Early-emerging cabbage maggot mows down and tunnels through roots of cole crops

Review management options for cabbage maggots, a sporadic yet damaging insect pest of cole crops.

Cabbage maggots feeding on the roots of kohlrabi. Photo: Ben Philips, MSU Extension.

Cabbage maggots feeding on the roots of kohlrabi. Photo: Ben Philips, MSU Extension.

Appearance and life cycle

Cabbage maggots (Delia radicum) overwinter as brown, hardened pupa in the first few inches of the soil surface. The first adult flies begin to emerge at 300 degree-days (base 43 degrees Fahrenheit); lilacs and yellow rockets are often blooming at this point. Peak flight and egglaying for this first generation is around 565 degree-days. Adult flies resemble houseflies, differentiated by their preference to fly close to the soil in fields where cole crops have been planted. Identifying this pest on appearance alone at any life stage is difficult.

Young plants in the Brassica family are targeted for egglaying. Adult flies lay tiny (0.3 inch), white, oval-shaped eggs on stems or in cracks in the soil near plants. The eggs require three to seven days to hatch into larvae, also known as maggots. When the larvae emerge, they burrow into the soil and feed on the roots of the young plants; feeding continues for two to three weeks. The larvae are small (less than 0.25 inch), white and lack distinct legs; the rear end is blunt, while the rest of the body tapers towards a pointed head.

These larvae will pupate under the soil, and after two to three weeks the next generation of adult flies will emerge. There are three generations per year in Michigan, with peak flights occurring at 565 degree-days, 1,476 degree-days and 2,652 degree-days. The first generation is generally the only one to cause major damage, as there are young transplants available early in the season and the pest does poorly in summer’s increasing temperatures. The final generation of flies will lay the eggs that will eventually pupate in the soil through winter and emerge next spring.

Damage

Larvae will damage roots of all sizes. Larvae completely consume smaller roots and tunnel into larger roots; larvae can also tunnel into stems. Secondary infections often occur in these tunnels, leading these areas to become brown and slimy. Looking at a field, plants under attack will not develop normally compared to their unaffected counterparts. They may appear blanched or stunted, and if damage is heavy, a hot day can cause wilting. On brassica crops where the leaves or heads are sold, plant death can occur, and on brassica crops like turnips, the tunneling into the marketable part renders it unmarketable. Seedlings are vulnerable to the worst damage, as the growing point can be damaged, preventing current and future water and nutrient uptake.

Cabbage damage

(A) On this cabbage, cabbage maggots have consumed all of the smaller roots, which will restrict water and nutrient uptake. Photo: Mary Ann Hansen, Virginia Polytechnic Institute and State University, Bugwood.org. (B) If the root is saleable portion of the crop, as is the case on this turnip, the tunneling can render produce unmarketable. Photo: Ben Werling, MSU Extension.

Similar species

The seedcorn and onion maggot are early season pests of other crops. The appearance of adults and larvae are similar, but generally each fly and its maggot are found primarily in the crop for which they are named. The maggots themselves are hard to distinguish without a microscope.

Wireworms also attack roots and are more prevalent on new ground coming out of long-term hay crops, CRP land or turf. Unlike any of the maggots, these grubs are cylindrical, brown, harder-bodied and larger than the maggots.

Management

Cabbage maggot is a sporadic pest in Michigan and simple measures can aid in keeping numbers low. Crop rotation helps reduce numbers of the first generation, and planting new fields 0.5 mile away from the previous year’s fields helps. Disking crop residue after harvest removes food sources for later generations.

Conditions that favor high cabbage maggot numbers include cool, wet springs and the presence of decaying organic matter (i.e., plowed down cover crop). If these conditions are present, consider an insecticide treatment at the time of planting (either seed or transplant) or planting after the first wave of adult flies has passed (Mid-May/565 degree-days).

MSU Enviroweather models for cabbage maggot can help determine when this date is in your area.

Unfortunately, the products available for cabbage maggot are limited, and resistance has evolved to some products in some areas. Always follow label directions as instructions can vary from crop to crop.

For current information on chemical treatment options, see Michigan State University Extension publication E0312, the Midwest Vegetable Production Guide.

Other resources

Learn about the Enviroweather seedcorn maggot tool:

Download tip sheet version of this article: Cabbage maggot of cole crops

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