Predators and parasites of temporary row crop pests: Agents of irreplaceable mortality or scavengers acting prior to other mortality factors?

Abstract

A study was conducted with the cabbage looper,Trichoplusia ni (Hubner), to determine: 1) whether naturally occurring entomophagous arthropods impart irreplaceable mortality to the cabbage looper in celery, and 2) whether entomophagous arthropods are present to impart such mortality toT. ni eggs and 1st instar larvae when the crop is sensitive to the pest. Predator evaluation involved 1) insecticidal + cage exclusion, 2) insecticidal exclusion alone, 3) D-vac removal of predators, and 4) sticky barrier exclusion. In all the exclusion regimes survival of eggs and larvae was higher than in the unexcluded control. Analysis of life table data was consistent with the hypothesis that some mortality of eggs due to parasitism byTrichogramma and of medium-larvae by other hymenopterous parasites was not replaceable by other mortality factors. The time of appearance of the first marketable petiole was correlated with both the height of the tallest petiole and the number of petioles on the plant. Peak densities ofTrichogramma and important predators occurred before the first marketable petiole appeared and declined to low levels as harvest approached.