Caterpillars on cole crops
- Cole crops include cabbage, cauliflower, broccoli, kale, rutabaga, radish, turnip and collard.
- The most common caterpillar pests of cole crops are imported cabbageworm, cabbage looper and diamondback moth.
- Imported cabbageworms are the most common and cause the most damage.
- The caterpillars of all three species feed between the large veins and midribs of cole crops.
- The older, larger caterpillars of all three species cause the most feeding damage.
- Treat caterpillars when they are still small and before they cause too much feeding damage.
How to identify them
Imported cabbageworm (Pieris rapae):
Adult butterflies are commonly seen flying around plants during the day.
- Adults are white butterflies with black spots on the forewings.
- Eggs are yellow and oblong, and are on both upper and lower sides of leaves.
- Caterpillars can grow up to 1 inch in length and are velvety green with faint yellow stripes running lengthwise down the back and sides.
- They move sluggishly when prodded.
Cabbage looper (Trichoplusia ni):
Adults are nocturnal moths with a 1½ inch wing span.
- Adult moths have mottled grayish brown wings.
- A small silvery white figure 8 is in the middle of each of the front wings.
- Eggs are creamy white, aspirin-shaped and about the size of a pin head.
- Eggs are laid on the undersides of the lower leaves.
- Caterpillars are pale green with narrow white lines running down each side.
- Full grown caterpillars are about 1½ inches in length.
Cabbage looper caterpillars have no legs in their middle sections and have a characteristic looping motion as they move across vegetation.
Diamondback moths (Plutella xylostella):
Adult moths are nocturnal flyers.
- Moths are light brown and slender.
- The folded wings show a pattern of three white diamonds.
- Eggs are laid near leaf veins on the leaf, and are creamy white and tiny.
- Caterpillars are light green, tapered at both ends and can grow up to 1/3 inch long.
- They wiggle vigorously when touched.
Diamondback caterpillars are much smaller than both imported cabbageworms and cabbage loopers.
Life cycle of caterpillars
All three species have similar life cycles.
- Eggs hatch into caterpillars and then damage plants.
- After feeding for weeks on cole crops, the larvae change into pupae in protected areas on the plants.
- Then they emerge as adults.
In the upper Midwest, they live through the winter in green pupal cases.
- Adults begin to appear in gardens in mid-May.
- They are a problem through the rest of the growing season.
- 3 to 5 overlapping generations a year.
They do not survive in the winter in the upper Midwest.
- Migrate from the south into Minnesota from early July to late August, but this time can vary from year to year.
- 1 to 3 generations a year during the growing season depending on their arrival time and late summer temperatures.
In the upper Midwest, they can live through the winter as adults in protected locations.
- Moths begin to appear in mid-May.
- Can be pests through the remainder of the growing season, but are usually less severe after spring.
- Generally 3 to 5 generations a year.
Damage caused by them
Imported cabbageworm and cabbage looper feeding
- Young caterpillars produce small holes in leaves that do not reach the upper leaf surface
- Larger caterpillars chew large, ragged holes in the leaves leaving the large veins intact
In cabbage, broccoli or cauliflower, larger caterpillars crawl toward the center and leave large amounts of frass (fecal matter).
Diamondback caterpillar feeding
- Start to feed inside the leaves, then move to the outside of the leaves.
- Eat all the leaf tissue except the upper layer, giving a windowpane look.
- Cole crops can tolerate some feeding damage.
Severe defoliation of young seedlings and transplants can cause distorted growth or even death and affect the head formation of cabbage, cauliflower, and broccoli.
Older plants can tolerate some defoliation, with little effect on yield. Do not allow defoliation to exceed 30 percent of leaves.
How to protect your garden from caterpillars
Check for caterpillars and their feeding damage on both sides of leaves of on cole crops. Check at least once a week right after planting and more often as the season progresses.
Make gardens unwelcoming to pests
- Destroy crop residue so that cabbageworms cannot find protection in winter.
- Remove weeds from the Brassicaceae family like wild mustard, peppergrass and shepherd's purse, as they are alternate hosts for these pests.
Use a physical barrier
Handpick and drop the caterpillars into a pail of soapy water to kill them.
Floating row covers made up of lightweight all-purpose garden fabric keep the adult moths from laying eggs on plants.
- Fit the row covers directly over garden plants or over metal hoops/wooden frame to cover the cole crops at seeding or transplanting.
- Remove row covers after harvesting the cole crop.
Natural enemies can control damage
Predators such as, paper wasps, and parasitic flies and wasps (e.g., the parasitic wasp, Cotesia glomerata) are natural enemies of cabbage looper, imported cabbageworm and diamondback moth.
- These small wasps and flies do not sting or bite people and occur naturally in gardens.
- They develop within the caterpillar, pupae or eggs, and eventually kill their hosts.
Pesticides are not very effective in killing older caterpillars.
Choose a low impact pesticide, like pyrethin, that is less toxic, and "easy" on natural enemies (i.e., paper wasps, parasitic flies and wasps) and pollinators such as bees and flies.
- Neem is a plant based pesticide and acts as an anti-feedant. Insects are not killed, but it causes them to stop feeding and they eventually die.
- Spinosad is derived from a naturally occurring soil-dwelling microorganism, provides excellent control, and does not harm natural enemies.
- Bacillius thuringiensis (Bt), a bacterium that occurs naturally in the soil is specific to caterpillars. Spray the product with good coverage, as it should be consumed by the caterpillars to be effective.
Conventional, or broad-spectrum pesticides, are longer lasting but they can kill natural enemies. Common examples of broad spectrum pesticides include permethrin, carbaryl, bifenthrin and lambda-cyhalothrin.
CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.
Be sure that the vegetable you wish to treat is listed on the label of the pesticide you intend to use. Also be sure to observe the number of days between pesticide application and when you can harvest your crop.
Reviewed in 2018