Management Strategies for Hessian Fly in Wheat

Hessian fly (Mayetiola destructor) is one of the most important pests of small grain crops in the United States and worldwide. In addition to little barley (Hordeum pusillum) being the only important wild host, crop hosts include wheat, barley, and annual rye, with wheat being the preferred host.¹ The purpose of this publication is to provide growers, consultants, and other agricultural professionals with background information on Hessian fly, describe its injury to wheat, and discuss management strategies.

Identification

The Hessian fly belongs to the family Cecidomyiidae. Adults are small flies with long legs and a dark body that is reminiscent of a mosquito. Females are about 1/8 in (4 mm) in length with a reddish color (figure 1). The brown or black males are smaller (1/10 in to 1/8 in or 2.5 mm to 3.5 mm) with longer antennae than females.^2,3 Eggs are long and cylindrical, 1/50 in (0.5 mm) in length, with a red coloration that darkens with age (figure 2).² Larvae (figure 3) have three stages (also known as instars), with size varying from 1/45 in to 1/15 in (0.56 mm to 1.70 mm) for first instars, to 1/15 in to 1/8 in (1.70 mm to 4.00 mm) for second instars.⁴ The third instar develops inside the second instar, which becomes a puparium where the final immature stage, the pupa, develops. The term “flaxseed” is often used to describe the pupa, as the pupa is dark brown and 1/13 in to 1/4 in (2 mm to 6 mm) long (figures 3 and 4).

Figure 1. Adult Hessian fly (Mayetiola destructor). Image credit: Scott Bauer, USDA Agricultural Research Service, Bugwood.org.

Figure 2. Hessian fly eggs (Mayetiola destructor). Image credit: Roger Ratcliffe, USDA Agricultural Research Service.

Figure 3. Hessian fly (Mayetiola destructor) larvae: second instar (A), third instar (B) and pupae (C). Note that second instars can be slightly translucid before turning white when they begin to feed. Image credit: Roger Ratcliffe, USDA Agricultural Research Service.

Figure 4. Hessian fly (Mayetiola destructor) pupa. Image credit: Pest and Diseases Image Library, Bugwood.org.

Life Cycle

Adult Hessian flies live for one to four days.⁵ Females lay between 250 to 300 eggs on the upper surface of wheat leaves.³ Eggs hatch after three to twelve days, depending on temperature.^3,6 Larvae then move towards the ground behind leaf sheaths until reaching the base of the plant, typically below the soil surface.² Feeding can then last between fourteen to thirty days, depending on environmental conditions. Pupation occurs under leaf sheaths, often located below the soil surface on young wheat tillers. Duration of the pupal stage can last between fourteen to twenty-one days. While most insects entering the pupal stage will emerge as adults within a short period of time, some individuals in a generation may not emerge for several months or years. Hessian fly generations are, therefore, referred to as broods rather than generations. As a cool season insect, the number of broods per year can vary from two in northern US regions to three to six broods in the southern regions.⁷ In South Carolina and the Southeastern United States, the insect oversummers as a pupa, with adult emergence beginning in September.⁷ Two to three fall broods, one winter brood, and one to two spring broods have been observed in South Carolina.⁷

Figure 5. Stunted growth and dark green leaves of wheat from Hessian fly (Mayetiola destructor) damage. Image credit: John C. French Sr. (Retired), Auburn University, Clemson University, and the University of Missouri, Bugwood.org.

