Problems in obtaining a description of the evolution of dimethoate resistance in Danish houseflies (Musca domestica)

Data on the evolution of a polygenically determined resistance to dimethoate, in populations of houseflies on Danish farms, were examined in order to see whether useful information about the processes involved could be obtained. Examination of the variance of tolerances in samples of flies suggested that there was considerable genetic variability for resistance before dimethoate was first used, but provided no other information. Before examining estimates of log LD⁵⁰ (LLD⁵⁰), the relationship between LLD⁵⁰ and the log weight of the flies assayed was investigated in an experiment in Denmark, and the standard error of LLD⁵⁰ estimates was estimated. Examination of LLD⁵⁰ estimates (adjusted for log weight) provided evidence of substantial migration of flies between farms, which produced large and unpredictable changes in tolerance levels on individual farms, and prevented extraction of further useful information from the data. However, it was concluded that even had migration been negligible, the data would not have provided information suitable for the construction of models of the evolution of this polygenically determined resistance. This conclusion is likely to hold for most existing data. Alternative approaches towards obtaining models of the evolution of polygenically determined resistances are suggested.