The carrot fly, or psila rosae are a garden pest that attacks mainly carrots, parsnips, celery and parsley. It is their larvae which cause the damage, beneath the soil, making it difficult to detect their presence before pulling up the crop, although the leaves above ground may become discolored. The carrot fly may have up to three life cycles each season.
The carrot fly lays its eggs in the soil next to the growing carrots. The first laying occurs in early spring and the larvae hatch within ten days. Initially they feed on the roots of the carrot but then burrow into the actual vegetable, creating little burrows which then allow mold to enter and make the whole carrot rotten. The larvae then develop into the tiny black carrot fly which flies very close to the ground. The whole process is repeated midsummer, and again in late autumn and the larvae remain in the ground, overwintering, during the winter months.
The main problem with carrot fly is that it often goes undetected until the vegetable is pulled up to eat. Many methods to prevent an infestation have been tried. The carrot fly is attracted by the scent of carrots, which it can detect from a large distance away. To prevent attracting them to the crop, carrots should be well spaced when planting, thus minimizing the need for thinning out. When harvesting the crop, carrots should be pulled at dusk, preferably on a windless evening, as the carrot flies are only active during the day.
Due to the fact that the fly can only fly about two feet (61 cm) from the ground, covering the plants with a protective mesh or two-foot high barrier around them, or planting them in pots which are kept high on a table, may be effective. Timing the planting of carrots to miss the three active periods of the carrot fly larvae may also help. Various methods of inter-planting with other crops, such as onions, have been suggested, but the efficacy of this has not been proven. It is thought that planting strong-smelling plants may make it more difficult for the fly to detect the presence of carrots.
When carrots have been attacked by the larvae, the leaves may turn an orange color, and then yellow. In many cases, though, the effect is only seen on the carrot, when picked. The carrot may be disfigured and the end often has little holes, or burrows, in it and may have turned black, due to mold. Some carrot varieties are more resistant to carrot fly than others.