Caterpillars in Brassica Crops
The major caterpillars on Brassicas – generally known as ‘worms in cabbage’ – include three species that differ in size and feeding habits, as well as how susceptible they are to certain insecticides. Getting acquainted with the pests helps you to know what kind of damage to expect and what to look for.
Imported cabbageworm; cabbage butterfly (Pieris rapae).
This familiar white butterfly can be seen in the daytime fluttering, around cole crop fields. Each forewing has a dark border and one or two round black spots. Eggs are laid singly on the underside of leaves, about 1/8 inch in length, light green and slightly elongated, standing upright. The larvae, called imported cabbageworm, is graygreen, slightly fuzzy, and sluggish. Feeding and resting occur on the underside of leaves, and larvae feed more heavily in the head of cabbage or broccoli as they grow. The overwintering stage is the crysalis (pupa), which is green or brown, smooth with three pointed ridges on its back. There are 3-4 generations per year. For more information on imported cabbageworm, click here.
Diamondback moth (Plutella xylostella).
These caterpillars are smaller, light green, appear more segmented and more pointed in shape. When disturbed they wiggle vigorously and may drop off the plant on a string of silk. Feeding causes small, round holes and tends to be spread across the foliage and not necessarily concentrated in the head. The adult moths are tiny (<1/2 inch), light brown, and rest with their wings folded together like a tent. They overwinter in crop residue, but may also enter the region by migrating from southern states. For more information on the diamondback moth, click here.
Cabbage looper (Trichoplusia ni)
Cabbage looper usually does not survive the winter in New England, and arrives in migratory flights from farther south. Generally numbers are not significant until late July or August, and some years they do not occur at all. However, earlier flights do occur, probably as a result of early migratory flights. Adult moths are mottled gray-brown, about 3/4 inch long, with a distinct round silver-white mark on each fore-wing. Since they fly at night, they are rarely seen unless monitored with pheromone traps. The cabbage looper caterpillar is light green, with wavy white or light yellow lines down the back and sides. Full-grown larvae reach 1 ½ to 2 inches. At rest or when disturbed, cabbage loopers of any size will raise the middle of their body in a characteristic “loop” shape. Eggs are round, light green or yellow, and laid underneath the foliage. Monitor caterpillar activity by field scouting. Feeding tends to create ragged, large holes in foliage, on both frame leaves and heads. For more information on cabbage loopers, click here.
Field scouting for caterpillars
It is especially important to check cabbage or broccoli plantings as they begin forming heads. Greens such as collards, kale, and Chinese cabbage should be scouted earlier, since all leaves are marketed. Check randomly-selected plants throughout the field looking for caterpillars or fresh feeding damage on the top or underside of leaves. Often it is easier to spot the feeding damage first, then find the caterpillar. Classify plants as infested (has one or more caterpillar) or non-infested, and calculate the percent of plants infested. In the Northeast, there is generally no need to treat young plants unless weather conditions delay plant development and at least 35% of them are infested with any of these pests. Treat plants between the start of heading and harvest if 20% or more of the plants are infested. The most critical time to scout and apply controls is just prior to head formation. Use a 10-15% threshold throughout the season for kale, collards and mustard. These thresholds are based on research trials that showed that use of the thresholds produces 98-100% clean heads, the equivalent of weekly sprays but with far fewer insecticide applications.
Do not use less than 50 gal spray material/A; higher volumes provide better coverage. Better coverage of lower leaf surfaces can be achieved by using drop nozzles. Use a spreader-sticker.
Diamondback moth (DBM) has become resistant to many synthetic and microbial insecticides. Even if you are getting excellent control of this pest with the materials presently being used, you should alternate between effective materials to retard development of resistance. Newer materials and the aizawai strain of Bacillus thuringiensis will usually provide better control of resistant DBM than older products. Use transplants grown in New England to avoid importing DBM that have already developed resistance to one or more classes of insecticides.
DBM has become resistant to many synthetic and microbial insecticides. Even if you are getting excellent control of this pest with the materials presently being used, you should alternate between effective materials to retard development of resistance. Newer materials and the aizawai strain of Bacillus thuringiensis will usually provide better control of resistant DBM than older products. Use transplants grown in New England to avoid importing DBM that have already developed resistance to one or more classes of insecticides.
-- R. Hazzard