Potent Ant Repellents Emitted from Nematode-Infected Insect Cadavers

Most known species of entomopathogenic nematodes (EPNs) are generalist obligate parasites of insects. They kill their hosts within days after infection and mortality is mainly caused by toxins produced by bacteria that co-infect the hosts and serve as food for the nematodes. For the successful infection and preservation of the cadaver the nematodes rely heavily on the bacteria, which, in turn, benefit from the symbiotic association via the effective transmission by the EPN.  The bacteria also help to protect the cadavers, which may include the production of repellent compounds.  Indeed, ants are known to avoid feeding on EPN-infected insect cadavers, most likely because they are repelled by volatiles that emanate from the cadavers [9-11]. Determining the source and nature of these repellents has proven to be a major challenge. Here, in a series of behavioral and chemical analytical experiments, we identified 2-heptadecanone and 2-hexadecanal as two volatile compounds that are emitted in substantial amounts by insect larvae infected by the EPN Steinernema feltiae, but not by uninfected larvae. When spiking sugar water with the two volatiles, they were confirmed to be highly repellent to the ant Lasius niger. Most probably, the mutualistic bacterium Xenorhabdus bovienii is the source of these repellents, and preliminary assays suggest they also repel other arthropods, such as mosquitoes. If so, the environmentally benign 2-heptadecanone and 2-hexadecanal could potentially be employed to ward off important pests.