Manitoba Agriculture, Food and Rural Initiatives

Cabbage maggot (sometimes called canola root maggot) is common throughout the canola growing areas of the Prairies.

Host Crops

This insect can be a serious pest of canola and other cruciferous crops, including mustard, cabbage, rutabaga, radishes, cauliflower and broccoli. Polish canola is more susceptible than Argentine canola.

Biology

The adult stage is a fly that emerges in the spring or early summer from overwintering cocoons in the soil. It looks like a common house fly but is smaller and ash grey in colour with a dark stripe along the back of the abdomen. A second adult emergence may occur later in the summer as the first generation completes its life cycle. Thus, maggots can be found throughout most of the growing season.

Each female fly will lay 50 to 200 eggs. The eggs are small (1 mm or 1/20 inch), white and elongated. They are laid at or near the base of host plants, usually just beneath the soil surface. The larval stage is a small, 6 to 10 mm (¼ to ½ inch) long, white, legless maggot. When the eggs hatch, the maggots move down into the soil and begin feeding on small roots and root hairs. Eventually, they tunnel into the plant's taproot. Feeding will last for three to four weeks, after which the maggots pupate in the root or in the soil. The pupae are elongated and brown, resembling small wheat kernels. Pupation occurs in the top 5 to 20 cm (2 to 8 inches) of soil.

May	June	July	August	September
Adults begin to emerge from cocoons	Egg-laying begins, some larval feeding	Egg-laying continues, larval feeding, and pupation	New adult flies emerge, lay eggs, larval feeding	Maggots pupate in soil

Scouting Techniques

Cabbage maggot infestations are more severe after a cool, wet spring. Larvae feed on feeder roots and tunnel into the taproot, producing brown streaks on the root. The lower leaves of infested plants often turn yellow, with severe damage resulting in arrested plant growth. Feeding damage may also promote disease, causing further stress on the plant.

The adults are most abundant in canola during June and early July. Heavy infestations in canola and mustard can delay blooming and cause severe lodging and yield losses. Infested canola roots are often darker than normal roots.

In the spring, use sticky traps or sweep nets to look for the adult fly. When searching for maggots later in the season, carefully pull several host plants with the taproot intact. Use a hand lens to look for evidence of maggot feeding on the root (feeding scars). Maggots will tunnel into the root and create channels along the outside. Check several plants in at least five locations in the field.

If the maggot is present, you may need to wait several minutes for it to emerge and show itself or you can cut into the root to assess maggot damage. Later in the season, you may have to screen the soil around the root to see if there are any puparia present.

Economic Thresholds

No action thresholds are available for cabbage maggots.

Control Tips

A moderate increase in canola seeding rates may reduce damage and yield loss experienced from maggot feeding. Heavier canola plant densities result in smaller basal stems that are less attractive to egg laying females.

Tillage prior to seeding can reduce the level of emergence of adult flies by moving pupae nearer to the soil surface where they are more susceptible to attack by natural enemies. Fall tillage can expose the pupae to severe environmental conditions over the winter. However, fall tillage can leave soils susceptible to erosion and should be used cautiously.