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Root Maggot

Root maggots are about 1/3 inch long, and the larvae feed on roots. The adults lay eggs on the leaves and stems of plants. When the larvae hatch they start eating roots and can destroy a root crop under the media.

Root maggots overwinter as pupae about 5-20 centimetres (2 - 8 in) beneath the soil surface. Adult flies emerge from early May through to July. Adults resemble house flies but are smaller (5 millimetres or 1/5 in), dark grey in color with a dark stripe along the back of the abdomen, and are covered with many black bristles. Eggs are laid in June, most commonly beneath the soil surface near the base of the plant; occasionally eggs are also laid on the lower stems and leaves of canola plants. During their 5- to 6-week life span, females lay 50-200 oval white eggs either singly or in small masses. Small, white, legless larvae (maggots) hatch in three to five days and then eat their way through canola roots, creating feeding tunnels. The maggots mature in about three weeks and measure 6 to 10 millimetres (1/4 in) in length.

Maggots inhabit roots of canola plants that are bolting, in flower or in the early pod ripening stages. Plants severely infested with maggots can turn pale green and appear stunted and wilted, especially on hot days. Roots damaged by larval tunneling are invaded by root rot fungi far more frequently than roots without larval damage. Infestations have been most severe in central and northwestern Alberta where 95-100 percent of plants in a field are frequently infested to some degree by root maggots. Varieties of Polish canola (B. rapa) are more susceptible than Argentine (B. napus) varieties. Yield losses can be as high as 52% for crops of B. rapa and 20% for crops of B. napus. Maggots prefer cool, moist environments; therefore, crops grown under cool, moist conditions are most susceptible.

If the length of the growing season is sufficient, or if growers can seed early in spring, producers should seed varieties of B. napus rather than B. rapa. If the length of the growing season is too short to grow B. napus, seeding at higher rates (approximately 9 pounds per acre rather than 5 pounds per acre) also reduces root maggot damage. At a given seeding rate, increasing row spacing when seeding to about 10 or 12 inches reduces damage to canola. Tillage prior to seeding (either in fall or spring) reduces the survival of overwintering root maggot pupae. Even though canola grown in zero tillage is damaged by root maggots to a greater degree than canola grown in conventional tillage, greater yields can still be achieved with zero tillage, because plants in this regime can better compensate for root maggot damage.

Root maggot lifecycle

Vegetable root maggots are the larva of a type of fly called a root maggot fly. There are several kinds with different preferred host plants. The eggs of these root eating insects are laid in the soil and hatch into larva. The larvas are the little worms you see on the roots of your plant. The larva will come to the surface to pupate and then they are adults who will start the process all over again. Eggs can survive the winter in the soil.

Root maggot infestation identification

If a plant is unexplainably stunted or if it starts to wilt for no reason, there may be vegetable root maggots in the soil. Root maggots are more likely to attack in cool weather.

The best way to tell is to gently lift the plant from the soil and examine their roots. If vegetable root maggots are the culprit, the roots will be eaten away or tunneled through in the case of larger rooted plants like turnips. And, of course, root maggot larva will be present.

Root maggots typically attack either legume plants (beans and peas) or cruciferous plants (cabbage, broccoli, turnips, radishes, etc.) but they are not exclusive to those plants and can be found on almost any type of vegetable.

Root maggot control

These root eating insects will stay in your garden beds and attack other plants unless you take steps to get rid of them. There are several things you can do for root maggot control.

The first thing to do is to get rid of infested plants. Dying plants will attract the root maggot fly and should either be disposed of in the trash or burned. Do not compost them. Once a plant is infested, it cannot be saved, but you can do several things to keep the next plants from becoming infected.

Organic root maggot control can be:

  • Dusting the plants with diatomaceous earth
  • Adding beneficial nematodes to the soil
  • Releasing predatory rove beetles into your garden
  • Covering plants with floating row covers
  • Solarizing infected beds

These pesky root eating insects can be stopped if you follow the above tips.

Prevention If eggs are on plant, the larvae can be prevented from reaching roots by slitting a 3" piece of tar paper and slipping it around the stem at ground level. Also, destroy damaged plants after harvest.

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