Distribution of insect attacks in biological control of weeds: Infestationof Centaurea virgata flowerheads by a gall fly

The success of biological control efforts to reduce weed density through re lease of insects may depend as much on the distribution of insect attacks a mong individual plants or plant parts as on the mean level of infestation. We used an index of dispersion to describe the distribution of Urophora qua drifasciata (Diptera: Tephritidae) galls among squarrose knapweed (Centaure a virgata) flowerheads at 18 west central Utah sites in the first 5 years f ollowing introduction of the biological control agent. Two thirds of the sa mples showed a Significantly aggregated distribution of galls among flowerh eads. Statistical analysis showed that site and year accounted for relative ly small proportions of the variance in the index of dispersion. The degree of gall aggregation among flowerheads was positively correlated with the m ean flowerhead quality (mean number of seeds per flowerhead; P = 0.013) and tended to be negatively correlated with the mean fly density per flowerhea d at a site in a given year (P = 0.097). Our data suggest that higher quali ty flowerheads, and possibly higher quality plants, are preferentially atta cked by U. quadrifasciata and therefore are more heavily subject to reduced reproductive potential through biological control. However, an aggregated distribution of By attacks may undercut the potential of the fly to reduce seed production by the weed population as a whole. Understanding both the d istribution of insect attacks among individual plants and the behavioral me chanisms producing such distribution patterns is important to the biologica l control of weeds. (C) 2001 Academic Press.