Forest defoliation by an invasive outbreak insect: Catastrophic consequences for a charismatic mega moth

Earth is now experiencing declines in insect abundance and diversity unparalleled in human history. The drivers underlying those declines are many, complex, and incompletely known. Here, using a natural experiment, we report the first test of the hypothesis that forest defoliation by an invasive outbreak insect compromises the fitness of a native insect via damage-induced increases in toxicity of the forest canopy. We demonstrate that defoliation by the invasive spongy moth (<i>Lymantria dispar</i>) elicits an average 8.4-fold increase in foliar defense expression among aspen (<i>Populus tremuloides</i>) genotypes. In turn, elevated defense dramatically reduces survivorship, feeding, and growth of a charismatic mega moth (<i>Anthereae polyphemus</i>). This work suggests that changes to the phytochemical landscape of forests, mediated by invasive outbreak insects, are likely to negatively impact native insects, with potential repercussions for community diversity and ecosystem function across expansive scales.