Feeding Spodoptera exigua (Lepidoptera: Noctuidae) larvae with bacteria increase larval juvenile hormone levels inhibiting cannibalism under certain conditions

Abstract

Feed quality influences insect cannibalistic behavior and gut microbial communities. The relationship between larval cannibalism and larval-midgut bacteria was determined in this study. Spodoptera exigua larvae fed with different artificial diets exhibited different cannibalism ratios. The RW-fed larvae had delayed cannibalistic behavior and reduced cannibalism ratio compared with those of the CS, CSK, SW, or CSW-fed larvae. The gut bacterial load of the CSK-fed larvae was the lowest among the five tested larval group. The 16S rRNA gene sequence of the bacteria in the midgut of S. exigua larvae fed with five diets indicated that Enterobacteriales was the prominent bacterial family in the RW-fed larvae. Fifteen culturable bacterial isolates were obtained from the midgut of the RW-fed larvae; of these, ten were belonging to Enterobacteriales. After administered to CS or CSK-fed S. exigua larvae, two bacteria isolates (SePC-12 and − 37) delayed cannibalistic behavior in both tested larval groups. Furthermore, CSK-fed larvae had lowest Juvenile hormone (JH) concentration, and they were more aggressive in intraspecific predation. But the loading of SePC-12 increased the JH hormone in the CSK-fed larvae and inhibited their cannibalism. The bacteria in the larval midgut involved in the JH levels stabilization, thereby regulating host larval cannibalistic behavior.