Seasonal patterns of parasitism and differential susceptibility among species in macrolepidopteran assemblages on Salix nigra (Marsh) and Acer negundo L.

Abstract 1. Previous research has demonstrated that parasitism of larval macrolepidoptera on Acer negundo (box elder) is greater than that of larvae on Salix nigra. 2. In this study, percentage larval parasitism of species on box elder and black willow was calculated for distinct portions of the season, to determine if the higher levels of parasitism on box elder resulted from greater parasitism during a particular portion of the season. 3. Larval parasitism was higher on box elder regardless of the portion of the season; however, the seasonal pattern of parasitism differed for each tree species, i.e. late-season box elder parasitism increased and late-season black willow parasitism decreased relative to early- and mid-season parasitism. 4. The degree to which higher levels of parasitism were associated with host and parasitoid traits such as average larval abundance, number of species, type of host, and parasitoid species load of macrolepidoptera, as well as average number of species of parasitoids during each portion of the season was determined. 5. In addition, the possibility that differences in the pattern of parasitism on box elder and black willow were caused by the presence of a greater number of numerically dominant parasitoid species in late-season box elder was investigated. 6. None of the variables, other than type of host species, were associated with higher levels of parasitism in late-season box elder. 7. Late-season, single-tree species on box elder (i.e. species that occurred only on box elder) were more heavily parasitised, compared with dual-tree species. This difference was not observed earlier in the season nor on black willow. 8. Further, single-tree species on late-season box elder included three numerically dominant species that suffered significantly greater larval parasitism than any other type of species on either of the two trees. 9. This study supported previous predictions, that suggested the importance of larval coloration: there was a higher proportion of green larvae among the most highly parasitised host species, i.e. single-tree, numerically dominant species.