Biological Control of Invasive Plants

What is Biological Control of Invasive Plants?

Biological control or “biocontrol” is the intentional introduction of insects, mammals or other organisms to reduce invasive species populations. Biocontrol agents against invasive plants can sometimes kill them outright, or reduce their ability to spread further and compete with other plants. This is achieved by targeting an invasive plant’s flowers, seeds, roots, foliage, or stems. 

Invasive plant biocontrol is most effective on large infestations, although it can take many years to make a significant impact. Biocontrol is an effective and important IPM tool, but it does not work in every situation and should not be expected to completely eradicate the target invasive plant. When possible, biocontrol should be integrated with other IPM management strategies such as chemical, mechanical, and cultural methods to improve overall control success. 

Request a Biological Control Agent Release

Biological Control Agent Research Process

Modern biocontrol permitting for release in the United States requires that potential agents must go through a multi-year, rigorous research process to determine their host-specificity status. Host-specificity means that biocontrol agents will only feed on the targeted invasive species and nothing else. The United States Department of Agriculture’s Animal and Plant Health Inspection Service - Plant Protection and Quarantine (USDA-APHIS-PPQ) is the federal agency responsible for providing the testing guidelines and authorizing the importation of biocontrol agents into the United States.

Spotted Knapweed

There have been ten species of biological control agents released in Wyoming for spotted and diffuse knapweed. The four most impactful species against spotted knapweed in Teton County are described below. To learn more about the other knapweed biocontrol agents, visit the Wyoming Weed & Pest Council’s Biological Control Storymap.

Spotted Knapweed Flower Weevils

The Lesser Knapweed Flower Weevil (Larinus minutus) and the Blunt Knapweed Flower Weevil (Larinus obtusus) are very similar in appearance and biology.  L. minutus prefers to feed on diffuse knapweed, while L. obtusus prefers spotted knapweed. Both weevils, however, will feed readily on both knapweed species and are well-established throughout Wyoming. These weevils work well with the Knapweed Root Weevil. 

Eggs are laid in the flowerbuds. Once hatched, the larvae consume the developing seeds, reducing knapweed seed production. These weevils then pupate in the seed head. In late summer or early fall, adult weevils chew their way out of the seed heads to overwinter in the soil, leaving noticeable exit holes. 

Knapweed Root Weevil

The Knapweed Root Weevil (Cyphocleonus achates) is a large, mottled weevil that will feed on both spotted and diffuse knapweed. They work well in conjunction with the Knapweed Flower Weevils and are well-established throughout Wyoming. C. achates larvae attack knapweed plants at the roots. Root feeding hinders the ability of knapweed to take up nutrients from the soil, and can result in plant mortality in young plants. When multiple larvae occupy a root, this can lead to a reduction in plant biomass and density of knapweed stands. Mid-summer, pupation occurs in the roots. Late summer, adults chew their way out of the root. Overwintering occurs in the larval stage.

Sulfur Knapweed Moth

The adult Sulfur Knapweed Moths (Agapeta zoegana) have bright yellow forewings with brown wing bands. Adults are short lived, mating within 24 hours after emergence in late summer and fall. Females lay eggs on stem crevices and leaves the next day. Newly-hatched larvae migrate to the crown and mine roots. Larval damage to the roots hinders knapweed’s ability to take up soil nutrients and reduces the biomass and density of infestations of mature plants. This can kill small plants if the roots are completely consumed. Larvae overwinter in the roots and continue to feed on those roots in the spring.

Banded Knapweed Gall Fly

The Banded Knapweed Gall Fly (Urophora affinis) adults have dark bodies and clear wings with horizontal brown bars. Adults emerge when knapweed is in the bud stage, and females lay eggs in immature flowers. Newly-hatched larvae tunnel towards the seed head base where they feed. This feeding triggers a hard, woody gall to form. Larval feeding destroys some seeds, reducing knapweed seed output. These galls also deprive nutrients from other plant parts, which causes stunting and reduced flower production. Larvae overwinter inside the flower head gall, pupate in the spring, and emerge as adults in mid to late spring.

Common Mullein

Mullein Seed Head Weevil

The Mullein Seed Head Weevil (Rhinusa tetra), previously referred to as Gymnetron tetrum, emerges as an adult in early spring. Adult females lay their eggs in the flowers. Once hatched, larvae feed on developing seeds. Larvae are cannibalistic and only two will survive inside one seed head capsule. Pupation occurs in the seed head and mature adults will eventually chew their way out, often causing the seed to die. Overwintering occurs in the adult stage.

St. Johnswort

St. Johnswort Inchworm

St. Johnswort Inchworm (Aplocera plagiata) larvae, or the "inchworms," are reddish brown and resemble twigs. The adult moths have gray wings with dark gray bands. There are up to two generations each season, and damage occurs when the larvae feed on foliage. This weakens St. Johnswort, but doesn’t kill plants outright. When there are large populations of A. plagiata, flower and seed production can be reduced. Overwintering occurs in the larval stage.

Klamath Beetles

Klamath Beetles (Chrysolina quadrigemina) are oval metallic beetles that emerge in early summer as St. Johnswort is flowering. They feed heavily on foliage before resting in the soil over the summer. Larval feeding can wipe out St. Johnswort populations. Adults can cause some defoliation as well. When the fall rains begin, the beetle becomes active again to feed on foliage. Females lay eggs on the leaves as St. Johnswort is senescing and overwinter in this stage. In the case there aren’t enough fall rains, however, adults will overwinter. From overwintering eggs emerge larvae in the spring to feed while St. Johnswort is bolting. Pupation occurs in the spring within the soil.

