Aphids in Wheat
The bird cherry oat (Rhopalosiphum padi), English grain (Sitobion avenae), greenbug (Schizaphis graminum), and corn leaf (Rhopalosiphum maidis) aphids are key pest species in small grains (wheat, barley, and cereal rye) in Kentucky for their role as vectors of barley yellow dwarf virus (BYDV).
Bird cherry oat (Figure 1) and English grain aphids overwinter as nymphs or adults, and they can start feeding (and potentially transmit viruses) when temperatures are above 45º F. They start probing plants when temperatures are greater than 45º F. At constant 50º F temperatures, these aphids may complete their life cycle in 28 to 30 days, whereas at 77º F, the life cycle can be shortened to approximately 8 days. Below 45º F, aphids are inactive, lethargic, and sheltered in soil crevices near the bases of wheat stems.
Early in February 2023, I collected some wheat plants that I took to the laboratory. At that time, I was not able to count any aphids but after 2 weeks in the laboratory, these plants were completely infested with bird cherry oat aphids. In addition, I scouted for aphids in some fields during the week of February 20, and I found aphids with averages ranging from 0 to 8.5 aphids per foot-row per field; these numbers are below the economic threshold to conduct sprays (Table 1). In some fields, I observed many lady beetles searching for aphids. At low aphid numbers, these natural enemies may be important and should be considered before conducting sprays (Figure 2).
In 2023, temperatures in February had been above 45⁰ F for many days, and there were a couple of days with temperatures higher than 60⁰ F (Figure 3). This pattern has occurred in many regions of the U.S. and maximum record temperatures have been reached in 2023 compared to previous years. Figure 3 shows the daily average temperatures for Princeton for various periods from October 1 to March 31 in several years. These data were evaluated for a 13-year period (2010-2023; not all data shown in Figure 3) but the trendline for the 2022-2023 (blue) shows that these temperatures are the highest compared to the others.
Given the circumstances mentioned above, with high temperatures (greater than 45⁰ F) in February, it is recommended that farmers and consultants scout commercial fields before conducting sprays for aphid management. If tallies are above the threshold levels indicated in Table 1, an insecticide spray needs to be considered. Prophylactic sprays are not recommended as they are not part of sustainable IPM programs.
To reduce insect pests and manage insecticide resistance, farmers should consider different tools for IPM programs. These include the use of tolerant cultivars, crop rotations, early/late planting to avoid pests, and scouting to monitor pests or the abundance/absence of natural enemies. If more than one application is needed, farmers should use insecticides with a different mode of action to avoid the buildup of aphid resistance. Also, when sprays are applied, use the rates established on the insecticide label; if lower rates are used, the possibility of insecticide resistance can increase.
Insecticide spray programs for aphid management should follow integrated pest management (IPM) practices instead of calendar-based programs. In Kentucky, the rest of the USA, and many other parts of the world, calendar-based insecticide programs continue to be used due to time constraints for scouting, and economic savings achieved by reducing trips and conducting combined applications of herbicides and fungicides across fields. However, as mentioned above, the use of IPM is recommended to avoid unnecessary applications of insecticides, avoid increased costs of production, reduce insecticide resistance, and reduce killing natural enemies that may be keeping pests populations at low levels.
By Raul T. Villanueva, Entomology Extension Entomology Specialist