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Genetic analyses of malathion resistance in Anopheles stephensi showed that the resistance gene concerned is on linkage group III, ca. 24 crossover units from the dieldrinresistance gene and 38 crossover units from the diamond palpus mutant. Three-point crosses, using the 2 insecticides in turn, indicated a gene sequence of dieldrin resistance-malathion resistance-diamond palpus. Resistance to malathion is expressed in 4th-instar homozygotes but is barely detectable in 4th-instar heterozygotes and is not expressed at all in 1st-instar heterozygotes. Malathion resistance is dominant in young adult heterozygotes but declines sharply as they age. This has implications for the accuracy of field-resistance testing using WHO kits and for the future use of malathion in vector control.
Mark Rowland (Fri,) studied this question.