It has been shown that gut microbes are very important for the behavior and development of Drosophila, as the beneficial microbes are involved in the identification of suitable feeding and egg-laying locations. However, in what way these associated gut microbes influence the fitness-related behaviors of Drosophila melanogaster remains unclear. Here, we show that D. melanogaster exhibits different behavioral preferences towards gut microbes. Both adults and larvae were attracted by the volatile compounds of Saccharomyces cerevisiae and Lactobacillus plantarum, but were repelled by Acetobacter malorum in behavioral assays, indicating that an olfactory mechanism is involved in these preference behaviors. While the attraction to yeast was governed by olfactory sensory neurons expressing the odorant co-receptor Orco, the observed behaviors towards the other microbes were retained in flies lacking this co-receptor. By experimentally manipulating the microbiota of the flies, we found that flies did not strive for a diverse microbiome by increasing their preference towards gut microbes that they had not experienced previously. Instead, in some cases, the flies even increased preference for the microbes on which they were reared. Furthermore, exposing Drosophila larvae to all three microbes promoted Drosophila development, while exposure to only S. cerevisiae and A. malorum resulted in the development of larger ovaries and in increased egg numbers in an oviposition assay. Thus, our study provides a better understanding of how gut microbes affect insect behavior and development, and offers an ecological rationale for preferences of flies for different microbes in their natural environment. The effect of gut microbiome on behavioral preference of fliesAfter showing that the flies become attracted to two of the microbe types and that all microbes positively affect the flies' oviposition 3

The silkworm’s Cat L-like gene, which encodes a lysosomal cathepsin L-like cysteine protease, is thought to be part of the insect’s innate immunity via an as-yet-undetermined mechanism. Assuming that the primary function of Cat L-like is microbial degradation in mature phagosomes, we hypothesise that the suppression of the Cat L-like gene expression would increase Bacillus thuringiensis (Bt) bacteraemia and toxicity in knockdown insects. Here, we performed a functional analysis of Cat L-like in larvae that were fed mulberry leaves contaminated with a commercial biopesticide formulation based on Bt kurstaki (Btk) (i.e., Dipel) to investigate its role in insect defence against a known entomopathogen. Exposure to sublethal doses of Dipel resulted in overexpression of the Cat L-like gene in insect haemolymph 24 and 48 h after exposure. RNA interference (RNAi)-mediated suppression of Cat L-like expression significantly increased the toxicity of Dipel to exposed larvae. Moreover, Btk replication was higher in RNAi insects, suggesting that Cat L-like cathepsin may be involved in a bacterial killing mechanism of haemocytes. Finally, our results confirm that Cat L-like protease is part of the antimicrobial defence of insects and suggest that it could be used as a target to increase the insecticidal efficacy of Bt-based biopesticides.

After ingestion by a susceptible insect and damaging its midgut epithelium, the bacterium Bacillus thuringiensis (Bt) reaches the insect blood (hemolymph), where it propagates despite the host’s antimicrobial defenses and induces insect death by acute septicemia. Although the hemolymph stage of the Bt toxic pathway is determinant for the infested insects’ fate, the response of Bt to hemolymph and the latter’s role in bacterial pathogenesis has been poorly explored.