The Hessian Fly:

A Pest of Wheat in North Carolina

John Van Duyn and Steve Bambara, Department of Entomology, NC State University
Randy Weisz, Department of Crop Science, NC State University

Why Has Hessian Fly Become a Problem?
In recent years, numerous North Carolina fields have suffered extensive losses because of Hessian fly infestations. Historically a wheat pest in the Midwest, changes in field-crop production including early planted wheat, increased adoption of no-tillage double-cropped soybeans, and the use of wheat as a cover crop for strip-tillage cotton and peanut production have permitted the Hessian fly to reach major pest status in North Carolina.

Hessian Fly Life Cycle
The adult Hessian fly is a small, long-legged, two-winged insect that resembles a small mosquito (Figure 1). It is one of many species of gnat-sized flies that may be found in wheat fields. The female Hessian fly adult is reddish-brown and black in color and about 1/8 inch long. The slightly smaller males are brown or black. The elliptical eggs are very small and colored orange. Eggs are deposited singly or end-to-end in “egg lines” between the veins on the upper surface of the young leaves (Figure 2).

Newly hatched larvae (maggots) are also orange for 4 or 5 days before turning white (Figure 3). As larvae mature, a translucent green stripe appears down the middle of the back. The maggot is about 1/4 inch long when full grown. The maggot transforms into an adult fly inside a dark brown case, or puparium, that resembles a flaxseed in size and shape. Newly formed puparia will be a lighter brown color that transforms to a mahogany brown color with age. Puparia, or "flaxseeds”, (Figure 4) are located under leaf-sheaths and usually below ground on young tillers, or below the joint in older plants.

Hessian fly can be found in small numbers in most wheat fields at harvest (Figure 5). If the wheat stubble is destroyed after harvest, the fly dies and the life cycle is broken. If, however, the wheat stubble is left in the field, the fly can survive as “flaxseeds” in the stubble through the summer. In late August and September, adults emerge from the “flaxseed” pupal stage and lay eggs on volunteer wheat, or on early planted cover-crop wheat. A first generation can be completed on these plants, and the next generation adults emerging from cover-crop or volunteer wheat plants can lay eggs on wheat planted for grain in October and November, before the weather turns cold enough to kill the adult flies. Often Hessian flies begin depositing eggs very soon after seedling emergence.

Figure 5. The movement of Hessian fly from one wheat for grain crop to another (wide lines). Early-planted cover-crop wheat plays a additional major role in Hessian fly population growth (thin lines).

Once established on a new wheat crop, Hessian fly eggs hatch within a few days, and the tiny maggots migrate into the whorl of small wheat plants but ultimately locate below ground at the base of the stem, where they enter the pupal stage. While feeding, the larvae damage the plants by rupturing leaf or stem cells. This can result in tiller stunting and dieback. A heavy infestation on early-stage plants may greatly reduce plant stand. A new generation of adults usually emerges in March depending on the weather, lay eggs, and produce new larvae that migrate to the stem joints where they feed and cause further damage. This spring damage may kill the wheat tiller but usually results in weakened stems, small heads, and poorly filled grain heads with low-quality kernels. Often, wheat lodges in seriously infested fields.

Management
Rotation: Because the Hessian fly life cycle is largely dependent upon the presence of wheat stubble, rotations which prevent new wheat from being planted into or near a previous wheat crop’s stubble will be the most effective way to prevent infestations. Growers should avoid planting wheat into last season’s wheat stubble! Continuous no-tillage wheat-double-cropped-soybeans may result in severe problems and should be avoided in Hessian fly problem areas. Additionally, since the Hessian fly is a weak flier, putting distance between the location of new wheat plantings and the previous season’s wheat fields can be a successful method of preventing new infestations. Although Hessian fly can become serious under other situations, most serious infestations occur when wheat is early planted into wheat stubble or into fields next to wheat stubble.

Tillage: Disking wheat stubble after harvest effectively kills Hessian fly. Planting soybean no-till into wheat stubble enhances Hessian fly survival by preserving the site where puparia spend the summer. Burning wheat straw will reduce puparia but many puparia are found below the soil surface. Therefore, burning is not as effective as disking.

