Movement to the plant of least resistance: Fitness, competition, and movement of soybean aphid (Aphis glycines Matsumura) biotypes in a mixed host-plant resistance environment

Susceptible refuge planting has been utilized successfully in the past to mitigate the evolution of virulent pest insects (able to overcome resistance) in response to pest resistant crop cultivars. Such refuges decrease the selective pressure imposed upon pests by providing habitat for non-virulent pests that act as a virulence sink during sexual reproduction. However, the efficacy of susceptible refuges is dubious against asexual pests where clonal amplification allows for the rapid selection for virulent clones. Despite this challenge susceptible refuges may provide some control by 1) providing habitat for non-virulence, 2) decreasing selection pressure, and 3) encouraging direct competition between virulent and non-virulent clones. In this study we investigated how these forces impact the comparative movement, fitness, and competition of virulent and non-virulent clones in the cyclically parthenogenic soybean aphid (Aphis glycines) across a gradient of selection regimes. Non-virulent (Biotype1) and virulent (biotype3) clones were reared in 4-plant microcosms consisting of 4:0, 3:1, 1:3, and 0:4 blends of susceptible and resistant s soybean. Resistant cultivars expressing the resistance genes Rag1 and Rag2, both individually and in concert, were utilized providing treatments of full resistance, non-resistance, and partial resistance to biotype3. In Rag1 and Rag2 treatments biotypes displayed significant movement from HPR to susceptible soybeans, irrespective of virulence. Additionally, biotype3 outcompeted biotype1 in mixed biotype infestations, becoming dominant increasing population share by 26%. Alternatively, the stacked 3:1 (HPR:Susceptible) treatment favored biotype1 with limited biotype3 movement and positive net growth in biotype1 populations, suggesting a potential control tactic against virulent soybean aphid.