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Cabbage Worms

by Celeste Welty    

Classification

Damage

All three species chew on leaves of cole crops. Very small diamondback larvae mine inside the leaf; older larvae of all three species chew holes through the leaves or graze the surface, leaving tissue-like windows in leaves. Much of the feeding is on the older leaves, which can be tolerated at moderate levels when these leaves will not be harvested. The most serious damage occurs when larvae feed directly on the harvestable part of the plant.

Hosts

• Head crops: cabbage, broccoli, cauliflower, Brussels sprouts.
• Leafy crops: collards, kale, mustard greens, turnip greens.
• Root crops: radish, turnip, rutabaga, kohlrabi.

Appearance and Life Cycle

The imported cabbage worm larva (Fig.1) is green and slightly fuzzy with a pale yellow stripe down its back. When prodded at the head end, it remains in place but sluggishly moves its head to one side.

Figure 1. Imported cabbageworw (a) adult, (b) egg, (c) larva, (d) chrysalis

The cabbage looper larva (Fig. 2) is smooth and green with several white stripes down its back. When it crawls, it arches its body in a loop. It is 1.5 inches long when fully grown.

Figure 2. Cabbage looper (a) adult, (b) larva, (c) cacoon, (d) egg "top view", (e) egg "side view"

The larva of the diamondback moth (Fig. 3) is smooth, green without stripes, and tapered at both ends. When prodded at the head end, it wiggles its whole body vigorously and often drops from the plant. It is 1/2 inch long when fully grown.

Diamondback moth (a) larva, (b) pupa, (c) cacoon, (d) adult "side view", (e) adult "top view"

All three species go through the typical life cycle of egg, larva (caterpillar), pupa, and adult. Imported cabbageworm is relatively predictable because it overwinters in Ohio, whereas the cabbage looper migrates from the southern USA and is thus variable in when it appears. Diamondback moth probably overwinters in Ohio but some also arrive by migration. Characteristics of the life stages for the three species are shown in Table 1.

Natural Enemies

Imported cabbageworm larvae are commonly parasitized by a wasp, Apanteles glomeratus, and pupae by another wasp, Pteromalus puparum. Larvae of the diamondback moth are parasitized by a wasp, Diadegma insularis. Larvae will already have caused some damage before these parasitoids kill them, but parasitoids break the life cycle by reducing the number of caterpillars changing to the adult stage. Wasps of the Trichogramma group are egg parasitoids that kill these pests before they hatch and begin feeding.

General predators that feed on cabbageworms as well as other pests are ground beetles, true bugs, syrphid fly larvae, spiders, lacewings, spined soldier beetles, yellowjackets, and paper wasps. Virus diseases can infect and decimate imported cabbageworm and cabbage looper populations.

Cultural and Physical Control

• Destroy crop debris after harvest.
• Remove weeds in the mustard family (wild mustard, wild radish, shepard's purse, peppergrass) from the area.
• Row covers prevent adults from laying eggs on plants.
• In small gardens, the worms can be hand-picked from plants.

Chemical Control

Insecticides are usually effective at controlling these pests, although populations of the diamondback moth in some areas have developed resistance to some insecticides.

The biological insecticide B.t. (Dipel, Vegetable Insect Attack) is very effective against all three species of cabbageworms in both backyard gardens and commercial planings. Because B.t. kills caterpillars but does not kill other insects, it allows natural enemies to survive and contribute to pest suppression.

Conventional insecticides used for cabbageworm control in backyard gardens include carbaryl (Sevin), diazinon, malathion, methoxychlor, and endosulfan (Thiodan). The botanical insecticide rotenone will control cabbageworms but, like the conventional insecticides, it is toxic to natural enemies as well as to pests.

NOTE:Disclaimer - This publication contains pesticide recommendations that are subject to change at any time. These recommendations are provided only as a guide. It is always the pesticide applicator's responsibility, by law, to read and follow all current label directions for the specific pesticide being used. Due to constantly changing labels and product registration, some of the recommendations given in this writing may no longer be legal by the time you read them. If any information in these recommendations disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned. The author, The Ohio State University and the Ohio State University Extension assume no liability resulting from the use of these recommendations.


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