Biology and Biological Control of Knapweed

Wilson, L. M. and C. B. Randall. 2003. Biology and Biological Control of Knapweed. USDA-Forest Service FHTET-2001-07. 2nd Edition.

Introduction

Overview

The knapweeds comprise a diverse complex of species that predominantly infest rangelands in the western United States and Canada. This manual considers the biological control of six species of knapweeds: 1) spotted knapweed, 2) diffuse knapweed, 3) squarrose knapweed, 4) meadow knapweed, 5) black knapweed, and 6) brown knapweed. Spotted knapweed, Centaurea stoebe, is perhaps the most widespread species, followed in abundance by diffuse knapweed, C. diffusa. A third species, squarrose knapweed, C. virgata var. squarrosa, has a more limited distribution in the West. Three less widespread species, meadow (C. pratensis), brown (C. jacea) and black knapweed (C. nigra), are host to some of the same biocontrol agents and thus are included. Another well known species is Russian knapweed, Acroptilon repens (formerly known as C. repens). While a serious rangeland weed itself, Russian knapweed is not considered in this manual because it is sufficiently different from the other knapweeds to be considered separately and it has a unique complex of biocontrol agents.

In the United States, approximately 5 million acres are infested with knapweeds. A highly competitive and invasive groups of weeds, knapweeds have adapted to a wide range of habitats and environmental conditions. Although some of the most common rangeland pests in the West, knapweeds also invade pastures and fields in the Midwest and Eastern states. Spotted knapweed, for example, is widespread throughout the United States and found in all but four states (see Chapter 1 for maps of knapweed distribution in the United States).

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

Among the myriad of weed control approaches to manage knapweed is biological control, a well-known and long-established tool in the United States and Canada. Biocontrol of knapweed is one of the earliest, and diverse biocontrol of weeds programs in North America. There is a lot of readily available information describing knapweed biocontrol in general terms. Lacking, however, is a publication that describes knapweeds and their many 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 suitable site, collecting and releasing the biocontrol agents, and follow-up monitoring of the agents and the knapweed.

This manual provides a practical reference for field workers and resource managers when implementing a biological control program for knapweed. It includes information on selecting a release site, collecting and releasing new agents, evaluating past releases, redistributing established biocontrol agents, and monitoring agents and vegetation after the release. The guidelines and timetables outlined in this manual are based on research and practical field experience, and can be used to maximize the success of your knapweed biological control program.

Biological Control of Weeds

Biological control is the deliberate use of naturally occurring organisms to limit the distribution and abundance of a target weed. These are natural enemies of the weed in its native range (i.e.: Europe) and include such organisms as insects, mites, nematodes, and fungi. Natural enemies are also referred to as biocontrol agents, bioagents, biological control organisms and weed herbivores. Plant-eating insects and other organisms may control weeds by killing the weed directly, by weakening or stressing the weed, and by destroying seeds, root, or stems - thereby weakening the weed and limiting its reproduction. Secondary infection from pathogens that invade feeding lesions is indirect damage.

There are a number of advantages to biological control of weeds. Biocontrol with carefully selected agents is not damaging to the environment: it provides long-term impacts on the target plant; it has limited side effects; it is directed to a specific weed or closely related group of weeds; it has nonrecurring costs, and biocontrol agents are self-perpetuating.

Historically, biological control works best on large infestations of a single weed species. It has been most successful on weeds that have been introduced into areas where their specialized natural enemies do not occur. In the United States, knapweed, leafy spurge, rush skeletonweed, tansy ragwort, purple loosestrife, thistles and St. Johnswort are a few examples of weeds with established biocontrol programs.

Knapweeds were introduced from Europe without the complex of organisms that regulate their population densities. In a system known as Classical Biological Control, these natural enemies are identified in knapweed’s native land, rigorously tested to determine what plants they eat (their host range), and finally imported and released into the environment.

It’s very important that the candidate insect and weed are in synchrony. When initiating a biological control program, natural enemies are collected from areas where the weed is native. Specific areas are chosen that are climatically similar to the area where the weed is to be controlled. Ecological and genetic studies are needed to ensure that the lifecycle of the potential biocontrol agent is the same as the lifecycle of the target weed. Potential biocontrol agents undergo 5 to 10 years of rigorous testing to ensure that they eat only the target weeds and will in fact die without the weed. This is known as host-specificity and is the ecological cornerstone of biocontrol of weeds.

These preliminary studies are important in order to:

• Have the best fit between bioagents and knapweed
• Preclude introduction of unapproved organisms
• Protect nontarget species, such as crop plants or rare and endangered plants
• 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 into the US for biological control of weeds. Rigorous 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.

While biocontrol is an effective and important weed management tool, it is not a panacea; it does not ‘fix’ the problem of knapweed. 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 manage weeds while keeping the desired plant community intact. The article listed in the Selected References section entitled, "Biological Control of Weeds," (McFadyen, 1998) provides a review, examples, and a discussion about the advantages and disadvantages of different approaches used to control weeds using biological methods.

About This Manual

This manual provides background information on each of the six knapweed species listed above, detailed descriptions of 13 knapweed biocontrol insects, and elements of a knapweed biocontrol program. The chapters are:

Chapter 1 provides detailed discussions of each of the knapweed species included in this manual. The species are identified by their 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 knapweed biocontrol agents (flies, moths, and beetles) and their basic biology, including information on identification and lifecycle of each of the knapweed biocontrol agents. Information in this chapter is particularly useful in being able to identify each biocontrol agent in the field. Eight species of seedhead feeders and five root borers are described.

Chapter 3 includes detailed elements of a knapweed 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 covers critical references from the comprehensive body of literature on knapweed biology, ecology, and biological control.

Appendices contains various insect release and monitoring forms, checklists, vegetation monitoring forms, and most important, a troubleshooting guide.

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