The seed that farmers plant may be an important source of viruses. But tests on soybean seed showed little viral contamination in 2000, Grau says. Forage legumes may harbor the viruses the aphids transmit. Therefore, planting soybeans near forage legumes may increase the risk of losses to the viruses. Plants that become infected with viruses after flowering suffer far less damage than those infected earlier.
Insecticides are most helpful if they delay virus transmission until after flowering.
Sprays can keep the aphid population in check, Wedberg says. Growers should begin monitoring the crop in spring. They may not see aphids on seedlings, he says. Last year the aphid population exploded during the late vegetative and early bloom stages when plants were growing most rapidly. Wedberg says sprays will have the greatest impact after the aphids develop colonies and before flowering is over.