Soil moisture effects on infectivity and persistence of the entomopathogenic nematodes Steinernema scarabaei, S. glaseri, Heterorhabditis zealandica, and H. bacteriophora

Abstract

We tested the effect of soil moisture on the performance of four entomopathogenic nematodes species that have recently shown promise for the control of white grubs, i.e., Heterorhabditis bacteriophora, H. zealandica, Steinernema scarabaei, and S. glaseri. Experiments for all four nematodes were conducted in sandy loam, for S. scarabaei also in loamy sand and silt loam. Infectivity was tested by exposing third-instar Japanese beetle, Popillia japonica, to nematodes in laboratory experiments and determining nematode establishment in the larvae and larval mortality. Nematode infectivity was the highest at moderate soil moistures (−10 to −100 kPa), and tended to be lower in wet (−1 kPa) and moderately dry (−1000 kPa) soil. In dry soil (−3000 kPa), only S. scarabaei showed some activity. S. scarabaei was active from −1 to −3000 kPa in all soil types but the range of highest activity was wider in loamy sand (−1 to −1000 kPa) than in loamy sand and silt loam (−10 to −100 kPa). Persistence was determined in laboratory experiments by baiting nematode-inoculated soil with larvae of the greater wax moth, Galleria mellonella. For both Heterorhabditis spp. persistence was short at −10 kPa, improved slightly at −100 kPa, significantly at −1000 kPa, and was the highest at −3000 kPa. Both Steinernema spp. persisted very well at −10 kPa. However, S. glaseri persistence was the shortest at −10 kPa but did not differ significantly at −100 to −3000 kPa, whereas S. scarabaei persistence was not affected by soil moisture. Our observations concur with previous observations on the effect of soil moisture on entomopathogenic nematodes but also show that moisture ranges for infectivity and persistence vary among species. Differences among species may be based on differences in size and behavioral and physiological adaptations.

Introduction

Entomopathogenic nematodes (Rhabditida: Heterorhabditidae and Steinernematidae) are lethal parasites of soil-dwelling insects that occur in natural and agricultural soils around the world (Hominick, 2002) and are used for the biological control of insect pests in several ornamental and crop production systems (Grewal et al., 2005). The infective juvenile (IJ) or dauer juvenile is the only free-living, non-parasitic stage of these nematodes (Poinar, 1990, Griffin et al., 2005). The purpose of this non-feeding IJ stage is to persist in the soil environment until it can infect a host. Using foraging strategies varying with species along a continuum from ambush to cruise foraging, the IJs seek out a host and invade it through natural openings or thin areas of the host's cuticle. The IJs ultimately penetrate into the host's body cavity where they release species-specific symbiotically associated bacteria, which they have carried in their intestinal tract. Bacteria and nematodes combine to overcome the host's immune response and kill it, typically within 1–3 days. The bacteria propagate and protect the cadaver from colonization by other microorganisms. The nematodes develop through one to three generations, feeding on the bacteria and host tissues metabolized by the bacteria. Depleting food resources in the host cadaver lead to the development of a new cohort of IJs that emerges from the host cadaver after 1–3 weeks to search for a new host.

The success of nematode applications for insect control in soil and the survival of naturally occurring nematode populations depends on the IJ's ability to disperse and persist until it can locate a host. Numerous intrinsic factors (e.g., behavioral, physiological) and extrinsic factors (e.g., temperatures, soil moisture, soil texture, RH, UV radiation) (Kaya, 1990, Smits, 1996) have been shown to affect IJ dispersal and persistence. Soil moisture is probably the most important variable affecting nematode performance and survival in the soil (Molyneux and Bedding, 1984, Kung et al., 1991, Koppenhöfer et al., 1995, Grant and Villani, 2003a, Grant and Villani, 2003b) because nematodes need a water film for effective locomotion (Wallace, 1958).

In soil, IJs move through the water film that coats the interstitial spaces or through water-filled pores wider in diameter than the nematodes’ body (Wallace, 1958). Nematode movement is optimal if the thickness of the water film is approximately half the thickness of the nematodes’ body (Wallace, 1958). As the soil dries, the water film becomes thinner and larger pores drain of water which increasingly restricts nematode movements. On the other hand, nematode movement can also be restricted if the interstitial spaces are completely filled with water (in water saturated soil) when the pores’ diameter is much greater than that of the nematodes, i.e., >200 μm (Quénéhervé and Chotte, 1996).

Several studies have indicated that the soil moisture range in which IJs can be active may differ among entomopathogenic nematode species (Koppenhöfer et al., 1995, Grant and Villani, 2003a, Grant and Villani, 2003b). IJs cannot survive rapid desiccation in laboratory experiments under low RH regimes (Simons and Poinar, 1973, Kung et al., 1991, Womersley, 1990), but they can persist considerable lengths of time in dry soil (Kung et al., 1991). The gradual desiccation in soil may provide IJs enough time to adapt physiologically into a partially desiccated and immobilized quiescence state (Womersley, 1990). Because nematode species may differ in their ability to adjust to low soil moisture, the effect of soil moisture on nematode persistence may also differ among nematode species (Kung et al., 1991, Grant and Villani, 2003b).

The objective of this study was to further elucidate the effect of soil moisture on entomopathogenic nematode performance with particular interest to nematodes species that have recently shown promise for the control of various species of white grub, the root-feeding larvae of scarab beetles (Coleoptera: Scarabaeidae) that are among the most serious pests of turfgrass and ornamental plants in the USA and many other countries (Potter, 1998, Vittum et al., 1999). Studies have indicated that nematode efficacy against white grubs in turfgrass is positively related to soil moisture and to irrigation volume and frequency after nematode application (Shetlar et al., 1988, Georgis and Gaugler, 1991, Grewal et al., 2004). However, turfgrass areas that can be affected by white grubs often do not have irrigation systems. This is particularly true for home lawns and lower budget athletic fields which at least for the foreseeable future will comprise the majority of turfgrass areas in which nematodes may be used for white grub control (Shapiro-Ilan et al., 2002). Therefore, it is important to find nematode species or strains that can infect hosts and persist well over a wider range of soil moisture conditions, particularly in lower soil moisture conditions.

We compared the effect of a range of soil moistures on the infectivity and persistence of Heterorhabditis bacteriophora GPS11 strain (Grewal et al., 2002, Grewal et al., 2004), H. zealandica X1 strain (Grewal et al., 2002, Grewal et al., 2004), and Steinernema scarabaei AMK001 strain (Koppenhöfer and Fuzy, 2003a, Koppenhöfer et al., 2004). A fourth species that is considered scarab-adapted, S. glaseri NC1 strain (Klein, 1990, Klein, 1993), was included because the large size of its IJs may play a significant role in its activity across different soil moistures. While the effect of soil moisture on nematode performance has never been studied previously for H. zealandica and S. scarabaei, both H. bacteriophora (Grant and Villani, 2003a, Grant and Villani, 2003b) (but not the GPS11 strain) and S. glaseri (Koppenhöfer et al., 1995, Grant and Villani, 2003a, Grant and Villani, 2003b) have been previously studied. Nevertheless, we considered it important to include these species to facilitate direct comparisons because our study used different methods than previous studies to measure nematode performance. First, for the measurement of nematode infectivity our study used true soil-dwelling insects (i.e., white grubs) which are also target insect pests for these nematode species rather than larvae of the greater wax moth, Galleria mellonella. Second, we used more detailed methods for the measurement of nematode persistence by using four rather than one or two baiting rounds and determining nematode establishment rather than wax moth mortality.