Wheat fly-free date is set for a reason

In years past, agronomists consistently reminded wheat growers of the Hessian fly-free date.

Today, not nearly as much fuss is made over this fall date. However, University of Illinois agronomist Steve Ebelhar says that doesn’t make it any less important. In fact, it might be more important these days than ever before.

“In the past 20 years, we’ve noticed a strong correlation between the fly-free date and the instances of barley yellow dwarf virus,” Ebelhar adds.

Key Points

• Fly-free date is more relevant than ever thanks to barley yellow dwarf virus.

• As many wrap up harvest early, there may be a temptation to get wheat in early.

• Consider seed pretreated with an insecticide for extra protection.


Barley yellow dwarf virus is typically vectored by aphid feeding. It just so happens the fly-free date corresponds fairly well with an aphid-free date.

Some, but not all, aphids are carriers of barley yellow dwarf virus, which can infect plants in the fall. However, it typically doesn’t show its face until spring. When it pops up, turning leaves yellow and then purple, Ebelhar says farmers should expect 10% to 25% less in yield. And, perhaps most importantly, there is no rescue treatment.

“Our research says we really don’t want farmers planting more than seven days before the fly-free date,” he adds.

If harvest wraps up as early as most think it will, planting early wheat could be a big temptation. For those faced with this dilemma, Ebelhar says at least consider planting seed pretreated with insecticide. While it may add $10 to $15 per acre in seed costs, he says research shows a clear advantage to early plant health, especially in southern Illinois.

Of course flies and viruses have nothing to do with the concern of putting on too much growth in the fall. Ebelhar says 3 to 4 inches is optimal. If stands are near 6 to 7 inches tall going into the winter, be prepared for the possibility of some winterkill.

Finally, Ebelhar notes that not every year will be like 2009, when things never really dried up enough to plant wheat. “You can plant two to three weeks after the fly-free date and get a good crop.”

In fact, last year some of his top yields were from a November planting date.

Hessian fly story

U of I agronomist Steve Ebelhar says the Hessian fly was brought to the U.S. when the nation contracted Hessian soldiers to fight in the Revolutionary War.

In the fall, flies descend upon freshly planted wheat fields and lay eggs. The maggots hatch and feed on the wheat seedlings. The feeding results in a stunted plant and stand reductions. When wheat grows in the spring, the plant has weakened stems and small, poorly filled grain heads. Lodging can also be a problem.

The fly-free date was set by agronomists to secure a time period when Hessian flies had entered dormancy. A frost isn’t a requirement — just prolonged cool temperatures.


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This article published in the October, 2010 edition of PRAIRIE FARMER.

All rights reserved. Copyright Farm Progress Cos. 2010.