Insects are well documented to possess a diverse microbiome that can mediate several interactions with plants. Caterpillars (Lepidoptera) possess bacteria that occur along the interface between plants and insects. In our study, we utilized the fall armyworm (Spodoptera frugiperda) and resistant and susceptible maize genotypes. Our objectives were to evaluate: 1. how a gut-damaging chemicals influences the performance of axenic and gnotobiotic fall armyworm; 2. if bacteria “leak” from the gut into the hemolymph and induce immune responses, and 3. if the isolates we assessed have direct pathogenicity. In order to complete these objectives, we used sterile and gnotobiotic methods using gamma-irradiated plant tissues. We reintroduced isolates of Enterococcus, Klebsiella, and Enterobacter isolated from field caterpillars, which have different dietary associations with the insect.
Results/Conclusions
Our results found that there was negative effect of bacteria on the growth of fall armyworm for both resistant and susceptible maize. However, Enterobacter had significantly greater impacts on larval growth and mortality compared to the other isolates. Enterobacter–inoculated larvae consuming resistant maize had both bacteria present in the hemolymph and elevated immune responses. Enterobacter injected into fall armyworm larvae reduced growth, but did not induce sepsis. Our results strongly suggest that plant defenses can not only act directly upon the insect, but also utilize the attacker’s microbiome. Reduced performance was a combined outcome of the insect counteracting direct actions of the plant defense and the immune-related challenges from commensal bacteria entering the blood. These results have major implications in how we evaluate and interpret plant-insect-microbiome interactions.