Despite its name, seedcorn maggot (Delia platura) is a generalist pest of large-seeded crops including field and sweet corn, soybeans, and a diversity of vegetable crops such as legumes, cucurbits, and crucifers. Seedcorn maggot larvae attack germinating seeds and seedlings and therefore pose a threat during a narrow window in the growing season. Attack by seedcorn maggot can be highly variable, with many fields receiving little to no damage while others may sustain substantial losses.
As a member of the family Anthomyiidae, seedcorn maggot is one of many Delia species that pose a threat to common crop plants. Introduced to the US from Europe in the 1800s, seedcorn maggot now has a global distribution with the greatest abundance in northern temperate regions (35-60°N). Seedcorn maggot adults are gray with black bristles and have large eyes that are typically reddish purple. In males, the eyes are nearly touching while in females the eyes are clearly separated.
Crops at Risk
- Corn (field corn, sweet corn)
- Crucifers (cabbage, broccoli, canola, etc.)
- Cucurbit crops (cucumber, melon and squash
- Legume crops (beans, peas and soybean)
Crop Risk Factors
Seedcorn maggot damage occurs when larvae burrow into germinating seeds and consume the seed germ or when they attack the underground portions of very young seedlings. Seeds that are planted early when the weather is still cool will have the slowest rate of development and therefore will be vulnerable to seedcorn maggot attack for an extended period. In warmer weather, seeds germinate quickly, thus escaping attack. The occurrence of seedcorn maggot is typically higher when the early growing season is cold and damp. Because females use decaying organic matter as an egg laying cue, fields treated with manure or plowed to incorporate hay or cover crops into the soil are thought to be at a greater risk of attack. However, there is a dearth of data actually testing the predictors of seedcorn maggot attack and therefore researchers at Cornell are investigating the role that these cues play in mediating seedcorn maggot preference in order to provide evidence based recommendations to growers.
If seedcorn maggot damage is substantial, replanting is the only option for growers. Therefore, control methods for seedcorn maggot must be preventative.
Options for cultural control include:
- Delaying planting until the ground is warmer to accelerate seedling emergence.
- Avoid planting in fields that have had manure or at least plowing in green manure and animal manure 2-3 weeks before planting to allow organic material to decompose.
- Planting during the window of time when the first generation of flies have pupated in the soil.
Seedcorn maggot risk is limited to seeds and Once plants have established they are no longer susceptible.
Seedcorn maggot overwinters as pupae in the soil and will not develop when temperatures are below 39℉. In the spring, once enough degree days have accumulated to complete their development, adults emerge, mate, and then seek egg laying sites. This generation poses the greatest economic threat to crops as cool temperatures delay seed germination, increasing the risk of damage. Eggs hatch in 2-4 days and the maggots seek and feed on seeds and very small seedlings. Once fully developed 12-14 days after hatching, maggots will pupate in the soil and emerge as adults 12-14 days later. This cycle repeats over 3-5 generations during the growing season before the final generation pupates in the soil to overwinter.
What are degree days?
Insects are cold blooded and therefore cannot develop when temperatures are below a minimum required temperature. This threshold temperature is used to calculate degree days, which are an important metric for predicting important events such as adult emergence. To estimate degree days you can simply subtract the threshold temperature from the average temperature of the day. For example, the temperature threshold for seedcorn maggot is 39℉. If on a given day the low was 40 and the high was 50, the average temperature of the day would be (40 + 50)/2 = 45℉. Now if we subtract the threshold temperature (39℉) from the average temperature (45℉), we find that 6 degree days accumulated on this day. A modeling approach can account for finer scale variation in temperature throughout the day to allow for a more accurate estimate of degree days. For the overwintering generation of seedcorn maggot, it is estimated that peak emergence occurs at 360 accumulated degree days.
The content for this page including photos and illustrations were provided by Haley Schroeder, PhD Student, Department of Entomology.