A study of the development of dieldrin-resistance in relation to acaricide pressure in a population of Boophilus microplus

Abstract

Dieldrin and aldrin preparations were used on separate properties where only arsenic had been used previously for tick control. Tick stocks from the properties before the commencement of the experiment showed no acaricide resistance. Dipping in dieldrin commenced on one property in November, and satisfactory control was obtained with the first three dippings at approximately 6-week intervals. After the fourth dipping, in late March, however, there was a marked survival of all stages of ticks, but particularly adults, although the vat concentration of dieldrin was almost as high as formerly and the deposits of the toxicant on the hair of the cattle were heavier. Progeny of female ticks which survived the fourth dipping were shown in laboratory tests to be highly resistant to dieldrin. Ticks present on the cattle at the fourth dipping could have included descendants of tick stocks which had undergone a maximum of four challenges with dieldrin, though most would have experienced only two to three. The fifth dipping was more effective than the fourth, which suggested irregular distribution of the dieldrin-resistant ticks in the pastures, possibly associated with periodical changes in the grazing of the cattle. The ticks present at the sixth dipping were highly resistant to dieldrin, and there was no protective period. Ticks from a culture established about 8 months after the inception of dipping were more than 900 times as resistant to dieldrin as the reference culture when tested by immersion. On another property a poor kill resulted from an initial dipping in 0.05% aldrin in early February, but high kills resulted from dippings in 0.10% aldrin in early March and early April. The tolerance of the ticks to aldrin after the conclusion of the experimental programme of five dippings was no higher than before its commencement. This could have been due to the fact that the reproducing populations of the tick received less severe and less frequent selection with the acaricide than during the course of the dieldrin trial. However, satisfactory results were still obtained by the owner with aldrin during the following tick season, and it is more likely that the contrasting results on the two properties were due to the presence of the resistance gene on the one property and its absence on the other.