There are four species of caterpillars that are major pests of brassicas in Massachusetts. Though they may look similar, these four pests have important distinctions among them that can affect management decisions. They differ in size and feeding habits, as well as their susceptibility to beneficial parasitoid insect species and certain insecticides. Getting acquainted with these pests will help you understand what kind of damage to expect and what to look for when scouting for their different life stages and biocontrols. Feeding damage by any of these caterpillars can reduce yield and marketability of both leafy and heading brassica crops.
Imported cabbageworm
Imported cabbageworm or cabbage butterfly (Pieris rapae) adults are a very familiar white butterfly that can be seen during the day fluttering around brassica fields. Each forewing has a dark border and one or two round black spots. Eggs are laid singly on the underside of leaves, standing upright. They are bullet-shaped, about 1/8-inch in length, and initially pale white, turning yellow as they mature. Larvae are gray-green, slightly fuzzy, and sluggish. They can be very well camouflaged, especially when they lie along the midrib of a leaf. Feeding and resting occur on the underside of leaves, and in cabbage or broccoli, the larvae feed more heavily in the head than on the other leaves. The overwintering stage is the chrysalis (pupa), which is green or brown and smooth, with three pointed ridges on its back. There are 3-4 generations per year.
Diamondback moth
Diamondback moth (Plutella xylostella) adults are tiny (< ½ inch long) brown moths that rest with their wings folded together like a tent. Historically, DBM did not overwinter in the Northeast, but caterpillars have been reported as early as May in recent years, suggesting that they may be overwintering in high tunnels or other warm spots. Regardless, this pest also blows in every year from warmer areas to our south. Adults are weak fliers, but populations are known to disperse long distances on wind and annually reinvade areas well into Canada. Eggs are laid singly or in small clusters. Caterpillars go through four instars and are small (< ½ inch when fully grown), light green, and appear segmented, with a forked end and pointed shape. When disturbed, they wiggle vigorously and may drop off the plant on a string of silk. Feeding causes small, round holes that don’t break through the top layer of leaf tissue, leaving translucent films across holes, called “windowpane” damage. Feeding tends to be spread across the foliage and is not necessarily concentrated in the head.
Cabbage looper (Trichoplusia ni)
Cabbage looper (Trichoplusia ni) usually does not survive the winter in New England and arrives in migratory flights from farther south. Generally, cabbage looper numbers are not high until late July or August, though earlier flights may occur in some years or they may not be found at all in others. Adult moths are mottled gray-brown, about ¾ inch long, with a distinct round silver-white mark on each forewing. Since they fly at night, they are rarely seen unless monitored with pheromone traps. If you want to know when moths arrive, use a wing trap baited with Trichoplusia ni lure, placed near the canopy. Eggs are round, pale green or yellow, and are laid singly underneath the foliage. The cabbage looper caterpillar is light green and smooth, with wavy white or light yellow lines down the back and sides and stubby legs at the tip of the abdomen. Full-grown larvae reach 1½ to 2 inches. Cabbage loopers of any size move like inchworms—by raising the middle of their body in a characteristic “loop” shape. Feeding tends to create large, ragged holes in foliage, on both frame leaves and heads. Cabbage looper also feeds on many non-brassicas including lettuce, celery, spinach, and chard, so when they do arrive, scout those crops as well as brassicas.
Cross-striped cabbageworm (Evergestis rimosalis):
Cross-striped cabbageworm (Evergestis rimosalis): Formerly restricted to the South, this insect is now a serious problem on brassica crops in southeastern New England. Cross-striped cabbageworm does not generally overwinter in New England but we saw a very early infestation on one farm in MA last spring, suggesting that the caterpillars overwintered in high tunnel brassica crops. One of the major differences between this insect and the other brassica caterpillars is that the eggs are laid in a cluster, and caterpillars feed in a group on one plant so that it’s riddled with big holes like buckshot. CSCW is closely related to European corn borer, and the adults are similar in shape and color: straw-colored with a little purple, and crossed by wavy lines. Since the adults fly at night, you will likely only notice the caterpillars and their damage. The clusters of 3 to 25 eggs are yellow, flattened, and attached to the lower leaf surfaces. The caterpillars are light bluish-grey on top and green underneath, with numerous black transverse bands across their backs and a yellow line down each side. Larvae grow to ¾ inch long in 2 to 3 weeks. There are 2 to 3 generations per year, but generally, numbers do not reach damaging levels until late summer. Larvae can produce small holes in leaves until only veins remain, feed in terminal buds and sprouts, or burrow into heads. Plants with larvae are often completely skeletonized while adjacent plants may be left undamaged.
