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Ann. appl. Biol. (1973), 73, 243-258 Printed in Great Britain

Chemosterilization of the cabbage root fly under field conditions BYS. FINCH AND G. SKINNER National Vegetable Research Station, Wellesbourne, Warwick SUMMARY

Removing hedgerow feeding sites from within 40, 80 or 160 m of cauli­ flower plots, failed to reduce populations of the cabbage root fly (Erioischia brassicae (Bouche)). 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-(1-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. INTRODUCTION

There are two principal methods for applying chemosterilants: the release of sterile males or the treatment of the existing populations (Borkovec, 1966). The latter was chosen for study at Wellesbourne because the problems associated with mass rearing and release of sterile flies are eliminated. These tasks are difficult when large natural populations or areas are involved. Chemosterilization of a field population attacking a limited area of crop will be successful only if movement of the flies is restricted. Cabbage root flies have been reported to disperse over only limited distances (Read, 1958; Mowat & Coaker, 1968; Hawkes, 1969). Hawkes (1969) estimated that mature females initially dispersed towards a brassica crop at a mean rate of II6 m/day and that populations dispersed within a crop at mean rates of only 8-20 m/day. Most flies at a crop stayed near to a hedgerow or barrier during the morning, moved into the crop to oviposit in the early afternoon and returned to the hedgerow in the late after­ noon (Hawkes, 1969). This behaviour suggested that bait containing chemosterilant I6

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CD -@, Brassica plots:©, Cage ai�� Chemosterilant: 6, 2 May-19 June (!st generation) 1970; , 20 June-14 Aug. (2nd generation) and 15 Aug.-2 Oct. (3rd generation) 1970:9, 24 Apr.· IR June (1st generation)l97!:-0. 19 Junc-6 Aug. (2nd generation) and 7 Aug.--24 Sept. (3rd generation)� I 971:,......., Length of hedgerow cleared of flowers:q>, Tip for brassica waste: �, Laboratory area: � . Brassica plots providing overwintering pupae 1969; f