Assessing the Impacts of Neonicotinoid Insecticides on the North American Monarch Butterfly (Danaus plexippus) in Comparative Frameworks Across Species, Time, and Trophic Levels
Open Access
- Author:
- Cibotti, Staci
- Graduate Program:
- Entomology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 09, 2022
- Committee Members:
- Gary Felton, Program Head/Chair
Rudolf Schilder, Co-Chair & Dissertation Advisor
John Tooker, Major Field Member
Carolyn Lowry, Outside Unit & Field Member
Jared Ali, Co-Chair & Dissertation Advisor - Keywords:
- monarch butterfly
milkweed
neonicotinoid
pesticide
painted lady butterfly
Danaus plexippus
Vanessa cardui
Asclepias syriaca
pollinator conservation
ecophysiology
chemical ecology - Abstract:
- Since their introduction in the 1990s, neonicotinoid pesticides have become one of the most widely used classes of insecticides in the world. Their high potency, water solubility, and long half-lives make them effective insecticides; however, these properties have also raised alarm regarding their potential environmental impacts. Neonicotinoids can leach in soil and become systemic in non-target plants. Once assimilated by the plant, they can translocate to floral rewards where they become available to a suite of pollinators. While the adverse effects of neonicotinoid exposure for bees are well documented, their potential impacts on other pollinating insects are not. Like many insect taxa, butterflies have seen substantial reductions in occurrence frequencies in recent decades. One of the most iconic examples of this is the North American monarch butterfly (Danaus plexippus), which has sustained approximately 80-95% reductions in its’ overwintering populations since the 1990s. Landscape-level studies have correlated neonicotinoid insecticide usage with reduced butterfly abundances and population indices for a number of species, including monarch butterflies, indicating that widespread use of these compounds may be contributing to the observed declines. However, a limited number of studies have assessed the morphological, physiological, and behavioral impacts of neonicotinoid exposure for monarchs and other butterfly species. Neonicotinoids can interfere with motor function and are known to reduce flight performance metrics in some other insects. Given that monarchs are a vagile and migratory species, even slight reductions in flight potential could have major implications for survival and reproduction. Neonicotinoids can also alter metabolic rates in some insect species. Such energetic changes could alter resource allocation in ways that impact fuel requirements during flight and migration, reproductive investment, and/or energy reserves necessary for surviving the overwintering period. Furthermore, neonicotinoids can influence host-plant interactions by altering the expression of important plant defense genes, subsequently impacting phytohormone concentrations and plant defense response. These changes in defense induction could impact the survival and performance of monarchs larvae, and ultimately shape plant-herbivore interactions. Therefore, the major aims of my dissertation are to assess whether plant-mediated neonicotinoid exposure 1) affects butterfly flight performance, 2) impacts morphology and physiology across life stages, and 3) alters host-plant defensive chemistry and subsequently plant-herbivore interactions. In chapter one, I discuss the value of improving our understanding of the causal mechanisms underlying insect stress responses across species, time, and trophic levels to develop more targeted tools and strategies for responding to insect population declines. Chapter two examines how adult exposure to field-relevant doses of clothianidin, a widely used neonicotinoid, affects the flight metabolic rate, flight distance, and velocity of monarch and painted lady (Vanessa cardui) butterflies. In chapter three, I quantify monarch utilization of common milkweed (Asclepias syriaca) plants adjacent to neonicotinoid-treated and untreated corn fields and examine the effects of larval clothianidin exposure on the morphology and physiology of monarchs across life stages, using metabolic rates as proxies for physiological performance. In chapter four, I asses the impacts of clothianidin uptake and metabolism on defense induction and defense characteristics in common milkweed plants. Finally, chapter five synthesizes the results of this dissertation, and discusses some remaining questions and potential future directions in this research area.