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Beginner

Scientific Name
Plutella xylostella

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

• Adult DBM are small (1 cm, 2/5 in. in length), greyish-brown moths with distinctive cream coloured, diamond shaped markings on their back 
• Normally fly quickly from plant to plant mostly at dawn and dusk
• Eggs are very small, round, and yellowish and deposited on both sides of host leaves
• Larvae are small, pale green caterpillars with tapered ends; the rear legs extend in a “V” from the body
• DBM larvae chew distinctive holes in the leaves called “windows”; they tend to chew through the leaf layers only as far as the opposite side of the leaf
• Pupa are enclosed in a gauze-like cocoon

Often Confused With
Imported cabbageworm
Cabbage looper

Period of Activity
The DBM has traditionally not overwintered in southern Ontario.  The majority of adult moths arrive from the south during May and June, often prior to the planting of cruciferous crops. However, there is evidence that in some cases, and during mild winters, some may survive here in Ontario. Depending upon temperatures, there may be 2- 4 generations per year in Ontario, with the average lifecycle being 28 days.

Scouting Notes
To monitor for DBM, 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. “Window-pane” holes in the leaves are a good sign of DBM presence. There are two ways to quantify DBM infestations, using Cabbage Looper Equivalents (CLE) or Percentage Infestation.

Thresholds

	CLE THRESHOLDS	% 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
Plutella xylostella

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

Identification

The adult DBM is a small brown moth about 1.5 cm (3/5 in.) in length. At rest, the wings are folded over the abdomen and three "diamond" shaped spots become visible along the centre back. Adult females lay minute, oval, yellowish to pale green eggs singly or in groups of two or three, on the underside of leaves. DBM larvae are small compared to ICW and CL. They reach only 1 cm (2/5 in.) at maturity. The green larvae are similar to small imported cabbageworm larvae but can be distinguished by two features:

• DBM larvae have two prolegs on the last segment (the rear) that are spread apart in a "V". The ICW also has two prolegs on this segment but they are tucked under the abdomen and give the rear end a rounded appearance. 
• The other distinguishing feature of DBM is their reaction to being touched. If they are poked with a pencil or brush, they wriggle frantically and will rapidly attach a silken thread to the leaf and dangle over the edge. ICW, on the other hand, are sluggish and when touched may or may not respond by slowly moving away from the object touching them.

Often Confused With
Imported cabbageworm
Cabbage looper

Biology
The majority of adult moths lay eggs on a wide range of cruciferous plants in the late spring and early summer. A female lays an average of 160 eggs during a lifespan of 2 weeks, though fecundity depends on the protein content of the host plant. After 5– 10 days very small larvae emerge and begin feeding on the upper and lower leaf surfaces. Depending upon temperatures larvae mature and pupate in 2– 3 weeks. There may be 2– 4 generations per year in Ontario. Once the moths become established in Ontario, there can be 4 or 5 generations in addition to the influx of moths migrating into the province on air currents. In very warm weather they can complete a life cycle in as little as 16 days, 28 days being the average.

Period of Activity
The DBM has traditionally not overwintered in southern Ontario.  The majority of adult moths arrive from the south during May and June, often prior to the planting of cruciferous crops. However, there is evidence that in some cases, and during mild winters, some may survive here in Ontario, especially around Lake Ontario and Lake Erie. Generations of DBM continue throughout the growing season until late September, or harvest.

Scouting Notes
To monitor for DBM, begin in the early summer using the 5 x 5 method. Count DBM larvae on at least 25 randomly selected plants throughout the field by scanning both sides of leaves and pulling back loose wrapper leaves. DBM larvae chew distinctive holes in the leaves called “windows”. They tend to chew through the inner-leaf layers, leaving the waxy epidermis, creating a “window-pane” hole. Fresh feeding damage and frass are good signs that large larvae are present.

Insecticides are less effective against large larvae. Begin application of insecticides when young larvae are found and continue on a 5- 10 day schedule, or as necessary for adequate protection of the crop. Scouting is an excellent way to assess the population of brassica-crop caterpillars.

There are 2 ways to quantify DBM infestations, using Cabbage Looper Equivalents (CLE) or Percentage Infestation.

Thresholds

	CLE THRESHOLDS	% 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

• Small caterpillars are always easier to control. 
• 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.
• The most common parasite of DBM in Ontario is the tiny wasp Diadegma insularis. This wasp lays its eggs in DBM larvae.  The eggs hatch and the wasp larva develops inside the caterpillar. When the time comes for the caterpillar to pupate, the wasp pupa develops and soon after an adult wasp emerges and the cycle begins again. Research has shown that fields where IPM is practised can have more than 50% parasite rates. Another parasite of the DBM is a tachinid fly; however, rate of parasitism by this fly is small, about 5%. 
• DBM control is a concern world-wide due to the ability of this pest to develop pesticide resistance.