Hessian fly does not attack oats but can develop on some grass weeds such as little barley and wild ryegrasses. Adult hessian flies are small black flies about the size of a mosquito. Adults live for about two days, during which time they mate. Females lay about 200 eggs in the grooves of the upperside of wheat leaves. Newly hatched larvae move down the leaf groove beyond the leaf sheath to the stem where they begin to feed on the leaf base.
Maggots become white after the first molt and appear greenish-white when fully grown.
The entire life cycle requires about 35 days at 70 F. Newly hatched larvae are exposed on the leaf surface and are susceptible to adverse disease and weather conditions, but once larvae move to the stem base they are protected from exposure. Maggots suck sap and stunt tillers, presumably by injecting a toxin into the plant. Feeding by a single larva for several days is sufficient to completely stunt or kill a vegetative tiller. Stunted vegetative tillers are dark green, do not elongate or produce new leaves, and usually die after the maggots pupate. Infested jointed stems are shorter and are weakened at the joint where feeding has occurred.