Cabbage Root Maggot

Delia radicum

Insectalso: Cabbage maggot, Root maggot

The cabbage-family counterpart of the onion maggot: a small fly whose white maggots tunnel into the roots of brassicas, wilting and killing seedlings and transplants. Cool, wet springs and beds that grew cabbage relatives the year before make it worst.

🔎 How to spot it

The maggots are small, white, legless, and blunt-ended, found burrowing in the roots and lower stem. The adult is a delicate, hump-backed gray fly a bit smaller than a housefly, and the eggs are tiny white ovals tucked into the soil at the base of the stem. Above ground, watch for plants that wilt in midday sun, with outer leaves turning yellow or bluish, then collapse.

🥀 Damage it causes

The maggots chew and tunnel through the roots and into the lower stem, cutting off water and nutrients so young plants wilt, stunt, and often die; survivors are set back and may form poor heads. Root crops in the family, such as radish and turnip, are tunneled and made unusable. Brassica seedlings and fresh transplants are the most vulnerable.

🛡️ Prevent it

Cover the crop with row cover from transplanting to block the egg-laying flies, but only on ground that did not grow brassicas last year, since the flies overwinter as pupae in that soil. Fit a disk or collar of cardboard flat on the soil around each stem to stop egg-laying at the base. Rotate brassicas, and delay planting past the first spring flight where you can.

🧯 If it is already here

There is no spray that reaches the maggots inside the root, so pull and destroy wilting, infested seedlings promptly to stop the maggots maturing, and rely on exclusion for the rest. Firm, healthy transplants set out a little later can outgrow light damage. Clean up and destroy crop roots at season end so pupae do not carry over.

💡 Good to know

Cabbage root maggot overwinters as pupae in soil where brassicas grew, which is why rotation plus row cover (on rotated ground) is the winning combination and why a cover laid over a bed that grew brassicas last year backfires. A simple stem collar that blocks the female from laying at the soil line is a classic, low-effort fix.