by Joel Grossman
FAMILY
GENUS AND SPECIES
RANGE
CROPS AFFECTED
Carrot maggot—Plants in carrot family (Umbelliferae). Cabbage maggot—Cole crops and other plants in cabbage family (Cruciferae).
Onion maggot—Onions and other plants in the Allium genus. Seed corn maggot—Any vegetable seedling; most damaging to peas and beans.
PREVENTION AND CONTROLS
resistant varieties; early and late plantings; crop rotation; fall tillage and debris removal; screens and row covers; mulches; disks and barriers; yellow sticky traps; beneficial insects and nematodes; diazinon.
Root maggots are fly larvae notorious for ruinous tunneling of root crops and for seedling destruction, particularly of early spring plantings of cole crops, peas and beans. The main culprits are the cabbage maggot, the carrot maggot, the onion maggot and the seed corn maggot. All kissing cousins in the fly order, Diptera, they cause similar damage, have similar life cycles and are prevented and controlled in much the same way.
The maggots overwinter as pupae in the soil. First-generation adult flies emerge in spring and lay 50 or more tiny white eggs per female on the soil surface within two inches of the plant stems. The white, yellow or brown maggots hatch out in a couple of days and burrow into the soil, where they eat small feeder roots before moving on to scrape out soft taproot tissue with their blunt, hook-like jaws. After three to five weeks of feeding, they pupate in the roots or in the soil. Adult flies emerge, mate and lay eggs. There can be two to four generations per season; in warm climates, root maggots are active throughout the year.
Maggots tunnel into the roots of carrots, onions, radishes, turnips, cabbage, beans, peas, broccoli, spinach, celery, dill and many other herbs and vegetables. Young crops (those under six weeks old) with soft root tissue are most vulnerable to lethal attack, particularly when cool, moist spring weather favors flies. If few seedlings emerge from a newly planted row of peas, beans or corn, dig up a few seeds and look for seedcorn maggots inside. If young plants seem slightly stunted or yellowed, or if they wilt on warm days, pull up a few and check the roots for maggot tunnels. By the time plants are six to eight weeks old, the root tissue is tougher and resists tunneling, so maggots scrape winding paths around the perimeter. Later in the season, also check the soil within an inch of roots for the brownish ¼-inch-long fly pupae.
Close encounters with root maggots will most likely be one of the following species:
Carrot rust fly. This European native is partial to carrot family [Umbelliferae] members like carrots, caraway, celery, coriander, dill, fennel, parsley and parsnips. Adults are flashy insects with shiny blue-black to dark green bodies and a pale yellow head and legs. First-generation (early to mid-May) adults fly at ground level; second-generation adults (early July to late August) cruise at carrot top height; a smaller third generation attacks plants in September. The 2/10-inch-long maggots chew their way through the roots, leaving a trail of reddish brown excrement. Injured plants often become infected with soft-rot bacteria.
Cabbage root fly. The cabbage maggot made it to North America from Europe as a stowaway in ship ballast. Up to four generations per year attack mustard family plants (Cruciferae) such as radish, turnip, broccoli, Brussels sprouts, cabbage and cauliflower from Alaska to California. The ¼-inch-long adults look like small, bristly, black-striped, gray houseflies. Cabbage maggots are particularly damaging during cool moist springs in northern areas. In upstate New York, an early tipoff of the dreaded first flight (typically mid-May) is flowering of yellow rocket (Barbarea vulgaris), Windsor cherry and Reine Claude and European plums. Onion fly. The adult flies are slender, long-legged, large-winged, pale gray insects resembling small houseflies. They attack only onions and other Allium species. The damaging first spring flight often coincides with the flowering of yellow rocket and ox-eye daisy. Onion maggots bore through underground stems, and can hollow out small bulbs. A third maggot generation attacking before onion harvest allows storage rot fungi to invade.
