Invasive and Exotic Species

Overview

Purple loosestrife, Lythrum salicaria (Figure 1), is a member of the Loosestrife family (Lythraceae). It is a large, perennial, wetland plant that can grow up to 9 feet (3 m) tall. It was introduced to the northeastern United States and Canada in the 1800s from Europe, probably as seed contained in soil used as ship’s ballast. It was also brought to North America for ornamental and medicinal purposes. Once established in North America, this invasive weed quickly spread in wetlands and waterways. It is reported in nine Canadian provinces and in the United States it occurs in all states except Florida and Hawaii (Figure 2). Although purple loosestrife is declared noxious in 23 states and 5 Canadian provinces, it is still sold as an ornamental plant in some places. In its native range, purple loosestrife occurs from the United Kingdom west to western Russia and from Finland in the north to southern Europe and northern Africa in the south.

Figure 1. Purple loosestrife. (UGA1291003)

Figure 2. Map of purple loosestrife distribution in the United States and Canada.

Purple loosestrife invades natural and disturbed wetlands, such as stream banks, lakeshores, marshes, bogs, fens, sedge meadows, canals, drainage ditches, reservoirs, riparian meadows, wet prairies, and sub-irrigated pastures. Established plants can tolerate drier sites, posing a threat to agricultural lands and pastures.

Once established, purple loosestrife quickly crowds out most native vegetation, such as sedges, rushes, cattails, and other wetland plant species. At high densities, purple loosestrife can create near-monocultures. In addition to the loss of native biodiversity, purple loosestrife harms waterfowl nesting habitat, has negative impacts on some amphibians and algal communities, reduces water flow and quality, inhibits transportation, and degrades hunting and fishing areas.

Successful management of purple loosestrife is an intensive process. Herbicides approved for use around water provide short-term control of small infestations or isolated plants, but may be impractical and uneconomical against large infestations. Pulling small, individual plants is feasible; pulling large plants is very difficult. Plowing or disking small infestations is also possible, but will require repeated treatments as plants will sprout from cut root pieces. Plowing large infestations is difficult and generally ineffective. Mowing, burning, and flooding can be effective management strategies, but are difficult, expensive, and time-consuming, and may have severe negative, long-term consequences for wetland communities.

The biological control program for purple loosestrife began in 1985 at the International Institute of Biological Control in Switzerland with the search for, and testing of, potential biological control agents. In 1992, two European leaf-feeding beetles and a root-feeding weevil were imported and released. An additional flower-feeding weevil was subsequently introduced in 1994, completing the complex of four insects for biocontrol of purple loosestrife in the United States and Canada. Today, biological control of purple loosestrife is one of the most widely implemented biocontrol of weeds programs in North America.

Biological Control of Weeds

Most invasive weeds in the United States are not native: they arrived with immigrants and commerce from Europe and, to a smaller extent, South Africa, Australia, and Asia. Some plant species were brought intentionally for medicinal purposes or to adorn gardens; others arrived as stowaways in ship’s ballast or clinging to livestock, clothing, and other goods. Exotic plant species continue to be introduced accidentally as contaminants of agricultural goods or purposefully through travel and commerce (particularly for gardening), and some of these species will likely prove to be invasive in the future. These immigrant plants are generally introduced without their natural enemies, a complex of plant-eating organisms that are specialized to feed only on a particular plant species. Lack of control by its natural enemies is one of the main factors explaining why non-native plant species become major pests in the new area of introduction.

Biological control of weeds is the deliberate use of naturally occurring organisms to limit the distribution and abundance of a target weed. Classical biological control uses host-specific, natural enemies from the plant’s native range (the terms “biological control” or “biocontrol” used throughout this manual refer to classical biological control). Natural enemies (also referred to as “biocontrol agents,” “bioagents,” and “biological control organisms”) can directly kill or severely damage plants by destroying seeds, roots, foliage, or stems. Their damage may limit weed reproduction or facilitate secondary infection from pathogens. These stresses reduce the weed’s ability to compete with other plants. The aim of weed biocontrol, therefore, is to restore at least a part of the ecological balance that limits the competitive ability of an invasive plant species in its native range.

There are a number of advantages to classical biological control of weeds. Biocontrol is selective of a specific weed or closely related group of weeds; it can provide long-term control; and the biocontrol agents are self-perpetuating, thus avoiding recurring acquisition, rearing, and reintroduction costs. The disadvantages of biocontrol of weeds are the high initial costs of a program and the uncertainty that the agents will be effective. Additional disadvantages include the risk of unintended, adverse impacts on other plant species (non-target effects) and that, once released, biological control agents cannot be retrieved. Because biocontrol agents are irretrievably introduced into the environment, they must be carefully selected and extensively studied before being approved for release.

