Cabbage Maggots

Cabbage maggots are a serious pest of plants in the cabbage family (Brassica).

What starts out as a single, tiny fly can turn into 300 squirming, gnawing slugs of destruction in just two or three days. And you may never see them before it is too late. Cauliflower and Brussels sprouts are more likely to be attacked, but broccoli, cabbage, turnips, radishes, rutabagas, and mustard plants are also vulnerable, especially in cool, wet weather.

Adult cabbage fly (James Lindsey) CC BY-SA 3.0

Cabbage maggot description

Cabbage maggots are the larval stage of the cabbage fly (Delia radicum). Also known as cabbage root flies, root flies, or turnip flies, this dark gray fly is half the size of your average house fly. Maggots are white, legless, and only 1/3 of an inch long, with a pointed head and a blunt behind. Eggs are white and 1 mm in diameter.

​Cabbage maggot lifecycle
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Eggs are laid in the soil, around the crown of host plants. When they hatch, larvae (maggots) start feeding on fibrous roots and burrowing into taproots. This feeding facilitates entry by pathogens that cause blackleg and bacterial soft rot, and it continues for three weeks before maggots pupate in the soil. In two to four weeks, adult flies emerge to feed on nectar and the whole cycle begins again.

Cabbage fly larvae on cauliflower stalk (Rasbak) CC BY-SA 3.0

Symptoms of cabbage maggot feeding

These pests are easy to miss, but the damage they cause is obvious. As seedlings and young plants struggle to survive, chlorosis (yellowing), stunting, and even death may occur. Infested root systems are sparse and taproots show obvious tunneling. Established plants are better able to withstand attack, but they will also show stunting and chlorosis. Wilting in the middle of the day can also be a sign of cabbage maggot feeding. Plants will perk back up by morning, but, within a few days, they will be dead. Dark green leaves may show a bluish tinge, but that can be a normal feature of some plant varieties, so don’t use it as a symptom unless other signs are visible. Infested plants should be removed and destroyed. Do not compost infested plants.

​Cabbage maggot control

If cabbage maggot carnage is something you need to control, spinosad is moderately effective, according to a 2015 UCANR study. Traditionally, fumigation was used, and organophosphates (diazinon and chlorpyrifos) were applied to soil in concentrations heavy enough to contaminate ground water and kill off non-target species. Obviously, more environmentally safe, sustainable methods are needed. Many of the other insecticides that performed the best in the study are unavailable to the home gardener, and for good reason. You can reduce the likelihood of infestation by using crop rotation, row covers, and proper irrigation.

If you are determined to protect your cole crops, you can also use brassica collars around the base of each plant. A brassica collar is simply a flat piece of plastic, thick cardboard, or heavy fabric that covers the soil around the base of a plant. Cabbage flies are unable to lay their eggs as close as they would like, and many of the larvae simply starve to death before they reach your plants. You can make your own brassica collar, just be sure that the collar opening can be enlarged as the plant grows.