They are tiny, tireless, hungry, and environmentally friendly – except to one invasive weed.
They’re called seedhead weevils, and the Missouri departments of Transportation and Conservation, along with the University of Missouri – Extension, are using them to help control Missouri's newest noxious weed – spotted knapweed. The weevils, Larinus minutus and Larinus obtusus or "Larrys" as some researchers like to call them, are small, flying, dark brown or gray bugs with a long snout. They can spread throughout a knapweed patch in a few years.
"Knapweed infestations occur in poor soils along rock cuts and steep slopes, and the weed produces an herbicide within its roots that kills nearby plants," explained MoDOT Roadside Manager Chris Shulse.
This noxious weed resembles the garden flower called bachelor’s buttons. Although pretty, if knapweed is not controlled, it can spread onto private lawns and pastures, where it significantly reduces the available forage. Knapweed is hard to eliminate with herbicides alone because it grows in difficult to reach areas, often extending beyond the roadside onto private property.
MoDOT began to release the weevils on roadsides in southern Missouri in 2008 and in northeast Missouri in 2009. They are typically released in July when knapweed is in bloom so the females can lay eggs on the flowers. When the eggs hatch, the larvae go to work, eating the flower and seeds, reducing the plant’s ability to reproduce. A single larva can eat every seed in a flower.
A second knapweed-controlling weevil, the root-boring Cyphocleonus achates, or “Cy” is larger and doesn’t fly well. Its color helps it blend into rocky ground. These insects are released in August, when females lay eggs on young knapweed plants. The larvae burrow into the roots, causing the plant to die within a few years. Researchers documented reductions as high as 99 percent in knapweed patches where “Cy” took up residence.
In addition to the 2009 release, MoDOT introduced the hungry bugs to infested areas last month.
“Although it will take a few years for the weevil populations to increase enough to make a difference, our tiny partners will no doubt play an important role in suppressing knapweed,” Shulse said.