Use Predatory Nematodes on Cabbage Maggots
Use juvenile stage parasitic nematodes at transplanting time. Beneficial nematodes are tiny soil-dwelling worms, available commercially, that find their way to the maggots, burrow inside them and reproduce. The maggots are killed by bacteria released by the nematodes. See Predatory Nematode Products in Yardener’s Helper.
Barriers For Cabbage Maggots
Plastic Row Covers - One of the most important uses of row covers (either plastic tunnels or polypropylene fleece) is for insect protection. The floating covers are very effective for keeping the fly that lays the eggs producing the cabbage maggot, away from vulnerable plants. The key is to be sure that the edges of the row cover are held down by soil or some device to prevent the insect from sneaking in under the cover.
Netting, Cheesecloth, Screening, Or Fleece - You can use a material for barriers that allows the sun, air, and rain to get through, but prevents the cabbage maggot fly from getting access to your plants. Nylon netting, fine screening, or fleece covering plants are all effective barriers, preventing the fly from laying eggs at the plant's stem. The fly can't get through the material, so it can't deposit eggs either on or at the base of the plants. No eggs means no cabbage maggot. Cover transplants or a newly seeded area with the preferred material immediately. Lay the barrier material directly on the plants, seal all the edges to the ground, and leave it on until the flies have ceased to lay eggs. Provide lots of extra material so when the plants grow larger they don't strain against the covering. See Fleece Products in Yardener’s Helper.
Diatomaceous Earth (D.E.) As A Barrier - Dust diatomaceous earth around the base of each plant to create a barrier for the cabbage maggot fly. See Yardener’s Tool Shed for Diatomaceous Earth Products
Alkaline Barrier - Cabbage maggots don't thrive in a strongly alkaline environment, so use wood ashes or powdered limestone to create that environment. If you notice maggot damage on plants, take the following steps:
Prior to attack, protect vulnerable seedlings with a mixture of 4 parts wood ash to 1 part each powdered limestone and rock phosphate. Lightly scratch « to 1 cup of this mixture into the soil in a 6-inch radius around each seedling, lightly working the mixture into the soil to avoid crusting. Or, mix a heaping teaspoon of wood ashes into soil about 1 inch deep around stems of vulnerable seedlings and plants. Firm soil around each plant and water.
After attack by cabbage maggots carefully remove the soil from around the roots of affected seedlings and mix ashes with it before placing it back around the plants. This technique will kill the maggots in about a week.
Soil drench: place 2 pounds of lime in a 5 gallon can, fill with water and let stand for 24 hours. Pour off the clear water into another container and in the afternoon, water with this clear water, about one cupful at the base of each plant. Do not soak the entire garden, or the earthworms will rebel. The plants under root maggot attack should recover
Collars Or Mats - Some form of matting can be placed around the base of vulnerable transplants to prevent flies from laying eggs that will become cabbage maggots. Options include:
Foam-rubber (carpet underlay) discs 5 to 10 inches in diameter, with small hole punched in middle, and a slit along the radius. They prevent the adult flies from laying eggs near the plant stems, and they provide places for aggregation of predatory ground beetles that eat maggot eggs and larvae that do beat the system. These barriers also act as a moisture-conserving mulch to promote tolerance of maggot damage by plants.
Tarpaper squares, 5 or 6 inches on a side, with a slit cut halfway into each. Place them around the base of each transplanted seedling. Press the slits flat to the soil.
Wax paper cups, inverted, work even better than tar paper. Cut a hole in the bottom, then cut a radial slit and fit the cup upside down into the soil and around the plant stem. Make sure the rim of the cup is set into the soil.
Intercropping Or Companions
Mint, tomato, rosemary, and sage have been reported to repel the cabbage maggot, but research has not confirmed the reports. In one study a cover crop of clover reduced cabbage root flies and increased yields. Companion planting for pest control is easy in the case of brassicas. Simply mix brassicas with any plants that belong to a different family. Such interplanting has reduced infestations of aphids and cabbage root fly by 60% compared to pure stands of the vegetables. Twice as many predators of cabbage pests were counted in the mixed stands. Egg laying by the root fly was reduced simply by virtue of the mixed planting. Interplanting was most effective when the brassicas were planted in single rows and when the intercrop provided at least 50% ground cover between rows.