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The Hessian fly or barley midge, Mayetiola destructor, is a species of fly that is a significant pest of cereal crops including wheat, barley and rye. Though a native of Asia it was transported into Europe and later into North America, supposedly in the straw bedding of Hessian troops during the American Revolution (1775–83). There are usually two generations a year but may be up to five. In the spring the dark-colored female lays about 250 to 300 reddish eggs on plants, usually where the stems are covered by leaves; the larvae feed on the sap and weaken the plants so that they cannot bear grain. The Hessian fly was described by Thomas Say in 1817. It is a very harmful insect. It mainly attacks the stem, although if it is really hungry it will eat any part of the plant it can find. In 1836 a severe infestation of Hessian flies resulted in a crop shortage aggravating the financial problems of farmers prior to the Panic of 1837.


Hessian flies overwinter as full-grown larvae or maggots inside protective cases called flaxseed because of their resemblance to real flax seeds. Once wheat begins to grow in the spring, adults emerge (usually April) and mate. Adult Hessian flies are small (less than 1/8 inch long), dark (female has an orange-red abdomen), long-legged, two-winged insects that resemble mosquitoes. Even though adults are weak fliers, they can be carried aloft for several miles by strong winds. Once a female has mated, she can lay 250 to 300 eggs in her short life span (4 days or less). The very small, slender, reddish eggs are laid end-to-end in small groups (2-15 eggs). The eggs are deposited in grooves on the upper leaf surface. Females prefer to deposit their eggs on several young plants, but most of the eggs may be laid on a single plant.  After hatching (3-10 days) from their eggs, legless Hessian fly maggots are initially a reddish color before turning a whitish color. They prefer to feed within grooves between the leaf sheaths and stem. Maggots feed by using their mouthparts like sandpaper on the leaf surface and sucking up plant juices that seep out. They become stationary once they begin to feed and they never cut into the leaf sheath or stem. Depending on the weather, the maggots usually reach their maximum size in two weeks. Then, their outer skin becomes loose and hardens into a brown, protective shell, or "flaxseed". The flaxseed stage persists during the summer until volunteer wheat or early-planted wheat is present. Annually, there are two generations in Missouri unless wet conditions permit an extra spring or summer generation. This insect will also attack barley and rye, but wheat remains its favorite host.

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