In this year of predicted tight margins for producing field crops, farmers are asking which expenses they may be able to cut — treated soybean seed might be an option.
Soybeans are often planted with seed treatments, however, integrated pest management (IPM) practices can be used for successful planting of untreated soybean seeds. The insect pest causing greatest concern for untreated soybean seed is the seedcorn maggot (SCM).
SCM larvae create damage by burrowing into soybean seeds and feeding on cotyledons and the hypocotyl of developing seedlings, thus causing possible decay and death. SCM overwinters as a pupae and develops through the adult, egg and larval stages very predictably according to heat unit, or insect degree day, accumulation. Tracking SCM degree days, available on the UW-Extension Ag Weather website, can help soybean and other crops growers avoid planting during the most vulnerable times for SCM feeding, thus providing an IPM tool for cultural (non-chemical) control of SCM.
In upper Midwestern states, SCM undergoes three generations within a season. Female adults (flies) are attracted to freshly tilled fields with decaying organic material—incorporated legumes and green cover crops are most attractive. No-till and minimum-till fields with no green vegetation are the least attractive but may have some vulnerability during times of peak SCM activity.
Peak emergence of adults (peak flight times) are when the greatest egg-laying in soil occurs. Eggs hatch within 2-4 days, at which time untreated seeds and seedlings are vulnerable to larval feeding damage. Avoiding the peak flights and planting near the time where SCM is entering its non-feeding pupal stage can be accomplished by monitoring SCM degree day accumulation.
The UW-Extension Ag Weather website uses data from weather stations to calculate daily accumulations of degree days for several crop insect pests, including SCM. In the Thermal Models section of the website, choose Seedcorn Maggot to view a map showing current accumulated SCM degree days for regions across Wisconsin and Minnesota. Or, choose the Generic Degree Day Calculator to input specific latitude and longitude information for the field/farm to calculate degree day accumulations for SCM or 17 other pest categories.
The following table shows the SCM Fahrenheit degree day (DD) accumulations associated with peak flights for each of the SCM population’s three generations. An additional 450 F days are required for development to progress through the egg, larval and beginning pupal stage. Again, planting between peak flights and more near onset of the pupal stage is the goal.
More detailed information on cultural control methods for SCM in soybeans, corn and vegetable crops, including identification, lifecycle and using SCM degree days to time plantings when using untreated seed, can be found in UWEX Publication A3972 Insect IPM in Organic Field Crops: Seedcorn Maggot.