Wheat Planting Considerations for Insects & Diseases

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Adult Hessian Fly.

• Decisions made before planting wheat are critical for effective season-long pest control.
• Variety selection should balance the factors required to meet overall cropping system objectives.
• The Hessian Fly-free planting date and other cultural factors remain very important.
• Seed treatments provide additional protection in high pressure environments.

Decisions made before planting wheat can have a large impact on the success of the season-long pest management program.  Perhaps the most important pre-season decision with any crop is variety selection.  Wheat varieties vary significantly in yield potential, agronomics, and defensive traits.  Varieties must be selected with consideration for the overall objectives of the cropping system – e.g. top end yield may be compromised for an earlier maturing variety that facilitates double-crop soybeans and offers a better disease package.  By contrast, full-season top yielding varieties may compromise disease tolerance traits, requiring additional crop protection inputs.  

Variety Selection
Obviously the first trait that you are looking for is yield.  The second trait you should look for is resistance to Fusarium head blight (FHB).  This is the most destructive disease encountered across wheat production systems in North America, and results in reduced yields in addition to mycotoxin (deoxynivalenol-DON) contamination of grains, requiring additional cleaning and potentially blending by elevators.  Resistant wheat cultivars reduce the colonization rate and degree of spread in wheat heads and reduce DON contamination.  This is important as healthy-looking kernels can contain elevated DON and these kernels cannot be removed from high quality grain.  Our modern wheat cultivars have much greater resistance to FHB than those released 5-10 years ago.  In addition, most of these cultivars do not come with a yield penalty as they did in the past.  Ratings can be found on company websites.  Universities also publish uniform FHB trials, which use misted nurseries to avoid disease escapes and provide more accurate FHB resistance ratings.  These ratings can be easily accessed at the ScabSmart website:  https://scabsmart.org/    

Other diseases that may not be as devastating but more commonly occurring in wheat production should be next on your list of variety traits to look for. These diseases include Septoria leaf blotch, Stagonospora leaf and glume blotch, powdery mildew (especially in high nitrogen management systems and manured fields) and stripe and leaf rust. Stripe rust is typically a concern if it blows into wheat production regions and conditions remain very cool and wet.  Keep in mind what traits your varieties have so you can determine what diseases to look out for during the season.

Planting Date
A second critical pre-season factor to consider is planting date.  The Hessian fly is a small fragile gnat that appears in late September, lives for just a few days, and lays eggs in wheat, barley, rye, and other native grasses.  Maggots hatch soon thereafter and feed on plant juices, which can lead to permanent stunting, chlorosis, and lodging.  Planting after the fly-free date is effective because the wheat emerges after most adult flies have perished and the egg laying period has ended. The fly free dates ranged from late September to mid-October across most wheat growing areas of the Midwest, so check with your local extension entomologist for more information.  Hessian fly is now much less of a problem in the Midwestern US than it was historically, due to the widespread cultural practice of planting after the fly-free date.  Mild fall weather can push back the fly-free date by several days, because adults will remain active longer into the fall. The primary target is to get wheat in the ground within 10 days to two weeks after the “fly-free date.”  As you go beyond this target, increase the seeding rate by as much as 30% to compensate for fewer Fall tillers.  

Planting after the fly-free date is a good cultural practice for several additional reasons:  

• Early planting can lead to excessive fall growth, which can increase susceptibility to winter-kill and development of some foliar diseases.  In some cases, fall infection can allow some diseases that typically do not survive winters to overwinter, such as stripe rust.   
• Late planting can result in poor tiller development and winter survival.
• Higher soil temperatures in early fall increase the activity of a host of soil-borne diseases, including seedling blights, as well as soil-borne viruses like soil-borne wheat mosaic virus (SBWMV) and wheat spindle streak mosaic virus (WSSMV, vectored by the fungus-like organism, Polymyxa graminis).
• Modern varieties have been bred for resistance to Hessian fly, but new Hessian fly biotypes are evolving for which no genetic resistance is available. 
• Aphid populations are often higher in earlier planted wheat.  Aphids directly reduce vigor and can vector the disease Barley Yellow Dwarf Virus (BYDV) in the fall.  
• Early planting increases severity of infection by wheat streak mosaic virus (WSMV), which is vectored by wheat curl mites that are more active at higher temperatures. 
• Early planted fields may be targets for beetles that produce white grubs (i.e. June, Japanese beetles, masked chafer).  These grubs could be problematic for subsequent soybean and corn planted in these fields, especially if three year grubs from June beetles establish.
   

Seed Treatments
Seed treatments are also an essential component of high management wheat programs.  Fungicidal seed treatments provide critical protection against soil-borne diseases (Pythium, Rhizoctonia, Fusarium, common bunt, and stinking smut), seed-borne diseases (loose smut), and even some early fall, foliar diseases (powdery mildew, rust, and Septoria).  Keep in mind that these last about 3 weeks give or take, and will not have activity in the Spring. 

Insecticidal seed treatments have been somewhat less consistent in insect control and yield response, but thiamethoxam and imidacloprid have been shown to provide good protection against Hessian fly, aphids, and below ground insects for a period of about 30 days after planting.  In general, consider an insecticide seed treatment if planting early for your region to minimize issues with these insect pests.  In the absence of insecticidal seed treatments, foliar insecticides can be used to manage aphids in the fall if warranted, but application timing is critical to effectively reduce damaging populations. Always refer to product labels for specific control recommendations.

Hessian fly larva and flax seed puparium 

Symptoms of Barley Yellow Dwarf Virus in wheat