Influence of variety on the within-plant distribution of cassava green spider mite (Acari : Tetranychidae), and leaf anatomical characteristics and chemical components in relation to varietal resistance

The within-plant distribution of the cassava green spider mite, Mononychell us tanajoa Bondar, and the anatomical characteristics and the chemical comp onents relating to varietal resistance of cassava, were studied using 11 ca ssava genotypes with varying levels of resistance for two dry seasons and o ne wet season. The results show that M. tanajoa aggregates on the top leave s of cassava at low levels of resistance as compared with a more even withi n-plant distribution at higher levels of resistance. Thus, for accurate sam pling of mite populations, it might be important to consider young and old leaves, not just the young leaves, in fields containing resistant and susce ptible cassava genotypes. Correlations between the anatomical characteristi cs and mite population density and damage scores were not consistent across months, either within or over seasons. This suggests that leaf anatomical characteristics may not be important in the varietal resistance of cassava to M. tanajoa. During the dry season, mite population density was positivel y correlated with leaf nitrogen, potassium and phosphorus and negatively co rrelated with leaf calcium and fat. Only calcium was negatively associated with mite damage at the peak of the dry season (January 1993 and 1994). Bre eding cassava genotypes with high levels of leaf calcium and fat, and low l evels of leaf nitrogen, potassium and phosphorus, may improve the level of resistance to M. tanajoa.