The Effects of Milkweed Induced Defense on Parasite Resistance in Monarch Butterflies, Danaus plexippus

Abstract

Many plants express induced defenses against herbivores through increasing the production of toxic secondary chemicals following damage. Phytochemical induction can directly or indirectly affect other organisms within the community. In tri-trophic systems, increased concentrations of plant toxins could be detrimental to plants if herbivores can sequester these toxins as protective chemicals for themselves. Thus, through trophic interactions, induction can lead to either positive or negative effects on plant fitness. We examined the effects of milkweed (Asclepias spp.) induced defenses on the resistance of monarch caterpillars (Danaus plexippus) to a protozoan parasite (Ophryocystis elektroscirrha). Milkweeds contain toxic secondary chemicals called cardenolides, higher concentrations of which are associated with reduced parasite growth. Previous work showed that declines in foliar cardenolides caused by aphid attack render monarch caterpillars more susceptible to infection. Here, we ask whether cardenolide induction by monarchs increases monarch resistance to disease. We subjected the high-cardenolide milkweed A. curassavica and the low-cardenolide A. syriaca to caterpillar grazing, and reared infected and uninfected caterpillars on these plants. As expected, monarchs suffered less parasite growth and disease when reared on A. curassavica than on A. syriaca. We also found that herbivory increased cardenolide concentrations in A. curassavica, but not A. syriaca. However, cardenolide induction in A. curassavica was insufficient to influence monarch resistance to the parasite. Our results suggest that interspecific variation in cardenolide concentration is a more important driver of parasite defense than plasticity via induced defenses in this tri-trophic system.

Impacts of larval host plant species on dispersal traits and free-flight energetics of adult butterflies

“…Although monarchs benefit from sequestration of milkweed cardenolides since they confer some level of protection against higher trophic levels 55 , 56 and infection 57 , at high levels these toxins negatively affect larval growth and survival 23 , 29 , 31 , 55 . To start examining if sequestration-associated exposure to cardenolides may also affect adult energy expenditure, we extracted and quantified cardenolide content of wings from adult specimens used in the respirometry trials.…”

Host Plant Species Mediates Impact of Neonicotinoid Exposure to Monarch Butterflies

“…Differences in cardenolide concentrations between these milkweed species are well-established in the literature, with swamp milkweed having consistently low cardenolides and tropical having consistently high. Common milkweed shows more variability, but consistently falls between this range [51][52][53][54]. We again examined monarch development and survival to the adult stage and tested grip strength [55] as an indicator of physical performance.…”

Transcriptomics of monarch butterflies (Danaus plexippus) reveals that toxic host plants alter expression of detoxification genes and down‐regulate a small number of immune genes

“…While parasite replication occurs during the larval and pupal stages, disease symptoms are expressed during the adult stage and include reduced emergence success, body mass, mating ability, flight ability and lifespan (Bradley & Altizer, ; de Roode, Gold, & Altizer, ; de Roode, Yates, & Altizer, ), with greater parasite loads resulting in greater fitness losses (de Roode et al, ; de Roode, Yates, et al, ). Interestingly, previous studies showed that larvae feeding on milkweeds with greater concentrations of cardenolides result in lower parasite infection, growth and virulence (Gowler, Leon, Hunter, & de Roode, ; Lefèvre, Oliver, Hunter, & de Roode, ; de Roode, Pedersen, et al, ; de Roode, Rarick, Mongue, Gerardo, & Hunter, ; Sternberg et al, ; Sternberg, de Roode, & Hunter, ; Tan, Tao, Hoang, Hunter, & de Roode, ; Tao, Gowler, Ahmad, Hunter, & de Roode, ; Tao, Hoang, et al, ). At present, however, it remains unclear how cardenolides, parasites and the monarch's immune system interact.…”