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.”
He offered these field monitoring tips: check field areas often before seedlings emerge as the worm has a very broad host range; check for egg masses or larvae on weeds around the field; and check nearby crops like alfalfa which can also be a large source for beet armyworms.
He said pheromone traps may help determine large populations and when growers should check for egg masses. When seedlings emerge, check several times a week for egg masses and young larvae. Treat if early instars are found on one in every 10 plants. When sampling for beet armyworms, he suggested sampling for cabbage loopers and diamondback moths and include in the total worm counts. He recommended treating before heading so the grower doesn't take a loss in the field.
Natwick said this growing season thus far has lacked whitefly vector diseases in broccoli, cabbage and cauliflower. That is good news since whiteflies suck out large quantities of plant sap needed by cole crops to grow. Whiteflies also drop honeydew that leads to mold and product stickiness.
“A standard here (in the low desert) for early plantings is to treat with a neonicotinoid in the soil at planting to offer good plant protection. When whitefly numbers are high, stunting occurs, plus serious discoloration of the broccoli stalk,” he noted. “We haven't seen much of that since the 1990s when we had high whitefly numbers and the neonicotinoids was not yet used as a tool. I still occasionally see some white stalk in broccoli.”
When possible, Natwick recommended planting cole crops at least a half-mile upwind from cotton or melon fields that often serve as large reservoirs for whitefly populations. This will reduce large migrating whitefly infestations in cole crops.
Another suggestion is to ask growers with cotton and melons nearby to remove the crops immediately after harvest. The longer cotton and melons remain on the farm, the more whiteflies are generated which can move into cole crops. Whitefly populations in the low desert tend to drop off after mid-October with cooler temperatures.
“Whiteflies are actually still out there so growers should still be concerned. Biological controls usually don't do much good in an annual cropping system,” Natwick said. “Existing parasites, plus the exotic ones brought in and released in the 1990s, really don't benefit cole crops. The adult whiteflies come into the field, lay their eggs, and the nymphs begin to develop. Where are the parasites? They don't follow quickly behind the migrating whitefly adults.”
More important than the biological control of whiteflies are predacious insects like lacewing larvae, big-eyed bugs, minute pirate bugs and lady beetles.