Cabbage worms in the garden? Here’s what to do

If your cole crops (cabbage, broccoli, Brussels sprouts, cauliflower, kohlrabi) are doing well, chances are the critters that chew holes into the leaves are also doing well. The most common and damaging cole crop caterpillars are imported cabbageworm, diamondback moth and cabbage looper. They are all from the Order Lepidoptera.

Imported cabbageworm eggs. | Photo by Lena Allen

You are more likely to see the damage than you are to see the larvae. Since control options are the same for each of these I will discuss each one and then cover control options.

Imported Cabbageworm

The imported cabbageworm is a medium-sized, white butterfly. They overwinter as pupae and emerge mid to late spring.

Imported cabbageworm adult, top; cabbage looper larva, bottom-right; diamondback moth cocoon, bottom-left.

Imported cabbageworms fly during the day and lay eggs singly on both sides of the leaves. The torpedo-shaped eggs hatch in four to eight days and they begin feeding, causing roundish holes in the leaves.

The larvae are about 1¼ inch long at full size. Their response when disturbed is sluggish. After two to three, they pupate on the host plant and emerge in one or two weeks as adults to begin all over again.

One generation takes three to six, depending on temperatures. There may be as many as four generations per year in the warmer areas of eastern Idaho – probably not this year.

Diamondback Moth

Diamondback moth adults are small and skinny. The wing tips scoop up at the end. Adult moths are active at night. The eggs are small and roundish.

The larva grows to about one-third of an inch long and tapers at each end. They feed on the underside of the leaves and often leave the upper membrane, creating a windowpane effect.
Caterpillars mature in 10 to 14 days. The cocoon is loose and silky.

The leaf feeding is usually not economically significant, but the larvae may damage young buds or growing points, which stunts the plant. The larvae may also burrow into broccoli or cauliflower flower buds. When disturbed, they wriggle rapidly or drop off, attached to a silken thread.
Diamondback moths are nocturnal flyers that are present all summer, but more common in spring and fall. There may be up to four generations a year.

Cabbage Looper

The cabbage looper larva moves like an inchworm. The mottled brown moths have a white “figure 8” or “Y” in the center of their wings. They are nocturnal flyers.

Cabbage loopers overwinter in southern areas and fly north for the summer.

Females lay round eggs about the size of a pinhead on the lower, outer leaves. Larvae feed on the upper surface between the large veins and midribs for four to five weeks producing large, irregular holes. They can bore into young cabbage heads, creating unmarketable produce. With two generations a year, the second generation is the one that causes the most damage.

Integrated Pest Management (IPM)

There are several natural enemies of these pests, so control should center on activities that do not harm the beneficial predators and parasitoids.

Cultural – Remove weeds that belong to the mustard family as they are alternate hosts for these pests. Physical – Cover the crop with a light row cover or insect netting. This should be done from the time of transplanting. Since cole crops are not dependent on pollination they can remain covered throughout the growing season—unless you collect your own seed. Biological – Encourage natural enemies such as paper wasps, ground beetles and parasitoids wasps.

Insecticides

Bacillus thuringiensis (Bt) (aizawai or kurstaki strains) are very host-specific to lepidopteran pests. The dust formulation is reported to be more effective.

Spinosad (some forms are approved for organic production) absorbs into the epidermis and is effective on leaf-feeding pests.

There are other products labeled for these pests. Be sure to follow the label.