Development of the overwintering stages of the carrot fly, Psila rosae (Diptera: Psilidae)

Abstract The overwintering stages of the carrot fly (Psila rosae (Fabricius)) were sampled at Wellesbourne, Warwickshire, during the winters of 1987, 1988, 1989 and 1990. Between 20% and 55% of the insects overwintered as pupae in diapause. The remaining insects stayed in the larval stage during the winter period and gave rise to non-diapausing pupae in the early spring. In May, adults emerged earlier, and over a shorter period, from insects that overwintered as larvae than those that overwintered as pupae. Diapause was most intense during late December and early January. In the laboratory, diapause was completed at temperatures of 5°C and above but was not completed at 0°C. When the overwintering stages were sampled in early February and maintained at constant temperatures in the laboratory, the overwintering pupae required from 114 days at 9°C to 44 days at 21.5°C to give rise to adults. In contrast, the overwintering larvae required only 91 days (9°C) and 27 days (21.5°C), respectively. Populations of overwintering P. rosae collected from Warwickshire and Norfolk had similar requirements for spring emergence but diapause pupae collected in Lancashire emerged over a longer period of time than those collected in Warwickshire. Comparisons with published data indicated that North American and European pupae had different requirements for the completion of diapause. Accumulation of weather data for a P. rosae forecast should start when temperatures are low and any date in early February would be suitable.