Background While global change research has greatly expanded in recent years, it remains unclear how environmental change will impact the mobility of many organisms. Flight is an important mode of transportation that affects ecological functions, including mate location, foraging, and migration. However, the effects of increasing temperature and diet quality on flight remain largely unknown. Here, we explore the effects of rearing temperature and larval diet quality on the flight ability of an iconic and ecologically threatened migratory insect, the monarch butterfly, Danaus plexippus. Experimental Design Monarch larvae were reared at two temperatures (25 °C and 28 °C) and on three milkweed species with varying phytochemistry (Asclepias incarnata, Asclepias syriaca, and Asclepias curassavica) in a fully factorial experiment. We tested flight ability using an automated flight mill, which measured cumulative flight distance, duration, and instantaneous velocity. Results Higher rearing temperatures reduced monarch flight ability, and larval diet quality influenced forewing morphology. Dietary milkweed with higher cardenolide concentrations (A. curassavica) induced shorter, wider forewings whereas milkweed with low to intermediate cardenolides (A. incarnata and A. syriaca) induced longer, narrower forewings, which are considered better for gliding flight used during migration. Implications for Insect Conservation Our results provide evidence that projected increases in temperature and the subsequent expansion of tropical milkweed (A. curassavica) into the central breeding range of eastern North American migratory monarchs could reduce migration success. Further research is needed to identify mechanisms explaining the effects of diet and temperature on monarch flight ability and fitness, to ensure that appropriate conservation strategies are employed to preserve migratory populations.

Hosts combat their parasites using mechanisms of resistance and tolerance, which together determine parasite virulence. Environmental factors, including diet, mediate the impact of parasites on hosts, with diet providing nutritional and medicinal properties. Here, we present the first evidence that ongoing environmental change decreases host tolerance and increases parasite virulence through a loss of dietary medicinal quality. Monarch butterflies use dietary toxins (cardenolides) to reduce the deleterious impacts of a protozoan parasite. We fed monarch larvae foliage from four milkweed species grown under either elevated or ambient CO , and measured changes in resistance, tolerance, and virulence. The most high-cardenolide milkweed species lost its medicinal properties under elevated CO ; monarch tolerance to infection decreased, and parasite virulence increased. Declines in medicinal quality were associated with declines in foliar concentrations of lipophilic cardenolides. Our results emphasize that global environmental change may influence parasite-host interactions through changes in the medicinal properties of plants.

Environmental change has the potential to influence trophic interactions by altering the defensive phenotype of prey. Here, we examine the effects of a pervasive environmental change driver, elevated atmospheric concentrations of CO2 (eCO2), on toxin sequestration and flight morphology of a specialist herbivore. We fed monarch butterfly larvae, Danaus plexippus, foliage from four milkweed, Asclepias, species of varying chemical defence profiles grown under either ambient or eCO2. We also infected a subset of these herbivores with a protozoan parasite, Ophryocystis elektroscirrha, to understand how infection and environmental change combine to alter herbivore defences. We measured changes in phytochemistry induced by eCO2 and assessed cardenolide, toxic steroid, sequestration and wing morphology of butterflies. Monarchs compensated for lower plant cardenolide concentrations under eCO2 by increasing cardenolide sequestration rate, maintaining similar cardenolide composition and concentrations in their wings under both CO2 treatments. We suggest that these increases in sequestration rate are a by‐product of compensatory feeding aimed at maintaining a nutritional target in response to declining dietary quality under eCO2. Monarch wings were more suitable for sustained flight (more elongated) when reared on plants grown under eCO2 or when reared on Asclepias syriaca or Asclepias incarnata rather than on Asclepias curassavica or Asclepias speciosa. Parasite infection engendered wings less suitable for sustained flight (wings became rounder) on three of four milkweed species. Wing loading (associated with powered flight) was higher on A. syriaca than on other milkweeds, whereas wing density was lower on A. curassavica. Monarchs that fed on high cardenolide milkweed developed rounder, thinner wings, which are less efficient at gliding flight. Ingesting foliage from milkweed high in cardenolides may provide protection from enemies through sequestration yet come at a cost to monarchs manifested as lower quality flight phenotypes: rounder, thinner wings with lower wing loading values. Small changes in morphology may have important consequences for enemy evasion and migration success in many animals. Energetic costs associated with alterations in defence and morphology may, therefore, have important consequences for trophic interactions in a changing world. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13270/suppinfo is available for this article.