Seedcorn maggot. The adult flies are slender, grayish brown insects resembling houseflies. These nondescript Clark Kents turn into Supermaggots during cold wet springs, clobbering sprouting pea and bean seeds in moist soils rich in decaying vegetation. Up to five fly generations can also attack cole crops, corn, melons and numerous other seedlings from Alaska to Florida. The seed corn maggot rarely damages crops after the early seedling stage.
Prevention
Transplants often recover from maggot damage, but seedling root crops usually succumb to heavy maggot attack, so prevention is the best strategy. Planting early, before root fly flights, means some plants may be large enough to escape serious damage when the flies emerge. Likewise, crops planted late may miss peak flights. Make a practice of removing heavily infested plants and crop debris to prevent establishing a breeding fly population in the garden. Till soil in the fall to destroy maggots before they overwinter. To minimize seedcorn maggot damage, allow green manures and organic matter to decompose in the soil for several weeks before planting, or use compost.
It's crucial to protect young seedlings from the egg-laying flies at all times; start plants indoors or in screened greenhouses or cold frames. Once seedlings are transplanted, screens and row covers spread at least six inches on both sides of the rows provide effective protection, provided you are rotating crops so you aren't trapping emerging flies. Don't plant onion, carrot family or cole crops two years in a row in the same spot, especially if you have had maggot problems.
Tarpaper disks three to six inches in diameter and other barriers laid flush to the ground and tight around plant stems at planting time prevent root maggots from entering the soil. Use adhesive tape to plug tiny cracks between plant stems and disks that flies could slip through to lay eggs.
Diazinon is the best choice among the commercially available pesticides, but it will also kill the natural predators. The granules must be mixed into the soil around the plant stems before the maggots attack; once they're inside roots, they are home safe. Adult flies avoid treated foliage.
Biological Control
The sole commercially available biological control—beneficial nematodes such as Steinernema carpocapsae and Heterorhabditis bacteriophora—provide only partial control. The nematodes attack the young root maggots as they are hatching out and attempting to reach roots; adult flies and older maggots escape. Thus, to be effective, nematode applications must start when root flies start laying eggs and multiple applications may be necessary. Yellow sticky traps near ground level (tilted at a 45o angle to avoid trapping beneficial syrphid flies) are an early warning system of imminent fly egg hatch. The traps also provide some control by trapping adult flies.
Canadian and European researchers recommend creating semi-permanent clover refuges to encourage the natural buildup of indigenous beneficial beetles, such as the rove beetle (Aleochara bilincata), which preys on the fly eggs as well as parasitizing the pupae. In Britain, experiments with interplanting at least half the area between rows with French beans, ryegrass (Lolium) and clover ground covers increased naturally occurring populations of Carabid ground beetles and rove beetles and reduced cabbage maggot populations by 60%. Last year, English researchers reported that releasing two Bembidion tetracolum ground beetles per plant controlled a large, usually damaging, spring generation of cabbage root flies. But the beetles are not yet commercially available.
Deterrents
Michigan State University researcher Jim Miller once shared some Mexican food with an onion farmer who joked that the meal was hot enough to kill onion flies. Miller, along with colleagues such as Richard Cowles, now at the University of California, Riverside, gave it a try, sprinkling herbs and spices like chili powder, paprika, black pepper, dill and ginger around onion stems. Hot spices repelled onion flies much better than the traditionally recommended wood ashes, but too much "hot stuff" is phytotoxic and will cause stunting and yellowing of seedlings. To avoid damaging plants, sprinkle spices over a plastic mulch or protective barrier laid close to seedling stems, rather than directly on crops or soil.
Joel Grossman is a former pest control advisor and freelance writer in Santa Monica, California.
Copyright NGA
Reprinted with permission HouseNet, Inc.
Pest Patrol - Root Maggots
Anthomyiidae
Carrot maggot [carrot rust fly] -- Psila rosae
Cabbage maggot [cabbage root fly] -- Delia radicum
Onion maggot [onion fly] -- Delia antiqua
Seed corn maggot—Delia platura
Throughout North America and worldwide, except for the seedcorn maggot, which is more a problem in northern latitudes