Desiccation survival of entomopathogenic nematodes of the genus Heterorhabditis

The present study aims at determining the desiccation tolerance of entomopathogenic nematodes from the genus Heterorhabditis recently isolated in Israel. We first verified the most suitable desiccation conditions that lead to induction of the anhydrobiotic state using Heterorhabditis bacteriophora HP88. After direct exposure of infective juveniles (IJs) to 97% and 93% r.h. for 96 h, the survival rate was > 70%. By contrast, exposing HP88 IJs to 88% and 85% r.h, resulted in poor survival (< 10%) or complete mortality. Following exposure to 97% and 93% r.h. for 24, 48, 72 or 96 h, survival ranged from 68% to 79% with no significant differences between the exposure periods. Stepwise reduction of r.h. conditions (97% > 93% > 88% > 85% r.h.) at a 24-h or 72-h interval resulted in enhanced survival (30% survival) of IJs at the final r.h. level compared with IJs which were either directly exposed to 85% r.h. (0% survival) or were preconditioned at the higher r.h. levels prior to exposure to 85% r.h. (15% survival). H. bacteriophora HP88 IJs were able to survive for at least 18 days after preconditioning. Al 97% r.h. nematode viability remained stable at 70-85% and at 93%, r.h. survival ranged between 37% and 60%. The data indicate that survival is influenced by rate of water removal from the nematode's body and a minimal relative humidity level (>93%). Substantial differences in survival ability were observed among IJs of 12 new heterorhabditid populations, isolated from different climatic regions in Israel, which were preconditioned at 97% r.h. for 72 h following by an additional 72 h at 93% r.h. Maximum survival was recorded with HIS-19 (64%), moderate (40-55%) survival was observed with seven isolates, including H. bacteriophora HP88, and five isolates displayed poor (<25%) desiccation tolerance.