Correlation between the Length of the Larval Period of Musca domestica L. and Resistance of Adult Flies to Insecticides1

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Abstract
A strain of houseflies moderately resistant to DDT was split into an early substrain and a late substrain by successive selection of the 1st half to pupate into 1 colony and the latter half to pupate into a 2nd colony. The period from oviposition to midpoint of pupation decreased slightly in the early substrain and increased about 50% in the late substrain. The pupating periods from 1st to last pupation were altered similarly. Resistance of the adults decreased slightly in the early substrain and increased 6-8 fold in the late substrain after 15 generations. Changes in susceptibility to DDD were more marked than to DDT, but were slight to methoxychlor and absent with lindane, aldrin, dieldrin and pyrethrins. Similar but numerically smaller changes occurred with a strain originally highly susceptible to DDT. Further selection of the late pupae led to a slow decrease in resistance to the DDT type of insecticides, so that after 53 generations the flies were more susceptible than an unselected check strain. Length of oviposition to midpoint of pupation period simultaneously decreased approximately to that of the check strain. Susceptibility to pyrethrins was unchanged and to aldrin and dieldrin was cut about in half.