The inheritance of DDT-resistance in the cattle tick, Boophilus microplus

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Abstract
Adults of a DDT-resistant strain of the cattle tick from central Queensland were crossed with adults of a susceptible reference strain, by means of cardboard mating boxes glued to the skins of cattle. F1, backcross, and F2 larvae were tested for resistance to DDT by enclosure of larvae in filter paper packets impregnated with oil solutions of pp'-DDT. F1 and backcross engorged adult females were tested for resistance by injection with oil solutions of pp'-DDT. There was no evidence of departure from a 1 : 1 ratio in the backcrosses or from a 1 : 2 : 1 ratio in the F2, and there was little difference between the compositions of the F1 reciprocal crosses or among the backcrosses derived from them. Therefore DDT resistance in this strain was considered to be due to a single, incompletely recessive, autosomal gene. Engorged nymphs of the resistant strain moulted later in vitro than nymphs of the susceptible strain, and resistant engorged adult females detached from the host later than susceptible engorged adult females. After 13 generations of DDT-free culturing of a multiresistant strain, the percentage of homozygous DDT-resistant ticks in the strain had fallen from a high level to about 55%. This figure remained constant for a further 10 generations.