Common insects affecting cabbage and cole crops

By for CAES News

By David Riley and Stormy Sparks
University of Georgia

As fall nears, so does the time to plant cole crops like cabbage, collards, greens, broccoli and Brussels sprouts. Unfortunately, we're not the only ones who like the taste of greens on a cool fall night. So do insects.

Cabbage, collards, cauliflower and broccoli are typically transplanted in Georgia, while mustard, kale, and turnip greens are often direct-seeded. Some seedling pests are avoided through transplanting.

Even so, young seedlings -- and particularly plants that will eventually form heads -- are very susceptible to damage. Typical pests of cole crop seedlings are flea beetles, cutworms and other pests in the soil. Flea beetles cause small shot holes in leaves.

Other common pests include seed-corn maggots, which attack the germinating seed and very young seedlings, and cutworms, which clip the plant off at the soil line shortly after it emerges. Once the plants are well established, or if you use transplants, these seedling pests cause very little damage.

Occasionally, small insects can be seen tunneling within the leaves of young plants. Pests that feed on the leaves and are of concern are mostly species of caterpillars. These pests can reduce plant growth if they eat enough foliage.

In summer and fall plantings, cabbage webworms can infest plants and attack the main stem. This damage is more severe in cabbage, because it can cause multiple heads.

The treatment timing for soil insects, such as seed-corn maggot, is usually preplanting. For cutworms and webworms it's when damage is first detected, and for defoliators it's at 10- percent defoliation. Early infestations of webworms should be treated when detected to prevent severe damage to the growing tip.

Midseason attacks

In crops such as leafy greens, where the harvested portion of the plant is the leaves, controlling pests that feed on leaf tissue is increasingly critical as the season progresses.

In cabbage, control of foliage feeders is less critical in young plants but becomes more critical at the cupping stage, when the head begins to form. In general, significant yield loss from Lepidoptera larvae can be prevented during midseason using a treatment threshold of three larvae per 10 plants. In other words, if larvae numbers exceed three in every 10 plants scouted, you're likely to have significant yield loss.

The main species of Lepidoptera that attack cole crops in Georgia are the diamondback moth, cabbage looper, imported cabbageworm, cross-striped cabbageworm and cabbage webworm.

Other insects that can reduce the quality of foliage in this growth stage are aphids, which secrete honeydew and promote sooty mold on leaves; sweet potato whiteflies, which can transmit geminiviruses that stunt plant growth; thrips, which can scar the leaf surface when they occur in large numbers; and other foliage feeders, such as the yellow-margined leaf- beetle.

Stinkbugs can often be found on the foliage of cole crops, too, but significant damage from these pests isn't usually reported in Georgia. Cabbage root aphids can be often found on developing turnip roots, too, but they don't usually cause enough damage to warrant soil treatments in the fall, when the pest gets into the cabbage root system. It might take out a few cabbage plants.

Late season attackers

Once the cabbage heads or cauliflower kurds are formed, the tolerance for damage or insect contamination in the harvested part of the plant dramatically declines. The preharvest interval for many insecticides limits the use of chemicals just before harvest, too.

The bottom line for late-season insect control in most cole crops is to control insects early in the growing cycle, keeping them from being a problem at the time of harvest.

Alton Sparks is a Cooperative Extension entomologist with the University of Georgia College of Agricultural and Environmental Sciences.