How To Identify Seedcorn Maggots and What To Do About Them

There are a variety of creepy crawlies that specialize in invading specific plant species and cause problems as quickly as you can say “Jack Rabbit.” One such pest is the seedcorn maggot.

They infest your corn in order to feed and further their lifecycle, leaving behind damage that can ruin your harvest. If you don’t do something about them, they can cause serious crop devastation.

Read on and discover what you need to know about this troublemaker and how to address it.

What Are Seedcorn Maggots?

Seedcorn maggots (Deltia platuraformerly Hylemya platura, H. cana, H. cilicruraand H. like) belong to the family of Anthomyiidae. They are the larval form of the bean seed fly.

This pest was introduced into the US from Europe in the 1800s and is now recorded throughout North America, along with the rest of the world, except Antarctica. It has become a serious pest in the Americas, Europe, and parts of Asia.

They become active in the spring through autumn, but this can vary a little depending on temperatures and food sources. Depending on the length of the season, they can produce up to five generations per year.

The maggots feed on the seeds in the ground and on young seedlings. They can cause seeds to fail to germinate and have been known to reduce crops by 60% or more. Despite the name, they eat far more than corn.

How To Identify Seedcorn Maggots

It’s difficult to identify that there is a problem with this larvae pest until it’s too late to save your corn.

They live in freshly tilled soil, compost, or recently mulched fields and gardens, particularly where corn, soybeans, alfalfaand much more are being propagated.

Both the adults and larvae look similar to their cousins, the cabbage maggot fly (D. root) and the onion maggot fly (D. antiqua). Seedcorn maggots appear earlier in the season than their close relatives.

Lifecycle

In order to combat any invasion from seedcorn maggots, it helps to know their lifecycle. The entire process from egg to reproductive adult fly can take about two to six weeks, depending upon environmental conditions.

The adult flies insert eggs into the soil through cracks or loose clumps. They are typically deposited at the base of the intended host plants or in close proximity to seeds in the ground.

The eggs are tiny, oval-shaped, and white. They are almost undetectable once in the soil. Provided the right conditions, the eggs hatch in approximately three to ten days.

The larvae emerge and start feeding on the seeds or seedlings in their proximity. This is the most damaging phase of their lifecycle, lasting up to four weeks, and the damage they do can be immense.

Seedcorn maggots are small, white to yellow in color, legless grubs. Typically, they measure about 1/5 to 1/8 inch in length. The maggots have a cylindrical shape with slender bodies that narrow towards the end.

Their hook-like mouth parts make it easy to chomp through seeds and plants. You may find more than one eating the same host seed or plant as they group together.

The seed maggot must metamorphose through several instar phases before entering the adult stage. Then, its outer skin hardens to begin the next key stage in its evolution.

Seedcorn maggots shed their mouth parts and form a protective cocoon. They remain in the soil for around two weeks before emerging as adults. They can remain dormant until the environmental conditions are favorable. When the environment suits, the dormant pupae will emerge.

The adult fly of the seedcorn maggot can be mistaken for a house fly but smaller. The adults are about 1/5 inch long. Two to three days following their emergence, the adults mate, and the females deposit eggs in soil debris to start the next generation.

Seedcorn Maggot Habitat

For the optimum development of the seedcorn maggot through to adulthood and reproductive activity needs to be a steady 60-74°F for at least 25 days.

The trigger temperature for emergence at the beginning of the season is 39°F as long as a food source is available.

Moist soil is preferable as the female lays her eggs in the soil, and she is unable to penetrate hard ground. She will go in search of soft soil or freshly applied organic matter.

Susceptible Plant Species

Don’t be lulled into thinking the seedcorn maggot is only interested in feeding on corn species. It’s equipped with an adaptable digestive system to substitute its diet with other varieties of plants.

They can also feed on soybeans, alfalfa, strawberries, cabbage, turnips, radishes, onions, peas, potatoes, squashand melons.

