The Role of Volatile Substances Emitted by Cultivated Plant's Roots in Indirect Defense Against Soil Herbivores

Plants in nature have developed many defense mechanisms to defend themselves against attacks by harmful organisms; these mechanisms are indirect and direct. When attacked by a harmful organism, many plant species release volatile substances that attract natural enemies of herbivores. Volatile substances have an important role in the tritrophic system consisting of a plant, a herbivore, and its natural enemy. They function as a kind of chemical signal (semiochemical) which directly influences both harmful pests and their natural enemy. Some of these substances appear on damaged as well as undamaged plants, while other substances are released in the case of me‐ chanic damage or feeding of a particular herbivore species. Volatile substances may repel a herbivore. Harmful pests have an important role in attracting natural enemies, as they also emit chemical signals that function as kairomones for natural enemies. In order to increase our knowledge in the field of indirect plant defense we studied che‐ mosensation of four entomopathogenic nematode species (Steinernema, Heterorhabdi‐ tis) to compounds released by insect (wireworms and grubs) damaged and undamaged potato and carrot roots, and mechanically damaged maize roots. The aim of our research was (1) to study the effect of different EPN foraging strategies (am‐ bush, intermediate, or cruise) toward the tested volatile compounds, (2) to determine whether chemotaxis is species-specific, and (3) to assess whether the volatile com‐ pounds from damaged and undamaged roots have any behavioral effects on the EPNs studied, and (4) if volatile compounds are a part of an indirect plant defense. Our results indicate that all of the tested EPN species exhibited attraction (or repul‐ sion) to volatiles, irrespective of their foraging strategy, and suggest that responses to distinct volatile cues are a species-specific characteristic. These results expand our knowledge of volatile compounds as cues, which may be used by EPNs to find hosts and for other aspects of navigation in soil.