How to Control Root Maggots in the Garden

Prevent an Infestation of the Damaging Larvae Before It Starts

How to Control Root Maggots in the Garden

The Spruce / Alison Czinkota

Root maggots are one of the garden pests where control is all about prevention. Once root maggots have invaded the soil and eaten the underground parts of garden crops, there is not much you can do to get rid of them. Getting a root maggot infestation under control promptly is crucial because it makes vegetables inedible and can kill plants. Plants weakened and damaged by root maggots are more susceptible to fungal or bacterial infections, such as root rot, and secondary infestations by other insects.

Root maggots are the larvae of different species of flies. Although common species such as onion maggot and cabbage maggot are named after their main host plant, they also feed on numerous other vegetables. 

Infestations tend to occur more commonly when the springtime weather is cool and wet.

Cabbage root maggot (Delia radicum) on the damaged root of oilseed rape (canola)
Cabbage root maggot (Delia radicum) on the damaged root of oilseed rape (canola)

Tomasz Klejdysz / Getty Images

What Do Root Maggots Look Like? 

While the damage is done by the root maggots, or larvae, and not the adult flies, for effective control. It's important to familiarize yourself with the all the stages of the insect’s life cycle. 

Life Cycle of Root Maggots

All root maggot species share the same life cycle. They live for two to five weeks and go through three or more generations per year depending on the climate: the first one in late to mid spring, the second in early summer, and the third in late summer.

The small, red-brownish pupae of the root maggots overwinter on the top, one to five inches of soil, near the roots and other debris of plants that were infested in the previous year or in the soil. As the soil gradually warms up in the spring, adult flies emerge over a span of four to eight weeks and instantly mate. 

Two to seven days after emerging, the females seek out host plants to lay their eggs. Preferred spots are the base of host plants, in soil cracks and clumps of soil, or plant stems. A female fly can lay over 100 eggs on a single plant over a couple of days. 

After three to ten days, the larvae of the root maggot hatch and immediately start to feed, burrow into the stems of the host plants, roots, root hairs, seeds, and any decaying organic matter. It is during this two-to four-week period that root maggots cause damage. Once it’s complete, they evacuate their feeding grounds and move to the soil to pupate, remaining close to the surface. After another two to four weeks, a new generation of adult flies emerges, and another life cycle begins. 

The last generation in the late summer remains in the soil where it overwinters as pupae to emerge the next spring. 

Adult fly of the onion root maggot (Delia antiqua)
Adult fly of the onion root maggot (Delia antiqua)

Tomasz Klejdysz / Getty Images

Appearance During Different Stages 

The eggs of the root maggots are about one-eighth of an inch, white, and oval. The larvae hatching from the eggs are about one-quarter inch long and yellowish-white to creamy white in color. Their legless bodies are cylinder-shaped without a distinct head. 

The pupae are about the same length as the larvae. Their oval bodies are enclosed in the hardened, red-brown larva skins. The emerging adult flies are about one-quarter inch long, slender, dark gray, or gray-brown in color. They are about half the size of the common house fly.

Adult fly of the cabbage root maggot (Delia radicum)
Adult fly of the cabbage root maggot (Delia radicum)

Tomasz Klejdysz / Getty Images

How to Get Rid of Root Maggots 

Once root maggots have invaded a plant to feed, there is no EPA-approved pesticides to get rid of them. At this point, your only option is to remove the entire plants and destroy them. Do not leave any infested plants in your garden, as they will lead to an even heavier infestation the same season or the next year. 

The only way to control maggots is prevention as explained below. 

Signs of Root Maggot Infestations 

How badly plants get damaged by root maggots depends on the crop, its variety, and the age of the plant. For example, red cabbage is less susceptible to root maggots than green cabbage

To the naked and untrained eye, it is difficult to distinguish the adult flies of the different root maggot species from one another as they look very similar. However, their presence near host plants can give you the necessary clues. 

Onion damaged by onion root maggots
Onion damaged by onion root maggots

Tomasz Klejdysz / Getty Images

Onion and Cabbage Maggot Damage

Onion maggots feed on onions, leeks, shallots, garlic, and chives. Cabbage maggots attack cauliflower, Brussels sprouts, radish, cabbage, broccoli, collards, kale, kohlrabi, turnip, other cruciferous vegetables, and sometimes also beets and celery. 

Seedlings and young plants are most vulnerable while healthy older plants can survive moderate infestations. A single maggot can annihilate up to 20 small seedlings

In older plants, discolored, light green or yellow and/or wilted leaves and stunted growth can be an indication that root maggots have already damaged the roots by boring numerous feeding tunnels into underground stems, roots, and bulbs. Root crops and bulbs such as rutabagas are discolored, deformed, debris-filled, and have external scars, all of which makes them inedible. Infested cabbage, cauliflower, and broccoli plants do not form heads. Eventually, severely damaged plants will die. 

If you suspect root maggots, pull up plants and inspect the roots. You won’t necessarily see the maggots, only the tunnels, as the maggots might have already moved on to the soil to pupate. In that case, you might see the reddish-brown pupae in the soil around the plant. 

