Insect Management

Cabbage Root Maggot Flies

Life cycle and monitoring

Cabbage Root Maggot flies (Delia radicum) spend the winter as small brown pupae in the soil.  Adults maggot flies emerge in spring and can travel considerable distance in search of host plants (1/2 to 1 mile). A good indicator of the start of cabbage root maggot flight is blooming of the common roadside weed, yellow rocket. This weed starts blossoming in late April or early May in New England, with the onset of warmer weather. (see photo) Root maggot flies can be captured on captured on yellow sticky traps placed in cabbage and onion fields at the same time.

Cabbage root maggot flies are rather delicate, hump-backed gray-brown flies, about 5-7 mm long.  You may see them moving from plant to plant in early crucifers. Onion maggot (Delia antiqua) and cabbage maggot (Delia radicum) flies look nearly identical but are likely to be found only near their host crop – brassicas, for cabbage maggot, and aliums, (primarily onion) for onion maggot.  There is a period of time (6-10 days) after flight begins before eggs are laid. Female flies flies seek out crucifer or onion plants to lay eggs at the base of the stem. When flight begins, its time to test your eyesight, and begin to scout cabbage and onion fields for root maggot eggs. 

Monitoring cabbage for eggs.

To check your field for eggs, look for the 1/8-inch long, torpedo-shaped white eggs that are laid along the stem, or on the soil next to the stem of young transplants. Often eggs are laid in neat rows, or inserted into the soil. They may be under a small clod of dirt near the stem. A pencil point helps stir the soil to look for them. Check 20-40 plants in the field, in groups of 2-4 plants. If you find more than an average of 1 egg/stem, it is likely to be a damaging population and a banded soil drench is recommended. Eggs may be more abundant in wetter areas of the field.

When eggs hatch, cabbage maggot larvae tunnels through or eat the roots off of plants in the cabbage family such as turnips, radish, cabbage, broccoli and cauliflower. In root crops you may find the tunnels or the maggot. In crops such as broccoli or cauliflower your first sign of problems is wilting of the plant on sunny days and then later the plants die. If you pull one up you will see the reason it is wilting is the roots are gone. You may still find the little white maggots feeding.

There are 3-4 generations per year. Because of the variation in time of adult emergence in the spring, and the variation in length of developmental stages, it is possible for maggots to be present much of the growing season. However, high temperatures and disease suppress populations in July and August. The major damage from this pest is found in the early plantings of broccoli, cauliflower and cabbage. Damage occurs again in the end of the season since the cooler temperatures of September and October favor development. At that time, the maggots damage fall-maturing plants such as rutabagas, turnips. Chinese cabbage, mustard, radish, rutabaga and turnip are even more attractive to cabbage maggot fly than cabbage, and may be more severely injured.

Degree days can also be used to predict emergence. According to a model developed by Dr. Tony Shelton’s lab at Cornell University, the first emergence of cabbage maggot occurs at 160 degree days+/- 8 degree days(DD) (base 40C). 25% emergence occurs at 203 DD +/- 3 DD, 50% emergence occurs at 251 DD +/-8 DD, 75% emergence occurs at 304 DD +/- 37 DD and 95 % emergence occurs at 387 DD +/- 8 DD. Degree day accumulations vary greatly across New England. Degree day information for some locations in New England can be obtained from National Ag Statistic Service in the Crop Weather Report published each Monday (http://www.usda.gov/nass/pubs/staterpt.htm#m).

Soil Drench.

Target the base of the plants and use at least 200 gallons of water per acre to help the insecticide penetrate to the root zone. As of 2006, chlorpyrifos (Lorsban) is the only chemical option for soil drench. This material does not move readily in soil after the application is made, so it is important to provide adequate water so that the material penetrates several inches into the soil when it is applied. Under dry soil conditions, additional water may be needed to penetrate the soil. See the 2006-2007 New England Vegetable Management Guide for more details. Adhere to preharvest intervals.

Re-scout the field five to 7 days after application to note whether eggs have hatched; if there are few maggots active, then the application was effective.  Because the materials are quite persistent in the soil, a second application is usually not needed.

If you make several spring plantings of Brassicas, scout each plantings (it takes about 15 minutes) to achieve the best timing for a soil drench.  You’ll be able to pply it when it is needed, and you can save the cost of application when it is not needed.  In the Connecticut Valley of Massachusetts, fields planted after May 15 often do not need treatment.

Floating Row Covers

Floating row covers provide an effective barrier against this pest. Use in a rotated field, as flies overwinter in soil after late season crucifers and could emerge under the cover if the same field has spring brassicas. Seal edges well. Replace cover after weeding operations.

Other cultural practices

Crop rotation contributes to keeping populations low. Fall tillage to bury crop residues and expose over-wintering pupae is also important. On a small scale, there are ways to prevent maggots from getting to the root zone. A small piece of plastic, slit to the middle, wrapped around the plant stem and overlapped, can be taped or covered with soil. Blue plastic mulch, intercropping with non-brassica plants, cardboard or tarpaper disks that fit tightly to the stem or a 1meter high vertical barrier around the crop are other possibilities for small-scale situations. On a larger scale, given healthy growth of the crop, cultivation that brings soil up around the stem may help encourage formation of adventitious roots from the stem, which can help compensate for root loss even if maggots are present.

Author: R. Hazzard, University of Massachusetts; selections drawn from Eric Sideman, Maine Organic Farming and Gardening Association.