Evaluating effects of virus infection on alfalfa yield by comparing yield, quality, and virus prevalence (SRAV, AMV, and BLRV) in insecticide-treated versus check plots​

Insect-vectored viruses are known to be common in alfalfa (Medicago sativa), but there remain gaps in our understanding of the impacts of such viruses on alfalfa yield and quality. Aphids (including Acrythosiphon pisum) for example vector several viruses, including Alfalfa mosaic virus and Bean leafroll Virus. A recently discovered virus, Snake River alfalfa virus, is hypothesized to be transmitted by Western flower thrips (Frankliniella occidentalis). We conducted a two-year field experiment in southern Idaho to clarify the potential role of insect-vectored viruses on alfalfa yield and quality. Experimental plots (in a randomized complete block design with 8 replicates) were treated with insecticides targeting these vectors. Plots were planted in 2023 and 2024, and the 2023 plots were evaluated over the second year as well. Counts of F. occidentalis and A. pisum were recorded weekly from stem samples over each season and yield and quality data were recorded over multiple cuttings each year. Despite significant reductions in thrips and aphid abundance with insecticide applications, yield differences were rarely observed between treated and non-treated. Insecticide-treated plots in the 2024 new seeding, however, did show higher yields. Quality parameters also did not differ between treatments in 2023. Quality data for 2024 as well as virus results are in progress. Thus far, results suggest that the value of managing aphids and thrips to protect yield and quality losses in alfalfa may be limited.