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A simple method is described of testing the susceptibility to acaricides of larvae of the cattle tick Boophilus microplus (Can.) in Queensland, Australia, by enclosing them in packets made from rectangular filter papers, folded once and secured along the remaining three sides by spring clips, that had been impregnated with Risella-oil solutions of DDT, dieldrin, or Bayer 21/199 (coumaphos), or with deposits from xylene solutions of Dilan. The response of susceptible engorged adult females to dieldrin was tested similarly. Five out of six experiments showed statistical evidence of repeatability of results in determination of relative DDT-resistance of larvae. There was a slight increase in susceptibility of non-resistant larvae to DDT with increasing age between 7 and 28 days for fresh packets but not for re-used packets. In three out of four tests, re-used packets were significantly more toxic (up to 1·40 times) to susceptible larvae than fresh packets. In one test a loss of toxicity occurred after storage of Bayer 21/199 packets for 23 days. The possible use of this method for measuring resistance in ticks is discussed.
Stone et al. (Mon,) studied this question.
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