Seedcorn maggots are small fly larvae that burrow into germinating seeds of corn, soybeans, and vegetables, destroying the embryo before the plant ever emerges. They’re one of the most common early-season soil pests in North American agriculture, and because the damage happens underground, growers often don’t realize there’s a problem until gaps appear in their rows. The entire life cycle from egg to adult takes just three to four weeks, meaning multiple generations can emerge in a single growing season.
What Seedcorn Maggots Look Like
The larvae are dirty white with a yellowish tint, legless, and tapered toward the head. Fully grown maggots measure roughly 1/4 inch long. The head end is pointed with two small black mouth hooks, while the rear end is blunt with two brown-to-black breathing pores. If you dig up a damaged seed and see a small, pale, wedge-shaped worm feeding inside it, you’re almost certainly looking at a seedcorn maggot.
The pupae are easier to spot in soil. They look like tiny dark brown or black barrels, about 1/4 inch long, and are usually found near roots or decaying seed. Adults are small grayish-brown flies, roughly half the size of a house fly, with three faint stripes running along the back. At rest, they hold their wings crossed over the abdomen rather than spread apart.
Life Cycle and Seasonal Timing
Adults emerge from overwintering pupae in the soil once temperatures warm in spring. Females lay tiny white eggs (about 1/16 inch long) in moist soil rich in organic matter, especially near freshly turned ground, decomposing crop residue, or recently applied manure. The eggs hatch in two to four days when soil temperatures reach at least 50°F. Larvae feed for about 21 days before pupating, and the pupal stage lasts at least 10 days. The full cycle requires a mean temperature of around 74°F sustained for 24 to 25 days.
Three generations typically emerge per year, and their timing can be predicted using degree-day models. Starting from January 1 with a base temperature of 39°F, peak adult emergence for each generation hits at these accumulated degree-day thresholds:
- First generation: 360 degree-days (Fahrenheit)
- Second generation: 1,080 degree-days
- Third generation: 1,800 degree-days
The first generation is the one that causes the most damage to spring-planted crops. By the time the second and third generations emerge, most seeds have already germinated and seedlings are large enough to tolerate some feeding.
How They Damage Crops
Seedcorn maggots attack seeds that are germinating or have just begun to sprout. They chew into the seed and consume the embryo, which prevents the plant from ever emerging. The most visible sign in a field is reduced stand: gaps in the row where seeds were planted but nothing came up. If you dig in those gaps, you’ll often find hollowed-out seed husks, sometimes with maggots or pupae still present nearby.
Even seeds that aren’t completely destroyed can produce weak, stunted seedlings. The feeding wounds also open the door to fungal and bacterial infections, which can cause damping off, a condition where young seedlings rot at the soil line and collapse. Corn and soybeans are the crops most commonly affected, but seedcorn maggots also attack beans, peas, cucumbers, melons, onions, and other large-seeded vegetables.
Fields Most at Risk
Seedcorn maggot problems are strongly linked to soil conditions at planting time. Fields with high levels of decaying organic matter are the biggest attractant. This includes fields where cover crops were recently terminated and incorporated, where green manure or animal manure was freshly applied, or where crop residue from the previous season is still breaking down. The adult flies are drawn to the odor of decomposing plant material and lay their eggs in that soil.
Cool, wet springs amplify the risk. When soil stays cold and damp, seeds germinate slowly, giving maggots more time to find and feed on them before the seedling can outgrow the vulnerable stage. Early planting dates in heavy, poorly drained soils are a particularly high-risk combination. No-till or minimum-till fields with heavy surface residue can also attract egg-laying flies.
Scouting and Confirming the Problem
If you notice patchy emergence or uneven stands a week or two after planting, seedcorn maggot should be on your list of suspects. To confirm, dig up seeds in the areas with poor stand. Look for the maggots themselves, the barrel-shaped pupae, or seeds that have been hollowed out and are mushy or discolored. Poorly developing seedlings with chewed or scarred roots are another sign.
It’s worth noting that other problems, like cold soil injury, herbicide damage, or wireworms, can also cause stand loss. Finding actual maggots or their characteristic feeding damage (a seed with the interior eaten away, leaving a papery shell) is the clearest confirmation.
Prevention Through Planting Practices
Timing is the most effective cultural tool. If you’ve incorporated a cover crop or applied manure, waiting as long as possible before planting gives that organic material time to decompose and become less attractive to egg-laying flies. A general guideline is to allow at least two to three weeks between incorporating green organic matter and planting seed into that soil.
Planting into warmer, drier soil also helps. Seeds that germinate quickly spend less time in the vulnerable window. Shallow planting in cold, wet conditions slows germination and extends exposure. Anything you can do to promote fast emergence, whether that’s waiting for warmer soil, using high-vigor seed, or improving drainage, reduces the odds of significant damage.
Seed Treatments and Insecticides
There is no rescue treatment for seedcorn maggot once larvae are feeding on your seeds. By the time you notice stand loss, the damage is done. Prevention is the only chemical option, and it comes in two forms: treated seed or soil-applied insecticides at planting.
Most commercially treated corn and soybean seed already includes an insecticide coating that provides protection against seedcorn maggot along with other soil-dwelling pests. These coatings use compounds in the neonicotinoid family that are absorbed by the germinating seed and kill larvae that try to feed on it. If you’re planting untreated seed in a high-risk field, a soil-applied insecticide can be banded in the furrow at planting, though this requires the right equipment on the planter.
For vegetable growers planting beans, peas, or cucurbits in small plots, treated seed is less universally available. In those situations, the cultural practices described above, particularly timing incorporation of organic matter and planting into warm soil, become even more important.
Replanting After Damage
When stand loss is severe enough to hurt yield, the decision to replant depends on how many plants survived and how late in the season it is. Walk the field and count surviving plants across multiple rows to estimate your actual population. For corn, losing 10 to 20 percent of a stand may not justify replanting if the remaining plants can partially compensate, but losses beyond that often do. Soybeans are more forgiving of thin stands because individual plants branch to fill gaps.
If you do replant, the risk of a second round of maggot damage is usually lower. By the time replanting happens, soil is warmer, seeds germinate faster, and the first generation of flies may have already finished laying eggs. Still, using treated seed for the replant is a reasonable precaution, especially if the field has heavy organic residue.