From the University of Connecticut

Under favorable growing conditions, cole crops can withstand substantial defoliation from cabbage “worms” as long as the harvestable portion of the plant is not damaged. To maintain quality for most markets, however, high caterpillar populations must be controlled once the harvestable portion of the plant is present.

To assess populations, examine 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, 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 percent 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. A common mistake is to base spray decisions on the number of Imported Cabbage Worm (ICW) butterflies flying about. The relative abundance of butterflies does not take into account the number of eggs actually laid, egg predation, or the presence of cabbage looper and diamondback caterpillars, so it does not accurately reflect future ICW larval infestations, crop damage, or help time insecticide sprays.

Suggested thresholds: 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. 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 when using the 20% threshold on spring and fall crops. Use of this 20% threshold provided the same quality 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.