Accumulated temperatures for predicting the time of emergence in the spring of the cabbage root fly,Delia radicum(L.) (Diptera: Anthomyiidae)

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Abstract
Laboratory experiments showed that 4°C, rather than 6°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, approximately 179 ± 8 soil day-degrees above 4°C or 230± 10 air day-degrees above 4°C were required for 50% emergence in the spring. In general, males emerged 25–50 day-degrees above 4°C (4–8 days) before females. A further 60–170 day-degrees above 4°(2–5 weeks) then elapsed before maximum fly activity occurred in new host crops.