Submitted to: Journal of Insect Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/16/2017
Publication Date: N/A
Citation: Interpretive Summary: The Indianmeal moth is susceptible to microbial toxins, and recently there have been reports that bacteria in the insect gut can affect how they respond to the toxins. Bacteria were eliminated by feeding moth larvae antibiotics in the diet, or eggs were sterilized and larvae were reared under sterile conditions. In both cases, susceptibility to toxins was significantly reduced, and markers of immunity were changed in larvae without gut bacteria. Antibiotic treatment completely disrupted the expression of one immunity gene, and larvae never progressed to pupae. These data suggest that gut bacteria are important to the toxin response and immune functions of mealworm larvae.
Technical Abstract: Plodia interpunctella (Hübner) is an important stored grain insect pest worldwide, and the first lepidopteran with reported resistance to Bacillus thuringiensis (Bt) toxins. Since gut bacteria may affect Bt insecticidal activity, we determined whether P. interpunctella lacking gut enterobacteria had differences in immune responses and susceptibility to a Bt formulation, Bactospeine. To clear the gut enterobacteria, third instar larvae were reared on artificial diet using antibiotics, or were from sterilized eggs and reared under sterile conditions, and larvae were fed diets with or without Bt. Mortality was significantly lower (p<0.05) in enterobacteria-free larvae (either fed antibiotics in diet or from sterilized eggs and diet) treated with Bt, compared to larvae with gut bacteria and treated with Bt. The number of hemocytes was lower in control and Bt-treated larvae, but was significantly higher (p<0.001) in larvae treated with antibiotics and Bt, and larvae from presterilized eggs and reared on sterile diet had the highest number of hemocytes. Phenoloxidase activity was significantly lower (p<0.05) in Bt-treated larvae from presterilized eggs reared on antibiotics for 24 h or in larvae reared on antibiotic-treated diets prior to Bt introduction compared to those fed control diet. Hemolin gene expression was reduced in larvae fed Bt diets compared to control. Expression of hemolin was not detected in larvae treated with antibiotics, and larvae never reached the pupal stage. Therefore, the loss of enterobacteria in P. interpunctella affected the host immune response and expression of the hemolin gene, and loss of enterobacteria greatly reduced susceptibility to Bt.