Seedcorn maggot fly (Delia platura)
Seedcorn maggot fly (Delia platura)

Valter Jacinto / Getty Images

Seedcorn Maggot Damage 

The seedcorn maggot and bean seed maggot, also called turnip magot, attack germinating seeds and seedlings of a wide range of vegetables. This includes lima beans, peas, soybeans, corn, cucumbers, melons, squash, lettuce, spinach, cruciferous vegetables, and onions. They are more difficult to detect than onion and cabbage maggots because they feed on the developing shoot before the seedling emerges. You only know which plants have a problem when seedlings don’t emerge, they are only in greatly reduced numbers, or if they are damaged. 

What Causes Root Maggots? 

The appearance of root maggots can be random—adult flies of root maggots are able to travel up to a mile to find suitable host plants. If you’ve had root maggots in previous gardening years, they will likely appear again. 

In addition, there are three factors that can foster root maggot infestations: 

A cool, wet spring with temperatures between 66 and 79 degrees provides ideal conditions for root maggot eggs in the soil to hatch. The eggs cannot survive when the temperatures in the top two to three inches of soil reach over 95 degrees Fahrenheit.

Root maggots are attracted by animal manure, green manure, and any other partially decayed organic matter. This does not mean that you have to forsake all the benefits that adding organic matter adds to your garden soil. You need to make sure it is fully decomposed humus, so it does not offer root maggots anything to feed on. 

Weeds in the cruciferous family, such as yellow rocket, if they are growing nearby in substantial numbers can also bring root maggots to your garden. The root maggots overwinter in the debris and roots of those weeds, then move to other feeding grounds in the spring. 

How to Prevent Root Maggots 

There is no single failproof method to prevent a root maggot infestation. With a combination of different methods, you increase the chances of effective pest control. 

Floating Row Covers

Protecting susceptible vegetables with floating row covers is the best way to prevent the adults from laying their eggs and thus starting the entire infestation cycle. To keep the cover in place, avoid crushing tender young plants, as well as allowing for plant growth. It is best to set up the cover as a high tunnel and secure it with hoops. 

The vegetables should be covered from the time you seed or transplant seedlings in cooler weather until the onset of hot summer weather, and again in the fall when temperatures get cooler. Root maggots might go through another life cycle before the winter. 

Make sure the floating row cover is made of a breathable textile and lets in water and plenty of sunlight. Polyethylene covers are not suitable because it becomes too hot for the plants, and the condensation can lead to foliage disease. 

A high tunnel protects vegetable plants against root maggots
A high tunnel protects vegetable plants against root maggots

bungoume / Getty Images

Crop Rotation

Crop rotation is crucial to prevent root maggots, otherwise the overwintering population in the soil will have an easy target. Because root maggots move around, it is not only imperative that you don’t plant susceptible crops in the same location as the year before, but also that you plant new crops as far away from the location of the previous year’s crop as possible. 

Diatomaceous Earth

Spreading diatomaceous earth around the base of the young seedlings and transplants helps to keep root maggots from attacking the plant. It needs to be reapplied after each rain. 

Planting Schedule

Adjusting the planting schedule of susceptible crops in order to avoid root maggot peak times is another way to prevent an infestation. Depending on your weather, delay planting until the end of May or beginning of June when the soil is too warm for the eggs to survive. 

Raised Beds

The soil in raised beds warms up faster and earlier, and stays drier than in ground-level garden beds. Planting susceptible crops in raised beds creates an environment that is less than ideal for root maggot eggs to hatch because they prefer cool, moist soil. 

Growing green cabbage and kohlrabi in a raised bed helps prevent root maggots
Growing green cabbage and kohlrabi in a raised bed helps prevent root maggots

TG 23 / Getty Images

Garden Sanitation

Thoroughly cleaning up your garden and removing any crop residues of all susceptible vegetables is essential to prevent root maggots in the future. 

Promptly after the harvest, dig out all the spent plants including their entire roots and destroy them or dispose of them in the garbage. Even if you do not detect any root maggot damage, the plants should never be composted. Till or dig up the area so any overlooked plant is exposed, which will discourage the pupae from overwintering in the soil. 

Monitor nearby areas for weeds in the cruciferous family which can serve the root maggots as host plants. Remove them with their roots and dispose of them safely, the same way as any crop plants. 

Biological Control

Root maggots have several predators, including ground beetles, carabid beetles, and rove beetles, as well as different species of parasitic wasp who attack the larvae and pupae. Predaceous mites consume the eggs and spiders, male dung flies, yellowjackets, digger wasps, and birds feed on the flies. Parasitic nematodes and parasitic fungi also reduce root maggots. However, these biological controls alone are not sufficient to keep root maggot populations under control.

  • What will kill root maggots?

    Once root maggots are feeding on a plant, there is no chemical to kill them. Root maggot control is all about prevention.

  • What do root maggots turn into?

    After they pupate in the soil, they turn into flying insects that look like a small version of common flies.

  • Can you save a plant from root maggots?

    When a plant is heavily infested, you cannot save it but you can control the spread of the pest to other plants.

Article Sources
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  1. Root Maggots. University of Idaho Extension.

  2. Root Maggots in Alaska Home Gardens. University of Alaska Fairbanks, Cooperative Extension Service.

  3. Root Maggots. The Ohio State University Extension Vegetable Crops, Veg Net.

  4. Cabbage Root Maggot. University of Massachusetts Amherst, Center for Agriculture, Food, and the Environment, UMass Extension Vegetable Program.