The use of felt traps to estimate oviposition by the cabbage root fly, Delia radicum (L.), and turnip root fly, Delia floralis (Fall.), was compared with soil sampling at seven localities between 1992 and 1994 in Denmark and Norway. In all, 281 comparisons were made, based on 6800 samples. In 4.6% of these comparisons no eggs were found by either method. In 16% of the comparisons, 2.0±0.41 (±S.E.) eggs were found per soil sample and no eggs were found in felt traps, whereas in 0.7% of the comparisons 0.10±0.03 eggs were found per felt trap and no eggs in soil samples. When eggs were found using both methods, the ratio between soil sampling and felt traps varied from 13.1±3.2 when the egg laying rate was very low to 1.8±0.2 at high egg laying rates. Regression analysis showed significant correlation between felt trap catches and soil sampling (P<0.001), whereas locality did not influence felt trap efficiency (P=0.096). The influence of the physical conditions of felt traps, i.e., whether they were clean and dry, were analysed in a single year. Tentative action thresholds for control measures against cabbage root flies and turnip root flies were developed for cauliflower, Brassica oleracea, convar. botrytis (L.) Alef., var. botrytis, based on data from the literature and the present results.

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1 Seven organically grown cabbage fields were surveyed for entomopathogenic nematodes in the autumn by baiting. Nematodes were obtained from three fields with bait larvae infection ranging from 1.3-4.0%. 2 Inoculation of Steinernema feltiae (Rhabditida: Steinernematidae) in the spring increased bait larvae infection to 16.0-32.0%. 3 Four different species were found (Steinernema affine, Steinernema bicornutum, S. feltiae and Steinernema C1). 4 The number of Delia radicum (L.) (Diptera: Anthomyiidae) puparia was significantly reduced at plants where S. feltiae had been inoculated. 5 Delia radicum parasitoids were also affected by S. feltiae. The results indicated that Aleochara species (Coleoptera: Staphylinidae) were more negatively affected than Trypliographa rapae (Westw.) (Hymenoptera: Cynipidae).

The susceptibility of pupating larvae of pollen beetles, Meligethes spp. Stephens (Coleoptera: Nitidulidae) and brassica pod midges, Dasyneura brassicae Winnertz (Diptera: Cecidomyidae) to entomopathogenic nematodes (Nematoda: Rhabditida) was studied in the laboratory. The results showed that brassica pod midge larvae were almost unaffected by the tested nematodes (Steinernema bicornutum, S. feltiae and Heterorhabditis bacteriophora) whereas successful pupation of pollen beetle larvae was reduced with increasing number of nematodes (S. bicornutum, S. carpocapsae, S. feltiae and H. bacteriophora). The exposed larvae had been collected in the field and some of the pollen beetle larvae were parasitised by parasitoid wasps. It appeared that parasitised larvae were less affected by nematodes than non-parasitised larvae.

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