Effects of diet and temperature on monarch butterfly wing morphology and flight ability

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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. 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. 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. 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.