Cabbage Moths on Brassica Crops

Identification

There are three different types of cabbage moths which can affect brassica crops. All three types feed on brassicas when they are in their life stage as green caterpillars and eat holes in brassica leaves but not leave veins and leave behind lots of poop (see photos below in damage section). As well all three types can be controlled using BT so it’s not absolutely critical to precisely identify which type of caterpillar you’re seeing.

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.
PHOTOS-by-GINAPP

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.
  • Adults lay eggs 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 long.

Cabbage looper caterpillars have no legs in their middle sections and make 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 grow up to 1/3 inch long, much smaller than imported cabbageworms and cabbage loopers.
  • They wiggle vigorously when touched.

Commonly Affected Crops

  • Any brassicas but especially…
  • Cabbage (green, red and napa)
  • Kale
  • Broccoli and Cauliflower

Damage

  • Cole crops can tolerate some feeding damage.
  • Young seedlings and transplants are most susceptible to injury.
  • Severe defoliation of young seedlings and transplants can cause distorted growth or death.
  • Extensive feeding can also prevent 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.

The caterpillars of all three species feed between the large veins and midribs of cole crops.

https://extension.umn.edu/yard-and-garden-insects/caterpillars-cole-crops#using-insecticides-3202063

Management Techniques

We scout for moths and caterpillars and spray BT when certain thresholds are reached. Please check in with David if you are seeing caterpillars, moths or evidence of their feeding damage. Some feeding damage is inevitable but it can quickly increase so early control is essential.