Herbivore-induced chemical defense in aspen suppress spongy moth development and viral immune response

The invasive spongy moth (Lymantria dispar) threatens North American forests through extensive, repeated defoliation that reduces tree growth and increases susceptibility to secondary pests and pathogens. Quaking aspen (Populus tremuloides), a preferred host of L. dispar, is the most widely distributed tree in North American forests that serves as a foundational species and deploys highly plastic chemical defenses against herbivory. This intraspecific phytochemical diversity shapes resistance and susceptibility within and among P. tremuloides populations through effects on herbivore behavior and physiology. In 2021, an L. dispar outbreak caused a complete defoliation in a P. tremuloides common garden in Wisconsin that contained >500 genotypes. The following year we observed high L. dispar larval mortality from a naturally-occurring entomopathogenic virus, potentially driven by sustained induction of tree defenses during early-season feeding. To investigate if induced plant defenses suppressed L. dispar immune responses, we collected leaf tissue across both years from trees affected by the defoliation event, extracted the phytochemical defense compounds, and incorporated those extracts into artificial diets to rear L. dispar larvae. Larvae weight gain, consumption, and frass were tracked to calculate a series of nutritional indices (e.g., relative growth rate, approximate digestibility). From each treatment group, half were challenged with an entomopathogenic virus and immunocompetency measurements were quantified. Together, these traits offer key insights as to how delayed induced tree defenses alter L. dispar performance and risk of virus-mediated mortality, which is critical for predicting population dynamics of an invasive insect species under predicted global change.