Assessment of damage of the green peach aphid, Myzus persicae (Sulzer) on flue-cured tobacco (Nicotiana tabacum L.).

Plants infested with aphids for 20 days from 35 DAT [days after transplanting] did not show any significant difference with the control on the area of 10th leaf, height of tobacco at maturity, final yield, grade index, crop value, and nicotine content of the leaf regardless of the time of aphid infestation. However, the area of the 15th leaf was reduced when infestation occurred at 35 DAT. Likewise, the dry weight of the above ground portion of the plant was reduced after the 20-day period of aphid feeding at 35 DAT. Moreover, tobacco plants that were infested with 20-25 aphids/plant from 35 DAT to the last harvest were the shortest and consistently gave the lowest cured yield, poorest quality leaves and, lowest crop value. Most of the harvested leaves were affected with sooty molds, chlorotic and had burnt appearance. Aphid feeding did not affect the nicotine content of the leaf. The reducing sugar content of the leaves, however, was reduced by aphids. All aphid-infested plants had lower reducing sugar than the control. The economic threshold level [ETL] of the aphid varied with the growth stages of the plant. The computed ETL were 7, 14 and 44 aphids/plant at 35, 55 and 75 DAT, respectively. This result needs to be validated in the field. Among the insecticides tested for aphid control, methamidophos was the most effective and profitable. It gave complete control of the aphids after 3 sprayings. Plants treated with this insecticide had the highest yield, best quality leaves, and highest crop value. Methomyl was the least effective and least profitable because it gave the highest cost of application and least profit. The benefit cost ratio derived from the insecticides used ranged from 0.81 to 6.61. Untreated plants had significantly lower reducing sugars than the treated ones. Insecticide application apparently controlled the aphids to cause any alteration on this compound.