Choosing Cover Crops: Serious Hessian fly infestations have occurred in areas where wheat for grain was planted near early-planted wheat for cover, or early planted wheat for dove hunting purposes. In cropping systems where cover crops are used, such as in strip-till cotton or peanut production, the use of other small grains besides wheat will reduce Hessian fly populations. Oats, rye, and triticale are not favorable for Hessian fly reproduction and do not serve as a nursery making them preferable over wheat for cover cropping in areas where wheat for grain is also produced. If triticale is used for cover cropping, varieties such as “Arcia,” that are adapted to NC, should be planted.

Delayed Planting: Because Hessian fly adults are killed by freezing temperatures, a traditional method for preventing Hessian fly infestation is to delay planting until after the first freeze (often called the fly free date). This concept has not worked well in North Carolina because an early freeze is not a dependable event. Often a “killing freeze” does not occur until December in many areas of North Carolina, after most growers need to have wheat planted for agronomic purposes.

Resistant Varieties: Many wheat varieties are advertised as having Hessian fly resistance. Unfortunately, in most cases this resistance is to a “biotype” or kind of Hessian fly that we do not have in North Carolina. To be effective here, wheat varieties must be specifically resistant to “Hessian fly biotype-L”. Wheat varieties with biotype-L resistance are listed in Table 1. This list is updated every year and can be found on the the North Carolina Small Grains Production Web Site at: http://www.smallgrains.ncsu.edu/. In most cases, varieties rated as “good” in Table 1 should provide enough protection to avoid economic loses due to Hessian Fly biotype-L. However, in areas with severe Hessian fly problems, the use of resistant varieties (those shown in Table 1 as “good”) may not be sufficient to prevent infestations from occurring.

Systemic Seed Treatments: The use of systemic insecticidal seed treatments (e.g. Gaucho® or Cruiser®) can be highly effective in protecting wheat seedlings from Hessian fly. These seed treatments are, however, expensive and should be used only after careful consideration of current production economics.

Foliar Insecticides: Long residual foliar pyrethroid insecticides (specifically Warrior®) applied shortly after wheat emerges (at the two to three leaf stage) have been very effective in controlling Hessian fly. Our tests have shown that if applied timely, Warrior will kill the adult flies, may kill the eggs on treated leaves, and may also kill freshly hatched larvae before they become embedded in the stems. Pyrethroids labeled for use against Hessian fly are limited to Warrior and Mustang Max™. At least three of the following conditions should be met before using a pyrethroid for early season Hessian fly control:

Fields that passed the winter with a significant Hessian fly infestation will also be attacked by the next generation of larvae recycling in the crop. Fields with low tiller counts should be examined in January or February for Hessian fly puparia. If a pyrethroid is applied as the flies emerge and lay eggs, usually in early March, a high level of control may be accomplished. The need for a pyrethroid treatment may be judged by examining the plants for puparia, to identify fields that will have high fly numbers, and later scouting heavily infested fields for eggs on the top surface of new leaves. Eggs are very small, about half the size of a period, and magnification may be needed. An experienced person with good eye sight can readily detect Hessian fly eggs, especially in direct sun-light since the eggs will shine. Egg counts of four or more per leaf may justify a pyrethroid application.

Effective Control: For a management program to be most effective, growers must implement a combination of all the techniques mentioned above in coordination with neighboring producers. For example, the efforts of a producer who rotates his wheat may be frustrated by a neighbor who plants wheat as a cover crop or who has a no-till double-crop soybean field adjacent to his farm. Careful selection of oats, rye, or triticale for cover cropping, avoiding planting wheat into or near old wheat stubble, the use of resistant varieties, and/or planting after the first frost will minimize Hessian fly damage.

For More Information
Please refer to the North Carolina Small Grains Production Web Site at: http://www.smallgrains.ncsu.edu/

Acknowledgments
This publication is supported by a grant from the NC Small Grain Growers Association, Inc. The association provides funds to supplement public appropriations and research programs at NC State University for the benefit of the small grain industry, general consumers, and the public at large. Special thanks to K.S. Pike, WSU, for Figs. 1, 4 and linked figures.

Recommendations for the use of agricultural chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by the NC Cooperative Extension Service nor discrimination against similar products or services not mentioned. Individuals who use agricultural chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage regulations and examine a current product label before applying any chemical. For assistance, contact your county Cooperative Extension Center.

Publication AG-368. Originally published Aug. 1986 by R.Brandenburg & J. VanDuyn, by the North Carolina Agricultural Extension Service. (Revisons Sept. 1995, Nov. 2003).