To ensure that the wheat you put into the ground get off to a healthy, prosperous start, you should aim to have your fields clean two weeks prior to planting. This short lapse in time ensures that your field is clear of not only volunteer wheat but also the many insects and diseases that can come along with it. If you used glyphosate to kill off any volunteer wheat, the two week lapse will ensure that no residuals affect your new crop.
Controlling volunteer wheat is of great importance prior to drilling, as volunteer wheat can pose numerous problems. Three common threats in the Southern Region are Wheat Streak Mosaic, Hessian Fly, and Barley Yellow Dwarf; all of which can be effectively managed by the timely control of volunteer wheat, which harbors the insects that cause these issues.
Wheat Streak Mosaic (WSM) has hit our area hard the past few years. Erik DeWolf, a wheat disease specialist for Kansas State University Extension, estimates that in 2012 alone, WSM reduced central Kansas farm income by roughly $34 million.
WSM is a virus carried by the wheat curl mite, a microscopic insect immune to insecticides. Wheat curl mites feed on volunteer wheat until a new stand of wheat is established, at which point they move into the new wheat and infect it with the WSM virus. WSM causes stunting and yellowing of plants and is detrimental to yield. Since the carrier mite is immune to insecticides, the only way to control WSM is to control volunteer wheat.
Hessian fly is another detriment that volunteer wheat harbors. Hessian flies typically have two generations per year, but can have more than 3. In the first generation, larvae live throughout summer months on wheat stubble and crowns from the previous crop. In the late summer and early fall, the larvae emerge as adults that lay eggs among the wheat. The new larvae then feed throughout the fall and emerge as the second generation in March and April where they lay up to 250-300 reddish eggs on the plants and start the cycle over again.
Maggots soon hatch from the eggs and it is the ever-hungry larvae that cause damage to your crops and release harmful toxins as they feed. These tiny larvae crawl to the crown of seedlings (just above the roots) and feed on plant juices after injecting their unique saliva. Feeding by one larva can permanently stunt plant growth. A larva will complete its growth before cold weather and pass the winter as puparium or “flaxseed.”
Plants affected in the fall by Hessian flies display a dark, almost bluish discoloration and often die. Plants affected in the spring show short, weak stems, which can eventually cause breakage just above the first or second node and lodging during harvest. In the spring, adult flies emerge from the “flaxseeds” and lay eggs on the leaves. Upon hatching, the maggots work their way under the leaf sheath near the node of these large plants. Their feeding at this site weakens the stem which results in the stalks breaking over before harvest. The maggots change into puparia about the time wheat heads out and they remain in the stubble as “flaxseeds” until fall.
Barley Yellow Dwarf (BYD), which is a virus carried and spread by the bird cherry oat aphid, negatively affects yield as well. The bird cherry oat aphid, which has a distinct dark olive green to greenish black appearance, infests volunteer wheat during the summer and moves into newly planted wheat in the fall. BYD appears with symptoms similar to nutrient deficiencies and so it can be easily misdiagnosed in its early stages. Symptoms appear in patches across your fields and first show up in older leaves as faint yellow-green blotches near the tip. These blotches can change color from yellow to purple to red. Plants affected early are stunted, reducing yield. BYD alone cost north-central Kansas farmers more than $14 million in 2012.
Fortunately, BYD is distinguishable from nutrient deficiency with simple crop scouting for the bird cherry oat aphid. Furthermore, the bird cherry oat aphid, unlike the wheat curl mite, is controllable via insecticide if volunteer wheat management doesn’t prevent their presence to begin with. The threshold for treatment is 20 aphids per tiller, though many growers will treat at a lower threshold, as the presence of any number can mean the transference of disease.
Even with the careful control of volunteer wheat before planting, you will also want to consider using a post-emergence herbicide. I will talk about this at greater length in a future article, but it is important to begin considering the restrictions that the herbicides may bring into your operation. Sulfonylurea chemistry and Phenoxy herbicides are the most commonly used post-emerge products in the Southern Region—specific restrictions depend on the products you utilize in your operation, but residual in soil can be as long as 14 months. Your herbicide choices will need to take into consideration if you plan to rotate to milo, double crop or even plant a cover crop. Sulfonylurea Tolerant Soybeans (STS), have been developed to tolerate certain sulfonylurea herbicides and STS beans even have a four month restriction behind a SU herbicide.
Volunteer wheat and weeds not only harbor unwanted insects and diseases, but they steal moisture and nutrients away from your crops as well. Careful management of volunteer wheat now and plans to control weeds in the near future will ensure that your crops receive the maximum amount of precious water and nutrients, which is value added to your fields.