Gut physiology and the bacterial community play crucial roles in insect susceptibility to
infections and
insecticides. Interactions among Colorado potato beetle Leptinotarsa decemlineata (Say), its bacterial associates, pathogens and
xenobiotics have been insufficiently studied. In this paper, we present our study of the survival, midgut histopathology, activity of digestive
enzymes and bacterial communities of L. decemlineata larvae under the influence of Bacillus thuringiensis var. tenebrionis (morrissoni) (Bt), a natural complex of
avermectins and a combination of both agents. Moreover, we estimated the impact of culturable enterobacteria on the susceptibility of the larvae to Bt and
avermectins. An additive effect between Bt and
avermectins was established regarding the mortality of the larvae. Both agents led to the destruction of midgut tissues, a decrease in the activity of
alpha-amylases and alkaline
proteinases, a decrease in the Spiroplasma leptinotarsae relative abundance and a strong elevation of Enterobacteriaceae abundance in the midgut. Moreover, an elevation of the enterobacterial CFU count was observed under the influence of Bt and
avermectins, and the greatest enhancement was observed after combined treatment. Insects pretreated with
antibiotics were less susceptible to Bt and
avermectins, but reintroduction of the predominant enterobacteria Enterobacter ludwigii, Citrobacter freundii and Serratia marcescens increased susceptibility to both agents. We suggest that enterobacteria play an important role in the acceleration of Bt
infection and
avermectin toxicoses in L. decemlineata and that the additive effect between Bt and
avermectin may be mediated by alterations in the bacterial community.