release Form

CONTACT PERSON: LEGAL LANDOWNER:

Name:____________________________________ Name: _________________________________

Address:__________________________________ Address: _______________________________

City:_____________________________________ City:___________________________________

State:_____________________________________ State:__________________________________

Phone: _______ - _________ - ______________ Phone: _______ - ________ - ____________

e-mail:____________________________________ e-mail: ________________________________

Road Map to Site

Site and Vegetation Map

Comments

Appendix 101
Biology and Biological Control of Yellow Starthistle

Appendix III: Standardized Impact Monitoring Protocol (SIMP)

Overview:

A critical part of successful weed biological control programs is a monitoring process to measure populations of 

biological control agents and the impact 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 to enable land managers to take a more active role in monitoring the progress and 

weed control ability of the yellow starthistle seed-feeding insects. 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 seed-feeding insects on yellow starthistle throughout the USA. 

Yellow starthistle:

Yellow starthistle is an herbaceous winter annual typically growing 1-3 feet tall from a 

deep taproot. Rosettes have deeply lobed leaves up to 6 inches long. Plants produce 

one or more rigid, upright stems with multiple spreading branches. Stem leaves are 

typically narrow, unlobed, and attach directly to plant stems by wings that run down the 

sides of the stems. All stems, leaves, and wings are densely covered with cobwebby 

hairs later in the season, resulting in a characteristic gray-green color. Flower heads 

are 0.7 inches across and consist of 10-80 bright yellow florets. Flower heads appear 

in late summer through early fall at branch tips and sometimes at leaf or branch axils. 

The bracts are spiny and up to 1 inch long. Seeds are oblong and up to 1/8 inch long. Plumed seeds are tan 

to brown, located at the center of the seed head, and have a ring of pappus consisting of fine, white bristles. 

Plumeless seeds are dark brown to black, fewer in number, occur around the periphery of the seed head, and 

lack pappus. A large plant is capable of producing up to 100,000 seeds that can remain viable in the soil for up 

to ten years (the majority germinate within three). Yellow starthistle is common in rangelands, along roadsides, 

and in other disturbed habitats such as hayfields, orchards, vineyards and abandoned areas.

Yellow Starthistle Biological Control Agents:

Bangasternus orientalis

The adult weevils overwinter in soil litter and resume activity in spring when yellow 

starthistle is bolting. Adults are 4-6 mm long with somewhat flattened, cylinder-shaped 

bodies and short snouts. Adult bodies are brown with yellow-white hairs that give them 

a mottled appearance. Though adults may feed on starthistle stem wings and leaves, 

the feeding is superficial and does not significantly damage the plant. Females lay eggs 

singly just below flower heads and cover them with dark caps. Females may lay up to 

470 eggs in a lifetime. Eggs are covered with a dark, protective substance. Hatching 

larvae tunnel through the stem to reach the flower head where they feed on bracts, 

receptacle tissue, and developing seeds. Seed consumption does not kill existing 

plants, but does help reduce the rate of spread of starthistle populations. Larvae are up to 6 mm long with 

white, C-shaped bodies and brown head capsules. Pupation occurs in chambers made of damaged seed and 

receptacle tissue. Adults emerge in late summer and overwinter in soil litter. There is one generation per year. 

Chaetorellia australis/Chaetorellia succinea

Chaetorellia succinea is not approved for redistribution.

Adults of both species emerge in early spring as yellow 

starthistle is beginning to bolt. Both species are very similar 

in appearance, with the exception of the spots on the thorax. 

Adults have tan bodies with black spots on the thorax; there 

are 8 spots on the thorax of C. australis while C. succinea 

has 10. Their eyes are multi-colored and metallic, and their 

wings are clear with thick brown bands. Males are 3-4 mm 

long; females are 4-6 mm, including ovipositors. Females lay 

eggs singly beneath bracts of closed buds. After hatching, 

larvae tunnel into capitula and feed on receptacle tissue and 

102 Appendix
Biology and Biological Control of Yellow Starthistle

Appendix III (cont.): Standardized Impact Monitoring Protocol (SIMP)

developing seeds through three instars. Seed consumption does not kill existing plants, but does help reduce 

the rate of spread of starthistle populations. Larvae are somewhat barrel-shaped (though slightly thicker at 

one end) and up to 4 mm long. They are white, turning yellowish with maturity, and lack a head capsule. 

Pupation occurs in barrel-shaped puparia within flower heads inside chambers made of pappus and chewed 

seeds. Adults emerge in early summer, mate, and lay eggs on more starthistle buds. Larvae of this generation 

overwinter within seed heads. There are usually two generations, though three generations are possible where 

the growing season is sufficiently long. First generation C. australis adults emerge one to a few weeks earlier 

than C. succinea, often too early to utilize starthistle. Consequently, they frequently use buds of the earlierflowering invasive weed bachelor’s button (Centaurea cyanus). The second generation of C. australis utilizes 

yellow starthistle.

