Cole Crop “Worms” (Caterpillar Pests)

Descriptions and Life History

The imported cabbageworm (ICW) adult is the familiar white butterfly often seen fluttering around cole crops 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 two to 10 days. They develop into full-grown larvae in two to four weeks, go through metamorphosis in the pupae or chrysalis stage and finally emerge as the next generation’s adults.

The ICW survives New England winters as a chrysalis and is usually the most common and the first of the three species you may encounter each season. It goes through three to five generations between April and October. The DBM survives the winter as an adult moth under the residue of previous crops and has four to six generations each year. CL moths arrive from the southern wintering sites on storm fronts, have up to three generations, and usually, do not become numerous enough to exceed action thresholds in New England before mid- to late summer.

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.

See ‘Identification of Cole Crop Pests’ for images

Natural, Biological and Cultural Controls

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 three 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

If provided with favorable environmental conditions for sustained growth, cole crops can withstand substantial defoliation as long as the harvestable portion of the plant is not damaged.

Research has shown that broccoli can tolerate the feeding of nine full-sized caterpillar larvae per plant before heading without an adverse effect on yield or quality. To maintain quality for most markets, however, high caterpillar populations must be controlled when the harvestable portion of the plant is present. Profits can be maximized and insecticide use minimized if weekly caterpillar scouting is conducted from the initiation of heading until harvest and thresholds are used to determine if a spray is necessary. Droughts, without irrigation, may stop plant growth and necessitate scouting and caterpillar control prior to heading.

Scout 25 plants per field in groups of three to four, being sure to check both the bottom and top of leaves for larvae. If a caterpillar is found, note the species and immediately move on to another plant. It is not necessary to find all the larvae on the plant, just determine whether the plant is infested. After scouting 25 plants, multiply the number of infested plants by four to determine the number of infested plants in the field. Choose plants at random; do not look for plants with feeding injury to scout. Also, do not count eggs, pupae or adults. Those insect stages suffer high mortality, do not feed on the crop, may not be killed by insecticides and will not accurately reflect future crop damage.
The most common mistake growers make is basing spray decisions on the relative number of ICW butterflies flying. The abundance of butterflies does not take into account the number of eggs actually laid, egg predation, or the presence of the other two pest species between butterfly flights. Nine years of cole crop scouting experience in Connecticut has demonstrated to me that the relative abundance of butterflies does not accurately reflect future ICW larval infestations, crop damage, or help time insecticide sprays with the vulnerable caterpillar stage.

  • Treat cabbage and broccoli with insecticides after heading only if 20% or more of the scouted plants are infested with caterpillars of any species.
  • Treat cauliflower after heading and before tying if 10% or more of the scouted plants are infested.
  • Treat collards, mustards and kale any time during plant development if 10% or more of the scouted plants are infested.

Note. These thresholds do not imply that insects will damage 10 or 20% of your crop. Research conducted at the University of Connecticut over a three year period resulted in 98 to 100% undamaged cabbage heads at harvest using the 20% threshold on spring and fall crops. Use of this 20% threshold provided the same quality but with far fewer insecticide applications, as cabbage sprayed weekly whether or not insects were present. Cauliflower and non-heading cole crops require a slightly lower (10%) threshold to maintain quality at harvest. These thresholds have been used successfully for nine years on Connecticut farms.

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.

For further reading see:

  • Boucher, T. J. and R. G. Adams.1993. Integrated Pest Management Guide for Connecticut Cole Crops. UConn Coop. Ext. Sys. Pub. 93-19. 21 pp.
  • Mahr, S.E.R, D.L. Mahr and J.A. Wyman. Biological Control of Insect Pests of Cabbage and Other Crucifers. Coop. Ext. North Central Regional Pub. 471. 54 pp.
  • Petzoldt, C. H. (Edt). A Grower’s Guide to Cabbage Pest Management in New York. Cornell Coop. Ext. Pub. 101a. 50 pp.

Written By: T. Jude Boucher, Vegetable Crops IPM Program Coordinator,
University of Connecticut Cooperative Extension System, 24 Hyde Ave., Vernon, CT 06066

Originally Published: Proceedings. 1995 New England Vegetable and Berry Conference and Trade Show. December 12-14, 1995, Sturbridge Host Hotel, Sturbridge, MA. Pp. 96-99

Reviewed by: Jude Boucher, UConn IPM, 2012

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.