COS 85-2 - Heat waves late in the season impair plant resistance to subsequent herbivory in common milkweed (Asclepias syriaca)

Abstract: Heat waves ­-sudden rises in temperatures that last a few days- have been showing unparalleled increases in frequency and intensity jointly with climate variability over the decades, and mounting evidence point toward that their impacts on plant physiology can be adverse and long-lasting. Nonetheless, whether such effects can also indirectly affect subsequent plant interactions with the insect community has been largely understudied. We hypothesize that the timing of a heat wave relative to plant age, and the persistence of its effects on the plant will disrupt plant-herbivore interactions. Furthermore, distinct downstream effects are predicted on plant-herbivore interactions above- and belowground after a heat wave due to differences in their temperature fluctuations. We used common milkweed (Asclepias syriaca) and in-field experimental heat waves to investigate how timing of heat waves relative to plant age (early or late) and their short-term (24h post-HW) or long-term effects (2 weeks post-HW) impact plant physiology. We also evaluated the indirect effects of heat waves on subsequent plant interactions with a leaf herbivore (monarch caterpillar, Danaus plexippus) and a root herbivore (red milkweed beetle larva, Tetraopes tetrophthalmus). We found that heatwaves had distinct direct impacts on plant growth and defense traits over the season, with short-term, transient greater plant growth across all plant developmental stages, and delayed induction of defenses in later stages. Subsequent herbivory showed herbivore-specific plant responses, with greater growth, biomass and defense production after root herbivory compared to leaf herbivory. Interestingly, growth and defense tended to tradeoff only after monarch feeding, and this effect was indirectly exacerbated in older plants that suffered heat stress, allowing monarch caterpillars to feed more and perform better. Therefore, we show that heat waves have stronger direct effects on plants, in which transient investment in greater growth may be beneficial when temperatures increase over short periods of time. Nonetheless, these heat wave-mediated shifts may be detrimental if herbivores are present, particularly in older plants in which growth-defense tradeoffs may emerge more often, especially in response to aboveground herbivores.