Osmotic survival of the entomopathogenic nematode Steinernema carpocapsae

The effect of different osmolytes on the viability and the effect of osmoti c pressure on the induction of a dormant state similar to that caused by a slow desiccation rate were evaluated in the entomopathogenic nematode Stein ertema carpocapsae 'All'. For both experiments, a high-temperature (45 degr ees C) assay (HTA) was employed. Exposing fresh infective juveniles to the HTA resulted in a drastic reduction in viability. Using the same assay, the mortality of desiccated nematodes was gradual, showing an enhanced ability to withstand high-temperature conditions. The patterns of decline in viabi lity in the evaporatively dehydrated and the osmotically desiccated nematod es were similar, Most of the salts tested in the screening assay caused hig h mortality levels among the nematodes within the first 24 h of exposure. I n contrast, the nonionic solutes tested did not hamper the viability of the infective juveniles. In these nonionic solutions, all nematodes were compl etely shrunk after 48 h. Furthermore, 72-h exposure to these solutions resu lted in an increase in heat tolerance similar to that of the evaporatively dehydrated nematodes. A substantial increase in heat tolerance was recorded in the treatments with glycerol solutions at concentrations from 2.2 to 3. 8 M. A similar effect was obtained by polyethylene glycol (PEG) 300 NW at c oncentrations ranging from 1.2 to 1.6 M. PEG 600 MW induced enhancement of heat tolerance at a concentration of 0.8 M. A high level of viability was a ttained among nematodes that were stored for 72 days following a gradual in crease in glycerol concentrations, Exposure of these nematodes to 45 degree s C in the HTA resulted in 87.3 +/- 4.7 and 49.2 +/- 3.9% survival after 4 and 8 h, respectively. Reduction in viability was observed among nematodes that were directly exposed to the glycerol solution over a Ig-day storage p eriod, With this treatment, survival levels of 72.7 +/- 3.9 and 26.5 +/- 4. 7% after 4 and 8 h, respectively, mere recorded in the HTA. Reduction in vi ability among nematodes stored in distilled water was noted after 36 days o f storage. Evaluation of nematode infectivity by two criteria (insect morta lity and invasion rate) indicated that infectivity of nematodes desiccated by gradual osmotic pressure induced by glycerol was similar to that of fres h nematodes after 54 days in storage at 25 degrees C. In comparison, infect ivity of nematodes stored in distilled water declined significantly compare d to that of fresh nematodes. (C) 2000 Academic Press.