Utilizing effective management tools in cole crops can reduce profit-robbing insects such as cabbage loopers, beet armyworms and whiteflies that can flat line growers’ bottom lines through reduced crop quantity and quality.
Eric Natwick of the University of California Cooperative Extension discussed the latest cole crop pest management techniques during the 17th annual Western Vegetable Crops Workshop in Yuma, Ariz., in late November.
One of the most common insects feeding on cole crops such as broccoli, cabbage, cauliflower and Brussels sprouts is the cabbage looper. Distinguishable from other cole crop worms, the looper’s up and down looping action is the dead giveaway. The insect is light green in color with a white stripe down the side.
Cabbage looper eggs are flat, lay singly on leaves, and grow up to 1-¼ inches long. Mature larvae create a loosely spun silk for pupating usually on the under side of older leaves. The adult moth is brownish in color with a figure eight design on the wings.
Natwick, Extension director and entomology advisor in the Imperial Valley, said cabbage loopers have several generations in the same year and continue to develop unlike the beet armyworm. “While the armyworm drops out of the picture when the weather cools down, cabbage loopers tend to hang around.”
While the cabbage looper occasionally damages seedlings, most damage occurs in the plant’s head development. Cabbage loopers start by eating ragged looking holes in leaves between thinning and early head development, seldom causing any economic impact. The real damage occurs when loopers start boring holes into cabbage leaves or the head, which also can also lead to plant contamination.
Natwick said, “Growers should check 25 plants at random in a field each week and look for white, pearly looking eggs and small larvae on the underside of lower leaves. Open up the damaged heads as necessary.” Treatments should be based on the number of healthy larvae present.
“During the period from thinning to heading, the plants can tolerate quite a bit of damage. When you get to heading, growers should treat even if one larva is found in 25 plants. One is really too much,” Natwick noted.
The adult beet armyworm moth has dusty colored wings and is smaller than the cabbage looper moth. Female beet armyworm moths lay eggs in egg masses underneath a covering of cottony-white scales. Up to 600 eggs can be laid in just three to seven days. The scaly covering protects eggs from predators and from occasionally used insecticides.
Caterpillars change from the egg to the pupate stage in two to three weeks, reaching about 1 ¼-inch long. Most are olive green in color but can vary from purple to almost a yellow. Small squiggly lines down the caterpillar’s back are visible with a 10X hand lens. A yellow stripe down the side of the body is another distinguishing feature.
“Beet armyworms are more abundant from August through October but are also found in the winter and spring,” Natwick said. “Numbers increase in the spring when temperatures get warmer.” While numbers may be decreasing, heads are developing in the field. This is the critical time to protect the crop,” he said.
When the eggs hatch, the first instar tends to feed in clusters. Many larvae are found in one area where they skeletonize the leaves. In the second and third instar, spreading out occurs on the plant as well as down the field rows.
Initially there may be just a few egg masses and infested plant numbers may appear to be limited. But as the population develops and the larvae move, nearly every plant will be infested unless treated.
“Unlike the cabbage looper, the beet armyworm can destroy seedlings because of the clustering of larvae and egg masses. The damage can include stunting, plus at the growing point so the plant will never produce a head,” Natwick explained. “Early treatment for the beet armyworm is more critical than the early treatment of cabbage looper.”