Larval exposure to Clothianidin lowers flight metabolic rates of monarch butterflies

The restoration of milkweed to agricultural landscapes is thought to be essential for helping bolster declining monarch (Danaus plexippus) populations. However, the rise of neonicotinoid seed treatments over the past three decades has severely increased the toxicity of these landscapes for insects. It is therefore crucial that we understand how monarchs utilize neonicotinoid contaminated plants and the subsequent effects of these compounds on their health and survival in order to create informed management decisions. In this study, we monitored monarch usage of milkweed plantings adjacent to both neonicotinoid-treated and untreated cornfields throughout the year and found that they utilized both with equal frequency. We then performed a laboratory study to examine the energetic costs of plant-mediated larval exposure by tracking metabolic rates across life stages using flow-through respirometry. Our results show that larval clothianidin exposure did not affect the resting metabolic rates of monarchs at any life stage; however, it did lower both the average and peak active, free-flight metabolic rates of adults. Our models also revealed a significant interaction between clothianidin treatment and sex, with larval clothianidin exposure lowering the peak, flight metabolic rates of males more drastically than for females. These findings suggest that larval exposure to clothianidin contaminated plants can have sublethal effects that carry over into adulthood and may adversely affect flight behavior and capacity, particularly for males. Further studies are needed to elucidate the implications of these findings on crucial aspects of monarch ecology, including their foraging, migratory, and reproductive potential.