Neonicotinoid Insecticides can Enhance Milkweed Vigor and Subsequently Impact Monarch Performance.

Milkweed plants in agricultural landscapes throughout the United States and southern Canada are believed to be vitally important for the imperiled monarch butterfly (Danaus plexippus) populations. However, studies have found that these plants often assimilate a slew of pesticides from the surrounding landscape, including highly potent and widely applied neonicotinoid insecticides. This has prompted concern over the potential impacts of these compounds on monarch populations and has created a growing interest in determining the direct lethal and sublethal consequences of exposure. Fewer studies have considered how neonicotinoids may interact with milkweed defensive chemistry to indirectly influence monarch performance. Here we addressed this question by investigating whether uptake of a widely used neonicotinoid insecticide, clothianidin, could alter milkweed (Asclepias syriaca) defense responses and subsequently impact monarch growth and feeding. We found that clothianidin-treated milkweed plants grew taller, and produced monarch larvae that weighed more and consumed more leaf tissue than larvae feeding on control plants. After five days of monarch herbivory, clothianidin-treated plants had higher levels of the phytohormone, jasmonic acid, but similar levels of salicylic acid relative to control plants. Neither latex nor cardenolide production was impacted by clothianidin assimilation. Overall, these findings indicate that clothianidin exposure can improve the vitality of common milkweed plants, and may subsequently impact monarch performance.