Factors that influence Hessian fly fall infestations
Which wheat fields are most likely to be infested with Hessian fly in the fall? It depends on residue management, variety, planting date, the presence of nearby volunteer wheat, the use of insecticide seed treatments, and crop rotation.
Residue management. Undisturbed stubble favors survival. Experience has shown that, where soil management practices allow, thorough incorporation of the stubble can be a useful management technique. Thorough incorporation must be stressed, however. In one study, flaxseeds buried 1 inch below the surface of the soil allowed 26 percent of the population to emerge, at 2 inches only 6 percent emerged, and none emerged where stubble was buried to a depth of 4 inches. In another study, it was determined that double discing was five times more effective than single discing. What about burning and grazing? Studies have shown that burning destroys flaxseeds present on the above-ground portion of the stem. A slow-moving fire is best, but stubble fires are often fast moving and affect top growth instead of burning out the crowns at or below the soil line where the majority of flaxseeds exist.
Variety. Often the best practice is to consider planting a resistant variety, where practicable. Growers should consider this option carefully during times when fly populations appear to be increasing, especially when the intention is to plant early for fall pasture and where other options are limited. Consult with your local K-State Research and Extension agent for more information on performance of varieties in your area. Or see K-State Research and Extension publication MF-991 Wheat Variety and Disease Insect Ratings, available at: http://www.ksre.ksu.edu/bookstore/pubs/MF991.pdf
Planting date. In theory, waiting to plant until the best pest management planting date (BPMP) allows time for the main fall brood of adult Hessian flies to emerge and die before wheat is planted. Without live wheat plants, emerging females are deprived of a place to lay eggs, minimizing fall infestation. There is still some risk if a nearby infestation exists and a secondary fall brood develops. The BPMP used to be called the “Hessian fly-free date,” but that older term is not accurate because it implies there will be no Hessian fly adults after that date – and that is not true.
The risk of fall infestation is almost always greater where wheat is planted before the BPMP date, especially during years favorable for fly development. Observance of the BPMP date also reduces the incidence of wheat streak mosaic and barley yellow dwarf viruses. The BPMP date strategy is based on studies conducted from 1918-1935, and BPMP dates are based on data collected more than 70 years ago, but are now being refined.
The relatively mild fall weather in recent years, along with a slight increase in average fall temperatures over the last 30 years, has reduced the effectiveness of using this date as a planting guide. In studies conducted in Sedgwick County, Kansas, during 2006 -2010 using a Hessian fly pheromone trap, adult flies were active until early December. It seemed that more adult flies were trapped after a rain. The impact of this extended Hessian fly activity on wheat or on fly population density is not known, but it is interesting to note that potential for Hessian fly infestation exists longer into the fall than historical data indicate. In addition, the BPMP date may not always present the best planting date for optimum yield, but on average, it correlates well. The BPMP date can be used on an individual-field basis but will be more effective when it is practiced area wide.
Planting too late is also risky. Growers may be surprised to learn that delaying planting too late in the fall can actually increase the risk of Hessian fly infestation. While late planting dates may protect the field against fall infestation, the result is smaller plants in the spring. And when the spring brood of flies is active in March or April, those females prefer younger plants for egg laying. Thus, if a source of infestation is nearby, very-late-planted wheat of a susceptible variety may suffer extensive damage from spring infestations.
Volunteer wheat. Volunteer wheat that is allowed to grow for two to three weeks, especially in wet summers, can enable the fly to produce an extra brood and infest the planted crop in greater numbers. Volunteer wheat not only increases the population but also may render other practices, such as planting after the BPMP date, less effective. The adult fly is capable of dispersing to adjacent fields to lay eggs, so it is vital to destroy volunteer wheat in the area at least two weeks before the planted crop germinates. This practice also helps reduce the incidence of wheat streak mosaic virus.
Insecticide seed treatments. Studies have shown that systemic seed treatments may provide some control of Hessian fly larvae for up to 30 days. Depending on when the wheat is planted, this may protect plants through the egg-laying period in fall, or at least shorten the period of vulnerability before cold weather stops adult emergence and larval feeding. In either case, Hessian fly impact is reduced.
Crop rotation. The Hessian fly has a limited host range and is not a migratory pest, so populations can be reduced by not planting wheat directly back into infested stubble.
Jeff Whitworth, Extension Entomologist
Holly Schwarting, Entomology Research Associate