Eustenopus villosus

Overwintering adults emerge in spring when yellow starthistle is bolting. They feed 

heavily on immature starthistle buds, leading to a high percentage of seed head 

abortion. Adults are 4-6 mm long with oblong, cylindrical bodies and long, slender 

snouts. Adult bodies are brown with lighter-colored longitudinal stripes and are 

covered with long hairs. Females chew holes in the sides of mature, closed buds 

in early to midsummer, lay eggs singly inside, and cap holes with a dark substance. 

Larvae hatch throughout summer and feed on developing seeds through three 

instars. Seed consumption does not kill existing plants, but does help reduce the rate 

of spread of starthistle populations. Larvae are up to 6 mm long with white, C-shaped 

bodies and brown head capsules. Pupation occurs in seed heads in chambers made of damaged seed and 

receptacle tissue. Adults emerge in late summer and overwinter in soil litter. There is one generation per year. 

Larinus curtus

Overwintering adults emerge in summer when yellow starthistle is in bud and flowering 

and feed on florets and pollen. Adults are 5-6 mm long with oval-shaped bodies and 

medium-length snouts. Adult bodies are brown with white hairs that give them a 

mottled appearance. The body hairs are often yellowish from yellow starthistle pollen. 

Females lay eggs singly at the base of florets. Larvae hatch in late summer and feed on 

developing seeds through three instars. Seed consumption does not kill existing plants, 

but does help reduce the rate of spread of starthistle populations. Larvae are up to 6 

mm long with white, C-shaped bodies and brown head capsules. Pupation occurs in 

seed heads in chambers made of damaged seed and receptacle tissue. Adults emerge 

in late summer and overwinter in the soil litter. There is one generation per year. 

Urophora sirunaseva

Adults emerge in spring as yellow starthistle is bolting and starting to form buds. 

Adults have black bodies with a yellow spot on the bottom part of the thorax. Their 

eyes are multi-colored and metallic, and their wings are clear with dark crossbands. 

Males are 3-4 mm long; females are 4-6 mm, including ovipositors. Females lay 

eggs on top of immature, closed buds. After hatching, larvae tunnel into the bud 

and feed on florets through three instars. When they reach the receptacle, a woody 

gall is formed around each larva; multiple galls may occur in one flower head. 

Floret feeding and gall formation reduce seed production, helping reduce the rate 

of spread of starthistle populations. Galls are believed to act as nutrient sinks, 

diverting plant resources away from regular plant function. Larvae are somewhat barrel-shaped (though 

slightly thicker at one end) and up to 4 mm long. They are white, turning yellowish with maturity, and 

lack a head capsule but have a dark anal plate. Pupation occurs in barrel-shaped puparia within 

the woody galls. Adults emerge in summer, mate, and lay eggs on more starthistle buds. Larvae of this 

generation overwinter within galls inside the seed heads. There are two generations per year. 

Appendix 103
Biology and Biological Control of Yellow Starthistle

Appendix III (cont.): Standardized Impact Monitoring Protocol (SIMP)

Monitoring:

The standardized biological control impact monitoring protocol is based upon a permanent 20 meter vegetation 

sampling transect randomly placed in a suitable (at least 1 acre) infestation of yellow starthistle and sweep 

net samples of yellow starthistle seed head insects. Annual vegetation sampling will allow researchers to 

characterize the plant community and the abundance and vigor of yellow starthistle. Sweep net samples of 

yellow starthistle seed-feeding adults will provide researchers with an estimate of their annual population levels.

Permanent Site Set-up:

To set up the vegetation monitoring transect, you will need: 

1) a 25 x 50 cm Daubenmire frame made from PVC 

(preferred) or rebar

2) a 20 m tape measure for the transect and plant height

3) 10 permanent markers (road whiskers and 16 penny 

nails – see picture at right)

4) a post (stake or piece of rebar) to monument the site 

(see pictures for examples of field equipment)

5) 30-45 minutes at the site during the appropriate time 

period for monitoring your desired seed head insect:

Bangasternus orientalis: Late May to late June

Chaetorellia australis: 1st gen: Mid-April to mid-May 

 2nd gen: June to mid-July

Chaetorellia succinea: 1st gen: May 

 2nd gen: Mid-June to July

Eustenopus villosus: June to July

Larinus curtus: Late June to July

Urophora sirunaseva: 1st gen: May 

 2nd gen: June to July

To set up the transect, place the 20 m tape randomly within the infestation. Mark the beginning of the 

transect with a post. Place permanent markers every 2 m (for a total of 10 markers) beginning at the 2 m 

mark and ending with the 20 m mark on the tape measure. Place the Daubenmire frame parallel to the tape 

on the 50 cm side with the permanent marker in the upper left corner starting at 2 m (see pictures). Refer 

to the following monitoring form for how to conduct monitoring. Repeat the frame placement at 2 m 

intervals for a total of 10 measurements (one at each permanent marker). 

