The Cabbage Maggot—Cabbages Aren't Its Only Target

The cabbage maggot has been in North America since the 19th century.

It's a serious pest of cruciferous crops such as canola, mustard, cabbage, rutabaga, radishes, cauliflower and broccoli.

Delia radicum is known variously as the cabbage fly, cabbage root fly, root fly or turnip fly.

The larvae of the cabbage root fly are called cabbage maggots or root maggots.

The adult looks a lot like a common house fly, but is tiny and grey in color with a dark stripe along the back.

Adults may be found from May to October.

Crop Damage

Infected areas will be more severe following cool, wet spring conditions.

The adults are most abundant in canola during summer months.

Heavy maggot infestations in canola and mustard can halt blooming and yield losses. Also, maggot damage may allow the entry of root rot fungi.

The Polish canola varieties are more vulnerable to maggot feeding than Argentine types.

Vegetables

Infested vegetable crops will be pale green and stunted, and may wilt on hot, dry days.

Damage and yield losses can be heavy for root crops such as radish and turnip.

Cabbage Maggot Stages

Eggs

Each female may lay 50 to 200 eggs.

Eggs are laid near the base of host plants usually just beneath the soil surface and will hatch in 3 to 10 days.

Larvae

After 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 on the roots will last for 3 to 4 weeks, after which the pupae will develop either in the root itself, or in the soil.

Pupae

The pupae (cocoons) look like small wheat kernels.

Keep an Eye on Them

Use sticky traps or sweep nets to look for the adult fly in the spring.

When searching for maggot infestations late in the season, carefully pull up several host plants with the taproot intact. Observe for evidence of maggot damage on the root. Maggots may tunnel into the root and create channels along the outside. Check several plants in at least five or more places.

If maggots are present, wait several minutes for them to emerge and show themselves. Cut into the root to assess for maggot damage.

Often, infested roots are darker in color than healthy, uninfested roots.

Control

Cool temperatures and wet soil conditions at the time of egg-laying are the main factors influencing the number of eggs. Infestations are more severe following a cool, wet spring, and are generally lower in hot, dry years.

There are a number of natural biological control agents available, including a fungus that attacks adult flies, a parasitic wasp and a parasitic beetle that will attack the pupae. In addition, large numbers of cabbage maggot eggs are eaten by rove beetles.

Another pest management option is to control cruciferous weed species such as shepherd's purse, stinkweed, wild mustard and flixweed that could act as host plants. This will result in decreased over-wintering populations.

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