Resistance of Okra-Leaf Cotton Genotypes to Twospotted Spider Mites (Acari: Tetranychidae)

Abstract

Previous research showed that twospotted spider mite, Tetranychus urticae Koch, populations increase more slowly on okra-leaf than normal-leaf cotton, Gossypium Hirsutum L., genotypes. The following four differences between genotypes that may underly this plant resistance were investigated: quicker exploitation of smaller leaves, reduced nutritional quality, unfavorable canopy temperature or humidity conditions, and reduced habitat favorable for oviposition and development. Okra cotton leaves were smaller (70% of area of normal cotton leaves), but spider mite populations diverged at low levels of leaf damage (9-35%), indicating that lower population growth rates occur on okra-leaf genotypes before leaves are exploited fully. Spider mites did not discriminate between okra-leaf and normal-leaf cotton in choice tests, developmental and reproductive parameters of mites reared on excised disks did not significantly differ between leaf types, and nitrogen and tannin levels were similar between genotypes. Temperature and humidity conditions in the upper canopy, where most spider mite populations occur, were not significantly different between genotypes. In the field, mites develop colonies along the major veins, and, in leaf folds of cotton leaves and in laboratory choice tests, mites laid more eggs on leaf sections with a major vein than leaf sections without. The mechanism of resistance was defined as nonpreference caused by less suitable feeding and oviposition sites on okra-leaf genotypes