Request for Comments: Input on Potential Use of Insects for Control of Spotted Knapweed in Michigan

Request for Comments: Input on Potential Use of Insects for Control of Spotted Knapweed in Michigan

The Michigan Department of Agriculture and Rural Development (MDARD) is requesting comments on permitting the release of two species of insect that are scientifically proven to help prevent the spread of spotted knapweed (Centaurea stoebe micranthos, also known as star thistle), an invasive weed. These insects - seedhead weevil (Larinus minutus) and root boring weevil (Cyphocleonus achates) - feed exclusively on spotted knapweed and die off after that plant is removed from the landscape. Input will help ensure any future decisions regarding the use of these insects balances needs of interested parties, ecological considerations, and scientific understanding.

Stakeholders can provide comments through an online survey. All comments must be submitted by 11:59 pm EST on Friday, March 6, 2026.

Regulatory Authority:

Michigan’s Insect Pest and Plant Diseases Act (Act 189 of 1931) requires a permit for any living insects shipped into or within Michigan. MDARD typically accepts federal PPQ 526 permits in fulfillment of this requirement. However, the federal government has determined that the insects considered in this proposal are low-risk, and it therefore no longer requires PPQ 526 permits to move said insects across state lines.

MDARD is therefore seeking public comment to consider all perspectives on whether and how to issue a state-level permit under Act 189 of 1931 to enable the importation into Michigan of certain species of weevil for spotted knapweed control.

Background:

Spotted knapweed (also known as star thistle) was introduced into the United States in 1890s through commercial seed imports from Eurasia. It is often found in heavily disturbed sites, such as roadsides, gravel pits, and agricultural field margins, but it also can be present in undisturbed dunes, dry prairies, oak and pine woodlands and rangeland. Knapweed can produce as many as 1,000 seeds per plant, which can be dispersed via wind by up to 5,000 seeds per square foot. The seeds can remain viable for up to eight years.

Positive Environmental Impacts of Spotted Knapweed:

Native pollinators have been observed feeding on spotted knapweed, and beekeepers rely on it as a crucial late-season nectar source, particularly in areas where other plants have stopped blooming. Removal or widespread control of spotted knapweed therefore may have a negative effect on already declining populations of native pollinators as well as the beekeeping industry.

Negative Environmental Impacts of Spotted Knapweed:

Spotted knapweed’s ability to out-compete native plants poses a significant threat to conservation efforts due to its highly invasive nature and multitude of negative ecological impacts:

• Displacement of native plants and reduced biodiversity: Spotted knapweed outcompetes native plants for resources. It produces chemicals that inhibit the growth and germination of surrounding plants, giving it a further competitive advantage and allowing it to form dense stands, effectively crowding out native vegetation and reducing overall plant diversity. This loss of native plant diversity adversely affects wildlife that rely on those native plants for food and shelter.[1]^,[2]^,[3]^,[4]
• Impacts on grazing resources: Wildlife and most livestock find spotted knapweed unpalatable, further limiting available food sources in infested areas.^10,11
• Degradation of soil and water resources: By displacing native grasses, spotted knapweed contributes to increased soil erosion, surface runoff, and stream sedimentation. Increased sedimentation can harm streams and aquatic organisms. Studies suggest that spotted knapweed can alter the composition and overall health of the soil, potentially impacting long-term ecosystem function.⁹
• Hindrance to restoration efforts: Spotted knapweed's ability to produce thousands of seeds that remain viable in the soil for years makes it difficult to eradicate. While various control methods exist (manual removal, herbicides, biological control), effectiveness is limited, and consistent, long-term management is often required.[5]^,[6],[7],[8]

Management Approaches:

The most effective control strategy for large infestations of spotted knapweed is an integrated approach using biological control insects, mechanical removal (hand-pulling or grazing), and herbicides. Biological control insects are used in many states to control large stands of spotted knapweed.[9]^,[10],[11] The most effective insects are a combination of seedhead (Larinus minutus) and root-boring (Cyphocleonus achates) weevils. These insects have been approved for release by U.S. Department of Agriculture (USDA) and only survive as long as knapweed is present. According to the USDA, the biological control insects selected for spotted knapweed have evolved to feed exclusively on the plant, posing no threat to other native plants or crops.

Mechanical and fire-based control methods, such as mowing and prescribed burning, have shown limited or inconsistent effectiveness in controlling spotted knapweed.[12]^,[13],[14] Mowing alone does not kill the plant and may even stimulate regrowth or flowering if not timed precisely.[15]^,[16]

Current Status of Biological Control Insects in Michigan

The seedhead weevil (Larinus minutus) is already widespread in Michigan, and permits have been approved for this species to be released in Michigan in the past.

The root boring weevil (Cyphocleonus achates), however, has not been shown to be widely distributed in Michigan, and permits have not been approved for release in Michigan other than for research in very limited areas. Data shows that the combination of both types of weevils provide the best spotted knapweed control.^2,7,[17]

Requests for permits for both species are commonly brought to MDARD. While permits for the seedhead weevil have been approved in the past, MDARD is now seeking public input to ensure that any future decisions regarding their use continue to reflect current stakeholder needs, scientific understanding, and ecological considerations.

Proposal:

MDARD is considering four approaches for permit approval:

1. Blanket Approval: Permits would be granted for the release of both biological control insects across all affected areas. This approach supports broader, more consistent management of spotted knapweed and may reduce administrative burden. However, it offers less flexibility to address site-specific concerns and assumes that current understanding and past stakeholder input are sufficient to guide implementation.
2. Conditional Approval: Permits would be approved on a case-by-case basis, with specific conditions tailored to each location to ensure the safety and effectiveness of any release of either of these biological control insects. This approach allows for a potential compromise, but it will require ongoing input and feedback from stakeholders via formation of work group.
3. Research-Only Use: Permits would be limited to controlled research settings or pilot projects in select locations. This approach allows for continued study of the insects’ effectiveness and ecological impact in Michigan without broader release. It prioritizes caution and data collection but may delay widespread management benefits.
4. No Approval: No permits would be issued for the release of either biological control insect at this time. This approach eliminates the risk of unintended ecological consequences but may limit available tools for managing spotted knapweed and could place greater reliance on chemical or mechanical control methods.

We Invite Your Comments: We encourage all community members, stakeholders, and experts to provide their comments, concerns, and suggestions regarding this proposal. Please complete MDARD’s online survey by 11:59 pm EST on Friday, March 6, 2026.