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.

M.Sc. / The extent of economic damage caused to the cotton variety Acala 1517/70 by the feeding activities of Tetranychus cinnabarinus (Boisduval) has been studies at the Loskop Research Station, situated in the Transvaal Middleveld. Special attention is given to the relative importance of the occurrence of spider mite infestations during the various stadia in the development of the cotton plant. the common method by which the extent of mite infestations is calculated, namely the conversion of adult female counts to female-days / leaf is used, and this unit of measure is compared to the percentage of leaf surface area over which visible damage extends (trial 1) as well as to the actual damaged leaf surface area. An index system is used to determine the percentage of leaf surface area over which visible damage extends (trial 1). The effectiveness of this procedure in determining the actual damaged leaf surface area is found to be sufficient, and is recommended as it appears to be a rapid and accurate method for evaluation during field trials. In accordance with quoted literature it is determined that the regression of both the leaf surface area over which damage extends (as determined by the index system as described) and the actual damaged leaf surface area against the female-days I leaf at a specific stage, are highly significant. It is confirmed that the use of either the percentage of leaf surface area over which damage extends or the actual damaged leaf surface area is valid. A method that is based on the use of female-days) leaf, but which in addition takes into account the stage (in days after planting) at which a particular spider mite count has been present is developed for the purposes of this study. It has been confirmed that this particular unit (damagedays IleaO has practical application value. It is pointed out that even in the early stages of population buildup there exists a highly significant relationship between both the femaledays / leaf and the damage-days Ileaf,and the yield masses. Significant reductions of up to 16 percent in yield were found in spite of the fact that notable spider mite infestations had not occurred prior to the flowering peak. By taking the value of the cotton yield into account and comparing it with the cost of control, an economic evaluation of the specific treatments is made. The following important conclusions were reached: • The confirmation of the profitability of applying an acaricide (triazophos) as soon as the first signs of a red spider mite infestation is discovered. • the irrelevance of programme spraying regardless of the infestation present, • the decrease in relative importance of red spider mide infestations in the later stages of the development of the cotton plant, and • the probability of spider mites affecting both the yield mass and quality if heavy infestations prevail after 19 weeks (133 days) after planting.

D.Sc. (Zoology) / The influence of some natural enemies and pesticides was studied on Tetranychus cinnabarinus (Boisduval), T. lombardinii Baker & Pritchard and T. ludeni Zacher (Acari: Tetranychidae). Two important predators were discovered, viz: Pardosa crassipalpis (Purcell) (Arachnida: Araneida) and Labidura riparia (Insecta: Dermaptera). Both are more susceptible to some acaricides, especially dimethoate, than the mites. It is proved that the injudicious use of these acaricides, also of DDT and carbaryl, can cause an outbreak of these mites. As to the biologies of the species, the appearance was noted of a fourth chrysalis stage during the development of three T. cinnabarinus females.