Evaluation of Partial Plant Sampling Procedures and Corresponding Action Thresholds for Management of Lepidoptera on Cabbage

Abstract

Within-plant sampling procedures, coupled with action thresholds for cabbage-feeding caterpillars, were evaluated for accuracy and efficiency in New York, Ontario, Canada, and Wisconsin in 1982. When results were averaged over the three sites, use of all thresholds required fewer insecticide applications than a fortnightly schedule, while the total percentage of marketable heads did not differ significantly. However, use of the thresholds based on feeding-damage evaluation and 75% of the plants infested resulted in significantly fewer marketable no. 1 heads than did the other treatments. Inspecting only the head plus 10 surrounding leaves required ca. 50% less time than inspecting the entire plant in two of the locations, yet the marketability, averaged over all locations, was reduced. In treatment plots over all locations, there was no significant correlation between the larval populations (as determined by cabbage looper equivalents) on the head plus 10 leaves and the number of larvae on the entire plant. Thus, the sampling procedures did not accurately predict that portion of the plant on which the larvae would predominantly occur, nor did they predict the abundance of larvae on the entire plant based on counts from the head and surrounding leaves.