Injury to Small Grain Crops

Feeding leads to dark green stunted wheat plants, as well as dead tillers and thin stands (figure 5). During reproductive wheat stages, larval feeding can weaken the stalk, leading to reduced grain filling, lodging, and reduced test weight. Studies in Georgia showed that for every percent increase in infested tillers, yield losses were 21.1 kg/ha (0.34 bu/ac) in the fall and 12.8 kg/ha (0.19 bu/ac) in the spring.⁸

Management

Resistant Varieties

Thirty-seven genes in wheat have been identified to confer Hessian fly resistance, resulting in many commercially available resistant wheat varieties.^9,10 However, resistant varieties quickly become ineffective, as Hessian fly can adapt to resistance genes in these varieties.¹¹ New resistant strains of Hessian flies can then cause outbreaks even on wheat varieties that were once classified as resistant. Evaluating the response of resistance genes among field collections of Hessian fly is therefore important to help growers make informed decisions when choosing varieties and to help breeding programs select resistance genes when developing new varieties.¹²

Planting Date

In northern US regions, delaying planting until after the first freeze (known as ‘fly-free date’) is recommended as a cultural practice to prevent Hessian fly infestations.¹³ However, this practice is not effective in the southeastern United States. To maximize yield potential, wheat should be planted from November 1 to December 1 in South Carolina.¹⁴ Delaying planting beyond these dates for Hessian fly management is not recommended due to the lack of dependable freezing temperatures in South Carolina and other states in the Southeastern United States.^3,15

Rotation

Since the insect oversummers in wheat stubble, continuous planting of wheat or a double crop of wheat and soybean should be avoided. Instead, crop rotations can help reduce Hessian fly infestations by planting wheat away from a previous season’s wheat stubble.³

Control of Volunteer Wheat and Tillage

When adult Hessian fly emerges in September, volunteer wheat is an important host that can bridge the gap between the emergence and fall planting of wheat.¹⁶ Removal of volunteer wheat can, therefore, help to reduce Hessian fly infestations. Tillage can help remove volunteer wheat and bury oversummering Hessian fly pupae, although studies have shown inconsistent effects of tillage on Hessian fly emergence.^17,18

Cover Crops

The use of cover crops other than wheat can help reduce Hessian fly infestations.³ Oats can be used as an alternative to wheat as a cover crop, as oats are less attractive for egg-laying and do not promote the buildup of populations relative to wheat.¹⁹

Biological Control

A range of natural enemies have been reported to reduce Hessian fly populations,²⁰ although rates of predation and parasitism are highly variable. For instance, in Texas, parasitism rates varied from 0% to 87.5%, with the wasp Homoporus destructor (Say) (Hymenoptera: Pteromalidae) being the most common species found.²¹ Other common species of parasitoids include the wasps Platygaster hiemalis (Forbes) (Platygastridae), Homoporus destructor (Say) (Pteromalidae), and Pediobius epigonus Walker (Eulophidae).^21–23 Although data are lacking from South Carolina, a study in Georgia showed parasitism rates of 55%, with P. hiemalis being the dominant species.²⁴

Chemical Control

Insecticides can be used for Hessian fly control either as a seed treatment or as a foliar application. Neonicotinoid seed treatments can be effective for Hessian fly control,²⁵ particularly at high rates for susceptible varieties.²⁶ Seeds can be treated with the systemic insecticides imidacloprid, thiamethoxam, or clothianidin (table 1). Low rates, or those used to manage aphids, are generally not sufficient for control of large infestations of Hessian fly. In addition, none of the seed treatments are 100% effective under very high pressure. Controlling Hessian fly with foliar applications of insecticides is challenging, as the application needs to be timed when adults are present and laying eggs.²⁷ Because of challenges associated with the use of insecticides, there are no economic thresholds to help make management decisions or recommended sampling plans.

Table 1. Insecticide seed treatments available with Hessian fly activity.

¹ REI = restricted-entry interval; PHI = preharvest interval.

Summary

Hessian flies are difficult to control with applications of insecticide. Integrating proactive control measures using resistant varieties or insecticide seed treatments (particularly if planting a variety that is known to be susceptible to Hessian fly) are the most effective approaches to minimize injury and yield loss. While the insect can be a sporadic pest in the Southeastern United States, outbreaks have occurred in recent years, which warrant careful consideration of the recommendations outlined in this publication.

References Cited

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