Patch structure, oviposition behavior, and the distribution of parasitism risk
To date, almost no experimental field studies have attempted to assess the factors that generate heterogeneity in the distribution of parasitism risk, a putative indicator of host-parasitoid stability. In this study, I examined the interaction between a planthopper Prokelisia crocea and its egg parasitoid Anagrus columbi among discrete patches of prairie cordgrass, Spartina pectinata. In particular, I examined how patch geography and host distribution within a patch influenced the distribution of adult parasitoids, parasitoid oviposition behavior, the proportion of hosts parasitized, and the aggregation of parasitism (CV2) and adult parasitoids. Based on six generations of census data, the distribution of parasitism was strongly aggregated within (CV2 = 3.58), but not among (CV2 = 0.58) cordgrass patches. Parasitism was also spatially and temporally density independent. To determine what influences the distribution of parasitism risk, I selected 26 discrete cordgrass patches, removed all sources of A. columbi, and then quantified the immigration and subsequent oviposition behavior of A. columbi colonists. I found that the number of immigrants significantly increased with patch size and decreased with patch isolation. Patch size had no influence on the per capita hosts parasitized per leaf, but there was a significant twofold increase in per capita attacks from the least to the most isolated patches. The isolation effect was likely due to an optimal oviposition response to dispersal distance by A. columbi. For these experimental patches, substantial within-patch aggregation of parasitism (CV2 = 1.63) did not translate into strong among-patch aggregation (CV2 = 0.13). Searching adult parasitoids were randomly distributed within and among patches and thus did not explain the high CV2 within patches. Interestingly, the aggregation of parasitism risk within a patch was significantly negatively correlated with patch size and positively correlated with patch isolation. The distribution of parasitism risk could be divided into two general components. The within-parasitoid component was attributable to individuals engaging in multiple ovipositions within a leaf and the distance-dependent oviposition response. The latter response was likely the cause for the variation in CV2 with respect to patch size and isolation. Within-parasitoid aggregation has no effect on host-parasitoid stability. The among-parasitoid component of aggregation appears to have been due to heterogeneity in the vulnerability of hosts and an edge effect (parasitism risk is 60% more heterogeneous at the edge than interior of a patch) and is in theory stabilizing. Consequently, a change in landscape structure that leads to an increase in cordgrass edge habitat may promote a more stable host-parasitoid interaction.