Effects of Leaf Surface Moisture and Relative Humidity on the Efficacy of Sugar Esters from Nicotiana gossei Against the Tobacco Aphid (Homoptera: Aphididae)

The effects of leaf surface moisture and relative humidity on the toxicity of sugar esters extracted from Nicotiana gossei Domin to the tobacco aphid, Myzus nicotianae Blackman, were examined in the laboratory and the field. The efficacy of sugar esters applied at a concentration of mg/ml against tobacco aphids was >5 times higher under moist conditions (either wet leaf surface or wet filter paper in petri dishes) than under dry conditions (dry leaf surface plus dry filter paper) in the laboratory. The effect of moisture on the efficacy of sugar esters was greater as the lethal concentration value increased from LC 30 to LC 90 . There was no significant difference in aphid mortality between the red and green color forms of tobacco aphids at the same moisture and relative humidity levels. Regression analysis showed a linear relationship between aphid mortality from sugar esters and relative humidity. The toxicity of sugar esters to aphids increased with higher relative humidity. Preliminary field evaluations also demonstrated increased aphid mortality under higher relative humidity.