By Pierce Paul, Andy Michel, Kelley Tilmon, Ohio State University Extension
With the recent warm temperatures, we have been receiving a few questions on the risk of aphids in wheat and the transmission of barley yellow dwarf virus (BYDV). How should growers prepare and gauge the risk of both aphid infestation and BYDV transmission?
First, aphid infestations that cause economic damage are rare in Ohio either in the autumn or spring. There are several species of aphids that infest wheat, and most cannot overwinter in Ohio (they migrate from the southern US). However, aphids can, under certain conditions, build in numbers and damage wheat by feeding on the plant during seedling stages. A suggested treatment threshold for aphid management in wheat is 50 aphids per linear foot of row. Given the warm temperatures, we recommend that growers scout wheat fields to see if any aphids are present.
Second, since economic feeding damage is rare, the larger concern is BYDV transmission. For aphids to successfully transmit the virus, they normally need between 12 and 30 hours of feeding to acquire the virus, and then 4 or more hours of feeding to transmit it. However, aphids are capable of acquiring the virus after feeding on infected plants for only 30 minutes and, once they acquire the virus, they can transmit it to healthy plants for the rest of their life. The typical symptoms of this disease are erect leaves with yellowish to reddish-purple tips. Yield reduction due to BYDV is generally greater when infections occur in the fall than in the spring. BYDV tends to be most severe in fields planted before the fly-free date in which aphid populations can reach high levels. However, some fields planted after the fly-free date may still have high levels of BYDV, most likely because of warm temperatures that kept aphids active for a longer time period. Recommended management tactic for BYDV are as follows: 1) plant varieties less susceptible to BYDV; 2) delay planting until after the Hessian fly safe date to avoid early fall infections; 3) balanced fertility; and 4) controlling volunteer wheat, barley, and oats.
Spraying insecticide to control aphids in an attempt to manage BYDV is open to discussion, and not a recommended tactic. The main reason is that only a few aphids are needed for successful BYDV transmission. Any aphids present prior to spraying may have already transmitted BYDV, while other aphids may continue to arrive in the field after spraying. When spraying insecticides to control aphids early, growers should know that the residual effect of the insecticide may not last long enough to protect against later aphid population buildup nor virus transmission. Though insecticides applied after infection will reduce the aphid population, it will not prevent the disease from developing once the plants have been infected. Keep in mind that insecticidal seed treatments might prevent establishment of early arriving aphid populations, but they have to feed to get a toxic dose. In feeding, the aphid may transmit BYDV and, once infections occur, there is very little that can be done.
There are situations where it is acceptable to spray for aphids, and where insecticide application might pay. These include: 1) wheat under drought stress with aphids present; 2) growing a variety known to be susceptible to BYD with aphids present; 3) wheat being grown for seed; 4) wheat that is highly intensively managed with a 100+ bushel potential yield; and 5) wheat planted before the fly-free date. However, for most growers, cost-effective control of BYDV may not be possible by aphid spraying.