Anjel M. Helms, Swayamjit Ray, Nina L. Matulis, Margaret C. Kuzemchak, William Grisales, John F. Tooker, Jared G. Ali
Many organisms rely on chemical cues to assess their risk of attack. Prey animals often avoid predator odours and some plants use chemical cues to detect the presence of herbivores, responding with enhanced defences. Entomopathogenic, or insect-killing, nematodes are important natural enemies of insects and can be used for biological control of insect pests. Together with their symbiotic bacteria, entomopathogenic nematodes attack and kill insects in the soil, frequently locating their prey using olfactory cues from herbivore-damaged plant roots. These predatory nematodes not only respond to chemical cues, they also produce cues that provide a source of chemical information in their environment.
In this study, we investigated plant and insect herbivore responses to olfactory cues from entomopathogenic nematodes and explored the ecological significance of such cues for interactions among plants, insect herbivores, and the natural enemies of those herbivores. We predicted that insect herbivores would detect odours from predatory nematodes as a potential threat and avoid these cues. We also anticipated that some plants might respond to olfactory cues from entomopathogenic nematodes, as these cues could inform plants about the presence of natural enemies that often aid in their defence against insect herbivores.
We found that insect cadavers infected with entomopathogenic nematodes produce characteristic odours different from those of uninfected dead insects. We also found that potato plants responded to the presence of these predatory nematodes or their odours by enhancing defences that reduced performance of Colorado potato beetles. Our findings also revealed that female Colorado potato beetles laid fewer eggs on plants when odours from entomopathogenic nematodes were present, indicating they detected these cues as warning of a threat to the performance and survival of their offspring. These findings suggest that chemical cues from entomopathogenic nematodes enhance plant defences and deter insect herbivores. By linking plant detection of chemical cues from organisms that play a beneficial role in their ecology with herbivore detection of cues from their natural enemies, we can gain additional insight into the complexity of adaptations in interactions among organisms at different trophic levels.