Seedcorn maggot

The seedcorn maggot – Deltia platura (Meigan), Diptera: Anthomiidae – is the larva of a fly. The gray adult is less than 1/4 inch in length and resembles a small slender housefly (Figure 1).

The small, headless, larvae are less than one-fourth of an inch long, and white to light yellow in color. The body is tapered toward the head end. There are no visible legs or head capsule (Figure 2).

The pupae are oval and orange (Figure 3). As they age, the pupae turn dark brown. Only the adult stage occurs aboveground.

Seedcorn maggot adult
Figure 1. Seedcorn maggot adult.
Seedcorn maggots
Figure 2. Seedcorn maggots on soybean cotyledons
Seedcorn maggot pupa
Figure 3. Seedcorn maggot pupa. Photo: Randy Revard

Where they live

This insect occurs worldwide.

Hosts

The seedcorn maggot has a wide host range. The related cabbage maggot Deltia radicum L. is limited to cruciferous crops.

In addition to feeding on decaying organic matter, the seedcorn maggot attacks the germinating seeds of corn, soybean, edible bean, melons and other large seeded crops belowground.

Onions, canola and many other diverse crops are also hosts.

Life cycle

In Minnesota, the seedcorn maggot overwinters as a pupa in the soil. There are at least two generations in Minnesota.

Pupa development occurs when spring soil temperatures are warmer than 3.9 degrees Celsius (39 degrees F). Adults emerge in late April to May.

Degree days

You can use degree-day (DD) development models to predict the timing of fly activity. Fifty percent of the overwintering generation emerges as adults when 206 DD (3.9 degrees Celsius base) / 371 DD (39 degrees Fahrenheit base) have accumulated using 2-inch soil temperatures.

Adults are attracted to newly tilled fields and decaying organic matter. Eggs are laid in moist soil.

Fields at highest risk of damage are those with heavy applications of solid livestock manure and where a green manure (including heavy weed infestations and live cover crops) has been incorporated.

Natural enemies

Natural enemies of the seed corn maggot include:

  • Ground beetles.

  • Predaceous nematodes.

  • Fungal diseases.

You’ve probably seen small flies that have died on tall plant foliage. These are often seedcorn maggot adults that have been killed by a fungus.

Scarring on cotyledons
Figure 4. Scarring on cotyledons caused by seedcorn maggot.

Damage 

Damage from this insect is often worse when crop emergence is slow.

Feeding injury results in seedling injury and stand loss. Corn seed and seedlings can be heavily infested, reducing the stand. However, field corn stand losses can be lower than in other crops.

This is because of the seedling’s structure and because corn is often planted before flies are active.

The maggots have weak, hook-like mouthparts and can attack corn seeds only after they’ve been softened by water uptake. In corn, the endosperm is usually attacked and some tunneling in belowground tissues may occur.

The maggots damage germinating soybean and other large-seeded dicots by tunneling into cotyledons and hypocotyls (Figure 4). When a soybean’s plumule is damaged, the plant may form two terminal shoots or “snake heads.”

Scouting

In high-value crops and higher risk situations, yellow sticky traps or cone traps baited with molasses can help to determine whether adults are present after tillage and before planting.

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Managing infestations

There are no rescue treatments for seedcorn maggot infestations. Where possible, avoid planting higher risk fields during peak adult activity.  

In addition to predicting when egg-laying flies are active, degree-day models can be helpful in predicting when the damaging life stages are no longer active. By waiting 2.5 or more weeks (250 DD; 3.9 degree C base) after incorporation of organic material, seedcorn maggot would be in the non-damaging pupal stage.

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Always read and follow the pesticide label. 

Bruce Potter, integrated pest management specialist, Southwest Research and Outreach Center

Reviewed in 2018

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