Fine scale spatial distributions of two entomopathogenic nematodes in a grassland soil

Entomopathogenic nematodes (EPNs) are widespread lethal parasites of many insect species and are known to play an important part in the regulation of soil food webs. We studied the fine scale (5 × 5 cm) distribution and spatial pattern of EPNs in three replicate 0.25 m2 plots of grassland. All soil was removed from plots (100 samples per plot at 0–5 cm and 100 samples at 5–10 cm depth) and the entire nematode community was extracted using a sieving/centrifugal flotation technique. Entomopathogenic nematode larvae were examined microscopically and identified to species. We found Steinernema feltiae in all three sampled plots and S. affine in two plots. Distribution and spatial pattern were analysed using Lloyd's index of patchiness and spatial analysis by distance indices (SADIE). Both methods showed that the two nematode species had aggregated distributions, and in the two plots where both species coexisted, SADIE showed a strong spatial association between the two species. In addition, we assigned all EPN larvae to one of four groups (I–IV) of increasing physiological age, based on the retention of the sheath and visible lipid content. Lloyds index identified group I as being the most aggregated, whereas SADIE identified group II as the most aggregated, probably because group I nematodes were aggregated at a scale finer than our sampling regime. Groups III and IV nematodes were found to have random distributions by SADIE, and Lloyd's index of patchiness had low values for these groups. Our findings are consistent with the hypothesis that when EPNs emerge from host insects they exist in large numbers of highly aggregated individuals, but as time after emergence increases, a combination of dispersal and death results in lower numbers of more randomly distributed individuals.