We combined polymerase chain reaction (PCR) amplification of DNA sequences and important morphological characters as a technique to differentiate nematode isolates in the genus Steinernema. Five decamer oligonucleotide primers were used to generate random amplified polymorphic DNA (RAPD) fragments from 11 nematode isolates. The primers generated 8–12 fragments, ranging from 220 to 1300 bp in size. Reproducible amplified DNA fragments of 11 isolates showed obviously inter- or intra-specific polymorphisms, enabling us to differentiate easily the nematode species and isolates. Combining RAPD–PCR fragments with the examination of morphological characters of infective juveniles and 1st-generation males, we identified isolate OH1S, collected from Newport, Oregon, as S.feltiae; isolate OS21, collected from Grants Pass, Oregon, belonged to a previously undescribed species. Our study may provide a rapid and reliable method for the identification of Steinernema nematodes.

An 8-point visual index was developed for Oil Red O staining of neutral lipids in infective juveniles (IJs) of Steinernema carpocapsae (All), S. riobravis (Biosys 355), S. feltiae (UK76) and S. glaseri (NC). The visual index was found to be a reliable and rapid method for determining the relative neutral lipid content of individual IJs and was validated quantitatively by gas chromatography. The relationship between neutral lipid utilization and infectivity of IJs stored in distilled water at 25°C was also investigated and the first quantitative results on neutral lipid utilization in entomopathogenic nematodes are reported. Neutral lipid contents of freshly harvested IJs of S. carpocapsae, S. riobravis, S. feltiae and S. glaseri were 31, 31, 24 and 26% dry wt, respectively. Steinernema carpocapsae showed a sigmoidal pattern for neutral lipid utilization while S. riobravis used neutral lipids at an almost constant rate. Survivorship of these two species ranged between 120 and 135 days, whereas S. feltiae and S. glaseri lived >450 days and had a slower rate of lipid utilization during a 260 day storage period. Oil Red O staining showed that individual IJs in each population utilized lipids at different rates, even though they had the same initial lipid index. The infectivity of S. riobravis, S. feltiae and S. glaseri declined with lipid utilization. In contrast, S. carpocapsae maintained a high level of infectivity even at relatively low lipid levels. Therefore, neutral lipid content was found to be a suitable indicator of infectivity for S. riobravis, S. feltiae and S. glaseri but not for S. carpocapsae.