Dalmatian Toadflax

Dalmatian Toadflax Stem Mining Weevil

The Dalmatian Toadflax Stem Mining Weevil (Mecinus janthiniformis) is a small and elongate, bluish-black weevil with a long snout. Adults emerge from the previous year’s toadflax stems in mid-spring. Throughout the spring, females will lay eggs within stems of new toadflax plants. Once hatched, larvae feed inside the stem. Stems are also where the larvae pupate and adults overwinter. M. janthiniformis are well-established throughout Wyoming. M. janthiniformis looks identical to the Yellow Toadflax Stem Mining Weevil (Mecinus janthinus), but is only effective on Dalmatian Toadflax. As the name suggests, the larvae mine the stems of Dalmatian Toadflax. Larval feeding inside the stems reduces water and nutrient transport in the plant, causing deformation, desiccation, and plant death. Adults will also feed on foliage, inhibiting photosynthesis, suppressing flowering and seed production, and stunting roots and shoots.

Yellow Toadflax

Yellow Toadflax Stem Mining Weevil

The Yellow Toadflax Stem Mining Weevil (Mecinus janthinus) is also a small and elongate, bluish-black weevil with a long snout. Adults emerge from the previous year’s toadflax stems in mid-spring. Throughout the spring, females will lay eggs within stems of new toadflax plants. Once hatched, larvae feed inside the stem. Stems are also where the larvae pupate and adults overwinter. Yellow toadflax is less common in Wyoming than Dalmatian, but populations of M. janthinus have established in Teton County. Mecinus janthinus looks identical to the Dalmatian Toadflax Stem Mining Weevil (M. janthiniformis), but is only effective on Yellow Toadflax. As the name suggests, the larvae mine the stems of Yellow Toadflax. Larval feeding inside the stems reduces water and nutrient transport in the plant, causing deformation, desiccation, and plant death. Adults will also feed on foliage, inhibiting photosynthesis, suppressing flowering and seed production, and stunting roots and shoots.

Canada & Musk Thistle

There have been nine species of biological control agents released in Wyoming for Canada, Bull, and Musk thistle. The five most impactful species are described below. To learn more about the other Canada thistle biocontrol agents, visit the Wyoming Weed & Pest Council’s Biological Control Storymap.

Canada Thistle Rust Fungus

The Canada Thistle Rust Fungus (Puccinia punctiformis) has spores that germinate on the leaves of Canada thistle. The fungus then travel from the leaves down through the stem to infect the roots. This can cause weakening or mortality of the plant. Infected plants will have orange-colored pustules on the leaves in spring, and dark, reddish-brown colored spores in the fall. Distribution of Canada thistle rust fungus has only occurred recently in Wyoming, and the determination of its effectiveness locally is ongoing. 

Canada Thistle Stem-Mining Weevil

The Canada Thistle Stem-Mining Weevil (Hadroplontus litura), formerly known as Ceutorhynchus litura, has a varied impact on Canada Thistle infestations. In some areas, impact has been negligible. In wet pastures and riparian areas, attack from the weevil has resulted in shoot mortality and broad declines in shoot density. Adults will feed on leaf and stem tissue, whereas larvae mine leaf veins, stems, and root crowns.

Canada Thistle Stem Gall Fly

The Canada Thistle Stem Gall Fly (Urophora cardui) is well-established throughout Wyoming. Although it has low impact on thistle density, it can be effective at stunting plants and reducing seed production. Larvae burrow into the stems, causing galls to form. Larval feeding within the galls diverts nutrients away from the rest of the plant. U. cardui prefers moist areas with patches of sun and shade with scattered host plants.

Thistle Seed Head Weevil

The Thistle Seed Head Weevil (Rhinocyllus conicus) was first introduced into the United States in 1969 for the control of Musk thistle. It is a very effective agent, but pre-release testing prior to 1969 determined that this weevil would also attack other thistle species. At that time, native thistle species were not valued as they are today. Social and environmental values have since shifted, resulting in changes to the criteria required before new biological control agents are approved for release, such as single host-specificity. In 2000, USDA APHIS PPQ revoked permission for interstate distribution of this agent. 

‍R. conicus was first released in Wyoming in 1976. It is well-established throughout the state today and likely already present on most Must thistle infestations. R. conicus females lay eggs on the flower heads. These eggs are covered by tan “warts,” which are unique to R. conicus and are an easy way to determine if this species is present. The larvae feed on developing seeds of Bull, Canada, and Musk thistle as well as several others. If one were to break open a Musk thistle flower head, larvae and pupae may be present inside. Because this weevil is already well-established but no longer approved for interstate movement, releases are no longer being conducted.

Thistle Crown Weevil

The Thistle Crown Weevil (Trichosirocalus horridus) is similar to the Thistle Seed Head Weevil in that it is also widely established throughout Wyoming but permissions for interstate movement have been revoked by USDA APHIS PPQ. This was also due to non-target feeding.

Females lay their eggs on the underside of young thistles, primarily Musk thistle but other species as well. Newly-hatched larvae feed on tissue at the root-stem junction. Larval feeding can reduce thistle vitality, reduce seed production, and alter growth. Adults are easily distinguishable from R. conicus by the noticeable spines covering bodies.