Entomopathogenic nematodes in agricultural cropping systems

Summary

The ecology of entomopathogenic nematodes in relation to cruciferous crops has been studied in Denmark. The main objectives were to obtain information about nematode occurrence, host range and regulatory effect on insects. This included a range of studies in the field and in the laboratory.
In one set of experiment, cabbage fields were surveyed and Steinernema affine, S. bicornutum, S. intermedium and S. feltiae were isolated. The presence of nematodes was correlated to pupal numbers of the cabbage root fly, D. radicum. The conclusion was that the presence of nematodes at cabbage plants did not reduce the number of pupae.
Another approach was to quantify the proportion of larvae or pupae of D. radicum infected with entomopathogenic nematodes during the growing season. For that purpose a total of 6000 individuals were collected. Of these, only four isolates (S. feltiae) were obtained. This was in accordance with the above conclusion, that entomopathogenic nematodes under natural conditions only play a minor role in the mortality of D. radicum.
The nematode isolates were used in bioassays in the laboratory to test the ability of different insects to support nematode propagation. The main insects studied were the cabbage root fly and the pollen beetle, Meligethes spp.. Both insect species produced nematodes but only S. feltiaeisolates were successful in propagating in D. radicum.
Further experiments in the laboratory concentrated on insect behaviour in relation to nematodes, side effect on parasitoids and the effect of test arena size on insect mortality. The results from these experiments were used to extrapolate results obtained in the laboratory to field conditions. Selected results from one or more of these experiments and from the experiments mentioned above will be given.