The seedcorn maggot is a perennial pest of the germinating seeds and young seedlings of a wide range of vegetable and agronomic crops. In addition to corn, seedcorn maggots (SCM) has a large host range including numerous common vegetable crops. SCM can cause economic damage to seeds of artichoke, beet, Brussels sprouts, cabbage, cantaloupe, carrot, cauliflower, cucumber, kale, lettuce, bean (lima, snap, red), onion, pea, pumpkin, tomato, and turnip. In high numbers SCM can decimate entire crop stands if left untreated. SCM can be an increasing problem when susceptible crop crops are planted in succession.

Appearance

Larvae are typical of many other fly maggots: 7 mm when fully grown, cream–colored, legless, and wedge-shaped. The head of the maggot is sharply pointed. SCM pupae are dark brown, 5 mm long, cylindrical in shape, and evenly rounded on both ends. SCM spends between 7-14 days in the pupal stage at normal summer time temperatures (~70°F). Adult flies resemble miniature houseflies. They are dark gray with black legs, 4-5 mm long and commonly hold their opaque wings flat over their abdomen when at rest.

Life cycle

Seed corn maggots overwinter as pupae in the soil. Adult flies begin emerging in the late spring, peak emergence for the first generation occurs in early to mid-May. Adult SCM often swarm over recently tilled fields or gardens. Preferred egg deposition sites are locations with germinating or decaying seeds, plant residue, incorporated green manures or where organic fertilizers have been recently applied. Adults often mate and lay eggs within 2-3 days of emergence. Eggs hatch 2-4 days later depending on soil temperature. The larval portion of the SCM life cycle occurs below ground over the course of a few weeks. Once hatched larvae burrow into the soil 6-8 cm to locate food resources. A complete life cycle for SCM can range from 15-77 days, in a typical season 16-21 days is sufficient. In Wisconsin, there are typically 3-5 generations per year.

Damage

Injury to plants is caused exclusively by the larval stages of SCM. Larvae will feed in the cotyledons and below-ground hypocotyl (stem) tissue of seedling plants, resulting in a variety of damage symptoms. Feeding damage in germinating seeds will often kill seedlings before they emerge. Poor germination or poor stands of susceptible crops may indicate a SCM problem. In addition to SCM other issues may compromise stand counts and germination. In cool, wet soils common fungal pathogens such as Pythium may similar patterns of irregular crop emergence. To best diagnose SCM damage dig up un-emerged seedlings to look for feeding directly on the seed or sprout tissue. Often seedlings will survive below ground feeding but emerge with damage to the first true leaves or have no leaves at all (often called “snakehead” seedlings). Direct damage to the hypocotyl will often appear yellow and wilted.

Scouting

Management for SCM is only effective when used in a preventative manner. Once direct larval damage is detected there is no control option for the pest. Therefore, there are no economic thresholds for this insect pest.

SCM forecasting models to more generally predict peak flight windows are very effective for the majority of growers. Models calculate flights by calculating cumulative degree days beginning when the ground thaws in the spring. Simple degree days are calculated using the general formula [(daily high (°F) + daily low (°F)) ÷ 2] – 39. Use a maximum high temperature of 86°F and a minimum of 39°F for a low. Cumulative degree days are calculated by keeping a total sum of daily degree days (omit negative values). In Wisconsin, peak flights of SCM occur at 360, 1080, 1800, 2520, and 3240 cumulative degree days. Minimum crop risk occurs 450 degree days after peak flight, when larvae have ceased feeding and begun pupation.

Documenting peak flights can help to forecast subsequent generations of SCM. Adult flies are attracted to volatiles of decaying organisms. Accurate flight information can be monitored using cone traps baited with fermenting molasses, alcohol, bloodmeal, or bonemeal. Sticky traps have also been used successfully to keep track of adult flights at the local scale and are easily obtainable from garden stores.

Field-scale SCM flights can be easily monitored for fly-free risk windows. Depending on desired planting date, place three or four yellow plastic dishpans filled with soapy water along the edge of the field or garden (typically at 100’ intervals). Every four to six days, remove and count the trapped flies and add fresh soapy water. Keeping a record of the number of flies trapped will indicate when fly numbers are building up or tapering off. Local scouting coupled with degree day forecasting models can be an extremely effective non-chemical tactic to avoid SCM infestations in time.

Management

Cultural control

Since adult SCM is attracted to decaying organic matter, do not plant susceptible crops in fields where animal or green manure has recently been incorporated. The faster planted seeds germinate and grow, the less opportunity the maggots have to damage the crop. There are a few strategies to hasten germination:

• Delay planting until soil temperatures are at least 50°F before planting most susceptible crops. Peas and radish may be planted when soil temperatures are above 40°F.
• Plant seeds as shallowly as agronomically possible to speed germination.
• Soak untreated pea and bean seeds in water for 2 hours before planting to soften the seed coat.

Biological control

Naturally occurring fungal pathogens are occasionally effective at increasing SCM mortality, particularly when flies are abundant and humidity is high. During a fungal epidemic dead or diseased flies can be seen clinging to the highest parts of plants along field edges.

Predacious ground beetles eat SCM eggs, larvae and pupae. Ground beetles can significantly reduce SCM numbers in certain systems. Because these soil-dwelling beetles are also susceptible to insecticides avoid using broadcast applications whenever possible.

Chemical control

SCM is a recurring issue for many producers. Two common chemical delivery techniques are available for SCM management: seed treatment or an at-plant soil application. Numerous combinations of insecticidal compound and fungicide are available as pre-plant seed treatments directly from the seed vendor. Many of these components for SCM management are reduced risk insecticides which have lower non-target impacts.

Refer to the UW-Extension publication Commercial Vegetable Production in Wisconsin (A3422) for a list of registered insecticides and management recommendations.

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