EVALUATION OF SELECTED SYNTHETIC INSECTICIDES AND BOTANICALS FOR THE CONTROL OF INSECT PESTS ON OKRA (Abelmoschus esculentus L. Moench) IN SOUTH WESTERN, NIGERIA

Abstract

A randomized complete block field experiment was conducted over three years to evaluate the insecticidal efficacy of selected synthetic insecticides and botanicals for the control of insect pests of okra (Abelmoschus esculentus L. Moench) at the Teaching and Research Farm of The Federal University of Technology, Akure (70 16'N, 150 12'E). The experiments consisted of four rates (0, 50, 75, 100, and 125%) of lambda-cyhalothrin, dichloropyrivos and carbaryl and two extraction methods comprising of hot and cold water extracts of Piper guineense (Schum and Thonn), Allium sativum (Linn.) and Azadirachta indica (A. Juss) including their mixtures at the rate of 50g per litre of water. The foregoing treatments were sprayed twice and thrice while assessments of insecticidal efficacy based on Henderson Tilton formular was conducted at 2, 4 and 6 weeks after treatment application. Data were generated on percent efficacy, perforations on leaves, plant height, leaf number per plant, stem girth, days to 50% flowering, number of pod per plant, pod weight, pod length and diameter, fresh pod yield, 100 seed weight, toxicity of the botanicals on okra flea beetle, fruits nutrient composition, effect of treatment materials on soil and cost implications of insecticide applications. Result obtained shows that all three botanicals (hot and cold water extracts) and their mixtures contained trace to appreciable amounts of saponin, tannin, and alkaloids. Synthetic insecticides and botanicals tested had significant effects on number of fruits per plant, number of fruits per plot, and number of pods per hectare and fruit yield. Lambda-cyhalothrin and dichloropyrivos performance exceeded cold and hot water extracts and their mixtures in all the parameters measured. Untreated control plot was the least. All the biocides were generally toxic to the flea beetles and there was a general decline in the rate of leaf destruction as the insecticide application continued. Lambda-cyhalothrin performance exceeded all other insecticides in most of the parameters measured. Sensitivity of okra to the carbamates was demonstrated in plant height and flower initiation. The high level of carbaryl toxicity suggests it can be used as an indicator for detection of phytotoxicity of insecticides on leafy vegetables. All insecticides had significant effects on all growth and yield parameters measured except pod length. Laboratory work revealed that all the botanicals exhibited systemic poisoning with sole extracts of A. indica and mixture of A. indica + P. guineense and mixture of A. sativum + P. guineense showing 90% and 81.97% efficacy respectively while sole extracts of P. guineense, A. sativum and mixtures of A. indica + P. guineense exhibited 71.75%, 41.75% and 45% efficacy respectively. Proximate composition and vitamins A and C content of okra were significantly enhanced but calcium and protein were inhibited by dichlorvos and lambda-cyhalothrin at 2 spray times while phosphorus was significantly inhibited with 2 and 3 sprays of the tested treatment materials compared with control. Differences in the soil type and weather conditions in the study area had a great influence on the performance of the botanicals, insect pests and okra. Soil status of the experimental site was negatively affected by insecticide treatments. The cost benefits analysis of the insecticide treatments at 2 and 3 spray times revealed that application of insecticides at 2 spray times was much more effective and profitable than 3 spray times. A rate between 75% and 100% concentration for synthetic insecticides and 50g per litre of hot water extraction method for A. indica, P. guineense and A. sativum and cold water extraction method for mixture of A. indica and P. guineense, A. indica and A. sativum, A. sativum and P. guineense is recommended. It can be concluded that A. indica, P. guineense and A. sativum (in order of efficacy) can be used as an alternative to the synthetic insecticides tested in this study for the control of flea beetles and other insect pests of okra. Further work on residual toxicity of the biocides investigated and ways and means of increasing their efficacy should be embarked on.