Kale, a leafy green belonging to the Brassica family, is a highly nutritious vegetable that is a prime target for various insect pests. Like relatives such as cabbage and broccoli, kale produces specialized chemical compounds called glucosinolates. These sulfur-containing compounds act as feeding stimulants for insects that have adapted to this plant defense mechanism, drawing in specific pests. Understanding the different types of damage helps identify the culprit and determine the most effective response.
The Main Chewing Pests That Leave Holes
The most common and visible damage to kale comes from chewing pests that leave noticeable holes. Three primary pests are responsible for this defoliation, each leaving a slightly different signature. The Imported Cabbage Worm, the larval stage of the Cabbage White butterfly, is a velvety green caterpillar with faint yellow stripes. These caterpillars typically feed near the center of the plant or along the midribs, leaving large, ragged holes and depositing small, dark green fecal pellets, known as frass.
Another voracious caterpillar is the Cabbage Looper, which is pale green with white stripes. It is recognized by its distinctive “looping” motion when it crawls, due to a lack of middle legs. The Looper is the larva of a nocturnal moth, and its feeding results in significant, irregular holes in the foliage. Both caterpillars can quickly skeletonize a plant, especially younger ones, if their population is left unchecked.
Flea Beetles, small, shiny black or bronze insects that jump when disturbed, create a third distinct type of chewing damage. These beetles target kale leaves, especially on seedlings and young plants, by chewing numerous tiny, round holes that resemble a shotgun pattern, often called “shotholes.” While established plants can often tolerate the damage, a severe infestation can stunt or kill vulnerable new transplants by reducing the leaf surface area available for photosynthesis.
Sap-Sucking and Hidden Pests
Not all kale damage involves holes; a different group of insects causes harm by piercing the plant’s tissue and extracting fluids. Aphids, particularly the gray-green Cabbage Aphid, are soft-bodied insects that cluster in dense colonies on the undersides of leaves and new growth. Their feeding causes the kale leaves to become stunted, curled, and yellowed as they extract the plant’s sap.
Aphids excrete a sticky, sugary substance called honeydew, which attracts black, sooty mold. This mold further inhibits the plant’s ability to photosynthesize. The Harlequin Cabbage Bug, a brightly colored, shield-shaped insect, is another sap-sucker. Its feeding causes white or yellow blotches, known as stipples, to appear on the leaves, which eventually turn brown and wilt.
Understanding Pest Attraction and Timing
The attraction of specific pests lies in kale’s chemical defense system. Glucosinolates release pungent, sulfurous compounds when the plant tissue is damaged. While these compounds deter generalist insects, they function as a powerful cue for specialized pests like the Cabbage Worm and Flea Beetle to feed and lay eggs.
Pest activity is strongly seasonal, and multiple generations of the most damaging pests can occur throughout the year. Cabbage White butterflies emerge and begin laying eggs in early spring, and subsequent generations of caterpillars continue to feed until late fall. Flea beetles are particularly active during warm, dry weather, and their population can explode quickly, overwhelming young plants during the initial growth phase.
Immediate Steps to Protect Kale
The most effective immediate defense against kale pests is daily inspection and physical exclusion. Floating row covers, which are fine mesh fabrics secured over the plants, act as a physical barrier. They prevent adult butterflies and moths from laying eggs on the leaves, and also block flying pests like flea beetles and Harlequin Bugs.
Regularly inspecting the undersides of the leaves is necessary, where many pests like to hide and lay their eggs. Hand-picking caterpillars, Harlequin Bugs, and egg masses and dropping them into a container of soapy water is a simple, highly effective control method for small gardens. Aphid colonies can often be dislodged and destroyed by a strong, targeted stream of water from a garden hose.
For more persistent or widespread infestations, a natural product like neem oil or insecticidal soap can be used to treat the plants. Neem oil disrupts the feeding and reproductive cycles of soft-bodied pests like aphids and young caterpillars, while insecticidal soap suffocates them on contact. Applying a fine dusting of diatomaceous earth, a naturally occurring, sharp powder, can also help deter flea beetles and crawling pests by damaging their exoskeletons.