Broccoli, like other brassicas, is a frequent target for several species of caterpillars commonly grouped as “broccoli worms.” These pests are the larval stage of moths and butterflies, and they cause significant damage to leaves and bore into the developing heads. A multi-pronged approach using biological and physical controls offers safe and effective solutions. Understanding the specific pest and applying targeted removal and prevention methods ensures a healthy yield.
Identifying the Common Pests
Effective management begins with correctly identifying the specific caterpillar species causing the damage. Three main types of larvae are responsible for most brassica damage.
The Imported Cabbage Worm is a velvety green caterpillar, reaching up to one inch in length, often with a faint yellow stripe. This caterpillar tends to be sluggish when disturbed and feeds near leaf veins, leaving large, chewed holes in the foliage.
The Cabbage Looper is a pale green larva up to one and a half inches long. It features a distinctive looping or “inchworm” movement because it lacks the middle pair of legs. This pest often feeds on the underside of lower leaves, sometimes creating “windowpane” damage where only a thin layer of transparent tissue remains.
The Diamondback Moth Larva is the smallest, reaching about one-third of an inch at maturity, with a segmented body that tapers at both ends. When touched, this small, light green larva wiggles vigorously and may drop from the plant on a silk thread.
Immediate Physical and Biological Controls
For an immediate response to an active infestation, physical removal and biological treatments are the most direct methods. Handpicking the caterpillars off the plants is highly effective for smaller gardens, especially when performed early in the morning when the larvae are less active. Inspect the undersides of leaves and near the central growing points where pests often hide, and drop any collected larvae into a bucket of soapy water.
A strong jet of water from a garden hose can also dislodge smaller larvae and eggs, especially those on the undersides of the leaves. This method is a quick way to reduce the population, though it may need to be repeated daily for a few days to catch newly hatched pests. For a more widespread infestation, the biological insecticide Bacillus thuringiensis (Bt) offers a targeted treatment option.
Bt, specifically the subspecies kurstaki (Btk), is a naturally occurring bacterium that is lethal to certain caterpillar larvae. When the caterpillar ingests the Btk spores and crystal proteins, the proteins are activated by the alkaline conditions in the insect’s gut. This process causes the gut lining to break down, leading the insect to stop feeding within minutes, with death occurring within one to three days. Btk must be applied when larvae are small, and repeated applications may be necessary to cover new growth.
Preventing Future Infestations
Long-term protection focuses on excluding the adult moths and butterflies before they can lay eggs on the plants. The most effective non-chemical method is the use of floating row covers, which are lightweight, spun-bonded fabrics that allow light, air, and water to pass through. These covers act as a physical barrier to prevent the adult insects from reaching the broccoli leaves. The covers should be installed immediately after transplanting and secured tightly around the edges with soil or weights to prevent the insects from crawling underneath.
Cultural practices, such as crop rotation, can disrupt the pest life cycle and reduce the likelihood of infestation. Avoiding planting brassicas in the same location year after year prevents pests that may have overwintered in the soil from immediately finding a host plant. Planting timing can also be adjusted to avoid the peak activity periods of the adult moths and butterflies.
Introducing certain companion plants can also contribute to an environment that deters pests. Herbs with strong scents, such as dill, thyme, and rosemary, are often planted near brassicas because their aroma may confuse the adult moths looking for a host plant. This practice complements the use of row covers and biological controls to create a more resilient garden ecosystem.