Carrot Rust Fly

This small (1/4") greenish black fly lays its eggs in the soil at the base of a carrot plant. When the larvae hatch they make their way down to the root and eat their way inside. These small maggots tunnel through the root for several weeks, causing rust colored lesions and rendering the root inedible.

This is the worst pest of carrots and in seriously affected areas (it is found throughout North America) they make it almost impossible to grow Carrots without protection.

Prevention

The first line of defense against this pest is hygiene. The flies are said to be able to detect the smell of damaged Carrot foliage from more than a mile away. Keep thinning and weeding (which bruises the foliage) to a minimum and never leave the foliage lying on the ground. Also don’t leave the remains of an infected Carrot crop in the ground right through the winter, as it can mean a big increase in the incidence of this pest. Always dig and compost old carrots.

If the fly is severe some kind of barrier is probably the best way to go. Row covers are the the most common solution to this problem, but it’s said that a simple plastic screen, 30 to 36˝ high, around the plants will work just as well. Apparently the flies always stay close to the ground and will try to go around the screen, but they won’t go over it (so long as the bed is no more than 36˝ wide).

There are two generations of flies each year, the first in late spring and another in late summer. It is possible to avoid them both by carefully timing the planting and harvesting.

There are now some Carrot Fly resistant varieties available. Apparently these work best if some nonresistant Carrots are sown next to them, to act as a trap crop.

Image: Whitney Cranshaw, Colorado State University, Bugwood.org