Insect parasites and plant parasitic nematode antagonists: What are entomopathogenic nematodes doing down there?

Entomopathogenic nematodes are obligate lethal parasites of soil-dwelling insects. Nearly 60 species have been described from two genera. They are used successfully as biological control agents in several cropping systems to reduce populations of soil insects. This success has lead to their commercial development and widespread use.  Their intended effect on soil foodwebs is to reduce the density of herbivorous insects, thereby protecting crop plants. Their observed effects are wide-ranging and only partially understood. We have observed and described a number of surprising interactions between entomopathogenic nematodes and other soil fauna in laboratory, greenhouse and field experiments. In typical agricultural settings, 2.5 billion entomopathogenic nematodes are applied per ha. for insect control. Applications at this rate impact insects, plant-parasitic nematodes and free-living nematodes in various ways. The interaction between plant-parasitic nematodes and entomopathogenic nematodes is especially unexpected because these nematodes do not compete for common resources nor do they interact directly in any way. Nevertheless, in some cases entomopathogenic nematode applications to turfgrass significantly reduce plant-parasitic nematode populations and increase free-living nematode populations. We attribute the reduction in plant-parasitic nematodes at least partially to compounds produced by symbiotic bacteria associated with entomopathogenic nematodes. These bacteria are produced in large quantities during an entomopathogenic nematode infection and we have shown cadavers of insects with ongoing infections to be repellent to plant-parasitic nematodes, and the cell-free extract of the bacteria in culture to be toxic to most nematodes other than their symbiotic partners. The response of free-living nematodes remains more enigmatic. Thus, entomopathogenic nematode applications can reduce root herbivory in two ways; by reducing insect root herbivore and plant-parasitic nematode populations.