The cabbage maggot is the larva of a fly that resembles a small, slender housefly. The adult lays white eggs in the soil right at the base of brassica seedlings, often during the cool, moist conditions of early spring. The eggs hatch into small white maggots that burrow down and tunnel into the roots, severing the plant's ability to take up water. The first sign above ground is a transplant that wilts in the sun, takes on a bluish or off-color cast, and simply stops growing. Young, recently transplanted brassicas are the most vulnerable, and a heavy attack can kill them outright.
How to identify the problem
Because the damage is underground, you diagnose cabbage maggots by ruling out the alternatives and then checking the roots. A transplant that wilts despite moist soil, looks bluish, and is stunted is a candidate. Lift it gently and look for slender white maggots and the brown tunnels they leave in the roots and lower stem. Unlike clubroot, which swells the roots, cabbage maggots eat them away, leaving chewed, scarred, often rotting root tissue.
How to control them, step by step
The plan
Cover at transplant
Lay a floating row cover over the bed the moment transplants go in or seeds emerge, sealing the edges with soil so the fly cannot reach the stem base to lay eggs. This is the single most effective step.
Fit stem collars
Slip a snug collar of cardboard or similar material flat on the soil around each stem. It blocks the fly from laying eggs right at the base where the maggots would enter.
Time around the flight
The first generation flies in cool early spring. Where pressure is high, delaying transplanting until after that first egg-laying flush lets plants establish past the most vulnerable stage.
Rotate brassicas
The flies overwinter as pupae in soil near where brassicas grew. Move brassicas to a new bed each year and clear old roots in fall to lower next season's population.
Protect through establishment
Keep the row cover on through the early weeks. Brassicas are wind-pollinated for our purposes here (we harvest leaves, heads, and roots), so unlike cucurbits you can leave the cover on far longer.
Row cover is the core of the plan
The most reliable defense is to keep the egg-laying fly away from the stem base entirely. A floating row cover sealed tightly at the edges from the day transplants go in physically excludes the fly during the cool spring weeks when it is most active, which is exactly when young brassicas are most vulnerable.
Which plants are at risk, and how to plan
Cabbage maggots attack the brassica family broadly: cabbage, broccoli, cauliflower, kale, brussels sprouts, and collards are all hosts, and the root brassicas turnip and radish can be ruined when maggots tunnel the edible root itself.
Timing is your biggest non-physical lever. Use the planting calendar for your ZIP to know when your brassica transplants go out, so you can have row cover sealed from day one, and to consider whether delaying a planting past the first cool-spring flight lowers your risk. Knowing your frost dates helps you avoid rushing transplants into the cold, slow-growing conditions that keep them in the vulnerable stage longer.
What are the signs of cabbage maggots?
Above ground, affected brassicas wilt in the sun despite moist soil, take on a bluish or off-color cast, and stall in growth, often dying if young. Below ground you will find slender white maggots and brown tunnels chewed into the roots and lower stem. The combination of a stunted, wilting transplant and tunneled, rotting roots confirms cabbage maggots.
How do I prevent cabbage maggots?
The most effective prevention is to exclude the egg-laying fly. Seal a floating row cover over brassicas from the day transplants go in, fit stem collars at each stem base, time plantings to avoid the first cool-spring flight where pressure is high, and rotate brassicas to a new bed each year. There is no way to reach the maggots once they are inside the roots, so prevention is everything.
Can I spray to kill cabbage maggots?
Not effectively. Once the maggots are tunneling inside the roots, they are protected from foliar sprays, which is why control centers on excluding the adult fly before it lays eggs rather than treating the larvae. Row cover, stem collars, timing, and rotation are the tools that actually work against this pest.
When are cabbage maggots most active?
The first and often most damaging generation flies in cool, moist early spring, exactly when many gardeners set out their first brassica transplants. That overlap is why young spring transplants suffer the worst. Delaying a planting past the first flush, where your conditions allow, lets plants establish past the most vulnerable stage.
The bottom line
Cabbage maggots do their damage underground, where no spray can reach them, so the whole game is keeping the egg-laying fly away from your young brassicas. Seal a floating row cover over transplants from day one, add stem collars, consider delaying past the first cool-spring flight, and rotate brassicas each year. Time it all against your planting calendar and frost dates, and your transplants establish before the maggots ever get a chance.
