Chemosterilization of the cabbage root fly under field conditions

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Abstract
SUMMARY: Removing hedgerow feeding sites from within 40, 80 or 160 m of cauliflower plots, failed to reduce populations of the cabbage root fly (Erioischia brassicae (Bouché)). Oviposition on the plots was unchanged but the efficiency of yellow water‐traps was improved. Traps along‐side cleared hedgerows caught more gravid than non‐gravid females. In cages, 94% of the eggs laid were sterile when both sterilant baits containing tepa [tris‐(i‐aziridinyl) phosphine oxide] and hedgerow flowers were present.In the field, maximum egg‐sterility rarely exceeded 30% when sterilant was present when natural sterility averaged 8%. Chemosterilization was an inadequate control since sufficient eggs and larvae survived to produce root‐damage indices of 49 % in contrast to 2 % on insecticide‐treated plants.Most eggs were sterile when chemosterilant baits were placed 0.75 m apart and bait was most effective when placed around the crop during the pre‐oviposition period and through the crop at the time of maximum oviposition. Increasing the concentration of tepa failed to increase the effectiveness of the bait but a higher proportion of the eggs laid were sterile on large than on small plots. When chemosterilant treatments were stopped, egg‐fertility gradually reverted to the natural level.Protracted immigration, an innate tendency for females to disperse, reductions in the competitiveness of sterile males, and the failure of males to re‐disperse once sterilized, appeared to be the main factors limiting the levels of sterility in root‐fly populations exposed to tepa‐baited lures in the field.