A survey of uneven-looking wheat fields in north Texas recently found that many were damaged by larvae of the Hessian fly, a small insect that feeds on plant sap.

Jim Swart surveyed fields in Hunt, Grayson, and Fannin counties and found Hessian fly damage throughout the area. Damage in some fields was so extensive that yield prospects were greatly reduced.

Allen Knutson and Jackie Risner surveyed fields in Grayson County and also found several fields heavily damaged by the insect. However, other fields had few or no Hessian flies.

Why some fields are heavily infested and others are not is probably related to planting date and wheat variety. The Hessian fly begins depositing eggs on wheat leaves in the early fall following rains. Wheat planted early is subject to this early egg-laying activity and warm weather in October and November allows up to two generations to develop before cold weather slows reproduction in December.

Later-planted wheat escapes the early egg laying, and with less time before cold weather, only one generation might be completed before December. Wheat planted in early to mid-October appeared to have more Hessian flies than wheat planted after mid-October.

Variety can also influence Hessian fly damage, as varieties vary in their genetic susceptibility to the pest. However, varieties resistant to common biotypes of Hessian fly have been found to be heavily infested. This finding suggests that a new, virulent Hessian fly biotype has increased in numbers in north Texas.

Although the cause of the outbreak of Hessian fly this spring is not known, one or several of the following could a factor:

(1) Weather conditions in the fall of 2006 (drought followed by area-wide rain in late summer) that favored emergence and survival of large numbers of Hessian fly and that coincided with wheat emergence.

(2) Warm weather in February-April, which allowed two generations of fly to develop, rather than one.

(3) Emergence of a new biotype that can attack resistant wheat. The specialists say they cannot confirm the presence of a new biotype at this time, but field surveys showing large infestations in wheat varieties that were thought to be resistant to local biotypes suggest the presence of a new biotype.

Hessian fly infestation can be confirmed by the presence of the small, white maggots and brown puparia behind the leaf sheaths at the base of tillers and at nodes on the stems. Infested tillers are usually stunted.

Hessian flies feeding at the nodes weakens the stem; such stems will later lodge, making harvest difficult. Feeding by Hessian fly results in reduced yield and quality due to stunted tillers, lodging, and poor grain fill. Heavily-infested fields can be seen from the highway. They are characterized by scattered tall plants and a crop of shorter plants (tillers) that appear darker green than the taller plants.

It is very difficult to estimate losses due to Hessian fly. The number of infested tillers, the number of larvae per tiller, and compensation by surviving tillers no doubt all influence how Hessian fly will impact yield, but these factors have not been quantified.

There is no remedy for Hessian fly-infested fields at this time. Hessian flies in these fields will oversummer in the field and re-infest planted wheat this fall. The most effective control practice is to plant wheat varieties with genetic resistance to Hessian fly. However, the number of such varieties adapted to north Texas, and which also have resistance to rust and other disease, is very limited.

The publication, “Hessian Fly in Wheat E-350,” lists wheat varieties with Hessian fly resistance. However, the specialists say they have found large numbers of Hessian fly on Pioneer 25R78, which is resistant to biotypes GP, A, B, and E.

Apparently, they say, a new biotype has become abundant in north Texas, with a need to re-evaluate varietal resistance. Samples of Hessian fly from these fields will be sent to Kansas State University for biotype determination.

Seed treatments — both Cruiser and Gaucho — are labeled for suppression of Hessian fly and may offer some protection during the fall. Destroying volunteer wheat several weeks prior to planting can also help reduce pest numbers.

Hessian fly adults are mosquito size and are weak flyers. However, they can be carried by the wind and are believed to move up to a mile. While crop rotation helps, Hessian flies oversummering in adjacent fields or in volunteer wheat can move into newly planted wheat fields in the fall.