Screening and Characterization of Resistance for Sawfly
Wheat stem sawfly (WSS, Cephus cinctus) has long been considered the most important pest of wheat in the northern Great Plains of North America. In recent years, damage from WSS has expanded to areas of Nebraska, Wyoming, and Colorado that had not previously been affected. Adult WSS emerge from previous year’s stubble from May to June and females lay their eggs inside wheat stems. Larvae feed on parenchyma tissues and bore through vascular bundles of the plant. When the plant is nearly mature, the larvae cut the base of the stem in preparation for overwintering diapause, before pupating in early summer. Infested stems show reduced grain quality and yield; cut stems usually lodge which can result in harvest loss and increased harvest costs. Since the larvae develop inside the stem, they are protected from the external environment including insecticides. Hence, host plant resistance has proven to be most effective way to manage WSS. Solid stem varieties of wheat have been shown to be effective in impeding larval development and movement, thus reducing larval survival. The primary source of stem solidness in wheat is under genetic control of a QTL on chromosome 3B (Qss.msub-3BL) originally derived from the Portuguese landrace wheat ‘S-615’. However, there are reports from Montana that suggest sawflies may be adapting to this source of stem solidness. In addition, the solid-stemmed cultivars may not be preferred by growers because of low yield (10-15% reduction) compared to hollow-stemmed cultivars and inconsistent solidness expression. While new sources of resistance have been identified on chromosomes 2A, 3A, and 5B (larval mortality), 2D (decreased oviposition), and 4A (host plant attractiveness to females), their ultimate effectiveness and durability has not yet been shown. Given the limited sources of resistance to WSS in the current elite germplasm, there is a need to evaluate diverse wheat genetic resources for additional alleles that can enhance development of WSS-resistant varieties.