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

BACKGROUND 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. RESULTS 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. CONCLUSION 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.