Solid stems provide resistance to the wheat stem sawfly (WSS, Cephus cinctus Nort.) in wheat (Triticum aestivum L.). A major quantitative trait locus, designated Qss.msubâ3BL, controls most of the variation for stem solidness. Two alleles that differ for temporal expression of stem solidness have been identified. An allele derived from the cultivar âRescueâ, designated Qss.msubâ3BL.b, results in solid stems throughout stem development. An allele derived from the cultivar âConanâ, designated Qss.msubâ3BL.c, causes high stem solidness early in plant development, but solidness declines as the plant matures. The objective of this study was to develop nearâisogenic lines (NILs) derived from markerâassisted backcrossing to compare the impact of Qss.msubâ3BL alleles in different genetic backgrounds on stem solidness, WSS resistance, and agronomic traits. The Conanâderived allele Qss.msubâ3BL.c resulted in lower WSS infestation than the Rescueâderived allele Qss.msubâ3BL.b in two genetic backgrounds. Both alleles showed a high level of early stem solidness, though this declined as the stem matured for NIL with the Conanâderived allele. The Conanâderived allele Qss.msubâ3BL.c resulted in lower WSS infestation and stem cutting than the allele for hollow stems in two genetic backgrounds. Based on yield trials grown over 2 yr, there were no significant differences in grain yield between the NIL pairs. Our results indicate the Conanâderived allele at Qss.msubâ3BL results in lower infestation of WSS and does not significantly affect agronomic performance, thus providing a novel source of resistance for the development of WSSâresistant cultivars.