Chemical control of the heartwater vectors.

This paper reviews available literature on the efficacy of acaricides against Amblyomma hebraeum and other tick species, and presents information on tests done with registered chemicals in the laboratory. Little published information is available on the efficacy of chemicals specifically against A. hebraeum. A host of formulations are registered for use as acaricides on cattle, sheep, and goats in South Africa and thus, by implication, against this species. Resistance has only been described to arsenic and toxaphene in Southern Africa; the other registered products are generally considered to be effective. In contrast, many efficacy tests of various chemicals in different formulations against other Amblyomma spp. have been described. These publications have mainly emanated from the USA, where bite-wounds of these ticks serve as oviposition sites for screwworm flies. In this paper, Amblyomma maculatum and Amblyomma variegatum are included as potential heartwater vectors. The acaricidal efficacy of a number of compounds, representative of different chemical classes, was tested in South Africa against an arsenic and organochlorine resistant strain of A. hebraeum. The engorged adult female immersion method was used. A disconcerting discovery was that several of these registered products failed to control this tick when used at their recommended concentrations. It is concluded that many chemicals which fail against A. hebraeum on cattle do so because of insufficient persistence. Exposure of this tick to lower levels of existing chemicals, but for longer periods, ought to provide satisfactory control for many years.