By Pam Willison
On a warm morning in mid-August, Pam Willison and Keith Hammer waded the Stillwater River to access the “Big Island” portion of Owen Sowerwine. The purpose of this visit was to release 200 knapweed root weevils into several patches of knapweed on the west side of the island. These biological control insects were coordinated through the Flathead Conservation District and collected and provided through the Montana Biocontrol Coordination Project, located in Missoula.
The adult weevils emerge in late summer to early fall, at which time they are captured, relocated, and released so they can breed and lay eggs on the root crown of the knapweed plant at the release site. Each female can lay more than one hundred eggs, which hatch into larvae that bore tunnels to the center of the root to overwinter. In early summer, the larvae pupate into adults, which emerge. The cycle begins again.
The weevils impact knapweed plants in several ways. Larval feeding can kill small plants. When multiple larvae occupy a root, the damage is greater, and this harms larger plants. In addition, openings created by weevils tunneling into the roots expose plants to bacterial and fungal infections. The impact is usually noticeable within a few years of release.
The Montana Biocontrol Coordination Project is a grassroots network created to help landowners and managers understand and access biological control agents for various invasive plants. In 2024, the project supported the release of 1.6 million insects to treat noxious weeds on 11,556 acres. This effort reduced the use of chemical treatments and provided an ongoing treatment method intended to be self-sustaining.
The Project collects and distributes biological control insects for a variety of noxious weeds, including spotted knapweed, leafy spurge, whitetop, yellow toadflax, dalmatian toadflax, and St. John’s wort. The biological control agents work in various ways, and some are more effective than others. Research is ongoing to identify additional agents for addressing other noxious weeds.
Here is hoping that the Owen Sowerwine knapweed root weevils have a productive hatch and survive the coming winter so they can do further damage to the knapweed next year.