Effect of rapid and gradual increase of osmotic stress on survival of entomopathogenic nematodes

The effect of rapid and gradual exposure of entomopathogenic nematodes to osmotic stresses on the induction of a dormant state was determined with the nematode Steinernema feltiae IS-6 infective juveniles (IJs). Rapid exposure of nematodes to glycerol at concentrations of 24% and 28% (w/w) caused the nematodes to enter a dormant state which was characterized by shrinking and impeded motility of all nematodes within 8 h. However, pre-exposure to gradually increasing glycerol concentrations of 5%, 10% and 18% at 4-h intervals resulted in dormancy after 4 h exposure to 24% glycerol. The total time of exposure to glycerol solution was 16 h in gradual osmotic stress. For nematodes exposed to 24% glycerol solution either rapidly or gradually, recovery occurred after 40 min in distilled water. Infectivity of osmotically stressed S. feltiae Us was evaluated by two criteria, insect mortality and invasion rate. The assays indicated that infectivity of nematodes desiccated by rapid and gradual osmotic stresses was similar to that of fresh nematodes. Rapid exposure of S. carpocapsae 'All', S. riobravis 'Texas', S. glaseri 'Nl' and Heterorhabditis bacteriophora HP88 Us to the 24% glycerol solution resulted in dormancy within 8 h. These treatments caused mortality of 48.4% and 11.7% among S. glaseri NI and H. bacteriophora HP88 Us, respectively. Similar effects were observed when these nematode species were exposed to increasing osmotic stress of 5%, 10% and 18% at 6-h intervals. Under these same conditions, mortality of H. bacteriophora HP88 and S. glaseri Nl IJs was 27.5% and 61.8%, respectively.