By John Tooker, Extension Specialist
While many Pennsylvania growers have never encountered Hessian flies, the past few years have seen an increasing number of outbreaks in eastern states in wheat (and to a lesser degree barley and rye).
Hessian fly has not been common in the recent past because most farmers plant wheat after “fly-free dates” — dates after which egg-laying Hessian fly adults are not likely to be active. For best estimates of fly-free dates in the state, see this Extension website. Recognize, however, that these fly-free dates are likely imperfect given the trends for warmer years, so proper date might be a little later. Also, if your farm is at a higher elevation, the date is probably a little earlier than the rest of your county — this is an inexact science at this point, so just do your best.
With great adoption of small grains as cover crop species, some growers have been planting wheat, rye, and barley in late-summer earlier. These early-planted fields are available for egg-laying female flies and then can foster populations of Hessian fly larvae that can then emerge as adults in spring and further infest fields. Insecticides are generally not effective for control of Hessian fly so the best tactics for farmers is to adhere to the fly-free dates and plant Hessian fly-resistant varieties of wheat, rye, or barley.
If you are in the practice of planting wheat, rye, or barley as cover crops well before the fly-free dates, it is probably prudent to assess local Hessian fly populations by scouting that early planted small grain cover crop (particularly wheat) for hessian fly-induced stunting, larvae and pupae. For more information on Hessian fly see this fact sheet.