Which Biocontrol Agents are Evaluated? Dalmatian and yellow toadflax biocontrol agents look nearly identical. Both are stem weevils in the genus Mecinus, but they are specific to their respective host. Dalmatian stem weevils (M. janthiniformis) are 3-6 mm long, and yellow stem weevils (M. janthinus) are 2.4-3.4 mm long. There can be an overlap in size, so size alone cannot be utilized to separate the two species. In general, species are differentiated based on the toadflax from which they are collected.
When Should Toadflax Sites Be Evaluated? In Montana, yellow toadflax stem weevils are typically evaluated in May, and Dalmatian stem weevils are evaluated mid-May to mid-June, when the weevil populations are at their peak. Weather and timing can impact evaluation results. To ensure the most accurate results, evaluations should occur when 1) temperature is above 65° F, 2) it is not raining, 3) the windspeed 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 Toadflax Sites Evaluated? Stem weevil populations are evaluated by calculating the average number of insects observed on toadflax during a 3-minute count, repeated six times.
Stem weevil populations are evaluated by calculating the average number of insects observed during six, 3-minute counts, repeated six times. The Site Evaluation Form is available on mtbiocontrol.org to guide land managers through this process.Are Toadflax Biocontrol Agents Established? We consider toadflax agents to be established if any number of stem 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 Toadflax Biocontrol Agents be Collected from this Site? If the number of stem weevils observed during six, 3-minute timed counts averages at least 20, and there is approximately one acre of moderately dense toadflax at the site, toadflax biocontrol agents can be collected.
Are Release Practices Working? Toadflax biocontrol agent establishment and success depend on unique site conditions. Sites that are heavily grazed or lack snow cover during extended periods of freezing temperatures may be at a higher risk of establishment failure. If releases have been made at a site and weevil counts do not meet the collection threshold, adjusting your release practices may help. Stem weevils should be released in one spot (not scattered) for three consecutive years. If the toadflax infestation is growing on a hill, agents should be released only at the bottom of the slope because they prefer to move uphill. One release of stem weevils (200-300 insects/release) is the suggested minimum release rate for a 1-5 acre toadflax infestation. Releasing 10 times or more than the suggested rate could increase the likelihood of developing a collectible population.
Do You Have Toadflax Biocontrol Sites That Should Be Evaluated? Montana Biocontrol Coordination Project can assist land managers by conducting site evaluations, hosting evaluation field trainings, and distributing monitoring supplies. Contact jleblow@missoulacounty.us to learn more.
