Cabbage Root Maggot

This small (1/4") gray fly lays its eggs in the soil at the base of Brassica plants. When the larvae hatch they work their way down to the roots and tunnel into them. They live in the roots for 3 - 4 weeks and then go in to the soil to pupate. If enough maggots get into a root they will severely stunt or kill the plant (they also make root crops inedible).

Usually the first symptom of infestation is that the plant wilts in sunny weather, even though it has lots of water. If this occurs, examine the root for the small white maggots, which look like small grains of rice.

Prevention

This the worst Brassica problem I have encountered and prevention is much easier than trying to cure infested plants. The easiest way to deal with these pests is to use row covers, which prevents the fly getting near enough to the plant to lay eggs.

Control

An effective control is to use 6˝ squares (or disks) of foam carpet backing. You cut a slit to the center of the square and put them around the stem. These work very well, because the foam can expand as the stem enlarges. These disks not only make it harder for the fly larvae to get into the root, but also provide refuge for predatory beetles that eat the eggs and larvae. These disks have achieved 70% control, which is as good as most pesticides. You don’t have to eliminate all of these maggots. Some damage is tolerable as long as it doesn’t seriously affect the crop.

Image: Mary Ann Hansen, Virginia Polytechnic Institute and State University, Bugwood.org