By Jill Leblow and Melissa Maggio, Montana Biocontrol Project (MTBCP)

Introduction

Basic guidelines for biocontrol site evaluations were introduced in the 2026 February Weed Post. Here we provide details for spotted knapweed (Centaurea stoebe).

Which Biocontrol Agents Are Evaluated?

Brownish, gray weevil in a spotted knapweed flower. Flower is white to lavender colored.

Spotted knapweed flower weevil. Photo: MTBCP.

Root weevils (Cyphocleonus achates) and flower weevils (Larinus spp.) are the two spotted knapweed biocontrol agents evaluated in Montana. There are two species of flower weevil that look nearly identical, and they are not separated during biocontrol evaluations.

When Should Spotted Knapweed Sites Be Evaluated?

Spotted knapweed biocontrol agents are evaluated when populations are at their peak. Flower weevils are typically evaluated in mid- to late-July and root weevils are evaluated in August. Weather and timing can impact evaluation results. To ensure the most accurate results, evaluations should only occur when 1) temperature is above 80°F, 2) it is not raining, 3) wind speed is no greater than the average for the area, and 4) the previous day didn’t receive heavy rain and wasn’t unseasonably cold.

How Are Spotted Knapweed Sites Evaluated?

Brown, speckled weevil sitting on the top of a purple flower.

Spotted knapweed root weevil. Photo: MTBCP.

Both weevils can be counted simultaneously but peak populations do not overlap. Prioritize the timing of the species you are most interested in. Conduct 10 sweeps with a canvas net, dump the contents into a tray, and count the number of flower weevils before they fly away, then count root weevils. Do this six times and calculate the average for each species.One sweep is defined as an 180° arc in one direction. The Site Evaluation Form guides land managers through this process.

Are Spotted Knapweed Biocontrol Agents Established?

We consider spotted knapweed agents to be established if any number of flower or root weevils are found during the site evaluation five or more years after initial release. This indicates they have produced new generations and can survive at the site.

Can Spotted Knapweed Biocontrol Agents Be Collected from this Site?

If the number of weevils observed during six sets of 10 sweeps averages at least 20 flower weevils and/or 4 root weevils, and there is at least one acre of moderately dense spotted knapweed at the site, biocontrol agents can be collected.

Are Release Practices Working?

A bunch of weeviles and flower parts laying on a flat surface.

Weevils dumbed into a tray for counting. Photo: MTBCP.

Spotted knapweed biocontrol agent establishment and success depend on unique site conditions. Root weevils are less likely to establish at sites that flood or sit above 5500 feet. If releases have been made at a site and weevil counts do not meet the respective collection thresholds, adjusting your release practices may help. Weevils should be released in one spot (not scattered) for three consecutive years. One release of flower weevils (200 insects/release) and/or root weevils (100 insects/release) is the suggested minimum release rate for a 1–5-acre infestation. Releasing 10 times or more than the suggested rate could increase the likelihood of developing a collectible population. Flower weevils are generally well-established in Montana and rarely require additional releases.

Do You Have Spotted Knapweed Biocontrol Sites That Should Be Evaluated?

MTBCP can assist land managers by conducting site evaluations, hosting evaluation field trainings, and distributing monitoring supplies. Contact jleblow@missoulacounty.us to learn more.

Further Information

For more information about the Monthly Weed Post, contact Extension Invasive Plant Specialist Jane Mangold. Past posts are available in the Monthly Weed Post Directory.

This weed post is also available as a printable PDF (264 KB).