Moths and their larvae commonly infest Brassica family plants, including cabbage, broccoli, and kale. This issue is caused by three primary pests: the imported cabbageworm, the cabbage looper, and the diamondback moth. These insects target cruciferous vegetables to lay eggs, leading to leaf damage once the larvae hatch. Understanding how to identify these pests and implementing safe, practical control measures is the most effective approach to protecting your harvest.
Recognizing the Pest and the Damage
The first step in control is correctly identifying the culprits, as they each have a slightly different appearance. The most frequently encountered pest is the imported cabbageworm, which is the larva of the common white butterfly with small black spots seen fluttering around gardens during the day. The worms themselves are velvety green and fuzzy, growing up to one inch long with a faint yellow stripe running down the back.
The cabbage looper is a pale green caterpillar that moves with a distinct “inchworm” motion because it lacks legs in the middle section of its body. The adult looper is a nocturnal, grayish-brown moth with a small, silvery figure eight or Y-shape mark on its wings. Diamondback moth larvae are the smallest of the group, growing to about one-third of an inch, and will wiggle when disturbed, often dropping from the plant on a silk thread.
The damage caused by these larvae is often the first sign of infestation. Large, ragged holes chewed into the outer leaves are characteristic of the imported cabbageworm and larger loopers. A telltale sign of looper and cabbageworm activity is the presence of dark, pellet-like droppings, called frass, left on the leaves. Diamondback moth larvae create a distinctive “window-pane” effect by consuming the leaf tissue but leaving a thin, transparent layer of wax intact.
Physical Barriers and Cultural Prevention Strategies
The most proactive solution involves using physical barriers and adjusting garden management practices. Floating row covers are effective because they physically prevent the adult moths and butterflies from landing on the plants to lay their eggs. These covers are made of lightweight, spun-bonded material that allows sunlight, air, and water to pass through freely.
To install a row cover, drape the material loosely over the plants or over wire hoops to allow room for growth, then secure the edges completely. Weighing them down with boards or rocks is necessary to prevent the adult insects from crawling underneath. Since brassica crops do not require insect pollination, these covers can be left in place for the entire growing season.
Manual inspection and removal is a powerful tool for small-scale gardens, especially when performed regularly. Checking the undersides of leaves for the small, oblong yellow eggs or the newly hatched larvae allows for removal before significant damage occurs.
Cultural practices like crop rotation and garden sanitation reduce pest pressure over time. Rotating the location of the Brassica family crops to a different section of the garden each year helps to break the pest’s life cycle. Removing all plant debris from the garden at the end of the season prevents overwintering sites for the pupae.
Companion planting can confuse the adult moths, making it difficult for them to locate the brassicas. Planting aromatic herbs near the susceptible crops releases scents that mask the chemical signals from the cabbage plants. Nasturtiums can also be used as a trap crop, as the moths are often drawn to lay their eggs on them instead of the main harvest.
- Dill
- Mint
- Thyme
- Sage
- Onions
Organic and Biological Spray Treatments
Once an infestation is established, targeted organic sprays can be used to control the larval stage. Bacillus thuringiensis (Bt) subspecies kurstaki (Btk) is a naturally occurring soil bacterium that is specific to caterpillars. When a caterpillar ingests Btk, the bacterial toxins bind to specific receptors in its gut, causing it to stop feeding.
The caterpillar will die from starvation within a few days. Because sunlight degrades the active ingredient, Btk should be applied late in the afternoon or early evening so the caterpillars can consume it during their nightly feeding period. Thorough coverage of both the top and underside of the leaves is necessary for the treatment to be effective.
Neem oil works as a repellant and an insect growth disruptor. It reduces the caterpillar’s appetite and interferes with the insect’s hormonal system, preventing it from maturing into an adult. Like Btk, neem oil should be applied during the cooler parts of the day, such as early morning or late evening, to prevent sun scorch on the leaves and to avoid direct contact with active pollinators.
Insecticidal soaps are a contact treatment option. These soaps work by disrupting the cell membranes of the larvae, leading to dehydration and death. Since insecticidal soaps only kill the pests they physically touch, complete coverage is essential, and repeat applications may be needed until the infestation is controlled.