Differential effects of clothianidin exposure on metabolic rates across life stages of <i>Danaus plexippus</i> (Lepidoptera: Nymphalidae)

The restoration of milkweed to agricultural landscapes is thought to be essential for bolstering declining monarch butterfly (Danaus plexippus) populations. However, the rise of neonicotinoid seed treatments in recent 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 their impacts on monarch health to better inform conservation efforts. We monitored monarch usage of milkweed (Asclepias syriaca) plantings adjacent to clothianidin-treated and untreated cornfields and found both were utilized with equal frequency. We then examined how plant-mediated larval clothianidin exposure affected monarch development, morphology, and energetics by tracking mortality rates, development times, body metrics, and metabolic rates across life stages. We found no difference in mortality rates or body metrics between the 2 treatment groups. Larvae feeding on clothianidin-treated plants required less time to reach pupation than those feeding on control plants, but there was no difference in the time between pupation and eclosion. Larval clothianidin exposure did not affect the resting metabolic rates of monarchs at any life stage; however, it lowered both the average and peak flight metabolic rates of adults, with the effects being stronger in males than females. These findings suggest that larval exposure to clothianidin-contaminated plants can have carry-over sublethal effects in adulthood, which may adversely affect flight capacity, particularly in males. Further studies are needed to elucidate the possible impacts on crucial aspects of monarch ecology, including their foraging, migratory, and reproductive potential.