104 Appendix
Biology and Biological Control of Yellow Starthistle

Appendix III (cont.): Standardized Impact Monitoring Protocol (SIMP)

Monitoring biological control agents is an essential component of a successful biological control program. Monitoring 

data can be used to accurately document the impact 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. When conducted annually, 

this monitoring data will document changes that occur over time.

Standardized Impact Monitoring Protocol (SIMP) Biological Control Monitoring Form

General Information:

Observer(s): Date: Landowner:

Permanent site? Y N Site name: Weed:

Biological control agent: Insect Stage:

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%

Perennial

Grass%

Bare 

ground%

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 site Insect count

1 10 sweeps repeated 6 times for (circle one):

Bangasternus orientalis Chaetorellia australis

Eustenopus villosus Chaetorellia succinea

Larinus minutus Urophora sirunaseva

2

3

4

5 1

6 2

7 3

8 4

9 5

10 6

Notes:

Appendix 105
Biology and Biological Control of Yellow Starthistle

Appendix III (cont.): Standardized Impact Monitoring Protocol (SIMP)

A step-by-step guide for completing the SIMP biological control 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?

• Biological control agent – Which biological control agent you are 

monitoring?

• Insect Stage – What is the developmental stage of the agent are 

you monitoring (egg, larva, nymph, pupa adult)?

• Lat/Long OR UTM – What are the GPS coordinates of the site 

you are monitoring? If UTM (preferred), what datum and year is 

your coordinate system?

Vegetation Cover (all in %, rows add up to 100%) – All percentages are to 

be estimated to the nearest 5%. If there is a trace of any of the vegetation 

you monitoring in the frame, round up to 5%. • Frame – Which frame number are you working on (1= 2m, 2= 

4m, ...,10 = 20m)?

• 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 perennial 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, 

2=4m,...,10=20m)?

• 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 of the 

target weed in the frame (in cm)?

Biological Control Agent

• Sampling location – Identify 6 sites at least 5 paces away from 

the vegetation transect but within the same weed infestation.

• # of insects per 10 sweeps – How many insects are in your net after 10 sweeps of the 

surrounding vegetation? Take one step between each sweep. Repeat 5 more times (for a total 

of 6 sweep sites, 60 sweeps) moving at least 2 steps away from the last sweep location

106 Appendix
Biology and Biological Control of Yellow Starthistle

Appendix IV: General Biological Control Agent Monitoring Form

SITE: ________________________________________ STATE: ____________ DATE: ___________ __________ ________

 year month day

Last name: ____________________________________ First name: ______________________________________________

GPS: Lat N ________°_________________’ Long W_________°_________________’ Elevation: ______________ ft m 

UTM: UTM Datum Zone: _____ UTM Year:__________ UTM Easting:________________ UTM Northing: _______________

TIME: _____________ TEMPERATURE: ____________ WEATHER: ________________________________________ 

1. Counting Biocontrol Agents

Perform a series of 25 sweeps, counting the adults of all six seed-feeding biocontrol insect species separately. 

Following each count, move to a new part of the infestation and repeat for a total of 5 counts. Record your 

counts in the table below, and then sum your measurements to determine the total number of adults observed.

Sample Bangasternus 

orientalis

Chaetorellia 

australis

Chaetorellia 

succinea

Eustenopus 

villosus

Larinus curtus Urophora 

sirunaseva

1

2

3

4

5

TOTAL

2. Estimating Visual Damage

Eustenopus villosus

Walk throughout the infestation and look closely at yellow starthistle seed heads to identify feeding damage 

by Eustenopus villosus (seed heads will have plugged holes between bracts; some seed heads will be aborted 

and “tipped”). Estimate the overall damage by circling the most appropriate choice in the list below.

• No E. villosus agent feeding observed

• Occasional, scattered feeding damage observed

• Conspicuous, widespread feeding damage observed

Puccinia jaceae var. solstitialis

Walk throughout the infestation and look closely at yellow starthistle foliage to identify rust infection (look 

for small yellow, orange, or rust-colored pustules or lesions). Estimate the overall infection level by circling 

the most appropriate choice in the list below.

