Differential parasitism of macrolepidopteran herbivores on two deciduous tree species

Patterns of larval parasitism among species in the macrolepidopteran assemb lages on two riparian tree species, Acer negundo L. (box elder) and Salix n igra (Marsh) (black willow) were examined. Larvae were collected throughout the growing season for five years and reared for parasitoid emergence. Tot al parasitism of larvae on box elder was significantly higher than that of larvae on black willow. Comparisons of parasitism levels among lepidopteran families showed that in five of seven families larval parasitism on box el der was significantly higher than on black willow. For species whose larvae were found on both tree species, total parasitism was significantly higher when the larvae were on box elder than when larvae of the same species wer e on black willow. In comparisons of species found on both tree species, la rvae in three of seven families suffered significantly higher levels of par asitism when on box elder than when on black willow. The roles of the funct ional/numerical responses of parasitoids, common and numerically dominant p arasitoid species, and plant volatiles are considered as causal mechanisms underlying differential parasitism but are not supported by the data.