Although many U.S. crop fields already have been harvested because of the drought, farmers still need to follow fly-free dates when planting wheat to avoid Hessian fly infestations in their winter crops, says a Purdue entomologist and Agricultural Research Service scientist.
Those dates already have passed in the northern Midwest, but can be as late as the end of October in the southern United States. Indiana's fly-free dates are as late as Oct. 9 in the southern part of the state.
"It's imperative that we don't jump the gun," Brandi Schemerhorn said. "It's tempting given the open fields early in the season to plant wheat early, but this could lead to disastrous consequences. The fly-free date is our main protection to avoid an infestation by the spring brood."
Hessian flies are small pests that are mosquito-like in appearance. There are at least two generations each year, one in fall and another in spring. The fall generation is more important economically because the larvae feed on green plant growth, including developing wheat crops.
Females can lay 150-300 eggs in a short time on the sides of wheat plants. The larvae will crawl downward into the whorl and begin feeding. Farmers inspecting their fields often miss the flies because they are hidden at the bottom of the plant.
"Just because you don't see them in the field doesn't mean they're not there," Schemerhorn said.
While the pests never enter the stem, they damage the plant so severely that infested stems usually break once the heads begin to fill. This, along with stunted growth caused by the flies, can lead to yield reductions.
Schemerhorn said planting after the fly-free date is the best way to prevent infestation. After these dates, most adult flies will die before the wheat emerges and, therefore, will not lay more eggs.
"Even if you spray insecticide, that's not going to get rid of them because the chemicals can't reach the larvae," she said.
Farmers observing the fly-free date for their regions could also help with other problems that their crops may experience.
"It has been shown that following the fly-free date helps reduce other wheat disease problems and reduces winter-kill from excessive growth," Schemerhorn said.
She offered other Hessian fly management tips:
* Clear fields of volunteer wheat. Flies can lay eggs in the early plants and rapidly build up their populations. Removing volunteer wheat before the emergence of the fall brood greatly reduces the insect reservoir for a spring infestation.
* Plant resistant wheat varieties, even after the fly-free date. Warmer temperatures in the late summer and early fall may extend fly activity beyond the normal fly-free date.
* After harvesting wheat in the spring, plow under the stubble instead of burning. Plowing fields after harvest destroys the fly. It buries adult flies and limits the number that can escape from the ground.
The Hessian fly is always around, but in some years the damage is more widespread than others. Schemerhorn said it's always a problem in the southeastern United States, but damage in the Midwest varies from year to year.
A map of fly-free dates across the Midwest can be found at http://extension.entm.purdue.edu/pestcrop/2012/issue25/graphics/popups/flyfreeDates.jpg.