The emergence of Delia radicum in relation to agricultural practice
Cabbage root fly passes through three generations a year in the south of England. The first generation peaks in late April to early May having overwintered in diapause. Recently, populations have been discovered in which the emergence of first generation flies was protracted, lasting until mid-July. The project aimed to investigate the incidence of late emergence in Devon and to examine the ways in which late emergers differed from early emergers in their progress through diapause. A new emergence trap was developed and used in conjunction with yellow water traps to monitor the emergence of cabbage root fly populations at locations of contrasting agricultural practice. Late emergence was widespread in Devon, occurring later (up to September), and in a greater proportion of some populations than any previously reported. The phenomenon appears to be a response to the planting of brassicas in June, as flies emerging at the normal time may not be able to locate a host crop. Emergence in a substantial proportion of one population was delayed for a year. This has not been been previously reported in cabbage root fly and represents a 'sit and wait' strategy associated with short-lived habitats, occurring in patches often separated by considerable distances but which frequently reappear in approximately the same location. A gas chromatography technique was developed which is capable of individually monitoring the respiration rates of large numbers of pupae. The results suggested that the temperature optimum for diapause development may r1se in late emergers, possibly above the threshold for postdiapause development. There was considerable 1ntrapopulat1on variation in temperature responses. Overall, the results suggest that the level of variation in temperature response and emergence times between and within populations will require careful local investigation with continuous reassessment of selection pressures, for an accurate prediction of cabbage root fly emergence.