Signs Of A Seedcorn Maggot Problem

Crop damage from a seedcorn maggot outbreak can begin even before seeds have germinated. Instead of big, healthy seedlings, your plants won’t emerge from the soil because they’ve been damaged by the maggots in the soil.

You might have empty areas in your rows or garden beds or missing patches where you planted seeds.

If you examine the seeds themselves, you might see hollowed-out or discolored seeds. The seeds might be rotten or have rotten areas.

For seedlings that do germinate, you might see wilting, yellowing, holes in the stems or roots, or a lack of roots. There may be delayed development and distorted growth or even the sudden collapse of the seedlings.

The maggots might also feed on the growing tips or cotyledons (the initial seedling leaves), cause Y-shaped growth or dead tips.

If you see any of these signs, look for any flies around the soil or dig up some soil and look for maggots.

Prevention

Once there is a seedcorn maggot outbreak, it can already be too late to control it. Prevention is the absolute key to ensuring that what is planted succeeds. Early crops are particularly susceptible.

1. Monitor

Scout your vegetable gardens, borders, and fields regularly to look for any evidence of pests. Overturn the soil here and there and examine it for maggots.

This is a crucial measure in detecting seedcorn maggot infestations early.

You can also place yellow sticky traps near the ground to capture the adults. If you see the adult flies, it’s time to treat preventatively, whether the maggots are present yet or not. The sticky cards won’t noticeably reduce adult populations, but it will alert you to the problem.

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2. Crop Rotation

Seedcorn flies prefer to lay their eggs in the soil near plants of the same family. By rotating crops, you can break the lifecycle and reduce the number of larvae in the soil.

Crop rotation is a smart technique no matter what you’re growing. When you skip a few years of growing a species in one area. it denies pests and pathogens a host and reduces the likelihood of problems.

3. Floating Row Covers

While it’s a little extra work to begin with, protection is definitely worthwhile. If you can cover the entire area with floating row covers, you stand a chance of completely preventing an infestation.

You should put floating row covers in place the day you plant and make sure the edges are tucked into the soil so the flies can’t find their way underneath.

Smaller covers like cloches aren’t as effective because the female can lay next to the cloche and the maggot can crawl right under. This is less likely to happen with floating row covers, especially if the edges are a few feet away from the plants.

4. Planting Techniques

The longer the germination period for the seeds, the higher the risk of a seedcorn maggot attack, simply because the seeds are in the ground longer.

Try to avoid sowing seeds close together and don’t till your soil unless necessary. When you till the soil, you break it up and give the females the perfect place to lay their eggs.

If you can, plant later in the season to avoid the egg-laying stage. Or, start seeds indoors and transplant out when the seedling is a nice and healthy size. At this point, it will be able to avoid the damage caused by the maggots.

Try to have a variety of species near your susceptible plants in order to attract beneficial insects and birds to help control the flies and maggots.

5. Treat Seeds

Treat the seeds with an insecticide when planting. That way, when a maggot goes to take a bite, they will be poisoned and killed. Use a product that contains pyrethrin. Use something like Bonide’s pyrethrin concentrate. Mix according to the instructions and dip the seeds.

Seedcorn Maggot Treatment

Because the maggots hide underground, they are difficult to treat. You should definitely rely on prevention and not treatments.

Entomophthora flies is a host-specific fungus that adult flies take up while feeding and die soon after consumption. The spores reproduce inside, causing the fly to hang upside down before it dies.

Sadly, it’s hard to get your hands on this fungus because its sensitive to its environment and difficult to grow. Check with your local extension office and they might be able to help you get your hands on it.

Introducing predatory nematodes to the soil can do some of the work of killing the maggots. Nematodes are microscopic roundworms that feed on many different maggot species. Pick up five million of them at Amazon. Follow the instructions carefully.

You might also opt to spray your garden with an insecticide to kill the adults, but keep in mind that this will also kill the beneficial insects in your garden and isn’t highly effective.