Soil moisture effects on entomopathogenic nematodes

The effect of soil moisture on entomopathogenic nematode virulence was examined in the laboratory. Objectives were to determine the virulence of several species and isolates of entomopathogenic nematodes at various soil moisture contents and temperatures, and after fluctuations in soil moisture. Studies included up to five isolates of entomopath ogenic nematodes: Heterorhabditis bacteriophora Poinar (Oswego and Tuscarora strains), Steinernema glaseri (Steiner) (NC1 strain), S. feltiae (Filipjev) (Biosys 369 strain), and S. carpocapsae (Weiser) (NY001 strain). Nematodes were applied to sandy loam soils ranging in soil moisture content from below the permanent wilting point of plants to near saturation. In all experiments, a rainfall or irrigation event was simulated by adding water to rehydrate soils to high moisture levels (near saturation). Nematode virulence was evaluated periodically by measuring insect mortality in Galleria mellonella (L.) larval bioassays, before and after rehydration. Nematode virulence increased with soil moisture content for all species and isolates tested. Our studies demonstrated that the virulence of entomopathogenic nematodes in low moisture conditions could be restored by rehydrating the soil. Insect mortality was generally low in low-moisture, nematode-infested soils before rehydration, but increased to high levels posthydration. Moisture effects were evident from the onset of each experiment, whereas the effect of soil temperature on nematode-induced insect mortality was delayed and nonsignificant until 14 wk after the initiation of the third experiment.