Linda M. Wilson, Cynthia Jette, John Connett, Joseph P. McCaffrey. 2003. Biology and Biological Control of Yellow Starthistle. USDA Forest Service FHTET-1998-17 2nd Ed.

Introduction

Overview Yellow starthistle (Centaurea solstitialis L.) is an invasive weed introduced from the Mediterranean region. It predominantly infests rangelands in the western United States. This second edition manual on the biological control of yellow starthistle is intended to give weed and land managers guidelines to develop and successfully establish a starthistle biocontrol program (Fig. 1).

Figure 1. Flowering yellow starthistle near Peck, Idaho (UGA1350002).

In the United States, approximately 18 million acres are infested with yellow starthistle. A highly competitive and invasive weed, yellow starthistle has adapted to a wide range of habitats and environmental conditions, mostly in California, Oregon, Washington and Idaho (see Fig. 4, page 7).

A large amount of information is available giving land managers useful tools to manage yellow starthistle by a variety of strategic methods. Chemical, cultural and mechanical methods used to control weeds all apply when managing yellow starthistle. However, most people recognize that yellow starthistle management on a large scale over the landscape requires well-planned, integrated programs that maximize the effective use of all weed management strategies in combination.

Among the myriad of weed control approaches to manage yellow starthistle is biological control, a well known and long-established tool in the United States and Canada. There is a lot of readily available information describing certain aspects of yellow starthistle biological control This manual provides a practical reference for field workers and resource managers that provides information on starthistle and its biocontrol agents, combined with a how-to, on-the-job reference that outlines, step-by-step, the process of establishing a biocontrol program, including selecting a release site, collecting and releasing agents, evaluating past releases, and monitoring agents and vegetation after the release. The guidelines and suggested timetables outlined in this manual are based on research and practical field experience, and can be used to maximize the success of your yellow starthistle biological control program.

Biological Control of Weeds

Biological control of weeds is the deliberate use of naturally occurring organisms to limit the distribution and abundance of a target weed. These organisms are the ‘natural enemies’ of the weed in its native range (i.e., the Mediterranean) and include such organisms as insects, mites, nematodes, and pathogens. Natural enemies are also referred to as biocontrol agents, bioagents, biological control organisms and weed herbivores. Insects and other organisms can be very damaging to a host plant. They might control the weed by killing it directly, by weakening the ability of the weed to compete, by destroying seeds, roots, or stems thus limiting reproduction in the weed, or by enabling secondary infection from pathogens to invade feeding lesions.

There are a number of advantages to biological control of weeds. Biocontrol with carefully selected agents is not damaging to the environment; it is directed to a specific weeds or closely related group of weeds, thus it is highly selective, it provides long-term impacts on the target plant, it has limited side effects, it has nonrecurring costs, and biocontrol agents are self-perpetuating. Also, once an agent is introduced, it reproduces without additional cost to the land manager.

Historically, biological control has worked best on large infestations of a single weed species. It has been most successfully used on weeds that have been introduced into areas where their specialized natural enemies do not occur. For example, most of our rangeland weeds are not native; they arrived with immigrants and commerce from Europe and elsewhere, sometimes brought intentionally to adorn future gardens in the New World or sometimes as ship ballast or as stowaways stuck to clothing and goods. These plants, destined to become major pests in their new home, were free of their complex of natural enemies from the old country, and thus enjoyed unrestrained expansion.

In a system known as Classical Biological Control, natural enemies are identified in the weeds native range, and rigorously tested to determine what plants they eat (their host range). These areas need to be climatically similar to the area where the weed is to be controlled. Ecological and genetic studies are conducted to ensure that the biology and lifecycle of the insect is closely synchronized with the host plant. Biocontrol agents selected for study undergo 5 to 10 years of rigorous testing to ensure that they have a very narrow range of suitable host plants. In order to qualify as a biocontrol agent, an insect is only allowed to eat and develop on yellow starthistle and in some cases on a few very closely related plant species. The most important precondition for an insect to be used as a biocontrol agent is that it will die without yellow starthistle.

These preliminary studies are important in order to:

• Have the best fit between bioagents and yellow starthistle
  
  
• Preclude introduction of unapproved organisms
  
  
• Protect non-target plant species
  
  
• Influence future assessments of risk
  
  
• Affect future evaluation processes

The USDA Animal and Plant Health Inspection Service (APHIS) is the governing agency responsible for authorizing the importation of an insect and other organisms for biological control of weeds. Laws and regulations are in place to minimize risks associated with introducing foreign organisms. Biocontrol researchers work closely with APHIS to maximize safety in biocontrol programs.

Although biocontrol is an effective and important weed management tool, it is not a panacea; it doesn’t "fix" the problem of yellow starthistle. The goal of biological control is not to eradicate starthistle but to reduce its competitive ability so that yellow starthistle-infested sites can be colonized by desirable plant species. In the most effective programs, biological control is used along with other methods of weed control. In fact, many land managers, ranchers, and farmers use integrated weed management systems, which combine more than one method to control weeds while keeping the plant community intact. The article listed under the Selected References chapter, entitled "Biological Control of Weeds", by R. McFadyen, provides a review, examples, and a discussion about advantages and disadvantages of different approaches used to control weeds using biological methods.

About This Manual

This manual provides background information on yellow starthistle and each of the six yellow starthistle biological control insects, and provides guidelines to establish and mange a starthistle biocontrol program. The chapters are:

Chapter 1 provides detailed description of yellow starthistle, including scientific name, description of the leaves, stems, flowers, seeds, and habitat and occurrence in the United States. Photographs, drawings, and distribution maps are also provided.

Chapter 2 features the six starthistle biocontrol agents; three species of flies and three species of beetles. Included is information on their biology, identification and lifecycle. This chapter is particularly useful for identifying each biocontrol agents in the field.

Chapter 3 includes detailed elements of a starthistle biocontrol program (planning, implementing, and evaluating). It encompasses techniques for all the agents. Included are guidelines for:

• Developing work schedules for field activities
  
  
• Selecting and preparing a release or nursery site
  
  

• Collecting, handling release, transporting and shipping biocontrol agents
  
  

• Monitoring agents and vegetation at the release site

Glossary defines technical terms essential in using and communicating about biological control effectively.

Selected References provides critical references from the comprehensive body of literature on yellow starthistle biology, ecology, and biological control.

Appendices A-G contains various insect release and monitoring forms, checklists, vegetation monitoring forms, and a troubleshooting guide.

• Appendix A: Troubleshooting Guide: When Things Go Wrong
  
  
• Appendix B: Sample Biocontrol Agent Release Form
  
  
• Appendix C: Monitoring Plan Questionnaire
  
  
• Appendix D: Biocontrol Monitoring Report
  
  
• Appendix E: Qualitative Monitoring Form
  
  
• Appendix F: Quadrat Density and Cover Data Form
  
  
• Appendix G: Macroplot Design for Measuring Density

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