Effects of insect-vectored viruses and vector management on alfalfa yield, quality, and virus prevalence

Damage to alfalfa, Medicago sativa L. (Fabales: Fabaceae) from insect pests and microbial pathogens has been extensively studied, but gaps remain in our understanding of the effects of insect-vectored viruses on yield and quality. Common alfalfa viruses transmitted by pea aphids, Acyrthosiphon pisum (Harris) (Hemiptera: Aphididae), include alfalfa mosaic virus (AMV, Alfamovirus), bean leafroll virus (BLRV, Polerovirus), and pea streak virus (PeSV, Carlavirus), but their yield effects are poorly understudied. In addition, the recent discovery of Snake River alfalfa virus (SRAV), which is widely distributed in alfalfa in the Northwest US, warrants investigation into its mode of transmission and possible effects on yield and quality. A 2-year field experiment compared a non-treated check versus a rotation of insecticides targeting pea aphids and western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae). Although insecticides reduced aphid and thrips densities, yield did not differ between treatments, except for one cutting in 1 year that showed slightly higher yield in the check. Insecticide-treated plots generally had lower densities of beneficial insects, which might have contributed to this difference. Forage quality metrics showed no clear differences between treatments. Virus prevalence was similarly unaffected by treatment, though higher AMV and PeSV prevalence was observed in the second crop year. Greenhouse experiments indicated that western flower thrips is not a vector of SRAV. Nevertheless, the high prevalence of SRAV and of aphid-transmitted viruses in older alfalfa stands underscore the need for further study on virus effects on alfalfa yield and quality over longer time periods.