Mayetiola destructor Say
Appearance and Life History
Hessian fly may pose a serious threat to wheat, barley, and rye with wheat being the preferred host. Hessian fly is not a problem in oats.
It is the immature, maggot stage of the Hessian fly that causes injury to plants. The maggot can damage seedlings in the fall and overwintered plants in the spring by rasping the lower stem tissue and sucking the sap which oozes from the wound.
Hessian fly passes the winter as a full-grown maggot within a brown protective case known as a puparia or flaxseed. The flaxseed is protected behind the leaf sheath at the bottom of the stem of newly seeded wheat or volunteer wheat plants. The maggot remains inactive during the winter.
In the spring, shortly after the wheat breaks dormancy and begins to grow again, the maggot changes within the flaxseed into a pupa, emerging in a week or two as a small [less than 1/8 inch (3 mm) in length] black fly.
The fly does not cause any damage to small grains, but within three or four days of emergence, the female lays many reddish-tinted eggs in the grooves on the upper side of wheat leaves. Young maggots hatch in 3 to 10 days and move down the stems behind the leaf sheaths. Once they reach the lower part of the stems, they begin feeding. Once feeding begins, they stay in the feeding site. The maggots never enter the stem. However, infested stems usually break once the heads begin to fill, leading to reductions in yield.
Within two weeks, the maggot develops into the flaxseed stage. Typically, the maggot will be in its protective flaxseed form for several weeks before harvest of the wheat. Since the flaxseed is located in the lower portion of the stem, harvesting has little effect.
The maggot remains within the flaxseed in the dry stubble throughout most of the summer, emerging as a mature fly in the late summer or early autumn. The cycle continues as eggs are laid on volunteer wheat or newly seeded plants in the autumn.
Maggot feeding on small seedlings in the autumn causes severe injury to plants. Infested wheat is stunted, dark green, and its leaves are broader than normal. Such injured plants will never grow past the four-leaf stage and generally die during the winter.
Survey wheat at two stages for evidence of Hessian fly. First, check wheat in the seedling stage in the autumn, 18 to 21 days after the plants emerge. In each of 5 areas of the field, carefully examine 20 stems. Record the number of stems that exhibit the symptoms of Hessian fly maggot feeding damage. Determine the percentage of infested stems from the sample of 100 stems.
Follow the same sampling pattern in the spring when the wheat heads just begin to fill. However, during your spring inspection(s), determine the number of broken stems out of the 100 stems sampled. Use this number to calculate the percentage of infested stems.
Small Grains Insect Control Recommendations: E-series 220-W (PDF)
No economic thresholds have been established for Hessian fly. Once the infestation is apparent, the damage has been done and no treatment will reduce the losses to the crop.
Until definite threshold guidelines are established, any level of Hessian fly infestation in this year's wheat crop demands that the next crop be planted to the most up-to-date Hessian fly-resistant varieties after the "fly- free" date for that particular locale (see map below).
The fly-free date marks the time when Hessian fly is (generally) no longer active in the fall. However, the continuation of warm weather into the late summer and early autumn may extend Hessian fly activity beyond the average fly-free date. Thus, it is essential to plant resistant wheat varieties, even after the fly-free date. Planting wheat after the fly-free date also reduces the impact of many wheat diseases.
Average Hessian fly-free dates in Indiana
If control is necessary, contact your state Cooperative Extension Service or click here for control materials and rates.