Abstract
M.Sc.
The possible resistance of the red spider mite, Tetranychus cinnabarinus, on cotton to the acaricides triazophos and profenofos was studied in the Groblersdal/Marble Hall area. During the 1980 and 1981 seasons T. cinnabarinus was collected from cotton in four districts where mite infestations had been controlled the preceeding two seasons with up to three triazophos sprays per year. A laboratory culture of T. cinnabarinus served as the control and the response of these mites to different concentrations of acaricides were compared with those of the four field cultures. The toxicity of triazophos and profenofos was evaluated by using the slide-dip technique and mortality readings were taken 24 hours after exposure to the acaricide solutions. The data were subjected to a pro bit analysis. Where pro bit lines could not be fitted second degree polinomial regressions were fitted through the log dose and empirical probits. During 1980 there was little difference between the susceptibility of the four field cultures to triazophos. However, the regression lines of all four cultures when compared with those of the control culture indicated resistance to triazophos. After these field cultures had been
reared under controlled conditions and not exposed to pesticides for a year all signs of
triazophos resistance disappeared. The four field cultures collected in 1981 showed great variability in their response to triazophos but no indication of resistance was observed. This may be because the recommended cotton pest control programme was more closely followed by the cotton producers in 1981 and fewer acaricide sprays were applied. The accuracy of the mortality readings made 24 hours after exposure to triazophos was investigated by taking readings at intervals of 8 hours. An analysis of the results indicated that the mortality curves calculated from all readings were not significantly different and consequently mortality readings were made every 24 hours after exposure to the acaricide solution. A second laboratory strain of T. cinnabarinus, obtained from the Fruit and Fruit Technology Research Institute in Stellenbosch, and regarded as susceptible to organo phosphates was also tested for resistance to triazophos. The regression lines obtained for this culture was similar to that obtained from the 1980 field cultures. This was taken as an indication of possible resistance and this culture was therefore not used as a con trol in this study. The control culture and the Groblersdal/Marble Hall field cultures did not differ significantly in their susceptibility to profenofos. However, profenofos is not widely used for the control of T. cinnabarinus on cotton and these results indicate that it can still be used effectively for the control of T. cinnabarinus. A third acaricide, amitraz, was also examined to determine its effect on T. cinnabarinus. The slide-dip technique was found to be unsuitable for the determination of mite resistance to amitraz. This acaricide is slow acting and an observation interval longer than 24 hours is required to obtain the necessary mortality data. It is difficult to exclude the effects of starvation when the slide-dip technique is used for prolonged periods and consequently amitraz was not tested further. An alternative method for the effective evaluation of this acaricide must be developed. From this study no definite evidence of resistance of T. cinnabarinus in the Grtoblersdal/Marble Hall cotton growing area to triazophos or profenofos was found. It was concluded that both can still be used effectively in the control programme for mites on cotton.