An ecological study of insect populations on cabbage in southern California

Abstract

An ecological study of insect populations on cabbage plantings was conducted over a two-year period in a coastal environment of southern California. Eleven orders of insects were represented in the collections. The Hymenoptera ranked first in the number of species represented, and the Collembola ranked first in number of individuals. Approximately 375 species of insects were collected, including about 125 each in the Hymenoptera and Diptera, and 50 in the Coleoptera. The cabbage aphid and the western flower thrips were the most numerous phytophagous species. Diaeretiella rapae (M’Intosh), a primary parasite of the cabbage aphid, was the most abundant parasitic species; the coccinellid, Hippodamia quinquesignata punctata Lec., and the syrphid, Allograpta obliqua (Say), were the two most common predators. The imported cabbageworm, cabbage looper, and diamondback moth were the most numerous lepidopterous larvae. Each year, the imported cabbageworm larval population peaked in August, the cabbage looper in September, and the diamondback moth in February and October. Their combined population averaged 15 larvae per plant in August. Twelve species of parasites were reared from the cabbage looper, nine from the imported cabbageworm, and five from the diamondback moth. Voria ruralis (Fallen), Pteromalus puparum (L.), and Diadegma insularis (Cresson) were the most common parasites on these three pests, respectively. Trichogramma pretiosum Riley was the only parasite reared from the lepidopterous eggs. Cabbage plants were severely injured by the cabbage aphid during the winter and spring months and by the imported cabbageworm and cabbage looper during the summer and early fall. Injury by the diamondback moth was minor in comparison. This study shows that although natural enemies of cruciferous crop pests should be conserved and augmented, current biological control methods alone are not adequate to control these insect pests under certain conditions without the supplemental use of pesticides.