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Posted 23 July 2007. PMN Crop News.

Appetite for Weeds

Tiny fly may help control aggressively invasive weed

University of Arkansas.

Fayetteville, Arkansas (July 9, 2007)--University of Arkansas Division of Agriculture scientists are enlisting the assistance of a tiny, winged ally in the battle against an aggressive weed that threatens pastures and native plants.


Spotted knapweed is an invasive species that originated in Eurasia and arrived in the United States in the Pacific Northwest, said Tim Kring, professor of entomology. The weed spread aggressively across northern states, then moved south and now covers some 7.5 million acres in the U.S. It displaces desirable forage in pastures, reduces native plant diversity, and increases surface water runoff and stream sediment.

It was first found in Arkansas in the mid-1940s, but recently has expanded dramatically into 20 counties, Kring said. It is most prevalent along Arkansas highways and field edges, but has recently invaded pastures, especially in Baxter, Carroll, Fulton, Madison and Washington counties. Without control, invaded pastures become useless because the cattle avoid feeding on the weed.

"Weed control in Arkansas has focused largely on herbicide-based management of weeds in agriculture and forestry," Kring said. "There has been no focused effort in biological control of invasive weed species."

Kring and Dagne Demisse, a graduate student from Ethiopia, are focusing on the biological control potential of a tiny seed head gall fly known as the UV knapweed seed head fly. A pattern of stripes on its wings that resembles the letters "UV" gives the insect its common name. The fly lays its eggs on the seed head and its larvae feed on knapweed seeds.

When spotted knapweed became a problem in northern states, scientists imported several insects from the weed's native Eurasian range for use as biological control agents. Of those, Kring said, only the UV fly has followed the weed to Arkansas.

"Having only one known biological control agent is fortunate, from a research point of view," Kring said. "It gives us the opportunity to study the effectiveness of this one insect in controlling the weed before we decide whether to investigate another."

The advantage of the UV knapweed fly is that much is already known about its feeding habits because it has already been studied in northern states, Kring said. However, much is yet to be known about how it lives and works in the South.

"Our study is aimed at determining the UV fly's potential for success in Arkansas, and to verify its minimal impact on native species."

Demisse has marked off test plots on the Arkansas Agricultural Research and Extension Center in Fayetteville, where spotted knapweed has already invaded the edge of a hay field. He uses netting to close off the fly's access to the seed heads of some knapweed plants and permit access to others.

"This allows us to see how effective the fly is by comparing the plants it can feed on with those it cannot," Demisse said.

He sweeps the plots with a net to track the presence of the UV fly.

The ultimate goal, Kring said, is to implement a knapweed biological control program in Arkansas by distributing the most effective and safe natural enemy throughout affected areas. Ideally, these will be beneficial insect species that are already established in the U.S. that are best suited for Arkansas' climate.

Dr. Tim Kring
Professor of Entomology