Selection of a pyrethrum‐resistant strain of the grain weevil, Calandra granaria L.

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Abstract
Adults of a field strain of the grain weevil, Calandra granaria L., initially twice as resistant to pyrethrins in oil solution as a laboratory standard strain, have been exposed to selection pressure with pyrethrins in 17 out of 22 generations in the course of five years. The strain has steadily increased in resistance to pyrethrins and is now 18 times as resistant as the standard. There has been a simultaneous increase in resistance to pyrethrins synergised with piperonyl butoxide of only X2.In the course of selection the beetles of the resistant strain became heavier in weight but not sufficiently to account for the resistance. They also became darker in colour but this change was shown to be physiologically unconnected with the increase in resistance to pyrethrins.

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