In fighting one of the most virulent of wheat pests, Hessian flies, producers are equipped with the rough equivalent of a well-stocked toolbox of pliers, wrenches and utility knives but nothing like a sledgehammer to deliver a knockout blow.
Hessian flies cannot be eradicated; they can only be understood. But by understanding them and employing a management system based on a knowledge of their life cycle, growers can substantially reduce the risk of Hessian fly damage.
As threats to wheat go, Hessian flies rank behind barley yellow dwarf year in and year out, but the effects of the mosquito-like flies are potentially devastating.
“Hessian flies get every grower’s attention because when you get a really bad infestation you end up with a field where there is no harvestable wheat and that either must be plowed under, harvested as hay or used as a cover crop,” says Kathy Flanders, a professor in the Department of Entomology and Plant Pathology at Auburn University and an Alabama Cooperative Extension System entomologist.
The species spends its summers in brown puparia that are called flaxseeds because they bear a close resemblance in color and shape to these seeds. They begin to emerge into adulthood in late August or early September.
“They come out looking for volunteer wheat or a couple of alternative hosts such as little barley,” Flanders says. “It’s the next generation that ends up infesting production wheat fields.”
Adults lay eggs on the leaves, and the maggots feed in protected areas on the plants.
”Depending on where you’re located in Alabama, you can have multiple generations — some occurring in late fall, others in late winter and early spring.”
With interest in wheat planting on the sharp rise in recent years, Flanders says producers should make themselves fully aware of the risks.
“As wheat acreage increases, the number of over-summering puparia also increase,” she says, “and as these adults come out and fly, the chances of their finding wheat is higher.”
For this reason, understanding the life cycle of the Hessian fly constitutes the frontline of control measures, Flanders says, who stresses that effective managements starts from the very beginning — during planting.
First rule of thumb
“The first rule of thumb is to avoid early planting, because that puts producers at higher risk not only for Hessian fly, but also for a disease known as barley yellow dwarf spread by aphids,” she says.
Farmers should plant during the recommended planting dates for wheat for their particular region. Planting too late results in loss of yield potential because of various agronomic factors.
The risks associated with this pest should be of special concern to producers who are not running rotation systems.
“If farmers are not rotating, if they’re running continuous wheat or soybean double-cropping systems, they should be concerned that the Hessian flies are going to be out in their fields and ready to start the cycle again each fall.”
Stubble burning is one option to address over-summering of winter pupae, but this only provides partial control.
“Some of them will escape by falling into cracks in the soil, but it is a practice that can mitigate the problem.”
Farmers who have not adopted no-till or minimal-tillage systems should also consider tilling immediately after wheat harvest as an added safeguard.
“A key concern is getting rid of any volunteer wheat that may act as hosts for the first generation that will work to build up future generations of the pests,” Flanders says.
Hunting plots are typically planted early and can serve as hosts of the first generation of flies. Consequently, these plots should be planted in something other than wheat, Flanders says.
Likewise, choosing something other than wheat as a cover crop reduces overall risk to production fields.
“As a general rule, save wheat for wheat production and plant wildlife plots and cover crops to non-hosts such as oats or ryegrass or some other alternative crop,” Flanders stresses.
Resistant varieties are yet another option for controlling Hessian flies, but, like other practices, they are no panacea.
“Right now, there are a few varieties — those that carry the H-13 gene — that carry more resistance than other varieties,” she says.
Older varieties carrying the H-7 and H-8 genes also appear to offer some benefit in some parts of the state, though the most effective ones are the handful equipped with the H-13 gene, Flanders says.
“If you can get your hands on one of those varieties they are a good management tool and don’t cost more,” she says.
Few insecticidal options are available to kill Hessian flies. High rates of approved seed treatments provide some control, but may not be economical.
The immature Hessian flies feed behind the leaf sheaths, so foliar spraying of insecticides has to be targeted toward adults. But predicting when the adults are present is difficult.
Flanders cites research in North Carolina demonstrating that spraying an insecticide immediately following wheat emergence may be another effective way to manage the flies, though at this point this is backed up with no hard evidence from tests in Alabama.