Although the effects of drought stress on plants are well documented, their indirect effects on insect herbivores are less understood. In this study, we investigated the indirect effects of drought on monarch larvae (Danaus plexippus) mediated by inter-specific variation in their host plants’ traits (Asclepias genus). Based on empirical evidence that drought alters plant defenses (latex and cardenolides) in some Asclepias species, we hypothesized that drought stress in host plants will negatively impact monarch larval survival. Additionally, because drought-resistant traits vary considerably among Asclepias species, we hypothesized that drought effects on larval survival would also vary according to host plants species. To test these hypotheses, we conducted bioassays with monarch larvae reared in fifteen different species of Asclepias assigned to drought and control treatments. The control treatment was standardized to an amount of water corresponding to the maximum stomatal conductance achieved by each Asclepias species and the drought treatment to an amount of water corresponding to a 50% reduction in stomatal conductance. Plants were under the drought treatment for a period of fourteen days. We placed individual neonates on each host plants and allowed them to feed for seven days after which we scored for larval survival.
Results/Conclusions
We detected an interactive effect of Asclepias species and drought treatment on larval survival (p = 0.04), indicating that drought effects on larval survival depended on the host plant species they feed upon. Across all milkweed species, drought effects on monarchs ranged from a 20-50% increase in survival in six Asclepias species to a 20-80% decrease in survival in three Asclepias species and no drought effect in the remaining species, demonstrating positive and negative effects of drought, respectively. While latex, caredenolides, and trichomes did not mediate drought effects, there was an association between milkweed species SLA and drought effects on monarch survival; drought increased monarch survival for species with low SLA (lowered milkweed resistance), while drought decreased survival for species with high SLA (increased resistance). These results indicate that drought effects on Asclepias are differentially transmitted by species and that these differences are correlated with drought-resistances traits within the Asclepias genus. As prolonged and intense periods of drought become increasingly common, this study highlights the importance of understanding the role of trophic interactions in the ability of herbivores to cope with drought.