Diamondback moth: A major pest of Michigan cabbage and other crucifer crops

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

The diamondback moth, Plutella xylostella, is one of the most destructive insect pests of cabbage and other crucifer crops throughout the world. The larvae, or caterpillars, of diamondback moth feed on the leaves of cabbage, broccoli, cauliflower, collards, kale, kohlrabi, Chinese cabbage and Brussels sprouts. All stages of plants are susceptible to diamondback moth feeding. The caterpillars can reach high densities and cause significant defoliation of the plants, as well as contamination and malformed head in cabbage, broccoli and cauliflower. Certain small, parasitic wasps are effective in regulating populations of diamondback moth caterpillars, but if these are absent or reduced in numbers, diamondback moth can become a serious problem. The diamondback moth has also developed widespread resistance to numerous insecticides, which has made chemical control of this pest extremely difficult.

Adult diamondback moths are small, grayish-brown moths, with a wingspan of about 3/5 of an inch. The moth has white markings on the forewings that form diamond-shaped spots on the back when the wings are folded, hence the common name, “diamondback moth” (see Photo 1). The caterpillar is yellowish-green in color and measures about 1/3 inch in length when fully grown (Photo 2). The body of the caterpillar is tapered at both ends, and is covered by fine, erect black hairs scattered over the body surface. When disturbed, caterpillars will wriggle vigorously and drop from a leaf by a silken thread.

Diamondback Moth
Photos 1 and 2. Left, diamondback moth pupa (top) and adult
moth (bottom). Right, fully grown diamondback moth caterpillar.

Adult diamondback moths overwinter in crucifer crop residues and emerge in late spring. After mating, females lay oval, yellowish-white eggs on the undersides of cabbage and other crucifer leaves. The caterpillars can be fully grown in 20-25 days, and pupate in a lace-like cocoon on the underside of a leaf (Photo 1). The adults emerge in about one to two weeks. There can be four to six generations of diamondback moths per year north of Interstate 70, so this pest is present throughout a large portion of the crucifer growing season in Michigan.

Trap cropping using plants such as collards has been shown to be a useful management strategy. Plant collards around the crucifer crop and destroy the diamondback moth caterpillars as they show up in the trap crop. Elimination of crop residues will prevent diamondback moths from overwintering. Encourage parasitic wasps in cabbage and other crucifer crop plantings by establishing plants on edges of the crop fields that will attract these wasps. If insecticide use becomes necessary, make the application before the population of caterpillars becomes large and difficult to manage. Small caterpillars are more easily killed by insecticides than large ones. It is critical to cover the undersides of crop leaves with insecticide, because caterpillars usually feed in that location. Scouting the crop every week and use of a pheromone trap to monitor adult activity are essential for effective chemical control of diamondback moth.

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