Selection at the Dieldrin Resistance Locus in Overwintering Populations of Lucilia-Cuprina (Wiedemann)

Abstract

Development was arrested in wandering L. cuprina larvae placed in the ground at Heidelberg, Victoria during May and June. During this overwintering period individuals resistant to the insecticide dieldrin were more severely selected against than at other times of the year. Over the 3 years of the study (1986-1988) the proportions of overwintering larvae that reached the adult stage of the life cycle were low (range 2.8-6.0%) compared to those at other times (52.8-80.0%). The rate of development was greatest during the late summer months. In laboratory controls the developmental times from egg to adult (13.0-14.0 days) and the proportions reaching the adult stage (78.0-96.0%) were consistent throughout the experiment. The larvae used in the experiment were of the F2 generation of an original cross between pure-breeding strains that were resistant (Rdl/Rdl) or susceptible (+ /+) to dieldrin. The frequency of the Rdl allele in adults emerging from the overwintering population was significantly lower (0.09-0.15) than at other times (0.44-0.52) when the result were similar to laboratory controls (0.43-0.53). Samples of pre-pupae placed in the ground in May 1988 were removed at 30-day intervals. A consistent decline in the proportion reaching the adult stage and in the Rdl frequency of these populations was observed with increasing time in the ground. Laboratory trials, in which pre-pupae were held at 8�C for periods of up to 11 weeks, showed similar trends to those observed in the field studies.