Why Are My Radishes Full of Tunnels and Maggots?

Pulling a radish that looks healthy from above, only to find it riddled with narrow tunnels and small creamy-white maggots inside, is one of the most discouraging findings in the radish bed. The culprit is cabbage root fly (Delia radicum), a pest that also attacks other brassica crops. The female fly lays her eggs at the base of the plant in the soil, and the hatching larvae burrow into the root. A heavily infested radish is completely inedible.

How to identify root maggot damage

Above-ground symptoms are subtle until damage is severe: wilting in dry weather (the damaged root cannot absorb water efficiently), stunted or yellowing plants. Below ground, the root shows small entry holes, sometimes with a brownish, mushy area around them, and small (3–5 mm) creamy-white legless maggots tunnelling through the root flesh. In severe infestations the root is almost entirely consumed internally.

Row cover as primary prevention

The cabbage root fly adult is a small grey fly that lands on the soil around the plant base to lay eggs. If a fine row cover or insect mesh (less than 1 mm mesh size) is placed over the bed immediately after sowing and sealed at the edges, the fly cannot access the soil to lay. Because radish does not need insect pollination, the cover can stay in place throughout the growing period. This single measure is nearly completely effective when correctly applied.

Brassica collar discs

Cardboard or fabric discs laid flat on the soil around each plant stem prevent the fly from accessing the soil immediately around the stem base — the preferred egg-laying site. These are more commonly used for brassicas where removing row cover for pollination is necessary, but they can supplement row cover for radish as an additional barrier.

Rotation and timing

Avoid growing radish and brassicas in the same bed year after year — rotation reduces the soil population of overwintering pupae. The first generation of root fly adults emerges in late April; sowings from mid-May onward in many regions avoid the peak first generation and experience less damage. Autumn sowings (August–September) often miss the main fly population entirely.