Margaret Murphy
Horticulture Educator for Chippewa, Dunn, & Eau Claire Counties
UW-Madison Extension

While out in the garden the other day, I spied this little garden foe –

The Imported Cabbageworm

I am a huge fan of collard greens. I grow them in my garden every year. These leafy greens are members of the Brassicaceae family together with many other cool-season crops such as Brussels sprouts, cabbage, cauliflower, kale, kohlrabi and broccoli. This group of plants is commonly referred to as “cole crops”.

Planting collards has taught me that members of this family can host a variety of insect pests. One in particular is the imported cabbageworm. You have probably seen the adult form of this caterpillar. It is the small, white butterfly with black spots on the forewings that flutters around your garden. The female will lay tiny, light-colored eggs often on the underside of leaves. The caterpillar is velvety green and has a faint yellow stripe down its back. When full grown it will reach about an inch in length and can slowly but efficiently chew its way through your crop.

Management of these pests begins with good surveillance. Now is the time to keep an eye out for the white butterfly flittering around the yard. Also, regularly check plant leaves for eggs, caterpillars or irregular-shaped holes. When feeding on cabbage, older larvae often crawl to the center of the plant and penetrate the head. On broccoli or cauliflower, they can forage on the developing flower. Cole crops can tolerate some insect damage but if treatment is needed, there are several approaches you can try.

Physical control: You can remove the caterpillars by hand and drop them in a pail of soapy water to kill them. I have tried this, and while it works, the task can get a bit daunting if you have a large crop. Physical controls can also involve barriers. I have had good luck using row covers, often called floating row covers. These are made out of very light-weight, synthetic material and have a “gauze-like” appearance. They allow air, sunlight and water through while creating a barrier to insect pests.

If you are new to floating row covers, they are easy to use. The cover “floats” directly on top of the crop. It can be secured using sod pins, boards, bricks, rocks, or soil. Provide enough slack so as the plants grow, they will just push the cover up. Or, alternatively, build a simple frame to support the row cover above the plants.

Natural control: There are a number of natural enemies that prey on the imported cabbageworm. Many are commonly found in our yards including paper wasps, and parasitic wasps and flies.

Chemical control: For most home gardeners, scouting for the caterpillars and removing them by hand is often the most effective method for control. There are chemicals that can be used to control them – when possible, select a control that is less impacting to natural enemies and pollinators such as Bacillus thuringiensis (Bt), a bacterium that targets caterpillars. Treat when caterpillars are first noticed. Whenever applying any type of insecticide, remember to read the label and follow all the instructions carefully.

For more information on common caterpillar pests of cole crops, see https://extension.umn.edu/yard-and-garden-insects/caterpillars-cole-crops.

This is also the time gardeners need to stay vigilant about checking their cucurbits (squash, melons, pumpkins, cucumbers, gourds) for summer pests. Here are the three main headliners looking for a cucurbit near you.

Squash bugs: These will attack all members of the Cucurbitaceae family but seem to prefer pumpkins and squash. They feed on the foliage by sucking out the sap but can also feed on the fruit. A severe infestation can cause leaves to wilt, turn brown and die. This makes them more of a concern when plants are seedlings and when they are flowering. The adult is unremarkable in appearance. It has a flat, dark grayish body about 5/8 of an inch long and 1/3 as wide. It can be easily confused with a stinkbug. Adult squash bugs are difficult to kill. Control of adults and any egg clusters found on the underside of leaves or on stems is usually done by picking them off by hand. As squash bugs like to gather under debris, you can also lure the adults by placing a board or heavy cardboard under the plant. Check in the morning and place those found hiding in a bucket of soapy water. Immature squash bugs (nymphs) can be controlled with insecticidal soap or spray. Read the label to ensure it works on the intended pest and can be used on your crop. As with any insecticide, please follow the directions carefully. Learn more at https://hort.extension.wisc.edu/articles/squash-bug-anasa-tristis/.  

Cucumber beetle: Both the spotted and the striped cucumber beetle feed on the leaves and stems of cucurbits and can kill or greatly curtail the growth of young plants. Adults will also eat the blossoms and fruit. As they forage, cucumber beetles can spread bacterial wilt. A non-chemical control for cucumber beetles is to use floating row covers. These covers create an effective barrier between the beetles and plants. Just remember to take the covers off when plants are in bloom to allow for pollination. If your crop is small enough, you can remove the beetles by hand. Good cultural practices such as eliminating weeds in and around the garden can help deter them as well. Insecticides are also available. Learn more at https://hort.extension.wisc.edu/articles/cucumber-beetles/.

Squash vine borer: The adult borer is a wasp-like moth with an orange

colored abdomen dotted with black spots. This moth is active during the day. It typically appears in late June or early July to lay its eggs primarily on squash and pumpkin plants. The moth lays her eggs individually on the base of plants, leaf stalks and vines. The eggs hatch in about a week. The larvae then bore into the plant stem to feed. Damage to the stem by the tunneling larvae blocks water flow to the rest of the plant causing it to wilt and eventually die. Now is the time to monitor for the presence of the adult borer. Early use of floating row covers will prevent this female pest from landing and laying eggs. Remove the row cover when plants flower to allow for pollination by bees. Chemical control includes applying an insecticide to the stem of the plants at the end of June or early July or when an adult borer is seen to kill the newly hatched larvae. Learn more at https://hort.extension.wisc.edu/articles/squash-vine-borer-melittia-curcurbitae/.

Have a garden question, visit https://hort.extension.wisc.edu/ask-a-gardening-question/ or contact your local County Extension office