A. R. Thompson

Diel eclosion rhythms in populations of wild and laboratory-reared cabbage root flies, Delia brassicae

The ability of cabbage root flies (Delia brassicae (Wied.) (Diptera, Anthomyiidae) to disnerse across host-cron and non-host-crop areas during “migratory flight” in the early part of adult life has been studied (Finch & Skinner, 1975) to assess more accurately the potential of controlling the pest on an area management basis. Complementary experiments were done in 1971 and 1972 to compare the eclosion rhythms of wild cabbage root flies and of flies reared in the laboratory at Wellesbourne (Finch & Coaker, 1969) to determine whether the behaviour of the latter had become adjusted to the artificial environment, making the flies unsuitable for the proposed field experiments. Laboratory experiment. On April 30, 197 1, 9790 laboratory-reared cabbage root fly pupae (Finch & Coaker, 1969) were placed under a 2.5cm layer of moist vermiculite in pots held in cages in a room maintained at 18” + lo and 65% f 5% R.H. Fluorescent lighting was used to extend ‘daylength’ to 16hr from ‘dawn’ at 04.OOhr until ‘dusk’ at 20.00 hr. For 4 days, flies which emerged on to the surface of the vermiculite during the photophase were counted hourly, removed and sexed. Field experiments. On April 26, 1971, 940 wild cabbage root fly pupae collected at Wellesbourne in December 1970 were placed on moist vermiculite in a pot filled with a further 2.5 cm layer of moist vermiculite. The pot was caged in a field with the rim level with the surrounding soil. For 4 days commencing on May 11, emerged flies were counted hourly from 05.30-21.30 hr. Batches of 2000 laboratory-reared pupae and 1000 wild pupae collected during the previous December at Wellesbourne and Great Rollright, Oxfordshire were caged separately at the same site on May 9, 1972. Emerged flies were counted from 05.00--19.00 hr for 19 days commencing on 10 May.