If you’ve noticed your brassica plants, like cabbage and cauliflower, suddenly struggling—perhaps showing yellowing leaves, wilting on sunny days, or early bolting—there’s a good chance your crops are under attack by cabbage root maggots (Delia radicum). These pests target the roots of brassica plants, including cabbage, cauliflower, turnips, and radishes, causing significant damage.
The Cabbage Root Maggot Life Cycle
Cabbage root flies, resembling houseflies, lay small white eggs on the soil near the base of brassica plants. There are typically two generations of these flies per year—one in late spring to early summer, and another in late summer or early autumn. Sometimes, a third generation may appear in the fall.
After about seven days, the eggs hatch into small, legless maggots resembling grains of rice. These maggots burrow into the soil and begin feeding on the roots of your plants, causing damage that can lead to stunted growth or even plant death. Root crops like radishes and rutabagas can also be affected, showing tunnel-like damage similar to that caused by carrot rust flies. While older plants may survive the infestation, their growth will likely be stunted due to damaged roots.
Once the maggots have fed sufficiently, they pupate in the soil and either emerge as adult flies after a few weeks or overwinter until the next spring.
Preventing Cabbage Root Maggots
1. Cabbage Collars
One effective way to protect your plants is by using cabbage collars. These simple barriers are placed around the stems of your brassica plants to prevent flies from laying eggs at the base. You can buy commercially-made collars, but they can be pricey, so making your own from cardboard or other materials is a great option.
To make your own, cut circles or squares about 10-15 cm (4-6 inches) in diameter, then slit the middle and cut a Y-shape. Place the collar around the plant stem and press it flat against the soil. If you’re using cardboard, it may need replacing over time, as it will rot. Though not completely foolproof, cabbage collars can help suppress weeds, keep roots cool, and maintain moisture levels.
2. Crop Rotation and Cleanup
After harvesting, remove any plants suspected of being infected and bury them deep in your compost heap to prevent the larvae from surviving. Crop rotation is also essential. Since cabbage root maggots are attracted to brassicas, rotating your crops each year can help reduce the pest population. However, adult flies will likely fly to nearby gardens, so be sure to protect your brassicas with fine mesh netting or row covers, which will keep the flies from laying eggs on your plants.
3. Using Mulch and Barriers
Mulching your brassica crops encourages the presence of beneficial beetles that feed on grubs. Additionally, spreading wood ash or diatomaceous earth around the stems can deter flies from laying eggs. If you suspect your plants are already affected, pile compost around the stems to encourage new root growth, giving your plants a chance to recover.
By incorporating these organic methods, you can help protect your brassicas from cabbage root maggots and ensure a healthier, more productive crop.