Spotted knapweed (Centuarea stoebe ssp. micranthos, Asteraceae) is an aggressive invasive knapweed that threatens pasture and rangelands of much of North America. The weed was first recorded in Arkansas in the mid 1940s, and is now found along roadsides, trails, pastures and many disturbed habitats in the Ozarks. A knapweed seed head fly, Urophora quadrifasciata (Tephritidae: Diptera), also called UV fly, was among 13 biocontrol agents introduced to North America to control the spread of spotted knapweed. The UV fly was the only exotic biocontrol agent recovered in 2006 from spotted knapweed patches in the Ozarks. The fly lays its eggs in developing flower heads of spotted knapweed, and has at least two generations per year. Larvae feed and develop on the seed heads of spotted knapweed and form galls. Because biotic and abiotic conditions in the Ozarks are different than the northwestern US, we are assessing the impact of the UV fly on spotted knapweed patches, using direct natural enemy evaluation techniques, to determine the number of flies, seeds and galls per capitulum and per stem of spotted knapweed.