Ecological consequences of systemic plant defense against insect herbivory

Insect herbivory induces plant defenses both locally in attacked plant tissues and/or systemically in non-attacked tissues. Systemic plant defense responses include aboveground herbivory altering belowground roots or belowground herbivory modifying aboveground plant tissues. Through induced systemic plant defenses, above- and belowground insect herbivores indirectly interact when feeding on a shared host plant. However, determining the systemic effects of herbivory on plant tissues and cascading consequences for ecological communities remains underexplored. I address this knowledge gap by investigating systemic plant defense against above- and belowground insect herbivory. First, I examined how belowground striped cucumber beetle (Acalymma vittatum) larval herbivory affects aboveground zucchini squash (Cucurbita pepo subsp. pepo) defenses and interactions with herbivores, including adult cucumber beetles and squash bugs (Anasa tristis). I discovered belowground herbivory repelled aboveground foraging herbivores, and I also found that larvae-damaged plants emitted higher amounts of a key plant volatile compound, (E)-β-ocimene, compared to non-damaged controls. These results indicate belowground herbivory systemically strengthens plant defenses and deters a suite of aboveground herbivores. Moreover, I also determined that neighboring zucchini squash plants can detect belowground larvae-induced changes in aboveground plant volatile emissions. Neighboring plants respond to the altered volatiles by enhancing resistance against aboveground squash bugs, but not adult beetles. My findings reveal a novel form of volatile-mediated plant communication about insect herbivory. Collectively, these findings underscore connections between above- and belowground ecological communities and expand our understanding of how plants detect and respond to the threat of insect herbivory.