Increase and Dispersal of Urophora affinis (Diptera: Tephritidae) on Spotted Knapweed in Western Montana

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Abstract
The percentage of spotted knapweed, Centaurea maculosa Lamarck, seed heads infested with Urophora affinis Frauenfeld and the average number of U. affinis galls per seed head increased significantly over a 4- to 6-year period at six sites in western Montana. U. affinis infestation percentages of 99% were recorded within 50 m of the fly release points at two sites in 1981. Differences in U. affinis numbers with respect to distance and direction were less consistent. Up to 24 galls were found in individual seed heads. Spider and rodent predation, in addition to an unusually cold winter, were possible factors adversely affecting U. affinis populations. No apparent correlation was observed between U. affinis numbers and spotted knapweed flower head density at sites where U. affinis was well established.