The larvae feed extensively on cruciferous vegetable crops such as broccoli, Brussels sprouts, cabbage, cauliflower, collards, horseradish, kale, kohlrabi, mustard greens, radishes, rutabagas, turnips, and turnip greens. They can also attack flowers that contain mustard oil, such as nasturtiums, and other members of the Brassicaceae family, such as sweet alyssum. The adult moths or butterflies sip nectar from flowers and don’t cause damage as the larvae do.
What Do Cabbage Worms Look Like?
It is useful to know what both the larvae and the adults of the cabbage worm look like, and be familiar with their life cycle because spotting near your plants most likely means that you’ll start seeing damage soon. While the life span and the number of generations per year varies, the life cycle of the three species is similar. The adults—butterflies or moths—feed on nectar and lay eggs on host plants. The eggs hatch into caterpillars or larvae that voraciously feed on host plants, then seek out a protected area on the plant to pupate. The pupae metamorphoses into adults and the life cycle starts again.
Imported Cabbageworm
The adult butterflies, which are out flying during the day are off-white with one or two grayish-black spots on each forewing. They have a wingspan of roughly 2 inches. The adults lay their eggs both on the upper side and the undersides of leaves. The eggs are yellow and oval-shaped. The velvety green larvae are up to 1 inch long when mature, which yellow stripes along the back and sides of their body. They feed on the host plants for about 15 days and move slowly when disturbed. There are about two to three generations per year.
Cabbage Looper
The adults of this species are nocturnal moths, so you won’t see them during the day. They have mottled grayish-brown wings with a 1.5-inch wingspan. They lay their eggs on the undersides of the lower leaves of host plants. The eggs are creamy white and about the size of a pin head. The larvae are pale green with thin white stripes lines running down each side of the body. The mature larvae are about 1.5 inches long. The larvae have no legs in their middle section. The feed on plants for two to four weeks. When they move, they make a looping motion—they have no legs in their middle section—hence their name. The number of generations ranges from two to three in cold climates to five to seven in hot climates.
Diamondback Moth
These nocturnal moths are light brown and slender with a wingspan of about 1 inch. They are named after the white diamond pattern on their folded wings. They lay their tiny, cream-colored eggs near the leaf veins. The mature larvae of the diamondback moth are only one-third inch long, much smaller than the larvae of the imported cabbageworms and cabbage loopers. The larvae are light green and tapered. When disturbed, they wiggle back and forth so rapidly that they often drop off the plants. How many days the larvae feed depends on the availability of food and the temperature. The number of generations per year varies greatly; in cold climates it’s about four and eight to 12 in southern locations.
Two Ways to Get Rid of Cabbage Worms
Check any brassicas frequently for the worms, especially if you have seen the adults nearby. The earlier you catch and get rid of them, the better. Treatment is always for the larval stage, not the adult moths or butterflies.
Treating Minor Populations
If there are just a few worms, hand-picking can take care of them. Collect them in a pail of soapy water to kill them. Another treatment for a minor population of cabbage worms is neem oil. While you won’t kill the insects directly, it prevents them from feeding on your plants. Sprinkling the plants with diatomaceous earth pierces the skin of the larvae and causes it to dry out, so they die. However, keep in mind that this abrasive substance also kills other beneficial insects.
Treating Infestations
If you are dealing with a major infestation, you might need to reach for chemicals. Instead of using broad-spectrum insecticides that also kill the cabbage worm’s natural enemies as well as pollinating insects, the better strategy is to use a low-risk and targeted insecticide. Treatment during the early life stages of the larvae is crucial as insecticides are less effective for larger, mature larvae. To kill the larvae, insecticides with these three substances as active ingredients work against cabbage worms:
Pyrethrin, an insecticide derived from the pyrethrum daisy Spinosad, a substance that is derived from a naturally occurring soil bacterium that is toxic to insects. Bacillus thuringiensis (Bt), also a bacterium that occurs naturally in the soil. Bt is produces proteins toxic to larvae when they consume it.
Whichever you use, it is important to get a get good coverage on the leaves when spraying. Make sure to follow the product label. As long as the pest is in your garden—and there can be many, overlapping generations—your crops are at risk, making repeated spraying necessary, especially following a rain.
Signs of Cabbage Worm Infestation
Because of their voracious appetite, an “infestation” can be as few as two or three worms per plant. Look for holes in the leaves as well as entry holes that the larvae burrowed into the heads of broccoli and cabbage. In particular, inspect the main leaf vein and the undersides of leaves because this is where the cabbage worms like to hang out. Their green color can make them difficult to spot especially when they first hatch out and are very small. As with all caterpillars, they will get bigger as they defoliate and devour your crop. As they feed, the larvae produce droppings (fecal matter) looks like green sawdust, another telltale sign of a cabbage worm presence. A serious infestation of cabbage worms can result in the death of the plant, since the more leaves that the cabbage worm manages to eat, the less the plant has the ability to photosynthesize.
What Causes Cabbage Worms?
Depending on the species, cabbage worms can overwinter in your area or migrate anew from warmer regions every summer. Imported cabbageworms overwinter as pupa or chrysalis in plant debris and emerge as adults in the spring. The cabbage loopers overwinter in the south and the adults migrate north in mid-summer.
How to Prevent Cabbage Worms
Good garden sanitation is the first thing you can do to prevent this pest. Remove any brassica crops, whether they were visibly infested or not, at the end of the season, to remove any places where the imported cabbage worms pupa could overwinter. Also remove any nearby weeds in the Brassicaceae family such as wild mustard, peppergrass, and shepherd’s purse as they could also serve the pest as an overwintering location. Cabbage worms have several natural enemies, including paper wasps, parasitic flies, and wasps that kill them. These insects are harmless to humans but of great value for biodiversity so make sure not to use any broad-spectrum insecticides that indiscriminately kill all the insects in your garden. To protect your plants with a mechanical barrier, use floating row covers made of lightweight all-purpose garden fabric, especially in spring and early summer when egg-laying activity is at its highest. Put the row covers in place when the seedlings emerge or at the time of transplanting and leave them up until after the harvest. To give the plants room to grow, and to keep the row cover in place, secure the fabric with metal hoops and clips. To prevent the worms from burrowing into cabbage heads, you can cover each head with a nylon stocking. It will expand as the cabbage grows so you can leave it on until harvest.