Natural enemies used in classical biological control of weeds include insects and mites, and sometimes nematodes and fungi. Beetles, flies, and moths are among the most commonly used insects. To be considered for release in the United States, insect biological control agents must eat and develop only on the target plant, and in some cases, on only a few closely related plant species. Also, the insect’s lifecycle should be closely matched, or synchronized, with the target plant’s. For example, when properly synchronized, foliage-feeding insects would be in the feeding stage when the weeds are actively growing, and root-feeding insects would be in the feeding stage when resources in below-ground tissues are at their maximum.

The most important precondition for an insect to be used as a biocontrol agent is that it will die without the target plant. This is known as host specificity, and is the ecological cornerstone of classical biological control of weeds. Potential biocontrol agents often undergo more than five years of rigorous testing to ensure that host-specificity requirements are met. These studies are important in order to:

• Have effective, safe biocontrol agents
• Introduce biocontrol agents that damage only the target plant
• Protect non-target plant species

The United States Department of Agriculture–Animal and Plant Health Inspection Service–Plant Protection and Quarantine (USDA-APHIS-PPQ) is the federal government agency responsible for authorizing the importation of biocontrol agents into quarantine. All states have their own final approval process to permit field release of plant biocontrol agents. The Technical Advisory Group (TAG), an expert committee of volunteers representing federal land management and environmental protection agencies, reviews all petitions to import new biocontrol agents, and makes recommendations to USDA-APHIS to aid their decision making process. Effective laws and regulations are in place to minimize the risks to native plant and animal communities associated with the introduction of exotic organisms to manage weeds. Weed biocontrol researchers work closely with USDA-APHIS-PPQ and TAG to further maximize the environmental safety of weed biocontrol programs.

Although weed biocontrol is an effective and important ecological management tool, it does not work in all cases and is not expected to completely eradicate a weed. For example, the expectation of classical biological control of purple loosestrife is to reduce the abundance of purple loosestrife to levels that do not damage the rest of the plant community. In general, classical biocontrol programs reduce a weed’s ability to compete with native plants. Often, biological control can be integrated with other methods of weed control—e.g., chemical or cultural control and livestock grazing. For more information on classical biological control of weeds we recommend reading the literature listed in the Selected References section of this manual.

About This Manual

This manual provides background information on purple loosestrife biology and on each of the four insects selected for biocontrol of purple loosestrife. It also provides guidelines to establish and manage a purple loosestrife biocontrol program. The manual is divided into the following chapters:

Chapter 1 provides a detailed description of purple loosestrife, including scientific name, description of the leaves, stems, flowers, seeds, habitat, and occurrence in the United States and Canada. Photographs, and drawings are provided. Closely related Lythrum species, as well as unrelated plant species often confused with purple loosestrife are also described.

Chapter 2 describes the four purple loosestrife biocontrol agents: two leaf beetles and two weevils. Included is information on biocontrol agent identification, lifecycles, and biology. This chapter is particularly useful for identifying biocontrol agents in the field.

Chapter 3 includes detailed information and guidelines on how to plan, implement, monitor, and evaluate an effective purple loosestrife biocontrol program incorporating one or more of the four biocontrol agents. Included are guidelines and methods for:

• Planning a purple loosestrife biocontrol program
• Selecting and preparing release sites
• Collecting, handling, transporting, shipping, and releasing purple loosestrifebiocontrol agents
• Monitoring of biocontrol agents and their effects on vegetation

Glossary defines technical terms essential to communicating specifics of purple loosestrife biological control.

Selected References are provided from the large body of literature on purple loosestrife biology, ecology, and biological control. Also included is a comprehensive list of Internet web sites that provide a wealth of practical information.

Appendix provides a troubleshooting guide, examples of insect release and monitoring forms, and vegetation monitoring forms. (These forms may be reproduced for use as needed.) These are, in order of appearance:

Troubleshooting Guide: When Things Go Wrong
Purple loosestrife Biocontrol Release Form
Monitoring Plan Questionnaire
Form 1: Purple Loosestrife Biocontrol Monitoring – Site Location
Form 2: Purple Loosestrife Biocontrol Monitoring – Biocontrol Agents
Form 3: Purple Loosestrife Biocontrol Monitoring – Vegetation
Form 4: Purple Loosestrife Biocontrol Monitoring – Associated Vegetation

Troubleshooting Guide:

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