Effects of low storage temperature on survival and infectivity of two Steinernema species (Nematoda : Steinernematidae)

Survival of two British steinernematids, one a strain of Steinemema feltiae (Filipjev) [ = S. bibionis (Bovien)], and the other an undescribed species designated Steinernema sp. (Nashes strain) was assessed after storage of infective juveniles in sterile sand at 5 "C and 15 "C and was measured by their motility (ability to be extracted by mini Whitehead trays). Infectivity after storage at the two temperatures was measured in bioassays with Galleria larvae at 15 "C and was documented as the number of nematodes that infected each larva. Bioassays used single larvae in stoppered plastic vials containing 25 ml of sand and 200-258 infective nematodes. The larvae were replaced in each via1 every 4 days until infections ceased. Survival of both nematode species was high in sand at 5 "C. However, the numbers that infected the Galleria larvae at 15 "C after storage at 5 "C produced a curve. That is, fewer nematodes infected as the period of storage increased, but after a time, the numbers that invaded the Gallerialarvae increased and eventually attained the original level. Survival of the Nashes strain was tested at 15 "C and the number of infective juveniles declined until few could be extracted from the sand after 16 weeks storage. The number of nematodes per host showed a similar pattern. It appears that storage at 5 "C induces most of the nematodes into a state in which they lose their ability to parasitise a host. Infectivity is regained after a period of cooling. These effects of storage temperature on infectivity were not reflected in the data for mortality of the insects, which generally remained high in al1tests. These studies should be repeated and extended to oth r species because entomopathogenic nematodes are frequently stored at low temperatures before use in experiments or in biocontrol programmes. This practice may affect virulence of the parasites and the period of storage may be critical.