Descriptions and Life History
There are now four economically important caterpillar pests that attack Brassica crops in New England: imported cabbageworm (ICW), diamondback moth (DBM), cabbage looper (CL) and cross-striped cabbageworm (CSCW). The imported cabbageworm (ICW) adult is the familiar white butterfly often seen fluttering around cole crops (cauliflower, broccoli, cabbage) and weeds during the spring and summer. The cabbage looper (CL) and the diamondback moths (DBM) are smaller, with dull coloration, and are less likely to be seen, as they primarily fly at night. Adult females of each species deposit 50 or more eggs, which hatch in 2 to 10 days. They develop into full-grown larvae in 2 to 4 weeks, go through metamorphosis in the pupae or chrysalis stage and finally emerge as the next generation’s adults.
ICW, DBM and CL lay their eggs singly, so you tend to find a single caterpillar, or just a few on one plant. However, the CSCW lays batches of eggs, so you may come across scattered plants that are completely skeletonized with 10 or 20 caterpillars on them. This is obviously an important difference when it comes to deciding when to take action (action threshold) against the pests.
From top left clockwise: DBM in a cocoon (Photo: Lyle Buss, University of Florida); ICW caterpillars (Photo: Adam Varenhorst); Cabbage looper adult and caterpillar (Photo: Adam Varenhorst); CSCW caterpillar and eggs (Photo: Arthur Raymond Cloyd, Kentucky State University).
The ICW survives New England winters as a chrysalis and is usually the most common and the first of the four species you may encounter each season. It goes through 3 to 5 generations between April and October. The DBM survives the winter as an adult moth under the residue of previous crops and has 4 to 6 generations each year. CL and CSCW moths arrive from the southern wintering sites on storm fronts. CL has up to three generations, and usually do not become numerous enough to exceed action thresholds in New England before mid- to late summer. CSCW has 2 to 3 generations per year and is most abundant on late-season plantings.
ICW larvae are slow-moving, velvety-green and eventually grow to a length of 1 1/4 inches. Larger ICW caterpillars have a thin, yellow line that runs lengthwise down the center of their backs.
DBM larvae only grow to 1/3 of an inch in length. Smaller DBM caterpillars have brown heads, which lighten as the larva matures, and eventually match the light-green body color. The body shape appears pointed on both ends. DBM larvae can be distinguished from other caterpillar pests by their habit of wiggling frantically when touched or prodded.
CL larvae are smooth, light-green caterpillars, with two white lines running the length of their backs and one narrower, white line on each side. They have no legs in the middle portion of their body and move with a distinctive inch-worm-like motion, arching the middle of their bodies upward before moving forward. CL larvae grow up to 1 1/2 inches in length.
Younger caterpillars of all three species prefer to feed on the underside of the foliage, but may migrate toward the plant head, as the larvae become larger and more mobile. These pests chew holes through the leaves of cole crops and may also stain or contaminate the harvestable portion of the plant with their feces or by their presence.
CSCW Larvae grow to 3/4 in. long in 2-3 weeks. CSCW caterpillars are light bluish-grey on top and green underneath, with numerous black bands across their backs and a yellow stripe down each side. CSCW caterpillars produce small holes in leaves until only veins remain, or target terminal buds and sprouts, or may burrow into heads. Plants with larvae present are often completely skeletonized, while adjacent plants may be left undamaged.
See ‘Identification of Cole Crop Pests’ for images
Natural, Biological and Cultural Controls
| Caterpillar Type | Natural Enemies | Cultural Controls | Others |
| Imported cabbageworm |
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| Cross-striped cabbageworm |
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| Cabbage looper |
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| Diamondback moth |
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The population of each of these cole crop pests is controlled somewhat by a complex of naturally occurring parasites and predators. The ICW is attacked by a granulosis virus, Paillotellavirus pieris, in cool, wet weather and the parasitic wasp, Apanteles glomeratus, is capable of killing up to 90% of the mature, summer generation larvae. Missouri researchers have successfully controlled ICW populations with releases of a combination of two exotic (foreign) parasitic wasps, Trichogramma evanescens and A. rebecula. Unfortunately, T. evanescens is not yet available commercially and does not survive New England winters. A. rebecula has been released at many sites across southern New England, where it often contributes to the control of this pest population, but has not yet spread between sites as expected. The ICW is easily controlled by applications of commercial formulations of the bacterium, Bacillus thuringiemis subspecies kurstaki (commonly known as BT), which is sold as Javelin, MYP, Biobit, Dipel, etc. BT products have the advantage of not harming the other beneficial insects mentioned.
The most important parasites of the DBM in the northeast are the two ichneumonid wasps, Diadegma insulare and Diadromus plutellae, which may kill up to 66% of the early season population. The parasitism rate may be enhanced with releases of the commercially available braconid wasp, Cotesia plutellae, or possibly in the future, with additional ichneumonid wasps which work well in other countries. Due to resistance problems, newer commercial BT products (e.g. Javelin) which use different combinations of the kurstaki endotoxins, or new subspecies such as aizawai (e.g. XenTari or Agree) may be more effective at controlling DBM larvae than older formulations.
