November Weather Is Suitable For Aphid Feeding & Virus Transmission On Wheat

DR. RAUL T. VILLANUEVA

PRINCETON, KENTUCKY

 Current Conditions

In Kentucky, small grain cereals, including wheat and barley, are planted after the fly-free dates established for the Hessian fly. These dates are October 4 and October 15 for the northernmost and southernmost latitudes of Kentucky, respectively. Since the beginning of October, most of the daily mean temperatures have been above 45⁰F, except during 3 to 4 days between October 30and November 2 (Figure 1). When temperatures are above 45⁰F, aphids become active and start to feed on plants. Thus, all of the small cereals planted so far might be at risk for aphid feeding and virus transmission by these insects. The risk also includes plants that had been planted with insecticide-treated seeds and those that germinated approximately 20 to 25 days ago (the average duration of the protection period for insecticide seed treatment).

Current Aphid Populations in Wheat

Although I only checked one research field and one commercial field that were planted by mid-October, I found populations near or above the threshold of 3 aphids per 1 ft-row for this stage of plants. In most cases, the aphids were winged (alate) aphids (Figure 2). These species include the bird cherry-oat (Rhopalosiphum padi), English grain aphid (Sitobion avenae), greenbug aphid (Schizaphis graminum) and corn leaf aphid (R. maidis).  The first three species are vectors of several viruses including barley yellow dwarf virus (BYDV). Studies have shown that when aphids feed on younger plants, these plants are more severely affected by the virus than plants that are at later stages of development when the aphids feed. As mentioned above, even when insecticide seed treatments were used, the protective effect of this treatment may have decreased or been gone 20 to 25 days after germination.

Management

Under the conditions described above, it is recommended that growers scout their fields. If untreated seeds were used, scouting should begin at the time of seed germination. If insecticide-treated seeds were used (this is done in most commercial fields in Kentucky), start to scout 20 to 25 days after planting. Scouting may be conducted during emergence (2-leaf stage when stand counts are evaluated), at the 4 to 5 leaf stage, then routinely every 1 or 2 weeks later. Scouts should examine three separate 1-foot lengths of row for each 20-acre location within a field. An additional site should be added for each additional 10 acres.

Insecticides may need to be sprayed to reduce aphid populations. Sprays need to be well-timed in the fall (if untreated insecticide seeds are used or warmer temperatures occur 25 days after insecticide-treated seeds are emerged), winter, or spring.

For insecticides registered in Kentucky, refer to the ENT-47 publication (Insecticide Recommendations for Small Grains) or contact your local county Extension agent. ∆

 DR. RAUL T. VILLANUEVA: University of Kentucky