The adult Hessian fly is a tiny, dark-colored insect about 1/8 inch long that resembles a large gnat or small mosquito. Females lay eggs on the seedling leaves, which hatch in three to seven days. Newly emerged maggots will move down to the crown of the plant and reside in the grooves of the leaf sheath and stem. They will feed on plant tissue by using their mouthparts like sandpaper and lap up juices that seep out.
Larvae are the only stage that damage wheat; adult flies do not feed. Maggots usually feed on the lower leaves and stems and reduce plant vigor. Infested plants become stunted and stiffly erect, and leaves are thickened with a bluish green color. A single maggot feeding on a plant for three days can stunt a young plant or tiller. Heavily damaged plants usually die during the winter.
Outbreaks of the Hessian fly have caused severe losses in wheat for the past 230 years in this country. It also may damage barley and rye, but not oats. Its damage was first observed in North America in the Long Island, NY area in the late 1770s.
It is not definitely known how the fly arrived from Europe or the time. My favorite story is the one that reports that the fly was found in the vicinity where Howe’s British troops were encamped. It was thought that Hessian soldiers in his army brought the pest from Europe in straw used for bedding; thus its name -- Hessian fly. The Hessian fly moved from Long Island approximately 20 miles annually until it reached the wheat growing region of the Great Plains. Problems associated with the pest and possible control methods are discussed at length in Thomas Jefferson’s agricultural writings.
Hessian flies caused a US shortage of wheat in 1836 that caused economic problems for farmers prior to the Panic of 1837. It was reported in Ohio agricultural reports as early as 1847. In the latter part of the 1800s Hessian fly infestations were specifically reported in Crawford, Defiance, Seneca, and Wood counties of Ohio. Serious outbreaks occurred in Ohio in 1895 and 1920. Parts of Indiana had serious outbreaks as late as the early 1960s.
A “fly free” date was observed as a possible control practice as early as the late 18th century in the Mid-Atlantic States. The Ohio Agricultural Experiment Station started using fly free dates for Ohio from 1911 to 1919. Nation-wide fly-free dates were established from studies conducted during1918-1935. The dates used today in Ohio and other states were a result of this research.
Resistant variety programs were initiated in our region in the 1940s. New varieties with Hessian fly resistant genes were heavily introduced between 1950 and 1983. The low incidence of damage from Hessian fly in Ohio today can be attributed to both host-plant resistance and planting date. However, over the past decade surveys have shown that the genes are no longer effective against a new emerging biotype of Hessian flies. With the loss of plant resistance, planting date will once again be our main defense against devastating yields losses to Hessian fly. With global warming, new research may be required and the dates adjusted as our climate changes.