Behavior of Tobacco Budworm Larvae on Flue-Cured Tobacco and Possibilities of Improving the Effectiveness of Insecticidal Treatments Applied Mechanically for Control1

Abstract

Damage to flue-cured tobacco by Heliothis virescens (F.) is seldom reduced more than 50% by conventional power sprays and dusts. In searching for possibilities of improving effectiveness of such treatments, the following items were investigated: (1) distribution, movement, and feeding sites of tobacco bud worm larvae developing on untreated field-growing plants; and (2) distribution of a conventional power spray on similar but uninfested plants. Time of day and weather conditions had little effect on the exposure of larvae to treatment. As the larvae aged over an 18-day period, they gradually moved to parts of the plant more exposed to treatment. However, larval damage increased, and 33% of the potential damage and 50% of the actual damage occurred by the time larvae were 6 days old. Thus, control was needed earlier in the larval period, when the exposure of larvae to treatment was near minimum. When larvae were 3 days old, 95% were within 6 nodes of the bud—41 % on the almost completely sheltered bud and 55% on the partially sheltered stalk and 6 small leaves below the bud. Most of the larvae on leaves were very close to the stalk. In addition, about half the larvae on the bud did not move from the bud for 24 or more hours, and, on the average, those that did moved only to the 3rd node below the bud. These data suggest that a greater concentration of an insecticide on the basal half of leaves, leaf axils, and stalk (the bud is almost completely sheltered) of the 6 terminal nodes should accelerate and increase the kill of larvae and thus reduce damage. As possible means of accomplishment, the following methods of mechanized chemical application directed from above the bud appear worthy of exploration: (1) gravity flow of a fine granular formulation in a 6-inch band, (2) spraying with a 15¼ injectortype full-cone nozzle, and (3) atomizing a waterless sprayable concentrate at a relatively low pressure. New conventional insecticides which exhibit greater contact and fuming properties than the residual materials currently in use and new systemic insecticides should also be evaluated.