Scout for aphids. Begin scouting for aphids in early July, and scout twice per week. Using a hand lens or magnifier, carefully inspect all leaves, stems and branches of the soybean plant. Check the undersides of leaves. Look for aphids themselves and signs of the insect, including white 'skins' that aphids shed and the honeydew they produce.
Estimate the extent of the pest problem. Calculate an average number of aphids per plant. If this average exceeds 250 insects per plant, consider applying an insecticide.
Apply insecticide. Always follow manufacturer's instructions. For best results, spray the entire soybean plant. Ground application is optimal as aerial application may reach only the upper canopy of the crop.
Maintain scouting and spraying. Keep checking the crop for aphids, and follow insecticide manufacturer's guidelines on when to re-spray. Do not spray too often, as pests may develop resistance.