MANAGEMENT OF Megalurothrips sjostedti TRYBOM ON COWPEA (Vigna unguiculata [L.] Walp.) WITH VOLATILE ORGANIC COMPOUNDS

Abstract

Cowpea is a vital plant protein in African diet. Megalurothrips sjostedti (Ms), is an insect pest of cowpea, which causes severe yield loss to the crop. Farmers manage Ms with conventional insecticides however, they are hazardous to the environment. Volatile Organic Compounds (VOC) developed into attract and kill mechanisms are safer alternatives but have been scarcely documented in cowpea insect pest management. Therefore, Headspace Volatiles (HsV) of cowpea cultivars and VOC were investigated for their attractant and repellant properties for Ms management. Response of Ms (n=60) to clean air and HsV from five Resistant Cultivars (RC): Moussa Local, Sewe, TVu1509, Sanzibanili, IT90K-277-2 and two Susceptible Cultivars (SC) - Ife brown and Vita7 at the flowering stage, was investigated in the laboratory with a Y-tube olfactometer. Choices of Ms (% response) were determined. The VOC of RC and SC were collected at Wageningen, The Netherlands, and Ibadan, Nigeria, using dynamic HsV collection method and profiled with a Gas Chromatography-Mass Spectrometry. Olfactory response of Ms (n=60) to eleven VOC: α-terpinene, γ-terpinene, (R)-(+)–limonene, tetradecane, sabienene, methyl saliscylate, hexadecane, dodecane, 1-tetradecene, nonanal and undecane were evaluated following standard procedures. Thrips responses (%) to VOC were determined. Three VOC baits: methyl salicylate, hexadecane and tetradecane, unbaited traps (Ut), lambda-cyhalothrin and untreated plot (control) were evaluated on cowpea plots (30x40 m2) in a Derived Savanna (Ibadan) and Guinea Savanna (Abomey-Calavi) Agro-Ecologies (AgE), established with Ife brown and Kpodiguegue cowpea cultivars, respectively. Fields were laid out in a randomised complete block design (r=3). Number of Ms and orders of insects in cowpea flowers and Sticky traps were recorded, respectively; Grain Yield (kg/ha) and Yield Losses (%YL) were determined. Data were analysed using descriptive statistics, Chi-square and ANOVA at α0.05. Attraction of Ms to HsV of cowpea cultivars relative to clean air was significantly higher in all the cultivars, ranging from 93.3% in Vita7 (χ2=45.07) to 76% in Sewe (χ2=17.07). The VOC identified in Wageningen and Ibadan were 68 and 29, respectively, belonging to 22 different classes of compounds. Attraction of Ms to VOC relative to clean air was significantly higher in the order: 80.0% in 1-tetradecene (χ2=16.20), 75.0% in methyl salicylate (χ2=15.00), 66.7% in γ-terpinene (χ2=6.67), 63.3 % in tetradecane (χ2=4.27) and 61.7% (χ2=3.27) in hexadecane, while nonanal with 9.0% (χ2=30.42) repelled Ms. In Abomey-Calavi, Ms was highest in tetradecane plot (212.2±93.33) and lowest in methyl salicylate (152.3±55.90). Also in Ibadan, tetradecane plot had the highest Ms (619.7±127.27), while lambda-cyhalothrin (198.9±127.85) had the lowest. Eight insect orders: Thysanoptera, Hemiptera, Hymenoptera, Diptera, Odonata, Coleoptera, Lepidoptera and Orthoptera, were identified on sticky traps. Grain yield ranged from 21,927.7 (Ut) to 15,163.6 (untreated) in Ibadan and 723.6 (lambda-cyhalothrin) to 432.8 (Ut) in Abomey-Calavi. Hexadecane elicited the lowest YL in Abomey-Calavi (15.9±14.2%) and Ibadan (3.8±4.4%). Resistant and susceptible cowpea cultivars were attractive to Megalurothrip ssjostedti. Methyl salicylate, 1-tetradecene, tetradecane and gamma terpenene attracted Megalurothrips sjostedti, while nonanal repelled it. Hexadecane minimised yield loss of cowpea in derived savanna and Guinea savanna agro-ecologies.