Bacillus thuringiensis Bioinsecticides Induce Developmental Defects in Non-Target Drosophila melanogaster Larvae

SIMPLE SUMMARY: The use of chemical pesticides is controversial mainly because of their detrimental effects on both the environment and human health. Public opinion along with government policies encourage reductions in their exploitation. An emerging solution is the use of biopesticides, which sales are currently increasing. Among biopesticides, the bacterium called Bacillus thuringiensis is the most used bioinsecticide both in organic and conventional farming to fight larval pests. One strain of Bacillus thuringiensis produces toxins that specifically kill lepidopteran (butterfly) larvae in two or three days. However, although not lethal for other insects such as bees or fruit flies, Bacillus thuringiensis may affect their development. Only scarce data are currently available on the unintended effects of Bacillus thuringiensis strains on insects other than those targeted. In this study, we characterized the adverse effects of Bacillus thuringiensis strains that target lepidopteran larvae, on the development of the fruit fly Drosophila melanogaster, a dipteran non-target insect. We showed that amounts of Bacillus thuringiensis in the nutritive medium close to the amounts found on vegetables after treatment induced developmental and growth defects in Drosophila larvae. We further showed that these effects are due to the disturbance of the larval intestinal physiology, reducing protein digestion. ABSTRACT: Bioinsecticides made from the bacterium Bacillus thuringiensis (Bt) are the bestselling bioinsecticide worldwide. Among Bt bioinsecticides, those based on the strain Bt subsp. kurstaki (Btk) are widely used in farming to specifically control pest lepidopteran larvae. Although there is much evidence of the lack of acute lethality of Btk products for non-target animals, only scarce data are available on their potential non-lethal developmental adverse effects. Using a concentration that could be reached in the field upon sprayings, we show that Btk products impair growth and developmental time of the non-target dipteran Drosophila melanogaster. We demonstrate that these effects are mediated by the synergy between Btk bacteria and Btk insecticidal toxins. We further show that Btk bioinsecticides trigger intestinal cell death and alter protein digestion without modifying the food intake and feeding behavior of the larvae. Interestingly, these harmful effects can be mitigated by a protein-rich diet or by adding the probiotic bacterium Lactobacillus plantarum into the food. Finally, we unravel two new cellular mechanisms allowing the larval midgut to maintain its integrity upon Btk aggression: First the flattening of surviving enterocytes and second, the generation of new immature cells arising from the adult midgut precursor cells. Together, these mechanisms participate to quickly fill in the holes left by the dying enterocytes.