In this method the toxicant is absorbed with blood serum onto a loose roll of chromatography paper; first-instar larvae of Lucilia cuprina (Wied.) are inserted into the air spaces of the roll and allowed to feed. Mortality of the larvae is assessed 24 h later. Typical results using diazinon, dichlofenthian and chlorfenvinphos against susceptible and resistant strains of L. cuprina showed larvae to have resistance factors 6-8 times higher than those for adults determined by established techniques.

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Two field strains of Lucilia cuprina (Wied.) of different susceptibilities to diazinon were selected over 25 generations by immersion of first-instar larvae in aqueous emulsions of diazinon for five minutes ; the resistance of the larvae and adult females was compared with a standard laboratory susceptible strain. Resistance increased rapidly in both strains, reaching a maximum for adult females (R.F. = 5–6) in generation 3 and for larvae (R.F. = 46–60) in generation 6. After removal of selection pressure at generation 8 the resistance factor fell to 20–24 by generation 25. Because the resistance mechanisms are known to place their carriers at a disadvantage in the absence of selection pressure, the practical implications of the rapid increase in resistance must await results of experiments with known genotypes.

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