Contribution of the Oulema spp. microbiome to insect survival rates after insecticide treatment

The digestive track of insects provides a nutrient-rich environment to numerous bacterial species, which affect a number of life processes of the insect, including reproduction, digestion of plant material, adaptation to new environmental conditions or resistance to insecticides. Cereal leaf beetle (CLB, Oulema spp., Coleoptera, Chrysomelidae) is one of the major cereal pests causing significant crop yield losses. Insecticide treatment are the main method of reducing damage caused by CLB. Withdrawal of many active substances of insecticides and repeated use of the same preparations has led to development insecticide resistance, including CLB. We investigate the effect of insecticides belonging to different chemical classes, with different mechanisms of action and varying concentrations of the active substance on structure, diversity, richness and function of bacteria at genus level. The experiments involved metagenomic analysis of the V3-V4 regions of the 16S ribosomal gene (16S rRNA) of DNA isolated from insecticide-treated and untreated CLB larvae (control). Exposure to insecticides changed significantly the structure of CLB larvae bacterial microbiome. Insecticide-treated larvae were characterized, on average, by higher biodiversity and richness of bacterial genera, compared to untreated insects. Each of the insecticides affected the decrease in the number of bacteria of the genus Pantoea, in addition, cypermethrin had a negative effect on bacteria of the genus Pseudomonas. In turn, insecticide-treated larvae showed an increase in the number of bacteria of the genus Stenotrophomonas and Acinetobacter, compared to control group of insects. The dominant bacterial genus in cypermethrin-treated larvae was Lactococcus, while in chlorpyrifos and flonicamid-treated insect was Pseudomonas. Reducing the number of bacteria had a statistically significant effect on decreasing the survival rate of insecticide-treated larvae. Obtained results demonstrate the need for investigation of the role of associated microbes in the insecticides degradation.