From January until mid-April, the entomology program for small cereals in Princeton monitored aphid populations in wheat planted on four different dates (from mid-September to December 1, 2024). Aphids were largely absent during this period. However, from mid-March to mid-April, an average of fewer than two aphids per 1-foot-row was recorded. Since the beginning of this week (April 21, 2025), an average of one winged foundress aphid and three nymphs have been observed on every other flag leaf in commercial fields across Lyon, Caldwell, and Livingston counties. A winged foundress aphid typically emerges in the spring from overwintering eggs and initiates an asexual reproductive cycle (known as parthenogenesis). These winged foundresses then disperse to secondary hosts, colonize new fields, and establish new colonies.

The majority of aphids observed was the English grain aphid (Sitobion avenae) (Figure 1). However, there are several species of aphids in small grain fields in Kentucky (see below). These species are a threat to wheat, rye, oat, or barley, not because of their direct feeding, but by transmitting yellow dwarf viruses (YDVs); all of these aphid species can vector YDVs.

Aphids & Yellow Dwarf Viruses

In Kentucky, the most important aphid species vectors of YDVs include the English grain aphid, the bird cherry-oat aphid (Rhopalosiphum padi), the corn leaf aphid (R. maidis), the rice soil aphid (R. padi), the greenbug (Schizaphis graminum), andthe rose-grain aphid (Metopolophium dirhodum). These aphid species differ in transmission efficiency and abundance in small cereal fields. During the winter wheat growing season, the most abundant aphids observed in Kentucky are the bird cherry-oat aphid and the English grain aphid.

Yellow dwarf viruses are economically important with yield losses that can reach 80% in cereals (wheat, barley, rye, etc.), as well as reduced yield quality. Non-cultivated grasses can also be affected. Yellow dwarf viruses are phloem-limited and are in the Solemoviridae and Tombusviridae families. In the Tombusviridae, YDVs include five species of barley yellow dwarf virus (BYDV): BYDV-PAV, BYDV-PAS, BYDV-MAV, BYDV-kerll, BYDV-kerlll. In the Solemoviridae, YDVs include two species of cereal yellow dwarf virus (CYDV-RPS, CYDV-RPV) and one of maize yellow dwarf virus (MYDV-RMV) and other species (i.e., BYDV-GPV, BYDV-SGV, BYDV-GPV and BYDV-SGV). The viruses in these groups tend to have broad, overlapping host ranges in the Poaceae grasses, with somewhat more specificity in vector relations.

What to Do?

Small grain farmers may be concerned about the risk of yellow dwarf virus infections transmitted by aphids. In response, they might consider applying insecticides to control aphid populations. However, most wheat in western Kentucky has already reached growth stages between 10.1 (heading) and 10.5 (flowering) or later. At these advanced growth stages, the potential for virus transmission remains high, but the impact on yield is minimal. Therefore, insecticide applications may not be necessary, as the crop is developed enough that any effect on yield would likely be negligible.

More Information

• Peters, et al. Ecology of Yellow Dwarf Viruses in Crops and Grasslands: Interactions in the Context of Climate Change. (link)
• Villanueva, Raul T. Scouting For Aphids in Winter Wheat Fields With or Without Insecticide Seed Treatment in March and April is Critical to Reduce BYDV (link)