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.
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.
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Authored by: John Tooker, Assistant Professor of Entomology
Last updated: August 2012