Hessian fly is known to exist in the Willamette Valley several decades ago but has not been identified as an agronomic problem in the region for many years. There have been recent identifications made in infected fields of both spring and winter wheat crops in Polk and Washington counties.
Scout within the wheat field for stunted plants with dark and wider leaves. Leaves may appear thickened, erect, and bluish-green in color. In cases of heavy infestations, the central stem is often missing. Infested stems usually break over at the time of head formation. Grain yields can be greatly reduced. The economic loss probably occurs when from 15 to 30 percent of the tillers are infested.
Look for small larvae that are small ~3/16 inch, greenish-white, legless, headless maggots underneath lower leaf sheaths. The pupal stage ~3/16 inch is represented by brown seed-like cases containing a maggot, often referred to as a “flaxseed.” The maggots or “flaxseeds” (puparium) present beneath the leaf sheath above the surface of the ground, as high as the second or third joint, are signs of Hessian fly. Adult flies have a red-brown to dusky-black body and dusky wings. They resemble mosquitoes in form and are about 1/6 inch long. They are short-lived and could be hard to find in the fields.
The Hessian fly usually has two to three partial generations per year, one to two in the spring and one in the late summer or early fall. Occasionally a second generation will occur in the spring. Usually, the flaxseed of the spring generation spends the summer in wheat stubble to produce another emergence of flies in late summer or early fall. Fall-generation flies deposit eggs on volunteer wheat or early-sown winter wheat. Larvae from such eggs overwinter in the flaxseed stage and produce flies in the spring.
Predisposing factors. Rain in late August or September, accompanied by cooler temperatures, are key factors that prompt pupation and adult emergence in the fall. In the spring a mean temperature of 50° F. is usually required before fly emergence begins. In most year’s very little emergence occurs before April. Warm weather emergence occurs before April. Warm, wet weather in early spring can lead to a heavy infestation in wheat seeded in late February or March.
- Planting resistant cultivars is the most effective approach to minimize losses to this pest. Consult your crop advisor, extension educator, or specialist to select recommended varieties for planting in your region.
- Deep plowing soon after harvest is helpful if soil conditions permit this practice. Plow wheat stubble soon after harvest to bury the flaxseed. Deep plowing prevents flies from emerging in the fall. Wheat stubble is a primary site where the Hessian fly both overwinters and over summers. It is most responsible for the spring populations. Plow under any volunteer wheat on which fall generation flies may deposit eggs.
- Use fly-free planting dates. Winter wheat seeded after mid-October is usually free of this pest. Spring wheat seeded behind failed fall-seeded wheat is especially prone to attack. Time fall-seeded cereals so that they do not emerge until after the Hessian fly flight period has ceased. Generally, wheat planted after the second week in October will avoid Hessian fly damage. Fall-seeded wheat usually suffers less injury than spring-seeded wheat. Generally, the fly prefers barley less than wheat.
- Seed treatments applied to wheat and barley seed may help control the Hessian fly. Please refer to PNW Insect Management Handbook for chemical control recommendations.