ACCUMULATED TEMPERATURES FOR PREDICTING THE TIME OF EMERGENCE IN THE SPRING OF THE CABBAGE ROOT FLY, DELIA-RADICUM (L) (DIPTERA, ANTHOMYIIDAE)

Laboratory experiments showed that 4.degree. C, rather than 6.degree. C as used previously by most authors, was a suitable base temperature for calculating the number of day-degrees C required to predict the time of emergence in the spring of Delia radicum (L.); 1 February was an appropriate date to start the accumulation, as more than 50% of the pupae had completed diapause by this time and were therefore able to proceed with their development. The time of 50% fly emergence in the spring was estimated more accurately from accumulated day-degrees 6 cm deep in the soil, the average depth of overwintering puparia, than from accumulated air day-degrees calculated from standard meteorological measurements of air temperature. At Wellesbourne, England [UK] approximately 179 .+-. 8 soil day-degrees above 4.degree. C or 230 .+-. 10 air day-degrees above 4.degree. C were required for 50% emergence in the spring. In general, males emerged 25-50 day-degrees above 4.degree. C (4-8 days) before females. A further 60-170 day-degrees above 4.degree. C (2-5 weeks) then elapsed before maximum fly activity occurred in new host crops.