Of all the insects that feast on bok choy, broccoli, Brussels sprouts, cabbage, cauliflower, collards, kale, mustard, radish, rutabaga, turnip and watercress, the cabbage looper often inflicts the most damage.
Not only are those brassicas sought by the caterpillar of Trichoplusia ni, but a multitude of other vegetables and several flower and field crops also serve as hosts of this pest.
The adult cabbage looper is a mottled grayish-brown moth with a silvery white figure eight on each front wing. It flies nocturnally but can be spotted during the day resting on the underside of leaves. The wingspan measures 1.5 inches. The females can lay 200 to 350 eggs, typically over 10 to 12 days. The pinhead-size, white, round eggs are usually laid near the outer edges of lower leaves. The eggs and larvae develop more slowly in cooler temperatures. Depending on the region and the weather, cabbage loopers may have numerous generations during the season.
The larvae hatch in three to six days and start feeding immediately. The larvae pass through five instars and feed for two to four weeks. The light green larva has two white stripes on each side. The early instars are only 0.125- to 0.25-inch long, while the late instars reach over 1.5 inches in length. Three pairs of slender legs are near the head; the hind end has three pairs of thick prolegs.
Unlike most of the other Lepidoptera insects, no legs are in the midsection. Consequently, their middles hump when the caterpillars move, causing the “looping” movement. Their larval stages all share that looping trait.
The pupae have light green coloration when young, and then turn dark brown when mature. They are 0.75 inch long and delicately wrapped in a cocoon of white threads. The pupae can usually be found on the underside of lower leaves. The cabbage looper overwinters in the pupal stage in southern states. Moths from the South migrate to northern states in July and August.
Pheromone traps can lure the adult moths. Trap catches enable growers to assess the extent of the pest population and determine treatment and timing to control caterpillars before they damage crops.
When young, the larvae feed between the veins on the undersides of lower leaves. As they progress, the larger caterpillars make ragged holes in the foliage, and then move to the center of the plant. The large cabbage loopers can burrow through a half-dozen layers of the tightly wrapped leaves of cabbage heads. Larvae in broccoli contaminate the florets, rendering them unmarketable. Their frass stains cauliflower heads. The feeding damage to the cruciferous leafy green vegetables, such as collards, kale and mustard, can easily cause them to be unfit for consumption. Other cruciferous vegetables, including radishes and turnips, can become sufficiently defoliated to stunt the crop.
Researchers, including those in Cornell and Oregon State, point out that scouting should be used as well as trap counts in evaluating the larvae population for spray decisions. Threshold recommendations for controls, however, depend on the crop and the market. Consult with your local extension staff for the latest information on controls as well as the pest monitoring system and results in your area.
The vegetable crop must also be considered. For example, many recommend treating broccoli and cauliflower about a week before harvest. Once larval development infests their heads, controls do not remove the contaminant.
Many natural enemies can frequently keep cabbage loopers under levels for economic damage, often until late pressure occurs at the time of crop heading. These biological controls include the egg parasite Trichogramma pretiosum; the larval parasites Hyposoter exiguae, Copidosoma truncatellum and Microplitis brassicae; parasitic tachinid flies Compsilura concinnata and Voria ruralis; and the parasitic wasp Encyrtidae.
Also, a nuclear polyhedrosis virus disease occurs sporadically. In some years this disease diminishes the cabbage looper population.
University of California (UC) Pest Management Guidelines advise monitoring for natural enemies of the cabbage looper before applying insecticides. Also, a selective insecticide such as Bacillus thuringiensis (Bt) avoids adversely impacting natural enemies. Jan van der Heide, Northeast sales and product development manager for Bejo Seeds, said that Bt controls cabbage looper larvae well. “Apply Bt at the first sign of eggs in the field,” he advised. “Caterpillars are best controlled when young.”
The Bt products with the subspecies kurstaki are specifically active against caterpillar pests. Their protein crystals kill the larva after it ingests the material treated by releasing toxins into its stomach. Good coverage, particularly on the underside of leaves, is essential. Evening application helps delay the breakdown of the product by sunlight. In addition, the pH of the water should be between 6.0 and 6.5.
Bt and the Entrust formulations of spinosad are acceptable for organic growers. If a serious infestation of large larvae threatens the crop, the UC Guidelines suggest that a carbamate or pyrethroid insecticide may be warranted.
Regardless of the control product employed, using alternative control strategies will help delay resistance development.
COVER PHOTO BY PEGGY GREB, COURTESY OF USDA-ARS