3: Microbial relationships shape the effectiveness of plant defenses in lepidopteran guts

Plants possess multiple classes of defenses that can jointly reduce insect performance, in some cases causing mortality. These disparate forms of defenses commonly target and disrupt gut tissues of herbivorous insects. Prior studies have demonstrated that defenses can mediate herbivore interactions with food-borne pathogens, where insect mortality from these agents can be altered by various defense substrate. Considering insect guts are naturally populated with bacteria and fungi, we hypothesized that herbivore interactions with plant defenses can likely extend to the resident components in the gut. In this particular study, we posited that by breaching the protective barriers in the insect gut, the effects of plant defenses can be magnified by opportunistic infections by gut microbiota. To test this, we used the lepidopteran fall armyworm (Spodoptera frugiperda), resistant and susceptible maize genotypes, and assemblages of bacteria isolated from field-collected fall armyworm as experimental models. Using sterile techniques, we demonstrate that residents in the fall armyworm gut heighten the effects of maize defenses that disrupt the fall armyworm peritrophic matrix. The extent of this effect is altered by different bacterial isolates, where some have heightened effects. The underlying mechanism appears to be mediating systematic invasion of gut microbiota to the body cavity. This process ultimately creates a metabolic dilemma for the herbivore to simultaneously exploit a well-defended host plant while also contending with potential microbial infections. Subsequent studies to determine gut microbiota influence plant-insect interactions with other forms defenses are currently ongoing.