Look for symptoms of damage, including leaf discolouration in host plants. 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.
Some 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. Trapper peas can tolerate higher levels. Plants infested before the flowers open recover without loss of yield. The threshold in alfalfa is 10 aphids per stem at 2 weeks before hay cuttings.