Nematicidal Effects of Four Terpenes Differ among Entomopathogenic Nematode Species

Entomopathogenic nematodes (EPNs) have been studied for more than half a century, and employed for insect pest management using augmentation, conservation, and classical biological control approaches. As obligate lethal parasitoids of insect larvae, EPN navigate a chemically complex soil environment and interact with their insect hosts, plants, and each other. EPN responses to various terpenes, such as herbivore-induced plant volatiles, have the potential to enhance EPN efficacy through their attraction. However, several of the terpenes are currently being formulated as biological fungicides, insecticides, and acaricides for above- or below-ground applications. We conducted laboratory experiments to investigate the possible nematicidal effect of four terpenes, carvacrol, geraniol, eugenol, and thymol, to two heterorhabditids and two steinernematid species. Each terpene showed nematicidal activity against at least two of the four EPN species, with carvacrol showing the strongest activity and Heterorhabditis bacteriophora the highest sensitivity. Despite the high sensitivity of both heterorhabditids and near-zero sensitivity of the steinernematids to thymol, carvacrol, and eugenol, an increasing effect was observed when steinermatid nematodes were exposed to geraniol, and a decreasing effect for heterorhabditids, with H. bacteriophora exhibiting higher mortality than H. indica . The virulence of the nematodes towards fourth instar Galleria mellonella was also tested after exposure to the median lethal doses of each terpene. No significant difference in virulence was observed between nematodes that were exposed or not exposed to sublethal doses. The experiments suggest that the tested terpenes have a strong effect on EPN viability, which should be considered when combining the two approaches in IPM. The terpenes did not have a universal effect on all nematode species, which merits further investigation, while virulence tests suggest that sublethal doses of these terpenes have no effect on the host-killing performance of EPNs.