Patterns in parasitism frequency explained by diet and immunity

material Appendix 1), and measured the activity of the enzyme, phenoloxidase, a key enzyme in the immune response of the melanization cascade (Beckage 2008, Gonzalez-Santoyo and Cordoba-Aguilar 2012). Prior work with these two species of caterpillars showed that amount of melanization on a bead injected into the hemocoel was a strong predictor of parasitism success (Smilanich et al. 2009b). If parasitoid success can be linked to variation in the immune response as measured by PO enzyme , which in turn can be linked to diff erences in host plant traits, such as phytochemical diversity (Richards et al. 2015), then plants, herbivores, and parasitoids are linked in a tri-trophic relationship where variation in the defensive chemistry of plants act as bottom-up determinants of biodiversity by modifying herbivore responses and aff ecting parasitoid abundance and richness (Bukovinszky et al. 2009). For the caterpillar species studied here, ( Eois apyraria and Eois nympha , Geometridae: Larentiinae), the PO activity was signifi cantly stronger when fed Piper imperiale foliage compared to P. cenocladum foliage (F 1,95 10.31, n 98, n 0.001, Fig. 1A), and the interaction between Piper species and the Eois caterpillars was non-signifi cant (F 1,91 2.39, n 98, p 0.125), although the raw eff ect size of host on E. nympha is greater than that on E. apyraria (Fig. 1A). Th is diff erence represented a 46% decrease in immunity for caterpillars on the poorer host. Concomitant with the strength of the immune response, we found that parasitism frequency (from the long-term rearing dataset) was higher on P. cenocladum (12%) compared to P. imperiale (1.7%) (parasitism by Piper species association, Χ 2 30.58, DF 1, p 0.001; Fig. 1B), supporting our hypothesis that the diff erences in parasitism rates are associated with variation in the herbivore immune response. Th is raw eff ect size is considered a very strong decline in parasitism compared to other studies that report ecological eff ects on parasitism rates for macrolepidopterans (Stireman et al. 2005), and these studies indicate that such changes in parasitism result in substantial ecological consequences (Stireman et al. 2005). Although we do not know the exact mechanism by which the effi cacy of the immune response is altered, the variation in the immune response is possibly due to the diff erence in Ecography 39: 001–003, 2016 doi: 10.1111/ecog.02498 © 2016 Th e Authors. Ecography © 2016 Nordic Society Oikos Subject Editor: Sarah Diamond. Editor-in-Chief: Miguel Ara ú jo. Accepted 15 June 2016