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Entomopathogenic nematodes (Rhabditida: Steinernematidae and Heterorhabditidae) (EPNs) are generalist obligate pathogens present in the soil of most ecosystems. They have the potential to infect a broad host range, yet the potential for endemic EPNs to regulate soil-dwelling insect populations has received limited attention. We investigated the population dynamics of endemic EPNs to determine their ability to regulate annual bluegrass weevil ( Listronotus maculicollis ) (Coleoptera: Curculionidae) populations, a major pest of turfgrass in the northeastern United States. Weekly sampling of nematode and L. maculicollis populations was conducted in untreated fairway transects on three golf courses in New Jersey between April and October of 2006 and 2007. Steinernema carpocapsae and Heterorhabditis bacteriophora were found infecting all weevil stages from third instar to teneral adults. Both EPN species exhibited a distinct seasonality, appearing in high densities in the weeks immediately following high densities of first generation weevils in the soil. A positive temporal relationship was observed between densities of EPN-infected weevils and weevil stages between third instar and pupa, but L. maculicollis generational mortality due to EPN infection was highly variable between years and sites, ranging between 0 and 50%. Although infection densities and larval densities were positively correlated, per capita mortality did not increase with increased weevil densities. EPN distribution dynamically cycled between aggregated and uniform throughout the season across fairways. Few significant relationships were found to support the hypothesis that weevil spatial dispersion influences EPN spatial dispersion. The variability in EPN seasonal occurrence and generational impact on L. maculicollis together with the lack of spatial association with L. maculicollis suggest an inability of endemic EPN populations to reliably regulate weevil populations on golf course fairways.

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.

Our objectives were to determine whether entomopathogenic nematode emergence from host cadavers is influenced by soil moisture, whether the nematodes can survive adverse desiccating conditions in the soil by remaining within the host cadaver, and whether differences in such an adaptation occur among species. In the first experiment, wax moth larvae killed by Steinernema glaseri, Steinernema carpocapsae, Steinernema riobravis , or Heterorhabditis bacteriophora were placed in soil water potentials ranging from −500 MPa (very dry) to −0.006 MPa (moist). No infective juveniles (Us) emerged from cadavers at −500 MPa, and only few S. glaseri and S. carpocapsae emerged at −40 MPa. Large numbers of IJs emerged at ≥ −5 MPa from cadavers containing S. carpocapsae, S. glaseri , or H. bacteriophora. S. riobrauis emerged only at ≥ −0.3 MPa. In the second experiment, cadavers were left in dry soil (−40 MPa) for various periods of time before being rehydrated. The number of Us emerging per cadaver and the infectivity of the emerged IJs were determined. IJ emergence declined with the time that the cadavers were left in dry soil. Regression analysis predicted that IJ emergence from cadavers with S. glaseri, S. carpocapsae, H. bacteriophora , or S. riobravis would stop after 27, 62, 80, and 111 days, respectively, in dry soil. We hypothesize that S. carpocapsae , a sit-and-wait forager, survives longer than S. glaseri because it is adapted to infect insects near the soil surface, whereas S. glaseri , an actively searching forager, is adapted to infect insects deeper in the soil profile. Cadavers colonized by S. carpocapsae , therefore, are more likely to be exposed to dehydrating conditions. H. bacteriophora , an actively searching forager, may survive longer within cadavers because the gummous consistency of its host cadavers retains moisture very efficiently. S. riobrauis may survive for considerable lengths of time within cadavers in adaptation to the subtropical, semiarid climate of its geographic area of origin.

Interactions between entomopathogenic nematode species with different foraging strategies were examined in the presence of two host species in soil. Based on differences in insect behavior and nematode pathogenicity, we hypothesized that black cutworm, Agrotis ipsilon (Hufnagel), larvae would be more likely to serve as hosts for Steinernema carpocapsae or Steinernema riobravis , whereas masked chafer grubs, Cyclocephala hirta LeConte, would be more likely to serve as hosts for Steinernema glaseri . In the laboratory, the highest mortality of and nematode penetration in A. ipsilon were observed for S. carpocapsae followed by S. riobravis and S. glaseri . In C. hirta , substantial mortality and nematode penetration were only observed for S. glaseri . After combined applications of two nematode species, S. carpocapsae dominated over S. glaseri in A. ipsilon , whereas S. glaseri and S. riobravis shared the host resources. In C. hirta , S. glaseri outcompeted each of the other species. In the greenhouse, containers with turfgrass were inoculated with S. glaseri , S. carpocapsae , or S. riobravis , or combinations of S. glaseri with either S. carpocapsae or S. riobravis . Four days later, each container received seven A. ipsilon and nine C. hirta larvae, and new insects were added at 30-day intervals. The densities of infective juvenile nematodes were monitored over 150 days by taking soil samples from each container. In the single species treatments, numbers of S. carpocapsae and S. riobravis increased after 30 days, decreased thereafter and remained low. Steinernema glaseri numbers fluctuated between low and high densities. In the combination of S. glaseri and S. carpocapsae , both species were depressed compared with the single species treatment. In the combination of S. glaseri and S. riobravis , both species coexisted and showed parallel fluctuations but S. glaseri dominated numerically. Our observations indicate that two entomopathogenic nematode species may successfully coexist in an area by having different foraging strategies that separate nematode species spatially, exhibiting host specificity, and occupying a different niche. In the field, patchy distribution and alternate hosts may also support nematode coexistence.