Studies on acaricide resistance in Rhipicephalus evertsi evertsi Neumann (Acarina: Ixodidae) in East Africa. Identification and inheritance of a resistance factor to organochlorines

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Abstract
Difficulties were experienced in determining the susceptibility status of populations of Rhipicephalus evertsi evertsi Neum. to organochlorine acaricides. A susceptible strain of this species was found at Maswa, Shinyanga District, Tanzania. Susceptibility for toxaphene, dieldrin and γ–BHC was assessed by means of an FAO recommended test method, the oil-based packet test. A homogeneously responding resistant strain was built up from larvae from Ngong, near Nairobi, Kenya, that had survived a discriminating dieldrin concentration of 0·2%. These showed resistance factors of 10·8 for toxaphene, 30 for γ-BHC and 95 for dieldrin. The F1 cross between susceptible and resistant populations responded homo-geneously and was intermediate in resistance between the parental strains, with resistance factors of 8·7, 9 and 8·1 for ithese compounds, respectively. Larvae from backcrosses of F1 adults with the parental strains showed stepped ld-p lines such as would be expected in a case of inheritance of a single gene allele with partial dominance for resistance. A population of R. e. evertsi near Dar es Salaam, Tanzania, was controlled by application of 0·3% toxaphene despite it having resistance factors similar to those of the F1 hybrids.