There are no aboveground symptoms, but a carrot root fly infestation can ruin your carrot crop, along with several other umbellifers. Carrot root fly (Psila rosae), also known as the carrot rust fly, is a pest found in most temperate regions.
Carrot root fly description and lifecycle
These tiny, fast flying pests are slender, greenish-black metallic flies. Their legs and head are yellow. Carrot rust flies tend to hang out in weeds and undergrowth near carrot fields. After mating, females head for the carrots, where they will lay their tiny, white eggs on the soil surface. Larvae are creamy white, approximately 1/3 of an inch long, and rather stiff, as far as maggots go. Orangish-brown pupal cases can be found in the soil.
In addition to feeding on carrots, carrot root flies also feed on caraway, celery, dill, fennel, parsley, and parsnips. Other plants, such as lettuce, endive, and chicory can serve as host plants in areas where the maggots are already in the soil.
Damage caused by carrot root flies
Like other root maggots, carrot root maggots feed on tiny root hairs and then tunnel along the surface and burrow into the root. These tunnels and burrows are then filled with mushy frass. The initial feeding causes stunting, and tunneling makes it easy for other pathogens to enter, which leads to several different types of rot and other problems.
Carrot root fly control
The best way to protect young plants from carrot root flies is to use row covers as soon as seeds are planted, and to leave the cover in place until it is time to harvest. Some people use window screen panels around raised beds to block this pest, while others use window screen material to create protective cones for individual plants.
There are resistant varieties of carrot, specifically ‘Flyaway’, which claim to thwart these pests, though I can’t vouch for its effectiveness, personally. Research has shown that intercropping alfalfa with carrots and other host plants, or mulching with alfalfa works to reduce carrot root fly infestations. Crop rotation can also be used to interrupt the carrot root fly lifecycle.
You can grow a surprising amount of food in your own yard. Ask me how!