First posted on 06-22-2012
by Fred Miller, University of Arkansas System Division of Agriculture
Over the last four years, entomologists from the University of Arkansas System Division of Agriculture have been releasing an army of tiny weevils to fight back the advance of an invasive weed that displaces desirable forage in pastures, reduces native plant diversity and increases surface water runoff and stream sediment.
Spotted knapweed originated in Eastern Europe, said Tim Kring, professor of entomology. It arrived in the U.S. Pacific Northwest some 100 years ago, most likely in the bilges of cargo ships, and began spreading toward the east and south. It spreads rapidly along roadways and began spreading aggressively in northwest Arkansas about 10 years ago. Now the weed reaches as far south as Alma and has followed U.S. Highway 412 as far east as Hardy.
“The weed has no natural predators in the U.S.” Kring said. “It can be controlled using herbicides, but that’s expensive and, across large ranges, it’s impractical.”
Kring, working with graduate students and research assistants, has been seeking to adapt biological controls in the form of beneficial insects that have proven effective in other parts of the country for use in Arkansas. His research has focused on two weevils from Eastern Europe — knapweed’s native range — that feed exclusively on this plant. One of the weevils feeds on knapweed roots, the other on the weed’s purple flowers.
“We’re attacking the plant from both ends,” Kring said.
The weevils are collected from established populations in Colorado. Kring began releasing the spotted knapweed flower weevil (Larinus minutus) in 2008 in test plots on the Division’s Arkansas Agricultural Research and Extension Center in Fayetteville and at other sites around northwest Arkansas. The knapweed root weevil (Cyphocleonus achates) was released the following year, and each year the insects were released at more sites.
The releases have demonstrated that the weevils thrive in the Arkansas climate and have a measurable impact on spotted knapweed.
Flower weevil populations are now established at 43 sites around northwest Arkansas and Kring is planning to expand the insect’s range in 2012. Graduate students Carey Minteer and Adam Alford and research technician Yong Jun Shen will collect the flower weevils from the established sites and release them across a wider area of the state.
“The weevils are easy to collect because they have a dropping response when frightened,” Minteer said. “You just brush the weeds and the insects fall into a bucket.”
Adult flower weevils overwinter on the ground, Kring said. “In the spring, they emerge and lay eggs in the weed’s seed heads. After hatching, a single larva will eat all the seeds in a single seed head.”
Kring said biological control of spotted knapweed takes time and will not eradicate the weed. “A successful outcome will mean that, within a decade, the weed will be reduced to a manageable level where the weevils are released. We’ll see low populations of the weed and a low, stable population of the weevils.”
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