Field scouting for caterpillars
It is especially important to check cabbage or broccoli plantings as they begin forming heads. Leafy greens such as collards and kale should be scouted earlier, since all leaves are marketed. Randomly select 25 plants throughout the field and check for caterpillars or fresh feeding damage on the top or underside of leaves. Look for black or green frass and tiny clustered feeding holes. It is often easier to spot the frass and feeding damage first before finding the caterpillar. Classify plants as infested (one or more caterpillars present) or non-infested, and calculate the percentage of plants infested. In the Northeast, there is generally no need to treat young plants until at least 35% are infested with caterpillars. Once heading crops (cabbage, broccoli, cauliflower) begin to form the head, a lower threshold should be used to protect the marketed head; treat heading crops after head formation if 15-20% or more of the plants are infested. The most critical time to scout and apply controls is just prior to head formation. For leafy crops like kale and collards, a 10-15% threshold should be used. Because cross-striped cabbageworm can be so destructive, a lower threshold should be used—treat when 5% of plants are infested with this pest.
Chemical Control
Use selective insecticides to protect beneficial insects that keep aphids under control, eat insect eggs and small caterpillars, and parasitize imported cabbageworm or diamondback moth. Selective products are often most effective when consumed with foliage, so coverage is important. Use at least 50 gallons of spray material per acre; higher volumes provide better coverage. Better coverage of lower leaf surfaces can also be achieved by using drop nozzles. Use a spreader-sticker to prevent sprays from rolling off of waxy leaves. The most effective materials include:
- Diamides (Group 28) including chlorantraniliprole (e.g. Coragen), cyantraniliprole (e.g. Exirel), and cyclaniliprole (e.g. Harvanta)
- Spinosyns (Group 5) including spinetoram (e.g. Radiant) and spinosad (e.g. Entrust) - also effective against flea beetles and onion thrips
- Bacillus thuringiensis (Group 11) including Bt aizawi (e.g. XenTari) and Bt kurstaki (e.g. Dipel DF, many other products) – these materials are highly selective and will ONLY affect caterpillars.
These materials and the aizawai strain of Bt will usually provide better control of resistant diamondback moth than older products. See the cabbage insect control section of the New England Vegetable Management Guide for additional synthetic and naturally derived products and more details.
Cultural and Biological Controls
Incorporate crop residues shortly after harvest to decrease movement to successive plantings and reduce overwintering populations. Populations are suppressed by a wide range of natural enemies, including several species of parasitic wasps. Diamondback moth eggs are parasitized by the ichneumonid wasp, Diadegma insulare, which occurs naturally in eastern North America. D. insulare females require sources of nectar, so maintain wildflower stands near brassica fields. Imported cabbageworm eggs are parasitized by the braconid wasp, Cotesia rubecula, which was introduced to New England from China in 1988 and is now established in Massachusetts. You may see their small white cocoons on brassica leaves. The chalcid wasp, Trichogramma brassicae, will lay its eggs in many species of caterpillar, including all of the brassica pests above (as well as non-target caterpillars, so be cautious if you are maintaining wildflowers that might attract endangered moths or butterflies). T. brassicae wasps are not found in New England but can be purchased from several biological control companies for release in brassica fields. The wasps arrive as pre-parasitized caterpillar eggs that are glued to cards that can be distributed throughout the crop. As of 2022, each card costs around $20-23, and contains about 100,000 wasps, which is enough for up to 1 acre. According to one source of T. brassicae wasps, IPM Labs Inc., some growers will release the wasps in lieu of using any kind of pesticide. Some growers release one card per acre per week for about 4 weeks, while others will release every week for the life of the crop.
These biological controls are compatible with many selective and lower impact sprays used for control of caterpillars (Bt, oils, soaps), especially because the wasps are protected from sprays when they are inside of host eggs. Another source, Evergreen Growers Supply, notes that Trichogramma wasps are more effective against moth species that lay their eggs in clusters, so they may be a good option if cross-striped cabbageworm has been a particular problem on your farm.