Parasitoids of the goldenrod gall moth: effects of scale on spatial density dependence.

A parasitoid assemblage consisting of four hymenopteran species caused larval and pupal mortality of the host Gnorimoschema gallaeosolidaginis. In the absence of discrete host patches, the relationship between densities of hosts and parasitized hosts was examined on scales of 0.25, 1, 4, 16 and 25 m2 within a 400 m2 plot and between ten 16 m2 plots within a 1·3 km2 area. Within the 400 m2 plot, positive density dependence was observed on scales of 1, 4 and 16 m2. Rates of parasitism were found to be density independent across the ten 16 m2 plots. The nature of parasitoid and host spatial patterns were examined at the 400 m2 plot by using Goodall's (1974) paired-quadrat variance function, and by fitting the observed distribution of galls and parasitized galls to the Poisson and negative binomial on three scales. Postive density dependence at the 400 m2 plot occurred in the context of a host density gradient and may indicate aggregation of search time by parasitoids. However, significant responses on scales of 4 and 16 m2 may only reflect the sum of parasitoid responses on lower scales because of this gradient. Predominance of significant responses on a 1 m2 scale may be related to the high degree of variation in host density, and limits to active searching range of parasitoids.