The Hessian fly, Mayetiola destructor (Say), is a pest of wheat and more rarely barley and rye. The fly usually has two generations per year in Michigan, one in the spring and the other in the fall. The summer and winter are spent as small, hard, black puparia (called "flax seeds") in the culms of the host small grains and grasses. Farmers very early found that fly damage could be avoided by planting their winter grains "late" in the season. Studies made early in this century found that the earliest date for planting to avoid the fly (the "fly-free dates") could be predicted and these dates have become an accepted practice.
Ruppel, Robert F.
"Emergence of Adult Hessian Flies (Diptera: Cecidomyiidae) From Overwintered Puparia,"
The Great Lakes Entomologist: Vol. 17
, Article 10.
Available at: http://scholar.valpo.edu/tgle/vol17/iss3/10