Aphids and yellow dwarf viruses of wheat
Bird cherry-oat aphids and greenbugs made the news this fall, with infestations in winter wheat fields reported across central and south-central South Dakota. Most fields planted in September had some level of infestation, and the dilemma was whether to apply an insecticide.
The recommendation is to treat with insecticides if there are on average, 20 bird cherry-oat aphids per plant or 15 to 25 per linear foot of row from seedling emergence in the fall to heading stage of wheat the following spring. The threshold for greenbugs is higher – 25 to 50 aphids per linear foot of row. These recommendations are based on feeding damage by the aphids, which causes yellowing of leaves and compromises plant growth. Temperatures recently dipping into the single digits has either made the insects go dormant or simply killed them.
Aphids are also known to transmit the viral diseases, Barley Yellow Dwarf (BYD) and Cereal Yellow Dwarf (CWD). Infections in the early growth stages are more serious than later. These diseases cause stunted growth and yellowing or reddening of the leaves, which can result in significant yield reductions. Once a plant has BYD or CYD, there is no treatment.
So why revisit the issue when nothing can be done? The best strategy for managing aphid infestations and potential yellow dwarf diseases is prevention, so the following information is for future reference. The best way to avoid aphid infestations in winter wheat in the fall is plant on or after the recommended date, 9/15 in northern South Dakota, 9/20 in the southern area. If a warm fall is forecast, later may be better. A warm fall can still allow aphid buildup in wheat planted at the recommended time. There is some level of BYD/CYD tolerance in a few varieties, but in wheat, the benefit is minimal.
If you must plant early to prevent wind erosion (fallow or prevented plant acres), insecticide seed treatments are effective in limiting aphid feeding and population buildup. Another approach for fallow or prevented plant acres is to plant a cover crop in August, grow some residue to prevent erosion, kill it at least 2 weeks before planting and plant during the recommended time. This strategy does hinge on available soil moisture, which can be a concern.
Foliar insecticides are not considered an effective control for the yellow dwarf viruses because the length of feeding time required by aphids to transmit the viruses can be as little as 30 minutes. If aphid infestations are detected early, insecticide treatments may limit the spread of the diseases, but this approach is not considered to be highly practical. In the fall, aphids tend to migrate below the soil line and getting insecticides to them is difficult. Aphids have been observed surviving insecticide applications on several occasions.
For winter wheat fields that had aphids this fall, one can expect some level of BYD and/or CYD virus next spring. That’s not guaranteed, as aphids are not 100% effective at vectoring the diseases, and not all aphids carry the virus, but it is likely some did. Winter wheat producers may want to watch their fields next spring for the telltale signs of the diseases; roughly circular patches of stunted plants with yellow and/or red leaves, especially around heading. How severely the disease(s) will affect yield depend on the weather and moisture conditions next spring and summer. Viral diseases restrict the uptake of moisture and nutrients. Stress from dry conditions and warm temperatures aggravate the symptoms of viral diseases and will make them very apparent. The yellow dwarf diseases were common in south-central SD in 2011, but plentiful rainfall and moderate temperatures in May and June minimized yield loss that year.