“Gammexane” and DDT, have many advantages over arsenic as tick killing agents. “Gammexane” dips have been used successfully to control the one host arsenic-resistant blue tick, B. decoloratus (Koch) but these dips have not been fully investigated for the control of two- and three-host ticks. The control of the three-host bont tick, A. hebraeum, a vector of “heartwater”, a disease of cattle, sheep and goats, is of great economic importance to South Africa. Larvae and nymphs seem to occur on cattle to a lesser degree than adults, but each stage shows a definite preference for particular sites on the host. For this reason, control measures have to be mainly directed against the adult stage. The effect of “Gammexane”, DDT and arsenical dips, and combinations of these, has been investigated, by making weekly counts of adults on treated and untreated groups of animals. Dipping has been compared with spraying, and the results have been examined in the light of chemical analyses and biological tests with the same samples. The experiments were spread over two consecutive years.
Preliminary experiments indicated that all treatments markedly reduced numbers of male bont ticks on the cattle. Weekly arsenical treatments with 0·16 per cent. As2O3 either by dipping or spraying did not reduce the numbers of females, nor did a composite dip-wash of 0·16 per cent. As2O3 and 50 p.p.m. gamma isomer. Dipping in 50 p.p.m. gamma isomer gave slightly better results against females than the above treatments. Encouraging results were obtained by spraying cattle with freshly diluted wash containing 50 p.p.m. gamma isomer, but dipping in 100 p.p.m. also gave satisfactory results. The relative ineffectiveness of dipping as compared with spraying, was found to be due to the loss of biological activity of hexachlorocyclohexane in dipping tanks, where complicating pollution factors appeared.
The results of the preliminary experiments were largely confirmed by the second series. Males always outnumbered females in collections where the ticks were removed week by week from cattle. The collections were taken to represent the rate at which cattle became re-infested, and formed a basis on which to gauge the effectiveness of treatments. Males increased in numbers week by week on other untreated control groups of cattle, and eventually greatly outnumbered the females. This suggested that males remained on the hosts longer, and were recorded more than once in the consecutive weekly counts.
All treatments reduced the numbers of males. Weekly treatments in 0·16 per cent. As2O3 did not reduce the numbers of females, nor did it prevent them from engorging. Some females laid after removal from cattle so treated but the eggs were sterile, whilst females in a similar state of engorgement, removed from untreated animals, laid fertile eggs. Arsenical treatments should thus eventually control bont ticks.
All “Gammexane” treatments appeared more effective than the arsenical treatments. Fresh dilutions of dispersible pastes and powders in the form of sprays gave better results than dipping in the same preparations at comparable concentrations. This was due to a loss in biological activity of the hexachlorocyclohexane as the washes became dirty with use in dipping tanks. This factor makes chemical determinations of dip-washes, based on total hydrolysable chlorine, of little value, unless these data are correlated with some suitable biological test. The addition of 0·03 per cent, copper sulphate in the wash did not prevent the loss of biological activity.
Oil emulsion dips, which were known to show a drop in the gamma isomer content with use in dipping tanks, were kept at the desired concentration and biologically active by adding fresh dip each week. In such cases both dip- and spray-washes gave satisfactory results when used at 100 p.p.m. gamma isomer. A combination of 50 p.p.m. gamma isomer and 0·16 per cent. As2O3 used as a dip-wash was not satisfactory in reducing the number of bont ticks, and little better than a fresh spray of 50 p.p.m. gamma isomer alone. The striking results given by a combination of a dispersible powder spray of 50 p.p.m. gamma isomer and 0·1 per cent. p,p′ DDT might be due to persistent action or repellent effect.
Arsenic is a stable substance and has been used for many years in dipping tanks to control ticks. It has disadvantages and in the case of the bont tick many females remained attached to the hosts when treated weekly in arsenic, although the engorged females laid sterile eggs. “Gammexane” preparations when used at 100 p.p.m. gave satisfactory results. These preparations, however, lost their biological activity in dipping tanks, and the best results were obtained when they were applied to cattle as fresh sprays.