Wildfire ash exposure reveals a disconnect between herbivore feeding behavior and fitness outcomes

Wildfires produce vast smoke and ash plumes that can be deposited miles beyond fire boundaries. Although ash effects on plant germination are well documented, impacts on higher trophic levels - such as insect herbivores - remain largely unknown, limiting our understanding of fire’s cascading effects on key species interactions. Plant-insect herbivore interactions shape plant populations and energy flow in many terrestrial ecosystems. These processes are dependent on insects’ tolerance and response to environmental stressors, making detoxification mechanisms central to maintaining ecosystem function. Although insect detoxification systems have been extensively studied in response to plant secondary metabolites, their behavioral and physiological responses to wildfire ash deposition remain largely unknown despite the current and predicted increase in the frequency and intensity of wildfires in the United States. To address this research gap, we tested: 1) three levels of foliar ash deposition on consumption patterns and foraging behaviors of three larval instars; and 2) ash amended diets on development time, feeding efficiency, baseline immune function, survival, and lipid reserves of a common specialist herbivore, Manduca sexta. Foliar ash deposition failed to alter caterpillar foraging behavior or leaf consumption, suggesting limited behavioral avoidance of ash. Rather, ash consumption slowed growth rate, increased mortality, and reduced adult moth lipid reserves while standing immunity was unchanged. These results suggest that resource diversion from growth towards maintenance and repair processes directly constrains growth rates and survival. Together, these results indicate that wildfire ash deposition threatens ecosystem functioning by reshaping herbivore performance and plant-herbivore interactions.