Egg - The thin, cylindrical egg is 0.4 to 0.5 mm long. Although uniformly glossy red when laid, it gradually becomes deeper red at one end and opaque white at the other.
Larva - The newly hatched maggot is red for 4 or 5 days after which it turns white. As the larva matures, a translucent green stripe appears down the middle of its back. The full grown maggot is 3.5 to 5.5 mm long and about 1 mm wide.
Pupa - The brown-headed pupa is basically white with a reddish tinge. The puparium or "flaxseed" within which the pupa is found is spindle-shaped, red to dark brown, and 2.5 to 6.2 mm in length.
Host Plants - Although wheat is the principle host plant, Hessian flies may also be found on rye, barley, and some wild grasses.
Damage - In the midwest, Hessian flies are one of the most destructive pests of wheat. The maggots burrow between the leaf sheath and the stem and extract juices from the plant. Fall-infested wheat usually dies during the winter. Spring-infested wheat produces grain but usually lodges before harvest. Economic infestations are uncommon in North Carolina largely due to proper planting date selection.
Life History - Typically, Hessian flies complete two generations per year, a spring brood and a fall brood, although as many as six generations a year have been reported. The pupae overwinter within puparia, the hardened skins of the last instar larvae. These puparia, known as the "flaxseed" stage, are located just below the surface near the crown of the plant. In spring (from March in Georgia and South Carolina to May in Michigan) the flies emerge from the flaxseed stage, deposit eggs on wheat, and die in 2 or 3 days. Maggots hatch from the eggs in 3 to 7 days, crawl down the leaves, and feed at the crown or joints along the stem. The maggots develop through three instars over a 25 to 30 day period, enter the flaxseed stage before harvest, and pass the summer in the stubble. In late August or September, second generation flies emerge and deposit eggs on volunteer wheat or early-sown winter wheat. Of the six or more Hessian fly races (biotypes) known to exist, two or more are likely to occur in any area where wheat is grown.
In areas of annual infestation, it may be beneficial to rotate crops so that wheat is not grown on the same land two years in succession. This practice reduces Hessian fly populations as well as those of other insects attacking small grain crops. For further control information, consult the current North Carolina Agicultural Chemicals Manual.