Entomopathogenic Nematode - Community Ecology

Community Ecology

Parasites can significantly affect their hosts, as well as the structure of the communities to which they and their hosts belong (Minchella and Scott 1991). Entomopathogenic nematodes have the potential to shape the populations of plants and host insects, as well as the species composition of the surrounding animal soil community.

Entomopathogenic nematodes affect populations of their insect hosts by killing and consuming individuals. When more EPNs are added to a field environment, typically at concentrations of 250,000 individuals per square metre, the population of host insects measurably decreases (Campbell et al. 1998, Strong et al. 1996). Agriculture exploits this finding, and the inundative release of EPNs can effectively control populations of soil insect pests in citrus, cranberries, turfgrass, and tree fruit (Lewis et al. 1998). If entomopathogenic nematodes suppress the population of insect root herbivores, they indirectly benefit plants by freeing them from grazing pressure. This is an example of a trophic cascade in which consumers at the top of the food web (nematodes) exert an influence on the abundance of resources (plants) at the bottom. The idea that plants can benefit from the application of their herbivore’s enemies is the principle behind biological control. Consequently, much of EPN biological research is driven by agricultural applications.

Examples of the top-down effects of entomopathogenic nematodes are not restricted to agricultural systems. Researchers at the Bodega Marine Laboratory examined the strong top-down effects that naturally occurring EPNs can have on their ecosystem (Strong et al. 1996). In a coastal shrubland food chain the native EPN, Heterorhabditis heplialus, parasitized ghost moth caterpillars, and ghost moth caterpillars consumed the roots of bush lupine. The presence H. heplialus correlated with lower caterpillar numbers and healthier plants. In addition, the researchers observed high mortality of bush lupine in the absence of EPNs. Old aerial photographs over the past 40 years indicated that the stands where nematodes were prevalent had little or no mass die-off of lupine. In stands with low nematode prevalence, however, the photos showed repeated lupine die-offs. These results implied that the nematode, as a natural enemy of the ghost moth caterpillar, protected the plant from damage. The authors even suggested that the interaction was strong enough to affect the population dynamics of bush lupine (Strong et al. 1996).

Not only do entomopathogenic nematodes affect their host insects, they can also change the species composition of the soil community. Many familiar animals like earthworms and insect grubs live in the soil, but smaller invertebrates such as mites, collembolans, and nematodes are also common. Aside from EPNs, the soil ecosystem includes predatory, bacteriovorous, fungivorous and plant parasitic nematode species. Since EPNs are applied in agricultural systems at a rate of 1,000,000 individuals per acre, the potential for unintended consequences on the soil ecosystem appears large. EPNs have not had an adverse effect on mite and collembolan populations (Georgis et al. 1991), yet there is strong evidence that they affect the species diversity of other nematodes. In a golf course ecosystem, the application of H. bacteriophora, an introduced nematode, significantly reduced the abundance, species richness, maturity, and diversity of the nematode community (Somaseker et al. 2002). EPNs had no effect on free-living nematodes. However, there was a reduction in the number of genera and abundance of plant-parasitic nematodes, which often remain enclosed within growths on the plant root. The mechanism by which insect parasitic nematodes have an effect on plant parasitic nematodes remains unknown. Although this effect is considered beneficial for agricultural systems where plant parasitic nematodes cause crop damage, it raises the question of what other effects are possible. Future research on the impacts EPNs have on soil communities will lead to greater understanding of these interactions.

In aboveground communities, EPNs have few side effects on other animals. One study reported that Steinernema felidae and Heterorhabditis megidis, when applied in a range of agricultural and natural habitats, had little impact on non-pest arthropods. Some minimal impacts did occur, however, on non-pest species of beetles and flies (Bathon 1996). Unlike chemical pesticides, EPNs are considered safe for humans and other vertebrates.