Seedcorn Maggot Activity Beginning in Southern Iowa

By Ashley Dean and Erin Hodgson

Seedcorn maggot larvae feed on germinating seeds or seedlings of corn and soybean (Photo 1). Feeding can delay development or kill the plant, and plant injury is especially prevalent during cool, wet springs when plants grow slowly. Even with variable weather so far this spring, seedcorn maggot development is on track with previous years. Forecasted temperatures suggest that seedcorn maggot will likely develop quickly in the next few weeks. Farmers thinking about planting should track growing degree days to reduce the risk of injury in high-risk fields, which include those with a history of seedcorn maggot injury, recently tilled fields, and fields where organic matter was recently incorporated (e.g., manure or cover crops).

Photo 1. Seedcorn maggot on soybean. Photo by University of Minnesota Extension.

Seedcorn maggot has a lower developmental threshold of 39°F and an upper threshold of 84°F. Peak adult emergence for the first generation occurs at 360 accumulated degree days (GDD) since January 1. Based on current GDD, the first generation of adult seedcorn maggot has likely emerged throughout much of southern Iowa (Figure 1).

Figure 1. Accumulated growing degree days (base 39°F) in Iowa from January 1 to April 9, 2023. Map courtesy of the Iowa Environmental Mesonet, Iowa State University Department of Agronomy.

It is recommended to avoid planting during peak adult emergence, because first-generation larvae typically emerge within a few days (414 GDD) and feed for several weeks (781 GDD). Remember that cool soil temperatures likely increase the risk of seedcorn maggot injury because seeds take longer to develop; warmer soil temperatures may be conducive to quick growth of seedlings and less injury. If possible, target planting during the “fly-free” period (781-1,051 GDD) in areas at higher risk for seedcorn maggot.

You can track GDD for seedcorn maggot by visiting the Pest Maps and Forecasting page and comparing key GDD to the map. Additionally, if you choose your nearest weather station on the right side of the page, you can see forecasted GDD for the next two weeks.

While no rescue treatments are available, cultural and chemical methods prior to or at planting can minimize risks. Read more about seedcorn maggot biology, risk factors, and management in this encyclopedia article. To confirm seedcorn maggot injury, check areas with stand loss and look for maggots, pupae, and damaged seeds (hollowed out seeds or poorly developing seedlings), and keep an eye out for other seedling pests, such as wireworms or grubs.