Manitoba Agriculture, Food and Rural Initiatives
Diamondback Moth I
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The diamondback moth is an occasional pest of canola
across the Prairies. Most damage is caused by feeding on the surface of filling and
maturing pods.
Host Crops
On the Prairies, diamondback moths are most frequently observed in canola fields. Other favoured food plants include broccoli, Brussels sprouts, cabbage, cauliflower, horseradish, commercial mustard, radish and turnip. They also feed on stinkweed, shepherd's purse and wild mustard.
Biology
The moths are small, 12 mm (½ inch) long, with an 18mm to 20mm (¾ inch) wing-span. They are grey or brown, with light tan marks on the margin of the forewing. While at rest, the wings are folded, roof-like, over the body. They hold their antennae forward when at rest.
This pest does not overwinter in the Prairies. It moves in from the U.S. on strong south winds on an unpredictable basis. A female may lay 100 to 300 scale-like, greenish white to creamy yellow eggs. The eggs are laid singly or in small groups. The mature larva is a small, pale green caterpillar, about 8 mm (5/16 inch) long. It has a narrow head and hindquarters (ie, somewhat spindle-shaped). It has five pairs of legs on the rear half of the body. The tail end of the larva has a forked appearance.
Larvae of the diamondback moth feed on the foliage from the seedling stage to harvest. They can greatly reduce the quality and yield of the crop. Older larvae feed on flowers, young pods, and the surface tissue of stems and mature pods, usually from mid-July to early August.
The larvae pupate in a loose, silken cocoon on the plant, with the new adult moths emerging a week or so later.
June July August Moths blown in and egg-laying begins More moths blown in and lay eggs, larval feeding New moths emerge, lay eggs, larval feeding
Scouting Techniques
When monitoring for diamondback moth larvae on canola, gently pull out a canola plant and count all the larvae present. Count larvae by taking whole plants and knocking on the hood of a car or on a light coloured jacket. The leaves of infested plants will be shot-holed.
Pheromone traps can be used to monitor moth activity. Although these traps cannot predict the size of an outbreak, the trap counts can provide an early warning of an infestation.
Economic Thresholds
It takes a severe infestation of small larvae to cause appreciable damage. Damage is most noticeable during dry summers. The action threshold for diamondback moths in podded canola is 200 to 300 larvae per square metre (sq. yd.). This is roughly equivalent to 20 to 30 larvae per 0.1 square meter (sq. ft.). There have been reports of up to 415 larvae per plant during severe infestations. If pod damage begins to show when the larvae are still small and numerous, control measures are definitely warranted.
Note that, if the moths are blown in from the south early or if the canola is sown late, the larvae may feed on the developing buds of canola. If this happens, the economic thresholds would be much lower than when larvae feed on the pods.
Control Tips
Check with provincial agricultural representative for timing of applications.
For further information, contact your GO representative.
