David Voegtlin, Center for Biodiversity, Illinois Natural History Survey, Champaign, Illinois 61820-6970, USA
Many aphid species that vector barley yellow dwarf virus are known to make extensive south to north migrations each spring. Some of the same species are capable of surviving the winter, especially a mild winter, considerably far north into the continent where winter wheat is planted. The contribution to local spread of BYDV by these two components of the vector population is not known except after a severe winter during which local populations are killed. The following spring local populations develop exclusively from migrants. Local population counts generate management decisions that might have been made differently had the identity and approximate number of aphids migrating in that direction been known. Population growth can be predicted from local counts, but major influxes into an area by potential vectors can modify predicted population levels significantly. Behavior and subsequently movement of virus of locally generated alate aphids may be different from migrants. Documenting fall movement on a large scale, coupled with winter weather data may also provide basic information for predicting regional populations.