Description
The cabbage root maggot (Delia radicum) is an early season pest of cruciferous crops. Adults are delicate, hump-backed flies, dark gray in color, with black stripes on the thorax. They are half the size of a house fly. Females lay their eggs near Cole crops, just below the soil surface. The small, white eggs are elongated with tapered ends. Cabbage maggot larvae are 1/3 inch long, white and legless. Their head is pointed and the rear end blunt. They feed on roots for approximately 3-4 weeks, creating extensive tunnels throughout the roots. This can kill or stunt plants and create openings for pathogens and resulting infections.
The cabbage maggot closely resembles the onion maggot fly (Delia antiqua) the seed corn maggot fly (Delia platera), the bean seed maggot (Delia florilegia) and the turnip root fly (Delia floralis). Accurate identification is difficult and requires a good key. The damage caused by the larvae is the same between species.
Target Crops
Cabbage root maggots feed on the roots of all cruciferous crops, but prefer cauliflower, brusell sprouts, turnip and radish. Small transplants and seedlings are the most susceptible and damage is worse during a cool, wet spring.
Life Cycle
In the early spring cabbage maggot flies emerge from the soil and the females lay their eggs close to host crops in the top few inches of soil. They are attracted to freshly tilled fields with decaying organic matter. Cool, moist conditions are best for egg survival and soil temperatures higher than 95°F will kill the eggs. The larvae hatch out and feed on roots of host plants for 3-4 weeks. They pupate in the soil for 2-4 weeks and emerge as adults. Cabbage maggots overwinter as pupae in the upper few inches of soil. There can be 2-4 generations per year depending on location.
Beneficial Insect Control
Natural enemies like ground beetles feed on cabbage maggot eggs, larvae and pupae and can cause high levels of mortality. Parasitic wasps and predatory mites can also aid in control. These are not sold commercially, but judicious use of pesticides and planting insectary crops can invite these natural enemies to help with pest control.
Steinernema feltiae—entomopathogenic nematodes can be applied to the soil to infect and kill cabbage maggot larvae at soil temperatures down to 50°F. Apply as a drench to transplants prior to setting in field and/or after transplanting. Nematodes need a moist soil environment to survive and should be applied at the time cabbage maggot eggs are hatching.
Introduction rates and release information
For nematodes on sponges, apply 1 million nematodes per 1000 sq feet of soil. For each square foot you will need .66 gallons of water as a carrier to saturate to a depth of 2”. Unless using your nematodes immediately, store in a refrigerator. Do not freeze! Read our Nematode Tech Sheet for specific instructions.
Insecticide Options
Several broad spectrum biopesticides applied as a soil drench can be effective. LalGuard M52 is composed of the spores of the fungus Metarhizium brunneum strain F52 and PFR-97 contains the fungus Isaria fumorosa. Azaguard, Molt-X and Azatin O contain the active ingredient Azadirachtin and act as insect growth regulators (IGR's). All can be utilized up to the day of harvest.
Cultural Control Tips
- All management strategies are preventative since reactive methods like foliar sprays are generally ineffective.
- Rotate crops to keep maggot populations low and distance them as far apart as possible.
- Cover recently seeded or transplanted crops with floating row covers as a barrier against cabbage root maggot flies. Do not use where brassicas were grown the previous year. As soil temperatures rise and first flight ends, crops grow larger and covers can be safely removed.
- Avoid manure applications before transplanting as this will attract adult flies.
- Since the first generation of cabbage maggots are the most damaging, planting seeds or transplants after the peak of egg hatching in the spring, or after the first flight, can provide some control. Plantings established after mid-June generally suffer less damage than earlier plantings.
- Till under fall crop residue immediately after harvest to expose and help reduce populations of overwintering pupae. Allow crop residue to dry and decompose completely.
- Diatomaceous earth may help reduce pest populations.
- Avoid hardening transplants near infested fields.
- Provide habitat and conservation techniques to preserve natural enemies.
Cabbage Root Maggot, UMass Amherst
Cabbage Maggot, Wisconsin Horticulture
Cabbage Maggot, Oregon State University VegNet