Identification of okra (Abelmoschus esculentus Moench; Malvaceae) insect pests in Kenya and evaluation of varieties for resistance to the cotton aphid, Aphis gossypii Glover

Eight okra varieties were tested in the field in Kenya in 1999 and 2000 to determine the primary insect pests, the effects of these pests on plant growth and yield, and to evaluate varieties for insect pest resistance. Cotton aphid (CA), Aphis gossypii Glover, was identified as the main okra insect pest in the study area causing yield losses of up to 50% in some varieties. Attempts to control CA with recommended doses of organophosphorous and pyrethroid insecticides resulted in increases in aphid abundance. Insecticide-treated plants had significantly more CA and corresponding yield losses than untreated plants.; Based on cotton aphid scores and plant growth parameters, two potentially resistant and two susceptible okra varieties were selected for further examination. Studies on the relationship between resistance/tolerance to CA and trichomes revealed that numbers and density of trichomes on the abaxial leaf surfaces, though significantly different among varieties, did not influence CA behaviour and physiology. No significant relationship was established between trichomes and aphid settling, length of the pre-reproductive period, nymph mortality, intrinsic rate of natural increase or reproduction. Analysis of okra leaf volatiles by gas chromatography-mass spectrometry revealed the presence of thirty-six compounds. Although there did not appear to be significant qualitative differences, leaf volatiles from the four okra varieties examined elicited varying levels of attraction in apterous A. gossypii. However, volatiles of all okra varieties did not show any repellence to cotton aphid. High Pressure Liquid Chromatography analyses of okra leaf non-volatiles revealed qualitative and quantitative differences in chemical compounds occurring in different varieties. Numbers of aphids settling to feed on Parafilm ® and leaf surfaces treated with non-volatile chemicals from different okra varieties were significantly different indicating that non-volatile chemicals may have been responsible for behavioural differences of aphids in relation to these varieties. In all varietal extracts, however, compounds did not show any toxicity to aphid nymphs.