Hessian Fly on Wheat

The low incidence of occurrence of and damage from Hessian fly in Pennsylvania can be attributed both host-plant resistance and planting date.
Hessian Fly on Wheat - Articles


John C. French Sr., Retired, Universities:Auburn, GA, Clemson and U of MO, Bugwood.org

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

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.

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."

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.

CountyFly-free Date
AdamsOctober 1
AlleghenySeptember 28
ArmstrongSeptember 28
BeaverSeptember 28
BedfordOctober 1
BerksSeptember 27
BlairOctober 1
BradfordSeptember 26
BucksSeptember 30
ButlerSeptember 28
CambriaSeptember 27
CameronSeptember 27
CarbonSeptember 27
CentreSeptember 27
ChesterSeptember 30
ClarionSeptember 28
ClearfieldSeptember 27
ClintonSeptember 27
ColumbiaSeptember 27
CrawfordSeptember 26
CumberlandOctober 1
DauphinSeptember 27
DelawareSeptember 30
ElkSeptember 27
ErieSeptember 26
FayetteSeptember 28
ForestSeptember 26
FranklinOctober 1
FultonOctober 1
GreeneOctober 1
HuntingdonOctober 1
IndianaSeptember 28
JeffersonSeptember 28
JuniataOctober 1
LackawannaSeptember 26
LancasterSeptember 30
LawrenceSeptember 28
LebanonSeptember 27
LehighSeptember 27
LuzerneSeptember 27
LycomingSeptember 27
McKeanSeptember 22
MercerSeptember 26
MifflinOctober 1
MonroeSeptember 27
MontgomerySeptember 30
MontourSeptember 27
NorthamptonSeptember 27
NorthumberlandSeptember 27
PerryOctober 1
PhiladelphiaSeptember 30
PikeSeptember 26
PotterSeptember 20
SchulykillSeptember 27
SnyderSeptember 27
SomersetSeptember 27
SullivanSeptember 25
SusquehannaSeptember 27
TiogaSeptember 26
UnionSeptember 27
VenangoSeptember 26
WarrenSeptember 26
WashingtonOctober 1
WayneSeptember 26
WestmorelandSeptember 28
WyomingSeptember 26
YorkOctober 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