The number of aphids is increasing in Spokane and Whitman county small grain fields, according to Washington State University researchers.
Reports include a relatively new aphid species, the cereal aphid, found in North America in 2013.
Aphids can damage small grain crops through direct injury, by sucking nutrients directly from plant tissue, and indirectly by transmitting diseases such as barley yellow dwarf virus, according to a report by WSU Extension regional specialist Diana Roberts and WSU entomology professor Dave Crowder.
Roberts speculated that weather or predator populations have led to the increase in aphid populations.
The last significant aphid infestation in spring wheat was in 2006. Infestations are sporadic, Roberts said.
“Which makes it a little tricky from our perspective,” she said, because it is difficult for researchers to establish a recommendation for the level of infestation at which farmers should apply insecticides.
“They need to know what density or population of aphids causes how many bushels of yield loss, and then they can calculate that out against their expected yield and the cost of the chemical,” Roberts said. “In some areas this year, the spring crops are not very good and so that affects whether or not people want to put more money into it.”
No economic thresholds have been determined for the cereal aphid, but Roberts and Crowder recommend the following steps for the other aphids:
• For Russian wheat aphid, farmers should spray an insecticide if more than 20 percent of tillers are infested, because the aphid can distort the head.
• For the bird-cherry oat aphid, a density of 20 aphids per tiller will lead to an estimated 5 percent yield loss, and a density of 40 aphids per tiller leads to a 9 percent yield loss. Heavy populations may cause a golden yellow streaking on the leaves.
Roberts and Crowder advise farmers not to confuse it with the white streaks caused by Russian wheat aphids. The oat aphid can also transmit barley yellow dwarf virus, particularly in young plants, so the economic threshold is 1 to 2 per head at the flag leaf stage. These thresholds are rarely reached, Roberts and Crowder say.
With the increase in aphids, insects that prey on them can also multiply quickly. Roberts cautions that spraying insecticides can also affect beneficial insects and predators.
No treatments are recommended for most aphid species after wheat enters the soft dough stage before maturity.
Most infestations begin in the south and move north with the maturity of the wheat crop, Roberts said. Aphids were first spotted in the Walla Walla area and along the Snake River, and have made their way north.
Roberts encourages farmers to report crop problems on the WSU Small Grains website.