Distribution of overwintering Russian wheat aphid (Homoptera: Aphididae) in furrow-irrigated small grains in western Colorado.

The Russian wheat aphid, Diuraphis noxia (Mordvilko), was sampled in spring 1989, fall 1989, and spring 1990 to document distribution and density within fields of small grains in western Colorado. In spring 1989, aphids were found throughout fields with east-west furrows, but plants located on the south-facing slope of the irrigation beds had the highest infestations. In fields with north-south furrows, aphids were found only on plants on south-facing slopes of irrigation ditches at either end of the field. In fall 1989, there was a higher density of aphids on the south-facing slopes of beds than at other sample sites in fields with east-west furrows whereas they were evenly distributed over beds in fields with north-south furrows. Infestations increased on the south slopes and middle of beds in fields with east-west furrows during the winter of 1989-1990 but remained unchanged in fields with north-south furrows. Temperatures were higher and heat unit accumulations greater on the south slope of the bed than at any other site.