Seed-corn maggots shouldn’t be much of a problem in soybeans, yet there were fields in Michigan and Indiana this past growing season where the infestations and ensuing damage was so bad that fields had to be replanted.

SCM feeding on soybeans is common, showing up as scars on the cotyledons. Plants typically grow out of the damage, says Chris DiFonzo, Michigan State University entomologist. While the level of stand loss in soy this year is unusual, it’s not a surprise given the recent trend to plant beans earlier, into cooler soils and closer in time to tillage or burndown operations.

“Early planting exposes soybeans to a riskier time in the SCM life cycle, and the larvae from such infestations are happy to attack germinating corn, soybean, vegetables and many other crops,” she says.

Don't plant soybeans so early, but if you do, plant them in fields free of decaying material, DiFonzo advises. “Flies eat decaying stuff in general,” she adds. “If there's nothing there to attract them, they're not coming. But if you work something into the soil, particularly something green — it can be weeds, it could be a cover crop, doesn't matter, they don't discriminate — and then you plant early and don't leave ample time for that to decay enough, it’s like ringing the dinner bell.”

SCM overwinters as pupae in the soil. Flies emerge early, typically in mid-April. The degree-day model for SCM is based on accumulating degree days from Jan. 1 based on a chilly 39 degrees F, DiFonzo says. Peak flight and egg laying, meaning an estimated 50% of flies have emerged, occurs at 355 degree days. Flies lay eggs in fields with freshly decaying stuff, which is why DiFonzo does not recommend planting for at least two to three weeks later. 

"Once maggots start feeding, damage is worse under cool soil conditions, because the vulnerable, below-ground growth is exposed to attack for a longer time,” she says. “They feed on the  decaying matter. They’re not necessarily targeting the seed emergence, it’s just there and they feed right through it.”

A calculator on the MSU EnviroWeather website predicts fly emergence and egg laying. Visit enviroweather.msu.edu, and click on the corn and seed-corn maggot tabs.

Tillage and risk

Tying agronomic situations to risk of SCM infestation, DiFonzo cited information provided by Ron Hammond, a retired Ohio State University field crops entomologist, who did many SCM efficacy trials.

The highest risk for infestation and crop damage is when tilling is done on alfalfa, another legume, living green grass and heavy weed growth, Hammond says. Moderate risk was associated with tilling soybeans and corn residue, while very low risk was observed with tilling bare soil. No risk was seen with no-till in all crops.

“Maggot risk occurs with tillage [even light tillage] of a maggot food source — the greener and fresher the better — near or just before planting under cool conditions [April], which results in a combination of attractive conditions for infestation and delayed emergence, so maggots have plenty of time to attack seed,” DiFonzo explains.

By not planting soybeans early and into ground with decaying matter, it can eliminate the need to use a neonicotinoid seed treatment on soybeans.

“You don’t have a choice about getting a neonic on corn, but you do have a choice when you rotate a field to soy,” says DiFonzo, who cautions not to rely on a neonicotinoid seed treatment to protect against maggot feeding as many of the reports of damage by SCM this past spring were in treated soybeans.

And a neonic is not recommended when heavily damaged fields are replanted. “A whole lot of money just flew out of growers’ pockets to replant with a seed treatment when the fly risk was past or low,” DiFonzo says. “It’s human to make decisions based on the last crisis instead of the current situation. As Churchill said, ‘Generals are always prepared to fight the last war.’”

If growers had waited two to three weeks before planting, the hit could have been avoided, she says. “Yes, maybe you're hitting some yield on the back end, but pay attention to the front end — get all your beans up with a good stand. Otherwise, you're just moving dollars around,” DiFonzo says.