Entomopathogenic nematodes (EPNs) have potential to control many soil-dwelling insect pests but have been limited in their usage, partly by their unpredictable field performance. Numerous abiotic and biotic factors are thought to contribute to this poor predictability, but the exact impacts and relative importance of these factors in affecting EPN performance in the field are not well understood. Previous studies have highlighted diverse interactions between EPNs and other members of the soil community, from plants and fungi to arthropods and annelids. These interactions may help or hinder EPNs in a variety of ways. However, current research has yet to determine how many of these interactions influence EPN performance under field conditions, specifically, if they contribute to the variability limiting EPN efficacy and wide-scale adoption. Here we outline current knowledge of these interactions as well as challenges and avenues for future research, such as greater integration of EPN research with soil animal and rhizosphere ecology, that will better elucidate the potential, limitations, and proper use of EPNs in pest management.
Since the discovery of entomopathogenic nematodes (EPNs) in 1923 and their first commercialization sixty years later, thirteen EPN species of the genera Steinernema and Heterorhabditis have been commercially cultivated and marketed for use in a wide array of agricultural and horticultural systems (Lacey et al., 2015). EPN infective juveniles (IJs) invade insect bodies through the mouth, anus, spiracles, or cuticle (Lewis et al., 2006; Wang and Gaugler, 1998) and release their bacterial symbionts (Xenorhabus spp. bacteria for Steinernema spp. nematodes, and Photorhabdus spp. bacteria for Heterorhabditis spp. nematodes) into the insect’s hemolymph. The bacteria release toxins to kill the insect, though sometimes are aided by venom proteins and anti-immune agents produced by the nematodes themselves (Lu et al., 2017), and proliferate in the cadaver. The IJs consume the bacteria, complete their development, and reproduce. After one or more generations of nematodes inside the cadaver, new IJs leave to seek new hosts in the soil (Kaya and Gaugler, 1993; Lewis and Clarke, 2012). When using EPNs against pests in agricultural and horticultural systems, IJs are commonly applied onto soil as an aqueous suspension sprayed through the same kind of device used to apply a chemical pesticide. Many other application techniques exist, such as applying EPNs through drip irrigation systems or applying them contained within bait capsules or already-infected insect cadavers (Shapiro-Ilan and Dolinski, 2015). EPNs can effectively provide a non-toxic, environmentally benign alternative to chemical insecticides for the control of some soil-dwelling pests, including white grubs (Grewal et al., 2005), citrus root weevil (Shapiro-Ilan et al., 2005, 2002), and mole crickets (Parkman et al., 1996). However, despite these and other successes (Georgis et al., 2006), extensive use of EPNs in agriculture is limited to a few markets, including citrus orchards (Shapiro-Ilan et al., 2005) and vegetables in greenhouses (Dolinski et al., 2012; Lacey and Georgis, 2012). Despite the safety of EPNs for humans, other vertebrates, and many non-target invertebrates (Georgis et al., 2006; Lacey and Georgis, 2012), their variable performance and persistence remain important factors restricting their adoption by pest managers (Georgis et al., 2006; Georgis and Gaugler, 1991; Shapiro-Ilan et al., 2002). For instance, many studies of EPN efficacy against scarab grubs in turf and nursery crop systems report highly variable control even within the same EPN species (Georgis et al., 2006; Grewal et al., 2005). Increasing knowledge of how abiotic and biotic soil characteristics affect and contribute to variability in EPN performance and persistence will be a crucial task to better optimize EPNs for soil pest management. In addition, while many applied EPNs do not persist in soil longer than a few weeks or months (Ebssa and Koppenhöfer, 2011; Gaugler et al., 1997; Smits, 1996; Susurluk and Ehlers, 2008), some native EPN populations, adapted to a particular environment, have been shown to persist over multiple years when isolated, cultured, and applied to the soil (Koppenhöfer and Fuzy, 2009; Shields et al., 1999). This interest in using persistent EPN populations underscores the need for understanding the biotic and abiotic factors that limit or promote EPN persistence.