Hessian fly returns to wheat fields

Hessian flies have not been a major problem for wheat growers in the upper Southeast for the past few years. This spring a few wheat fields in the northern corner of North Carolina and southern Virginia have been devastated by Hessian flies.

Keith Balderson, county Extension agent in Essex County, Va., says the few fields in which he found Hessian fly were severely damaged by the insect. “We won’t know for sure until the wheat is combined, but I believe these growers will lose more than 30 bushels of wheat per acre to Hessian flies,” he says.

“I don’t think this is a widespread problem in Virginia by any means, but where these insects are popping up, the damage can be severe. If you have a raggedy looking wheat field, it is a good idea to take a close look at the crop. It is easy to pass this damage off as freeze damage, but a close look at the tillers will tell you if Hessian fly is the problem,” Balderson concludes.

Ames Herbert, professor of entomology at Virginia Tech and Virginia IPM director says, controlling Hessian fly is a matter of timing. If you can catch the larvae as they are moving down the plant, Hessian fly is fairly easy to control. Once they get deep into the plant, they are much more difficult to control.

Most of the spring damage comes in secondary tillers. A close look at the secondary tillers in wheat infected by Hessian flies often will reveal as man as 14-15 larvae at the base of flax seed where the fly laid its egg on the wheat stem. These eggs hatch, larvae crawl down and feed up and down the stem, and then pupate in flax seed. If that happens in the fall, it kills the main stem. If it happens in the spring, it is more damaging to secondary tillers.

The biggest threat is when volunteer wheat comes up in soybeans in a double-crop system. If the volunteer wheat isn’t killed and soybeans are planted back into the soybean stubble in the fall, Hessian flies that survive over the summer in the volunteer wheat, then infect the new fall wheat, Herbert explains.

North Carolina State University Entomologist John Van Duyn agrees summer carry-over of Hessian fly is largely dependent on the presence of wheat stubble.

Rotations which prevent new wheat from being planted into, or near, a previous wheat crop’s stubble is an effective tactic for reducing damaging infestations, he says.

Growers should avoid planting wheat into last season’s wheat stubble, Van Duyn stresses. “Continuous no-tillage, wheat-double-cropped-soybeans is a recipe for making a Hessian fly problem. Since Hessian fly is a weak flier, putting distance between the location of new wheat plantings and the previous season’s wheat fields is also important in avoiding economically damaging fly.”

Both spring and fall problems with Hessian fly can be avoided if the production system is properly planned, Van Duyn says.

Hessian fly aestivates (warm temperature hibernation) during the summer, as pupae, in wheat residue. Flies emerge in the early-to-mid-fall. Early emerging flies (e.g. September) may lay eggs on volunteer wheat plants in the fields of the previous season’s wheat crop.

Hessian fly may also infest a few species of wild grasses or very early-planted wheat, planted for cover crop or dove hunting fields.

New flies, from these early fall sources, may infest newly planted wheat later in the fall (e.g. in October). Also, late emerging flies from the previous wheat crop may contribute to these infestations.

The sporadic problems in Essex County are hard to explain, Herbert says. It occurred in fields following corn. It was an early, but not real early variety. There was just nothing that stood out to make this field susceptible, Herbert notes.

Hessian fly adults are killed by freezing temperatures and a traditional method for preventing Hessian fly infestation is to delay planting until after the first freeze (often called the fly free date).

This concept has not been entirely satisfactory in North Carolina because an early freeze is not a dependable event. However, it does work much of the time by avoiding active populations of flies or reducing the time flies are active within fields of wheat seedlings.

The expanded use of no-till systems to grow wheat, corn and soybeans may be a contributing factor to Hessian fly problems. Disking wheat stubble after harvest effectively kills Hessian fly.

Planting soybeans no-till into wheat stubble enhances Hessian fly survival by preserving the site where pupa spend the summer.

Burning wheat straw will reduce pupa but many are found below the soil surface. Therefore, burning is not as effective as disking.

Though damage in the Carolinas and Virginia has been sporadic the past few years, widespread outbreaks have been reported in other parts of the country.

In Texas, Southwest Farm Press Editor Ron Smith reports Hessian fly infestations have been detected in north Texas from the northeast corner near the Oklahoma State line, and westward. Infestations vary widely, with some fields showing limited potential for damage, and others facing severe yield loss, Smith says.

The USDA reports wheat farmers in Indiana saw the first Hessian fly infestation in more than a decade.

Over the last two years, the flies have also shown up in Missouri for the first time ever, signaling a westward expansion of the world's most destructive wheat pest.

Having mostly lurked outside of wheat fields for the past 45 years, the flies have recently undergone genetic changes that enable them to make major inroads into wheat fields once again.

ARS entomologist Brandon Schemerhorn is sampling Hessian flies across the United States in 2007 to determine their genetic variability. The greater the variability, the more resistance genes will need to be bred into wheat varieties to counter the fly's genes.

Whether the sporadic outbreaks in Virginia and North Carolina are part of a more widespread problem or just a seasonal occurrence remains to be seen. More careful scouting of wheat in the fall and spring is the first line of defense against Hessian fly.

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