Synergist effects of early planting dates and Russian wheat aphid resistant varieties on aphid populations in Colorado, Kansas, and Nebraska

The Russian wheat aphid, Diuraphis noxia (Kurdjumov)(Hemiptera: Aphididae) (RWA) is an important pest in the U. S. Western Plains, causing hundreds of millions of dollars of losses to wheat and barley production through reduced yields and increased pesticide treatment costs. The objectives of this research were to evaluate the performance of two RWA-resistant barley varieties under two planting dates conditions, and to evaluate the effects of these varieties on RWA and natural enemy populations. The research was conducted in three experimental barley fields located in Colorado, Kansas, and Nebraska during 2007, 2008 and 2009. The experimental design used was a split-plot design with two main plot treatments (early and normal planting dates). Four split plot treatments (barley varieties) were randomized within each main treatment plot. Variety treatments included the Stoneham and Sydney RWA-barley resistant varieties, and the susceptible variety Otis under thiamethoxam-protected and unprotected regimes. Sampling of RWA and natural enemy populations was conducted on four dates from late May through early July. RWA populations in early planting date (1st week of March) plots were significantly lower in 2007-2009 for the Colorado and Kansas sites. Within early planting date samples, RWA-resistant varieties yielded RWA populations similar to those in insecticide-treated Otis plots at the Colorado and Kansas sites. Very low RWA populations were present at the Nebraska site on both planting dates, resulting in no differences in RWA populations between varieties. Early planting dates and RWA-resistant varieties did not affect the natural occurrence of RWA biological control agents.