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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.
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