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.