• No P. jaceae var. solstitialis infection observed

• Occasional, scattered infection observed

• Conspicuous, widespread infection observed

Notes: ________________________________________________________________________________________

_____________________________________________________________________________________________

_____________________________________________________________________________________________

Appendix 107
Biology and Biological Control of Yellow Starthistle

Appendix V: Yellow Starthistle Qualitative Monitoring Form

SITE: __________________________________ STATE: _________________ DATE: __________ __________ _______

 year month day

Last name: ______________________________________ First name: __________________________________________

GPS: Lat N ________°_________________’ Long W_________°_________________’ Elevation: ______________ ft m 

UTM: UTM Datum Zone: _____ UTM Year:__________ UTM Easting:________________ UTM Northing: _______________

TIME: _____________ TEMPERATURE: ____________ WEATHER: ________________________________________ 

Biocontrol Agent:___________________________Year of release: ______________________________

Cover class estimate by plant category (Overall infestation, ücheck one for each row)

Plant Group 0% 1-5% 6-25% 26-50% 51-75% 76-95% 96-100%

Yellow starthistle

Annual grasses

Perennial grasses

Forbs

Shrubs

Trees

Dominant Plant Species on Site:______________________________________________________________

_____________________________________________________________________________________

_____________________________________________________________________________________

Other Noxious Weeds:______________________________________________________________________

______________________________________________________________________________

Estimate yellow starthistle density class (check one) Yellow starthistle phenology class 

Flowering plants/m2 Yellow starthistle distribution Starthistle stage Estimated percent

0 Isolated Seedling

1-25 Scattered Rosette

26-50 Scattered-Patchy Bolting

50-75 Patchy Flowering

>75 Continuous Senescent

Comments/Observations:

108 Appendix
Biology and Biological Control of Yellow Starthistle

Appendix VI: Yellow Starthistle Quantitative Monitoring Form

SITE: __________________________________ STATE: ______________ DATE: ___________ ___________ _________

 year month day

Last name: _______________________________________ First name: _______________________________________

GPS: Lat N ________°_________________’ Long W_________°_________________’ Elevation: ______________ ft m 

UTM: UTM Datum Zone: _____ UTM Year:__________ UTM Easting:________________ UTM Northing: _______________

TIME: _____________ TEMPERATURE: ____________ WEATHER: ________________________________________ 

Chart A: Cover Class

Class % Cover Class % Cover Class % Cover Class % Cover

0 0 2 6-25 % 4 51-75 % 6 95-100 %

1 1-5 % 3 26-50 % 5 76-95 %

Quad

#

Yellow Starthistle Plants Cover Class

(use Chart A)

# 

Plants

Height (cm) of 4 tallest

stems

StarthistleAnnual 

Grass

Peren. 

Grass Forbs Shrubs Litter Bare ground

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

Notes: __________________________________________________________________________________________________

________________________________________________________________________________________________________

________________________________________________________________________________________________________

________________________________________________________________________________________________________

Appendix 109
Biology and Biological Control of Yellow Starthistle

Appendix VI (cont.): Yellow Starthistle Quantitative Monitoring Form

110 Appendix

Materials needed: 1-meter measuring stick, 1.0 m2 (3.5 ft2) 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 yellow starthistle plant attributes. Monitoring is easier with two people, 

one to make the observations and the other to record data. A transect is made in year 1, with 20 permanent 

markers inserted along the transect (one marker for each quadrat). All 20 quadrats are re-measured in 

subsequent site visits (preferably once per year).

1) Site information: Fill out the site information at the top of the form. 

2) Position the quadrat: Locate the transect using the GPS coordinates and the permanent markers. Position 

the quadrat frame along the transect, as close to the ground as possible, along that transect line. Gently 

arrange vegetation so that plant parts are either within or outside (rather than underneath) the frame, but be 

sure not to damage the plants. The quadrat should be in the same location as the previous year’s quadrat. 

4) Count stems: Count the number of yellow starthistle plants, beginning at one corner of the quadrat and 

working systematically across the quadrat. 

5) Measure stems: Select the four tallest yellow starthistle plants in each quadrat (if there are fewer than 

four plants/quadrat, measure all that are present). Measure the stem height to the closest cm. 

6) Estimate percent cover: Standing over the frame, estimate how much of the quadrat is covered by 

yellow starthistle. Then estimate the percent cover of annual grasses, perennial grasses, other forbs, shrubs, 

plant litter, and bare ground. Use cover estimates in Chart A to estimate the percent cover class. 

7) Measure entire transect: Repeat steps 2-6 20 times at the 20 different quadrats.

8) Other observations: Record any general observations or useful information such as disturbances, 

grazing, fire, etc., for the sample quadrat or the site in general.