By: Heather Rhoades
Root maggots can be a pain for any gardener who is trying to grow almost any kind of root vegetables or cole crops in their garden. While the root maggot fly is more of a problem in some parts of the country than others, they can affect almost any gardener. Knowing the symptoms of root maggots and control methods will help you keep this troublesome pest out of your garden.
Root maggots get their name from the fact that they attack the roots of root vegetables such as:
They also like cole crops such as:
The root maggots are the larva of several species of root maggot flies. Despite the fact that they are of different species, however, the root maggots look the same and are treated and controlled the same. Root maggots are white and about ¼ of an inch (6 mm.) long. Often an infestation will not be spotted until after damage is done. Damage shows up in the form of holes or tunnels in the roots or tubers of the plant. In a heavy infestation, the plant itself may wilt or turn yellow.
While the damage to root crops by root maggots is unsightly, the parts of the root crop than have not been bored into by the root maggot can still be eaten. Simply cut away the damaged areas.
The most common method for root maggot treatment is biological/organic control. Common organic cures for root maggot include spreading diatomaceous earth around the plants while they are seedlings, floating row covers over seedlings, and using natural predators of root maggots such as Heterorhabditidae or Steinernematidae nematodes and rove beetles to kill the root maggots. Root maggot organic control is most commonly used due to the fact that these pests feed on plants that will be eaten by people.
Chemicals can also be used as a root maggot treatment. Pesticides will only be effective during specific points in the growing season, as once the maggots have penetrated the root of the plant, it is difficult for chemicals to reach the pests. If you will be using pesticides for root maggot control, apply weekly during the first eight to ten weeks of spring.
As with many other pests, prevention of root maggots is much better than controlling root maggots. Make sure to regularly rotate crops that can be affected by root maggots, especially in beds where you have had problems with them in the past. Remove dead vegetation from the garden each fall and make sure to destroy (not compost) any plants that were infested with root maggots.
Also, if you find that you are having an ongoing problem with root maggots, consider cutting back on the amount of organic material you have in your garden soil, particularly manure. Root maggot flies prefer to lay eggs in soil that is high in organic material, especially manure based organic material.
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You would be hard-pressed to distinguish a root maggot fly from a regular, small housefly, unless you observed the insect laying eggs in early spring. Female root maggot flies can lay more than 100 eggs at the base of a single plant over several days. The eggs hatch in 10 days or fewer, and the young larvae burrow into the plant roots, eating as they go. This dinner kills and damages vegetables and sometimes entire crops. The larvae mature, emerge as adults, mate and the females lay more eggs.
Once you notice damage from root maggots it's too late to treat them. Protect your vegetables by preventing or removing conditions that favor root maggots.
If you regularly have experienced root maggot problems then you probably will see them again.
Row covers are an effective option to prevent adult flies from getting near the plants to lay eggs.
Do not place row covers if onions or other root vegetables were planted in the same area the previous year. Root maggots live through the winter as pupae in the soil near their target plants. Placing a row cover will trap adults that hatch from the pupae and it will no longer protect the plants from the flies.
Practice crop rotation to minimize this issue: plant susceptible crops in different areas of your garden or alternate seasons when you grow them.
There is no pesticide available as a pre-plant treatment for cabbage and onion maggots.
Jeffrey Hahn, Extension entomologist and Suzanne Wold-Burkness, College of Food, Agriculture and Natural Resource Sciences
If you already have root maggots, all is not lost—although the pesticide-based control methods are admittedly more effective than the organic ones. If you want to try organic first, though, there are a few ways to go about it.
Nematodes are also an effective maggot control method in many areas and can be purchased for home use.
The sole “conventional”, or inorganic, way to control a cabbage root maggot population is through pesticides. These are more effective than the organic methods by far. Because root maggots live primarily under the soil, most pesticides will need to be spread with a thorough watering to help the pesticides soak in.
These pests overwinter as brown pupae in the soil near the roots of fall crops.
The flies emerge from the soil in early spring in the north, and in the fall and spring in areas with warmer climates, such as California. They can travel as far as a mile to find host plants!
The adults will feed on pollen and nectar for 10 days or so, and then they lay their eggs at the base of the plants. The larvae will hatch in about a week.
After hatching, the larvae tunnel through the soil to the root system, starting with the fibrous roots. They can completely destroy root systems.
Older larvae may tunnel into the stems of plants as well.
The larvae pupate in the burrows they leave while digesting the the root material, and then emerge in 2-3 weeks to start the cycle over again.
Cabbage worms are another common pest of the cabbage family. Click here to deter imported cabbageworms.