Seedcorn maggot degree days and corn and soybean risk in cool, wet soils

Seedcorn maggot degree days and corn and soybean risk in cool, wet soils

Eileen Cullen, Extension Entomologist

One of the earliest potential pests of corn and soybean is seedcorn maggot, Delia platura. Seedcorn maggot is a soil insect pest of seeds and emerging seed leaves. Corn and soybean (and vegetable crops) are more susceptible to seedcorn maggot damage under conditions that delay emergence, such as cool spring temperatures and wet field conditions that cause soil surface crusting.

This article, website links, and attached Extension fact sheets offer degree-day phenology model information for seedcorn maggot adult flight peak.  The information is particularly applicable to conventional fields in the absence of seed treatment, and organic production systems. Planting as close as possible to the ‘fly-free’ period between seedcorn maggot generations will minimize risk and serves as the primary cultural control for this spring soil insect pest.

This year, late planting and cool wet weather limit grower options to delay planting any further to avoid peak seedcorn maggot flight. However, the information is very useful to plant around seedcorn maggot peak flight to the degree possible in fields at risk. Risk is relatively higher this year as first generation seedcorn maggot will peak over the next couple of weeks where corn and soybean planting are underway. If weather conditions promote timely crop emergence after planting, this can also help minimize risk.

Nearly all conventional corn and much of the soybean seed come coated with a neonicotinoid insecticide seed treatment that offers protection from seedcorn maggot.

Fields at risk for seedcorn maggot include fields without insecticide seed treatment. Small grain or legume green cover crop incorporation, weed cultivation, and manured fields with decomposing fresh organic matter increase attractiveness of fields to adult seedcorn maggot flies at spring planting.

Seedcorn maggot overwinters in the soil as pupae. Adult flies emerge in spring once the ground has thawed and sufficient heat units, or degree days, have accumulated. Peak spring emergence of the overwintered (first) generation will occur at 360 Fahrenheit degree days (base 39°F) or 200 Celsius degree days (base 3.9°C). Most of the seedcorn maggot eggs will be laid during peak flight, and degree days can be used to avoid planting during this peak flight.

To track seedcorn maggot degree days for your area, click here or visit:

You can also view seedcorn maggot degree days for Wisconsin on The IPM Toolkit app available to users for free for iPhones and iPads through the University of Wisconsin’s Integrated Pest Management Program. From the app choose the “Publications” tab, then scroll down to the bottom of the publications list. Click on Beta test – Seedcorn maggot thermal model, first-generation peak adult emergence at 360 DD using the “quick look” icon.


First generation seedcorn maggot adult flights have peaked in southern and southwestern Wisconsin. Follow the daily degree day accumulations on the Wisconsin map at the link above to check your area. First generation peak is occurring over the next week or more (depending on temperatures) as you move north on the degree day map.

Seedcorn maggot has three generations in the Midwest. The first generation occurs in spring when field corn and soybean seeds are usually most susceptible to larval feeding damage, particularly under cool wet soil conditions. During subsequent generations, established corn and soybean seedlings and young plants are no longer vulnerable to seedcorn maggot.

For more information on calculating seedcorn maggot degree days for your area, the following UW Extension resources are available free online:

Holm, K. and E. Cullen. 2012. Insect IPM in Organic Field Crops: Seedcorn maggot. UW Extension Publication A3972-01.

*Publication A3972-01 content is applicable to conventional non-seed treated fields and provides step by step instructions on how to calculate degree days using  the UW Extension Ag Weather website.

Delahaut, K. 2007. Seed corn maggot. UW Extension Publication A3820.

To learn more about insect degree days , click here