Cereal leaf beetle‐associated bacteria enhance the survival of their host upon insecticide treatments and respond differently to insecticides with different modes of action

The cereal leaf beetle (CLB, Oulema melanopus) is one of the major cereal pests. The effect of insecticides belonging to different chemical classes, with different mechanisms of action and the active substances' concentrations on the CLB bacterial microbiome, was investigated. Targeted metagenomic analysis of the V3–V4 regions of the 16S ribosomal gene was used to determine the composition of the CLB bacterial microbiome. Each of the insecticides caused a decrease in the abundance of bacteria of the genus Pantoea, and an increase in the abundance of bacteria of the genus Stenotrophomonas, Acinetobacter, compared to untreated insects. After cypermethrin application, a decrease in the relative abundance of bacteria of the genus Pseudomonas was noted. The dominant bacterial genera in cypermethrin-treated larvae were Lactococcus, Pantoea, while in insects exposed to chlorpyrifos or flonicamid it was Pseudomonas. Insecticide-treated larvae were characterized, on average, by higher biodiversity and richness of bacterial genera, compared to untreated insects. The depletion of CLB-associated bacteria resulted in a decrease in larval survival, especially after cypermethrin and chlorpyrifos treatments. The use of a metagenome-based functional prediction approach revealed a higher predicted function of bacterial acetyl-CoA C-acetyltransferase in flonicamid and chlorpyrifos-treated larvae and tRNA dimethyltransferase in cypermethrin-treated insects than in untreated insects.