Colonies of bird cherry-oat aphid were spotted in winter wheat at Draper in South Dakota last week. As winter wheat is heading toward booting stage, careful monitoring of populations by frequent scouting is highly recommended. Cereal aphids are the most common sucking pests of wheat in South Dakota. There are three different species that are frequently spotted in South Dakota: bird cherry-oat aphid, English grain aphid, and greenbug. Among the three, bird cherry-oat aphid appears first in the season as it has the ability to overwinter in South Dakota whereas the other two fly into the state from the south and thus appear late in the spring season.
The bird cherry-oat aphid is dark green with a rusty patch at the tail end near the cornicles (spine like structures); greenbug is bright green with a dark stripe on its back; and the English grain aphid is plain green to brown with no markings and long black cornicles (Figure 1). Although severe losses due to aphid feeding in South Dakota do not occur often, aphids have the potential to cause significant yield losses when favorable climate is coupled with high aphid populations.
Aphids have a tremendous ability to multiply as they reproduce without mating and can double their populations in 2 to 3 days. Damage due to aphids is two-pronged: direct damage caused by sucking on plant sap weakening the plant and affecting the yield and indirect damage by transmitting barley yellow dwarf virus. Of all three species, greenbug is known to cause the most damage as they inject a toxin during feeding that causes yellowing and stunting of plants.
Scouting should be done by taking aphid counts on individual stems by looking on both sides of the leaves. Usually aphid colonies start from lower leaves and the easy trick is to look for ants that get attracted to the sweet secretions of aphids. Economic thresholds for cereal aphids have been determined for all the three species (Table 1). However, irrespective of species, if 85% of sampled stems have one or more aphids at early stages of wheat development or 12 -15 aphids per stem at or before heading pesticide treatments are recommended. Before making spray decisions check for predators such as lady beetles, lacewings, hover flies, minute pirate bugs, and parasitic wasps that play a major role in suppressing aphid populations.
For the list of insecticides that are labelled for application on wheat aphids in South Dakota, view the SDSU Extension 2013 South Dakota Crop Protection Guide: Wheat Insecticides Table publication.