Aphids have been showing up in small grains recently, primarily in wheat and oats. These insects can be problematic because they transmit Barley Yellow Dwarf Virus (BYDV). Bird cherry-oat aphids (Figure 1) are the primary species responsible for vectoring BYDV, but other aphids like the English grain aphid can transmit it as well. Aphids acquire the virus when feeding on an infected plant, then can transmit BYDV to other plants as they move through the field feeding. The potential for yield loss is higher when plants are infected in earlier growth stages. Infected plants may be stunted, have heads that do not emerge, and have lower yields.
Aphid numbers will vary from season to season. The number of aphids present in the field during the fall depends on multiple factors. Planting date, the size of aphid flights, and the weather are all factors that affect infestation numbers in a grain field. The risk of BYDV infection depends on the number of these early arriving aphids that have the virus. If many of the colonizing aphids are carrying BYVD, the risk of virus spread is much higher.
Aphids are not always easy to spot in the field. They are small and often found feeding on the underside of the leaves. When the weather is colder, they move down to the base of the plant or underground. Therefore, a warm day with temperatures above 60°F is the best time to spot aphids on the leaves. A sweep net can also be useful to detect the presence of aphids, but will not always catch the aphids at the base of the plant or in the ground.
Symptoms of BYDV
There is a lot of variability in the symptoms associated with BYDV infection. BYDV is typically diagnosed using both field symptoms and the presence of aphid vectors in the field. The infected plants can be discolored or stunted. Infected plants often occur in small patches surrounded by healthy plants. If the infection is widespread due to high aphid populations and virus transmission, patches can be larger. Leaves with BYDV may be yellow, red, or purple with variation between crops. Infected oats can turn red or purple while wheat leaves may turn red or yellow (Figure 2). The Plant Diagnostic Laboratory at Auburn can diagnose BYDV from plant samples if you are unsure.
The best strategy to avoid problems with aphids and BYVD is to plant within the recommended planting date for your region. Seed treatments that contain imidacloprid, thiamethoxam, or clothianidin can help reduce fall infestations of aphids and therefore virus transmission. If aphids are present, it is important to control them in late winter before it warms up and the populations begin to increase. Foliar insecticide applications are effective at reducing aphid populations in late January and early February. Refer to the ACES Small Grains IPM Guide for chemical recommendations and rates.