Why Are There Holes and Tunnels in My Carrots?

You slice into a promising carrot and find it riddled with rusty-brown tunnels and holes, often packed with crumbly muck. This is the work of the carrot rust fly — usually just called carrot fly — the single most damaging carrot pest there is. The maggots tunnel through the roots, ruining them. The good news is that this pest is highly preventable with the right barrier. Let me explain the damage and how to stop it.

Recognising carrot fly damage

The signature is rusty-brown tunnels winding through the root, especially around the lower part and tip, often with the surrounding flesh discoloured and rotting. The small creamy-yellow maggots may still be present. Above ground, an early sign is the foliage taking on a reddish or bronze tint and the plants looking stunted, as the larvae damage the roots. The adult is a small, low-flying fly; it lays eggs in the soil near carrots, and the hatching larvae burrow into the roots to feed. There are usually two or more generations a year, so damage can appear from early summer onward.

The barrier defence

The most effective control exploits the fly's behaviour: the female flies low to the ground, under about 60 cm, to find carrots by scent. So a physical barrier works brilliantly. Surround the carrot bed with a fine insect-mesh fence about 60 to 75 cm high, or — even better — cover the crop entirely with horticultural fleece or fine insect mesh laid over hoops, sealed at the edges, from sowing onward. This physically denies the fly access to lay her eggs. Covering the crop is the single most reliable way to grow clean carrots, and many gardeners never grow carrots uncovered because of this pest.

Reduce the scent that attracts them

Carrot fly is drawn by the smell of carrot foliage, which is released strongly when leaves are bruised — above all during thinning. So thin carrots in the evening when flies are less active, do it on a still day, remove the thinnings completely (do not leave them lying about), and firm the soil back down afterward to seal in the scent. Sowing thinly to reduce the need for thinning helps. Some growers also interplant with strongly scented alliums like onions to mask the carrot smell, with mixed results.

Timing, rotation and varieties

You can also dodge the fly. The first generation flies in late spring, so sowing carrots a little later, or harvesting early crops before the larvae build up, avoids the worst. Rotate carrots to a new spot each year, since pupae overwinter in the soil where carrots grew, and clear the bed of old roots. Resistant or partially resistant varieties exist and suffer less damage. Combine a physical barrier, careful low-scent thinning, rotation and good timing, and you can keep carrot fly from tunnelling your crop.