Cabbage Maggot

Delia radicum

Insectalso: Cabbage root fly, Cabbage root maggot

The maggot of a gray fly that attacks the roots of cabbage and its relatives, especially cool-season and early transplants. Because the larvae feed underground on the roots, the first sign is often a plant that suddenly wilts, stunts, or topples for no visible reason above the soil.

🔎 How to spot it

The adult is a delicate dark-gray fly about a quarter inch long, covered in fine black hairs and easily mistaken for a small house fly or the seedcorn maggot fly. It lays small white torpedo-shaped eggs at the base of the stem and in the soil right next to young plants. The maggots are small, legless, yellowish-white grubs with pointed heads and a blunt rear, found tunneling in the roots when an affected plant is lifted.

🥀 Damage it causes

The maggots tunnel into the feeder roots and the main taproot, cutting off water and nutrient uptake so plants wilt, yellow, grow poorly, or die, with seedlings and new transplants hit hardest. The wounds also let in rot organisms such as blackleg and soft rot. Root crops in the family, like radish and turnip, are scarred and made unmarketable.

🛡️ Prevent it

Rotate brassicas to a new spot each year and clean up crop residue, since the fly overwinters as pupae in the soil where the crop grew. Fit a collar of cardboard, felt, or similar material snugly around the base of each transplant so females cannot lay eggs against the stem. Float row cover sealed at the edges over the bed from transplanting to block the egg-laying flies, and avoid setting transplants out during the peak of the first spring flight.

🧯 If it is already here

There is no effective rescue once maggots are in the roots, so the controls are preventive: stem collars, row cover, and rotation. Drenching the root zone with beneficial nematodes such as Steinernema feltiae can reduce larvae in the soil. Yellow sticky cards or water pan traps at the bed edge in early spring tell you when the flies are active so covers go on in time.

💡 Good to know

Because all the damage is underground and irreversible, this is a pest you beat by keeping the fly from ever laying eggs at the stem. Collars and row cover on early transplants are the backbone of home-garden control, with crop rotation breaking the cycle year to year.