Mechanisms of coexistence of two sympatric entomopathogenic nematodes, Steinernema affine and S. kraussei (Nematoda: Steinernematidae), in a central European oak woodland soil

Various aspects of the coexistence of two sympatric entomopathogenic nematode species Steinernema affine and Steinernema kraussei were studied in the field and laboratory. The seasonal dynamics of both nematodes and local insects, and nematode vertical and horizontal distribution were observed in the field in 2007. A series of subsequent laboratory experiments studied the host range of both species and their competition in selected experimental and natural hosts. S. affine was dominant in the natural habitat reaching a mean density of 25,000 infective juveniles per square meter and being several times more abundant than the latter species. Both nematode species had a patchy distribution and occurred in all vertical strata of the tested upper 10 cm soil profile, however S. affine was more abundant in the top 4 cm whereas S. kraussei preferred lower soil layers (6–10 cm). No evident spatial relationship between the two species was found. In laboratory bioassays both species were able to attack and multiply in the same spectrum of local insects from various dipteran and coleopteran families that probably serve as natural hosts of entomopathogenic nematodes. No evidence suggesting any host differentiation between the two species was found. Competition experiments showed a strong dominance of S. affine in all tested insects and its superiority appears to be universal. Generally it seems that both species share an ecological niche and thus the avoidance of competition with the latter species seems to be a crucial factor for S. kraussei. Patchy distribution and implicit differences in horizontal distribution probably markedly contribute to the coexistence of both species.