Bases of Resistance Among Cowpea Genotypes to the Bean Flower, Thrips, Megalurothrips sjostedti (Trybom)(Thysanoptera~ Thripidae)
ABSTRACT: The bases of resistance of cowpea to the flower thrips Megalurothrips sjostedti (Trybom) were explored among 13 assorted cowpea genotypes employing a range of techniques including scanning electron microscopy. The causes of resistance in
genotypes TVx 3236 and other genotypes previously identified as tolerant to thrips appeared to be related to a number of biochemical and biophysical factors. The phenolic content of flower buds was negatively and significantly correlated with thrips damage (r = 0.60) and thrips populations (r = 0.71); indicating a close association between high phenol content and reduced populations and therefore damage. Total nitrogen content did not appear to influence resistance to thrips. Biophysical factors including trichome vesture’ and cell compactness of calyx tissue offered the most fidelity with the trend of resistance/susceptibility among the tested genotypes. Trichome vesture was negatively and
significantly correlated with thrips damage (r = -0.60) but varied widely among the cultivars. Trichome exudate was negatively and significantly correlated with thrips damage (r = -0.64) and positively, highly significantly correlated with phenolic content (r = 0.80). Anatomical studies revealed that the calyx of the resistant genotypes TVx 3236 had more closely packed cells with reduced air spaces. The volume of air spaces available for oviposition in the susceptible genotype VITA 7 was nearly four times that of TVx 3236. Plant vigor via dry matter assessment and, unprotected flower counts are discussed as potential important indicators of field resistance.