HOST MATTER: Parasite infection drives chemical reconfiguration of decomposing host and shapes insect behavioural responses

Host–parasite interactions shape ecosystems by driving species dynamics, yet their influence is typically considered in terms of host mortality rather than the chemical environments generated during infection and decomposition. Here, we show that parasitic infection actively reconfigures the chemical phenotype of decomposing hosts, reshaping ecological interactions through volatile‐mediated signalling. Using soil‐dwelling parasites‐entomopathogenic nematodes (EPNs) and their symbiotic bacteria‐as a model system, we integrate behavioural assays, comprehensive volatolomic characterization and CO 2 quantification across three insect orders and two EPN species over four post‐mortem intervals. We demonstrate that EPN‐infection generates a distinct and conserved volatile signature dominated by carboxylic acids, sulphur‐containing compounds and unsaturated hydrocarbons, accompanied by elevated CO 2 emissions. These infection‐derived blends elicit contrasting behavioural responses: they repel non‐natural hosts while consistently attracting natural hosts, indicating that infected cadavers function as chemically structured information hubs. In contrast, frozen‐killed insects exhibit fermentation‐type profiles, and bacteria‐only infection produce simplified alkane‐dominated blends, demonstrating that the full chemical phenotype emerges from the nematode–bacteria complex. Our findings reveal parasitic infection as a process that reorganizes decomposition chemistry with measurable behavioural consequences, uncovering a previously underappreciated mechanism by which parasites structure soil food webs and influence multitrophic interactions. Read the free Plain Language Summary for this article on the Journal blog.