Repellent, irritant and toxic effects of essential oil constituents on Bemisia tabaci (Gennadius)

Whiteflies, Bemisia tabaci, are a widespread pest in agriculture, causing crop loss up to 100% by direct and indirect damage. Controlling this pest has proven difficult due to the fact that they stay underside the leaves and their ability to become resistant to conventional pesticides rapidly. Also, environmental and health concerns associated with the use of synthetic pesticides are rising. Therefore, new methods should be investigated to protect crops from B. tabaci. Insect proof nets (IPNs) create a physical barrier between the crop and a pest insect, but this alone is not suitable against small insects like B. tabaci. A possible solution is combining insect proof nets (IPN) with a naturally occurring repellent. Essential oils, mixtures of volatile secondary metabolites of plants, have been shown to have repellent and toxic abilities against many pest insects, including whiteflies. This study investigates the role of the different compounds in four essential oils; Cumin (Cuminum cyminum), Cinnamon (Cinnalonum zeylanicum), Lemongrass (Cymbopogon citratus) and Citronella grass (Cymbopogon winterianus), to see if the constituents are toxic, repellent or irritant. This is also used as a screening for compounds that seem promising to be used in combination with IPNs in the field. This should be a highly repellent, but not very toxic compound, to repel B. tabaci but with a reduced risk of the fast development of resistance. In this study I found that most constituents of essential oils have one dominant role (i.e. they are for example toxic or repellent but not both) and the effects of these different compounds combined add up in the mixture of the essential oil. As for the essential oil of citronella grass, this oil is less toxic than some of its individual compounds, suggesting interactions between the compounds when mixed. The most promising compounds to be used against B. tabaci in the field, based on their high repellency and low toxicity, are cinnamaldehyde (repellent at <0.084 mg/L and toxic at 8.4 mg/L) and linalool (repellent at 0.006 mg/L but with unknown toxicity). (Résumé d'auteur