a1 Rothamsted Experimental Station, Harpenden, Herts, AL5 2JQ, UK
An inexpensive and versatile method for maintaining age-structured Musca domestica L. populations for studies on the evolution of insecticide resistance is described. Adult flies are kept in spacious aluminium cages in which age-structuring is maintained by the thrice-weekly addition of pupae bred from eggs collected from within the cages. The population size is regulated in a density-independent manner by constraining the input of pupae to that necessary to maintain the required equilibrium density of adults within a cage. Adult numbers are monitored by photographing from outside the cage flies settled on a grid etched on the rear wall, and by converting this grid count to an estimate of population size using a calculated regression line. Observed changes in fly numbers in a cage accorded well with those predicted by a computer model using empirical data on larval productivity, and the emergence and survivorship schedules of adult flies. Since a variety of insecticide control regimes can be applied within the cage, this system enables an adequate yet tractable simulation of selection for resistance by insecticides under field conditions.
(Received September 30 1985)