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Beginner

Scientific Name
Pieris rapae (L.)

There are 3 major caterpillar pests of crucifer crops in Ontario. The imported cabbageworm (ICW), the cabbage looper (CL), and the diamondback moth (DBM).

Identification

• The adult is an off-white butterfly with black spots on its wings 
  It is often seen flying during the day in and around brassica fields
• Eggs can be found on the underside of leaves; the small, bullet shaped eggs are white when first laid but gradually turn yellow as they mature
• Larvae are velvet-green with a slender yellow stripe down the middle of the back and a broken yellow stripe along the sides
• Pupae are tapered at one end and light green to light brown
• aterpillars feed on the foliage of Brassica; they chew large ragged holes in the leaves and, if not controlled, can seriously interfere with photosynthesis

Often Confused With
Cabbage looper
Diamondback moth

Period of Activity
Adults emerge mid- to late May. The butterflies are most active on warm, sunny days when wind speeds are low between 10 am and 1 pm. ICW can continue to plague crucifers until the first hard frost in the fall.

Scouting Notes
To monitor for ICW, use the 5 x 5 method. Make sure that one stop is along the border, but no more than one. Remember to look on the undersides of leaves for eggs and caterpillars. Fresh feeding damage and frass are good signs that large larvae are present. There are two ways to quantify ICW infestations, using Cabbage Looper Equivalents (CLE) or Percentage Infestation.

Thresholds

	CLE THRESHOLD	% INFESTATION THRESHOLDS
CABBAGE	0.3	20-30% before head fill 10-15% after head fill
CAULIFLOWER/ BROCCOLI	0.2-0.3	20-30% before heading 5-10% after heading

Advanced

Scientific Name
Pieris rapae (L.)

There are 3 major caterpillar pests of crucifer crops in Ontario. The imported cabbageworm (ICW), Pieris rapae (L.), the cabbage looper (CL), Trichoplusia ni (Hubner) and the diamondback moth (DBM), Plutella xylostella (L.). Only the cabbageworm overwinters in Ontario whereas the cabbage looper and diamondback moth migrate into Ontario during the spring.

Identification
The adult ICW is an off-white butterfly with 2 or 3 black spots on its wings and with a wingspan of about 4.4 cm (1.7 in.). Small, bullet shaped eggs are white when first laid but gradually turn yellow as they mature. Larvae are sluggish, velvet-green caterpillars which can grow quickly depending upon temperatures. These caterpillars also have a slender yellow stripe down the middle of the back and a broken yellow stripe along the sides. The ICW larvae also have 2 prolegs on the last segment that are tucked under the abdomen and give the rear end a rounded appearance. This is a distinguishing feature between the ICW and the extended V-shaped prolegs of the DBM. The well camouflaged ICW larvae are about 3 cm (1 1/5 in.) long at maturity, at which point they enter the pupal stage. The pupa, a green chrysalid, which is tapered at one end and light green to light brown is attached to lower leaves, buildings or fence posts by a silken pad.

ICW larvae generally feed on the underside of leaves, while more mature larvae chew large, irregular holes throughout the plant. Dark green pellets of “frass” about 3 mm (1/8 in.) in length can be found near feeding holes. 

Often Confused With
Cabbage looper
Diamondback moth

Biology
The cabbageworm overwinters in Ontario. Each female ICW is capable of laying between 200 and 300 eggs. These eggs are usually found on the bottom surface of the leaves near the leaf margins as the female positions herself on the upper surface and curves her abdomen under the leaf to lay the egg. The number of eggs laid depends on the weather.  Many eggs are laid on warm sunny days and few, if any, are laid on cool, cloudy days. Between the egg and pupal stage there is considerable mortality. It has been estimated that only 10- 20% of the oviposited eggs actually reach adulthood. Given the large number of eggs oviposited by each female, a significant population does survive. Depending on temperatures, eggs hatch in 3– 7 days and the larvae begin feeding. The larvae grow quickly to maturity in about 2 weeks. Once mature, larvae enter the pupal stage which lasts about 2 weeks and the new adults emerge. There are 3 generations of cabbageworm in Ontario. 

Period of Activity
Adults emerge mid-to-late May and can often be seen flying during the day around brassica crop fields. The butterflies are most active on warm, sunny days when wind speeds are low between 10 am and 1 pm. ICW can continue to plague crucifers until the first hard frost in the fall. 

Scouting Notes
To monitor, count ICW larvae on at least 25 randomly selected plants throughout the field by scanning the under-sides of leaves and pulling back loose wrapper leaves. Make sure one stop is along the border, but no more than one. Fresh feeding damage and frass are good signs that large larvae are present. Choose 5 sites in each block, examining 5 plants at each site. There are two ways to quantify ICW infestations, using Cabbage Looper Equivalents (CLE) or Percentage Infestation.   

Thresholds

	CLE THRESHOLD	% INFESTATION THRESHOLDS
CABBAGE	0.3	20-30% before head fill 10-15% after head fill
CAULIFLOWER/ BROCCOLI	0.2-0.3	20-30% before heading 5-10% after heading

Management Notes

• The ICW can cause serious leaf feeding damage to cabbage, cauliflower, broccoli, Brussel’s sprouts and other cruciferous plants. Large amounts of dark frass can stain cauliflower heads and make cabbage and broccoli unmarketable. Cabbage for processing can tolerate more feeding damage because many of the outer leaves are removed.
• Small caterpillars are always easier to control. Monitor early!
• Caterpillars feeding directly on the heads or growing points are a more serious threat than those feeding on the outer leaves. 
• When plants are small, damage has a more profound effect on
  yield and head size. With that said, caterpillar control is more   
  critical just before harvest because of feeding damage and the
  presence of frass.
• There are several natural predators and parasites of ICW in Ontario. Syrphid fly larvae (Syrphus spp). are frequent predators of eggs and larvae. A small wasp, Cotesia glomerata (= Apanteles glomeratus), parasitizes the larvae and pupae.  C. glomerata kills full-grown larva and its yellowish, cigar-shaped larvae can be seen adjacent to shrivelled caterpillars.  The parasitic fly, Phryxe vulgaris, lays its eggs on small ICW caterpillars. Once hatched, the fly maggots develop inside infected ICW caterpillars. After the caterpillar pupates the mature fly maggot emerges, killing the pupating ICW. Late in the season, it is not unusual to see bald-faced hornets carry off large larvae to their nests. 
• The larvae are also plagued by a virus disease referred to as "wilt". Caterpillars with this disease become pale and sluggish and eventually die as blackened carcasses hanging from leaves.