Seedcorn Maggots Likely Active throughout Iowa

April 12, 2024
ICM News

Seedcorn maggot larvae feed on germinating seeds or seedlings of corn and soybean (Photo 1). Feeding can slow development or kill the plant, and plant injury is more prevalent during cool, wet springs when plants grow slowly. Even though most of Iowa has been in a drought, recent rain events have likely created pockets of damp soil that are ideal for developing larvae. High risk fields 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). Farmers thinking about planting should track growing degree days, plant during the fly-free period, and avoid planting into cool, wet soils.

Seedcorn maggot.
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 the state (Figure 1), and development has been accelerated compared to recent years due to warm conditions early this year. Keep in mind that development is likely overestimated slightly because air temperatures are used for the calculation while much of seedcorn maggot development is based on soil temperatures.

Growing degree days map.
Figure 1. Accumulated growing degree days (base 39°F) in Iowa from January 1 to April 10, 2024. (Map courtesy of the Iowa Environmental Mesonet, Iowa State University Department of Agronomy.)

It is recommended to avoid planting during peak adult emergence since first-generation larvae typically emerge within a few days (414 GDD) and feed for several weeks (781 GDD). Cool soil temperatures likely increase the risk of seedcorn maggot injury because seeds take longer to develop; warmer soil temperatures are conducive to quick growth of seedlings and less injury. If possible, target planting during the “fly-free” period (781-1,051 GDD for the first generation) in areas at higher risk for seedcorn maggot. Because development is ahead of schedule this year, planting may not coincide with the highest risk of seedcorn maggot injury (when first generation adults are laying eggs). Keep an eye on GDD accumulation and compare with Table 1 in our encyclopedia article to determine the fly-free period for subsequent generations of 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.

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on April 12, 2024. The information contained within may not be the most current and accurate depending on when it is accessed.


Ashley Dean Agriculture and Natural Resources Extension Specialist II

Ashley is an education extension specialist for field crop entomology at Iowa State University. She coordinates the Iowa Moth Trapping Network, the Regional Corn Rootworm Monitoring Network, and the Iowa Pest Alert Network. She also develops educational resources for field crop pests in Iowa and ...

Erin Hodgson Professor

Dr. Erin Hodgson started working in the Department of Entomology, now the Department of Plant Pathology, Entomology, and Microbiology, at Iowa State University in 2009. She is a professor with extension and research responsibilities in corn and soybeans. She has a general background in integrated...