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

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.