In wet years, a nuclear polyhedrosis virus, Borrelinavirus sp., has been reported to provide up to 40% CL mortality in NY and complete control in Minnesota. Parasitic wasps contribute only marginally (< 7%) to the mortality of this pest in the northeast. The egg parasites T. pretiosum and T. evanescens help control the CL in the southwest but do not survive the winter in the northeast. T pretiosum purchased commercially tends to produce lower kill rates on CL than natural populations because insectaries rear the parasite on the eggs of other hosts, for economic reasons. A larval parasite, C. marginiventris, is also commercially available for CL control. Recommendations from Wisconsin call for releasing 200-800 individuals per week for CL control on cole crops. Most BT products work well on CL.
Heavy rains dislodge and drown many of the smaller caterpillars of all species and severe droughts have been associated with lower populations. In addition, predators make substantial contributions toward the natural control of these pests. These include: ground (i.e. Pterostichus melanarius), rove and ladybird beetles, spiders, lacewing larvae, flower fly (Syrphid) larvae, several true bugs (Nabis sp.), paper wasps (Polistes) and birds.
Floating row-covers can be used to exclude cabbage maggot flies, flea beetles, slugs and caterpillars from spring-planted cole crops. The crop should be covered immediately after transplanting operations are completed and the edges of the row covers should be sealed with soil to exclude migrating caterpillars. Use of row-covers should be combined with crop rotation and good weed (crucifer) control practices to keep cabbage maggot flies from emerging under the covers. Row covers are not recommended for summer plantings as excessive heat and humidity under the covers may result in reduced yields.
Scouting and Action Thresholds
For ICW, DBM and CL: Scout fields by checking leaves (top and bottom) on 25 plants across the field. 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 chemical controls is just prior to head formation. Use a 10-15% threshold throughout the season for kale, collards, mustard, and other leafy greens.
For CSCW: Spray if 5% of the plants are infested with CSC.
The abundance of butterflies does not necessarily reflect the number of eggs actually laid. Therefore, spray decisions should not be based on the number of ICW butterflies seen flying in the field.
Spray Strategies and Resistance Management
DBM and CL are considered highly resistant insects. In some countries where chemical use has been high, there is no insecticide left capable of controlling the DBM. Resistance management techniques should be followed on all farms, whether organic or traditional, to help preserve the useful life of important pest control tools. Techniques include alternating among families of insecticides, applying the materials correctly, and using alternative controls, scouting and thresholds to minimize spray applications.
The most effective insecticide rotation for managing all three cole crop caterpillar pests in New England is a BT product (e.g. Javelin) alternated with a broad-spectrum chemical like a synthetic pyrethroid (e.g. Pounce) or a natural pyrethrin (e.g. Pyrenone). If you already have highly resistant DBM on your farm you may need to alternate between newer chemicals such as thiodicarb (e.g. Larvin) and aizawai BT products (e.g. Agree or XenTari).
Use BT alone in the spring on the first couple generations of ICW. This pest is easy to control with BT, and delaying the application of nonspecific chemical insecticides will allow the natural parasites and predators of aphids, thrips, cabbage maggots, DBM and ICW to become more numerous. Begin alternating between BT and broad spectrum insecticides in late summer or fall if DBM or CL larvae become consistently more numerous than the ICW. Synthetic pyrethroids provide a quicker kill and a longer residual period of control than BT products. Therefore, it may be beneficial to switch to a pyrethroid if a spray is necessary within a week or so of harvest, especially late in the season when beneficial insect populations decline naturally.
Many of the modern insecticides are contact or stomach poisons with short residual periods of activity. Both BT products and chemical insecticides need to be applied to the bottom side of leaves where young caterpillars tend to feed. This can best be accomplished with electrostatic sprayers or booms equipped with both over-the-row and drop nozzles. High volumes of water (>40 gal. /acre) and the use of spreader-stickers and hollow cone nozzles improve coverage to the lower leaf surface.
The performance of BT products and pyrethroids is highly temperature dependent. BT works best above 80o F, and the pyrethroids are most effective below 75o F. Tank mixing these two types of insecticides is not recommended, as it may hasten resistance to both materials. If possible, apply insecticides during the evening to delay the breakdown of residues by high daytime temperatures and UV light.
- Use 50 gal or more spray material/A; higher volumes provide better coverage.
- Drop nozzles improves the spray coverage of lower leaf surface.
- Use a spreader-sticker.
- Use selective materials to spare beneficials that help control aphids and caterpillars. See New England Vegetable Management Guide for the most recent list of recommended sprays.
- 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, 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.
For further reading:
- McKeag, L., Hazzard, R., & Scheufele, S. (2022, June 1). Caterpillars in Brassica Crops. UMass Extension Vegetable Program. Retrieved February 5, 2024, from https://ag.umass.edu/vegetable/fact-sheets/caterpillars-in-brassica-crops
By Maggie Ng and Shuresh Ghimire, 2024
This information was developed for conditions in the Northeast. Use in other geographical areas may be inappropriate.
The information in this document is for educational purposes only. The recommendations contained are based on the best available knowledge at the time of publication. Any reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended. The Cooperative Extension System does not guarantee or warrant the standard of any product referenced or imply approval of the product to the exclusion of others which also may be available. The University of Connecticut, Cooperative Extension System, College of Agriculture, Health and Natural Resources is an equal opportunity program provider and employer.