Seasonal and Spatial Dynamics of Alate Aphid Dispersal in Snap Bean Fields in Proximity to Alfalfa and Implications for Virus Management

Alfalfa is a source for viruses that may be acquired by aphids and transmitted to snap bean, Phaseolus vulgaris L. Snap bean fields in proximity to alfalfa could have an increased risk of virus infection. Knowledge of the abundance and temporal and spatial dispersal patterns of commonly encountered aphids in commercial snap bean fields, varying in distance from alfalfa, could provide insight into this risk. Alate aphids were monitored using water pan traps in snap bean and alfalfa fields that were adjacent to or >1 km away from each other. The pea aphid, Acyrthosiphon pisum (Harris), was the most common aphid species captured in early-planted snap bean fields in 2002 and 2003 (56 and 23% of total, respectively), whereas the corn leaf aphid, Rhopalosiphum maidis (Fitch), also was common in 2003 (15% of total). In contrast, the yellow clover aphid, Therioaphis trifolii (Monell), and soybean aphid, Aphis glycines Matsumura, were the most abundant species trapped in late-planted snap bean fields in 2002 (77% of total) and 2003 (64% of total), respectively. These species were prevalent in traps in alfalfa as well. The abundance and temporal dispersal patterns of these species in snap beans adjacent to and >1 km away from alfalfa were similar, suggesting that the risk for virus infection may not be affected by proximity to alfalfa. A similar number of alate aphids also were captured along snap bean field edges and field centers, regardless of their proximity to alfalfa. This suggests that the aphids dispersed into snap bean randomly rather than directionally from the field edge. The implication of these results is that separating snap bean fields from alfalfa or using crop borders/barriers are not likely to be successful virus management strategie