Parasitoids on the loose experimental lack of support of the parasitoid movement hypothesis

Forest fragmentation can disrupt important ecological processes both within and among species. Of particular interest is the extent to which fragmentation decouples economically important pest species from their parasites. In an influential paper, Roland and Taylor (1997) advanced the hypothesis that forest fragmentation at a relatively fine spatial scale would disrupt the ability of parasitoids to aggregate in response to host density, thereby facilitating the build-up of host populations at isolated sites. Our study provides the first experimental test of this notion. We augmented existing densities of forest tent caterpillar individuals at two types of sites: isolated forest fragments and continuous tracts of forest. Of the four parasitoid species studied by Roland and Taylor (1997), two were encountered in our experiment. For the sarcophagid fly Arachnidomyia aldrichi and the tachinid Carcelia malacosomae, we found no difference in parasitism rates among forest fragments and continuous forest tracts. A third tachinid species unique to this study, Lespesia frenchii , is known to be a broad generalist with respect to host choice. In this species, we observed significantly elevated parasitism in forest fragments, and higher parasitism at the forest edge than within the forest. All of these findings suggest complex effects of forest fragmentation on parasitoid movement, and warn against broad generalizations from observational studies. Forest fragmentation may affect movement patterns in a multitude of ways, significantly obscuring or modifying the simplest mechanisms proposed by Roland and Taylor (1997).