# Glucosinolate hydrolysis products suppress entomopathogenic nematodes in vitro but do not protect sequestering flea beetle larvae in vivo

## TL;DR

Flea beetle larvae use glucosinolate compounds to defend against nematodes, but this defense doesn't protect them from infection, though it affects nematode bacteria.

## Contribution

Shows that while glucosinolate products harm nematodes and their bacteria in vitro, they don't protect larvae in vivo, and host plant traits influence nematode interactions.

## Key findings

- Glucosinolate hydrolysis products in flea beetle larvae suppress nematode movement and bacterial growth in vitro.
- Reducing these compounds in larvae does not increase their susceptibility to nematode infection.
- Host plant type affects larval susceptibility to nematodes and bacterial symbiont abundance.

## Abstract

The efficacy of entomopathogenic nematodes (EPNs) in the biological control of insect pests can be influenced by the host's chemical defenses. Phyllotreta flea beetles, among the most destructive pests of Brassica crops, deploy highly reactive glucosinolate hydrolysis products as a defense against natural enemies. Here, we investigate the susceptibility of EPNs and their symbiotic bacteria to glucosinolate hydrolysis products and assess how this defense shapes the interaction between the horseradish flea beetle, Phyllotreta armoraciae, and EPNs.

Glucosinolate hydrolysis products were detected in uninjured P. armoraciae larvae but not in adults, and their levels were unaffected by EPN infection. EPNs and their bacterial symbionts were susceptible to glucosinolate hydrolysis products in vitro, with EPN immotility rates ranging from 35% to 96% and bacterial growth suppression from 20% to 85% at biologically relevant concentrations. However, reducing the levels of glucosinolate hydrolysis products in larvae, either by silencing myrosinase gene expression or by feeding on different Arabidopsis genotypes, did not make them more susceptible to EPNs. Nevertheless, the food plant influenced larval susceptibility to EPNs and the relative abundance of EPN bacterial symbionts in infected larvae.

Although glucosinolate hydrolysis products are toxic to EPNs and their symbiotic bacteria, they did not protect P. armoraciae larvae from EPN infection. However, the larval food plant influenced EPN susceptibility and bacterial community composition, highlighting the role of host plant traits in shaping insect–EPN interactions. These findings provide new insights into the limitations of EPN‐based biocontrol against glucosinolate‐sequestering pests. © 2026 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

The flea beetle's glucosinolate‐based chemical defense fails to protect larvae from nematode infection. However, the defense inhibits the nematode's symbiotic bacteria, thereby potentially impairing nematode reproduction and biocontrol success.