Dieldrin‐resistant cabbage root fly in England

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Abstract
SUMMARY: The existence of dieldrin resistance in Erioischia brassicae has been investigated in the main areas in England and Wales where brassicas are grown for human consumption. Dieldrin‐susceptible and dieldrin‐resistant flies were collected and bred as separate strains in the laboratory. In toxicity tests adult flies were treated with topically applied doses of insecticide in microdrops of cellosolve, and larvae were dipped in solutions of insecticide in 70% acetone and 30% water. Adults of the laboratory resistant strain showed a high level of resistance to dieldrin and aldrin and a lower level of resistance to γ‐BHC. There was no cross‐resistance with chlorfenvinphos or diazinon. Larvae of the resistant strain were shown to be resistant to dieldrin.Dieldrin resistance has been found in four areas in southern England, but not in a fifth major area, and resistant flies did not increase as rapidly in some areas as others. Sufficient migration of flies appeared to take place to produce similar percentages of resistant flies over areas of several square miles, but considerable differences were found in fly populations about 10 miles apart. It is suggested that the speed of development of insecticide resistance is affected by the proportion of brassica crops which are treated in an area, the amounts of insecticide applied to individual crops, and hence the insecticidal pressure applied to the E. brassicae population.

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