| Macrocentrus grandii, a biological control agent imported into the US in the 1920s, is one of the most common parasitoids collected from European corn borers, Ostrinia nubilalis, in Minnesota. With the increasing use of transgenic (Bt) corn to control corn borers, there is interest in whether M. grandii will play a role in how quickly a Bt-resistant gene could spread in the corn borer population. Whether M. grandii can influence the evolution of Bt-resistance depends on how the parasitoid responds to differences in host density. My research focused on understanding the spatial distribution of hosts and the dispersal behavior of the parasitoid to determine whether density-dependent parasitism could be scale-dependent. The first goal was to assess whether hosts were randomly distributed or aggregated, and whether the answer to this question was affected by the spatial scale of the analysis. The second goal was to understand the dispersal behavior of the parasitoid, in terms of how far parasitoids move within a field, and whether their movement was affected by differences in host density. Results showed that corn borers were aggregated at each of the spatial scales analyzed, and that female M. grandii could disperse beyond 200 m, farther than the average range of spatial autocorrelation in its host. In addition, M. grandii dispersing from a central location were just as likely to be located in adjacent Bt fields as adjacent non- Bt fields, which could result in a higher ratio of parasitoids to corn borers in Bt fields than non-Bt fields. These results suggest that both the ability to detect density-dependent parasitism of corn borers by M. grandii and the type of density dependence found may be scale dependent, and that the probability of a corn borer larva being parasitized in a Bt field may not be the same as the parasitism risk faced by a corn borer in a non-Bt field. |
