An evaluation of 17 chemicals was conducted in the field at Mwea-Tebere Agricultural Research Station to ascertain their efficacy against the red spider mites Tetranychus sp. attacking cotton in Eastern Kenya. The study became necessary owing to the inadequate control of red spider mites by the recommended acaricide, Dimethoate. Of the chemicals that performed well against the red spider mites, Kelthane, Akar, Morestan and Neoron were considered to be the most suitable replacements for Dimethoate because of their low levels of mammalian oral toxicity. Judged from the overall performance against the red spider mites, these acaricides were also about twice as effective as Dimethoate.

Field investigations were conducted at Mwea-Tebere Agricultural Research Station, situated in Central Province of Kenya, to evaluate the effects of the infestation of cotton by red spider mites, Tetranychus sp., on its growth and yield components. The results obtained revealed that mite damage adversely affected vegetative growth as measured by the amount of leaf and plant dry matter, number of leaves per plant, leaf surface area and mean size of leaves which were reduced by 67.07, 48.46, 68.18 and 60.37% respectively.

In addition to these effects, it was estimated that seed cotton yields could be reduced by 46.93% per plant due to mite injury. Seed development was adversely affected through the reduction of their weight (seed index), oil content and viability by 9.30, 10.07 and 36.67%, respectively. Similarly, mite damage to cotton plants caused the reduction of lint weight (lint index) and lint ginning percentage by 15.42 and 4.5%, respectively.