Rag virulence at a low frequency among soybean aphids (Aphis glycines) occurring in Wisconsin

Soybean aphid (Aphis glycines), a pest of soybean and native of Asia, invaded North America sometime before 2000 and rapidly became the most significant insect pest of soybean in the upper Midwest. Plant resistance, a key component of soybean aphid management, has undergone significant development in the past decade, and several resistance (Rag) genes have been identified. However, the efficacy of Rag genes in suppressing aphid populations has been challenged by the occurrence of soybean aphids capable of overcoming Rag gene-mediated resistance. Though the occurrence of these Rag virulent biotypes poses a serious threat to effective and sustainable management of soybean aphid, little is known about their current abundance in North America. The objective of this research was to determine the abundance of Rag virulent soybean aphids in Wisconsin. Soybean aphids were collected from Wisconsin the summers of 2012 and 2013, and assayed for Rag1, Rag2 and Rag1+2 virulence using no-choice tests in a greenhouse. One clone from Monroe Co. in 2012 performed similarly to biotype 4, three clones spread across four counties in 2013 performed similarly to biotype 2, and nine others had varying degrees of Rag virulence. Rag virulence in 2013 was observed in 32% of the sampled sites and was accounted for by just 4.5% of sampled clones, though likely a conservative estimate. Virulence assay results are discussed in light of current questions surrounding the biology, ecology and population genetics of soybean aphid.