After characterization of the natural spread of necrosis-inducing Bean yellow mosaic potyvirus (family Potyviridae, genus Potyvirus, BYMV^N), nonpersistently transmitted from clover, Trifolium repens L., to an adjacent field of snap bean, Phaseolus vulgaris L., in western Oregon, we established a study site enabling us to investigate the virus reservoir, to observe en masse transmission of BYMV^N to bean plants, and to identify aphid species associated with virus spread. Colonies of Myzus persicae (Sulzer), Acyrthosiphon pisum (Harris), and Aphis fabae Scopoli associated with virus spread were established in an insectary and shown to vector this virus. Although Nearctaphis bakeri (Cowen) comprised 68% of aphid alatae taken from bean leaves during virus spread, we were unable to show that this species could vector the virus by using the same methods that were successful for the other species. Instead, we found that when two distinct N. bakeri colonies unexpectedly emerged from the roots of T. repens BYMV^N source plants (WZwc #6 and #11) that were present in the laboratory (insectary), these aphids transmitted BYMV^N at rates comparable with those of M. persicae and A. pisum. Transmission of BYMV^N also occurred with two other N. bakeri colonies maintained for 4 mo on Trifolium pratense L. (NZwc Sch 3B and Sch 7C) BYMV^N source plants. Each of these four BYMV^N transmission successes also demonstrated an unprecedented once-only transmission of BYMV^N by N. bakeri colonies. Our experience with western Oregon N. bakeri colonies was compared with descriptions of this native North American species after its 1960–1980s arrival in France, Germany, and Italy.