Bacteria associated with the cereal leaf beetle act as the insect's allies in adapting to protease inhibitors, but impair its development in laboratory condition.

Oulema melanopus [L.] (cereal leaf beetle, CLB) is one of the most serious cereal pests. Plant protease inhibitors (PIs) are known for their insecticidal properties. The role of CLB-associated bacteria in insect adaptation to PIs is not yet known. We investigated the role of CLB-associated bacteria in adaptation to PIs, and whether the reduction of bacteria will affect the CLB development. We found a decrease in proteases activity in insects with a diminished bacterial community compared to those with an intact bacterial community. Thus, the study showed that the CLB-associated bacteria participate in the adaptation of CLB larvae to PIs. On the other hand, regardless of the type of PI used, ultimately a higher survival rates were recorded for larvae with a reduced bacterial community compared to insects with a natural microbiome in laboratory conditions. In such conditions, higher larval survival rates and a higher percentage of larvae reaching the pupal and imago stages were recorded in insects whose bacterial community was reduced. Since the CLB bacterial microbiome showed a negative impact on the development of the insect's host and its survival in response to PIs in laboratory conditions, it can be concluded that CLB-associated bacteria can be an ally of its insect host, but also an adversary when conditions are not optimal for symbiosis.