Severe infestations may appear as large, bronzy or brown patches in the field, often easily visible from the road. Later in the season, these patches may appear as thin stands. In younger plants, aphids may cluster near the base of the plant, on the upper sides of the leaves. In older plants, check inside leaf whorls for the aphids. Collect plants by cutting the roots below the crown.
Agronomists routinely take stem samples for aphid counts.
Cut the stem close to the ground and then hit it sharply against a stiff piece of white paper or into a white pan. This dislodges the aphids so they can be counted. While counting aphids, note the presence or absence of predators, parasitized aphid mummies, or aphids killed by fungus. The threshold for cereals is 12-15 cereal aphids per stem prior to the heading stage. The threshold for canaryseed is 20-30 cereal aphids per tiller between the heading and soft dough stage.