# 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

## TL;DR

Bacteria in cereal leaf beetles help them adapt to plant defenses but hinder their development in lab settings.

## Contribution

This study reveals the dual role of CLB-associated bacteria in adaptation to protease inhibitors and insect development.

## Key findings

- CLB larvae with reduced bacterial communities showed decreased protease activity.
- Larvae with reduced bacterial communities had higher survival and development rates in lab conditions.
- The bacterial microbiome can act as both an ally and adversary to CLB under different conditions.

## Abstract

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.

The online version contains supplementary material available at 10.1038/s41598-025-23674-9.