Cabbage worms present a challenge for gardeners growing plants in the Brassica family, which includes cabbage, broccoli, kale, and collard greens. These pests, primarily the imported cabbageworm (Pieris rapae) and the cabbage looper (Trichoplusia ni), are caterpillars whose feeding habits can quickly ruin a harvest. Gardeners can employ a variety of non-toxic, natural control methods to protect their crops. Effective management relies on early detection, physical exclusion, cultural practices, and targeted organic treatments.
Identifying Cabbage Worms and Their Activity
Accurate identification of the pest and its activity is the first step toward effective control. The adult imported cabbageworm is a small white butterfly, often called the cabbage white, with one or two black spots on each forewing, frequently seen fluttering near Brassica crops. These butterflies lay tiny, pale yellow or cream-colored eggs that are oblong and have vertical ridges; they are typically laid singly on the undersides of leaves.
The imported cabbageworm is a velvety green caterpillar with a faint, thin yellow stripe running down its back. Cabbage loopers are also green but can be distinguished by their “looping” or “inchworm” movement, caused by a lack of middle legs. Both larvae cause similar damage, chewing irregular, ragged holes in the leaves, especially in the central growing points and developing heads of the plants. A clear sign of their presence is the dark green or brownish fecal matter, known as frass, left scattered on the leaves.
Physical Barriers and Cultural Practices for Prevention
The most effective long-term strategy for preventing cabbage worm damage is to physically block the adult butterflies from laying their eggs. Floating row covers, made of lightweight, permeable fabric, create a physical barrier that allows light, air, and water to pass through while excluding the pest. The cover should be draped loosely over the plants or supported by hoops to allow for growth, and the edges must be completely sealed with soil, boards, or pins to prevent the butterflies from crawling underneath. Since Brassica crops do not require insect pollination, the row cover can remain in place for the entire growing season.
Employing sound cultural practices also helps to minimize the pest population. Crop rotation is a straightforward defense, requiring gardeners to avoid planting Brassicas in the same location year after year. This technique prevents pests that may have overwintered in the soil from immediately accessing new plants. Strategically timing a planting can also help, as planting very early in the spring or very late in the summer can allow plants to mature outside the peak activity periods of the adult butterflies.
Companion planting is another deterrent, using aromatic plants to confuse the adult female butterflies. Strong-smelling herbs such as dill, thyme, rosemary, or mint, when interplanted near Brassicas, can mask the scent of the host plants, making it difficult for the butterflies to locate a suitable egg-laying site. Some gardeners also use plants like nasturtiums as a trap crop, which draws the pest away from the Brassica plants. Regularly inspecting and removing any eggs or larvae found on these trap plants is necessary to prevent the population from establishing itself and spreading.
Targeted Organic Treatments
Once an infestation is present, several organic treatments are effective at controlling the larvae. The most targeted biological control is the application of Bacillus thuringiensis subspecies kurstaki, commonly abbreviated as Bt-k. This naturally occurring soil bacterium is harmless to humans, pets, and beneficial insects, but is lethal to caterpillars.
Bt-k must be ingested by the caterpillar to work, releasing a protein toxin that paralyzes the digestive system. The larvae stop feeding almost immediately after consuming the treated foliage and die within a few days. For maximum effectiveness, Bt-k should be applied when the larvae are small, typically less than a half-inch long, as the treatment is less effective on larger caterpillars.
Application timing is important, and it is best to spray Bt-k in the late afternoon or early evening because the bacterium degrades quickly when exposed to strong sunlight. Reapplication is necessary after a heavy rain or if overhead watering has occurred, as the treatment can be washed away. Other sprays, such as Neem oil or insecticidal soap, can also be used, particularly against very young larvae. For small-scale gardens, the simplest solution remains handpicking; daily inspection and removal provides instant control.