Penn State Extension


Hessian Fly on Wheat

During the past decade, damage to wheat by Hessian fly has been minimal in Pennsylvania. Outbreaks of this historic pest, however, have occurred in states to our south such as North Carolina and Virginia, possibly because growers have gotten lax about planting after the fly-free dates and/or increased use of small grain cover crops that are planted earlier in the year than a cash crop would be and then are not harvested.  

The low incidence of occurrence of and damage from Hessian fly in Pennsylvania can be attributed both host-plant resistance and planting date.  Many varieties of wheat commonly used in Pennsylvania possess either complete or moderate resistance to attack by Hessian fly. Also helpful in holding down fly damage has been the cultural practice of planting wheat after the "fly-free" date in late September or early October (see below for these dates).  Continued diligence in using resistant varieties and planting late should prevent Hessian fly from returning as a major pest of wheat in Pennsylvania.

Life History

Image of hessian fly eggs in wheat stalk.

Hessian fly eggs. #512-11. Oregon State University in cooperation with EPA. 

Hessian fly pupa. #512-25. Oregon State University in cooperation with EPA. 

There are two generations per year, one in early fall and one in the spring. There may also be a third generation in July and August if moisture conditions are high enough for volunteer wheat to sprout and grow. When a summer or supplementary brood does develop, early-sown non-resistant wheat is often severely damaged.

Flies (Fig. 1) of the fall brood tend to appear in late September and live for just a few days. They lay their eggs on the leaves of young grasses, including small grains. Wheat appears to be preferred, but flies will also lay eggs on barley and rye and some native grasses.  Planting after the fly-free date is an effective control method because it avoids having large numbers of young potential host plants available to receive eggs when adult flies are active. If the flies emerge and die off before the new wheat plants emerge, the crop cannot be infested.  Maggots can survive on native grasses, but populations do not appear to thrive until they find small grain hosts, particularly wheat.

Hessian fly adult


Fig. 1. Hessian fly adult. Photo from the USDA.

Maggots soon hatch from the eggs, and these tiny larvae crawl to the crown of seedlings (just above the roots) and feed on plant juices after injecting their unique saliva. Feeding by one larva can permanently stunt plant growth (Fig. 2).  A larva will complete its growth before cold weather and pass the winter as puparium or "flaxseed."

hessian fly-infested vs. healthy wheat plant

Fig. 2. A hessian-fly infested wheat plant on left compared to a health plant on the right.  Photo by John Tooker


In the spring, adult flies emerge from the "flaxseeds" and lay eggs on the leaves. Upon hatching, the maggots work their way under the leaf sheath near the node of these large plants. Their feeding at this site weakens the stem which results in the stalks breaking over before harvest. The maggots change into puparia about the time wheat heads out and they remain in the stubble as "flaxseeds" until fall.


High mortality of Hessian fly from parasites is sporadic in the spring generations but cannot be relied upon for the high level of control we desire. Therefore we suggest the use of fly resistant varieties where feasible or plant wheat after the "fly-free date". The fly-free date for each county in Pennsylvania is listed below.  It must be acknowledged, however, that these dates may be somewhat 'soft' given the warmer temperatures that we have been experiencing in recent years. To be more conservative, growers should consider planting even a week or so later if possible.

County & Fly-free Date


County & Fly-free Date

Adams October 1
Lackawanna September 26
Allegheny September 28
Lancaster September 30
Armstrong September 28
Lawrence September 28
Beaver September 28
Lebanon September 27
Bedford October 1
Lehigh September 27
Berks September 27
Luzerne September 27
Blair October 1
Lycoming September 27
Bradford September 26
McKean September 22
Bucks September 30
Mercer September 26
Butler September 28
Mifflin October 1
Cambria September 27
Monroe September 27
Cameron September 27
Montgomery September 30
Carbon September 27
Montour September 27
Centre September 27
Northampton September 27
Chester September 30
Northumberland September 27
Clarion September 28
Perry October 1
Clearfield September 27
Philadelphia September 30
Clinton September 27
Pike September 26
Columbia September 27
Potter September 20
Crawford September 26
Schulykill September 27
Cumberland October 1
Snyder September 27
Dauphin September 27
Somerset September 27
Delaware September 30
Sullivan September 25
Elk September 27
Susquehanna September 27
Erie September 26
Tioga September 26
Fayette September 28
Union September 27
Forest September 26
Venango September 26
Franklin October 1
Warren September 26
Fulton October 1
Washington October 1
Greene October 1
Wayne September 26
Huntingdon October 1
Westmoreland September 28
Indiana September 28
Wyoming September 26
Jefferson September 28
York October 1
Juniata October 1


Pesticides are poisonous. Read and follow directions and safety precautions on labels. Handle carefully and store in original labeled containers out of the reach of children, pets, and livestock. Dispose of empty containers right away, in a safe manner and place. Do not contaminate forage, streams, or ponds.

Authored by: John Tooker, Assistant Professor of Entomology

Last updated: August 2012

Penn State College of Agricultural Sciences research, extension, and resident education programs are funded in part by Pennsylvania counties, the Commonwealth of Pennsylvania, and the U.S. Department of Agriculture.

Visit Penn State Extension on the web:

Where trade names appear, no discrimination is intended, and no endorsement by Penn State Cooperative Extension is implied.

This publication is available in alternative media on request.

Penn State is an equal opportunity, affirmative action employer, and is committed to providing employment opportunities to minorities, women, veterans, individuals with disabilities, and other protected groups. Nondiscrimination.

© The Pennsylvania State University 2019

Related content