Abstract
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.