Monarch butterfly success in the Great Basin, USA: A tritrophic perspective

Western monarch butterfly (Danaus plexippus) populations are starkly declining, with climate change, summer breeding habitat loss, and increased pesticide prevalence as contributing factors. Yet, the influence of tritrophic interactions on monarch larval development and survival has been little explored. Food-plant species identity can be important to specialist herbivores because plants influence trophic interactions by affecting both developmental timing and predator recruitment. To investigate how milkweed (Asclepias spp.) species identity affects monarch-predator and monarch-parasitoid dynamics, we grew two of the most widespread species of milkweed in the West (A. fascicularis and A. speciosa) in a common garden in Reno, NV. Second-instar monarch larvae were randomly assigned to factorial combinations of the two milkweed species and either predator exclusion cages or mock controls. Larvae were monitored daily until they reached the fifth instar, at which point all larvae were enclosed to complete development. Predation had a strong impact on larval success: un-caged monarch larvae were less likely to survive than caged larvae. Moreover, larvae took longer to develop and were less likely to survive on A. speciosa than on A. fascicularis. Emerging adults reared on A. fascicularis were also heavier, with larger, more elongated wings that are better suited to long-distance migration. This suggests that larvae reared on A. speciosa are less suited to return to overwintering grounds in California in the fall. These results suggest that monarch fitness is influenced by tritrophic interactions, and thus that the species of milkweed being used for habitat restoration should be considered in conservation planning.