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Introduction
Wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae), is indigenous to North America (Lesieur et al. 2016) and has a broad host range of native grasses (Cockrell et al. 2017). It quickly adapted to spring-seeded wheat crops (including Triticum aestivum Linnaeus and Triticum durum Desfontaines (Poaceae)) that came with the expansion of agriculture on the Great Plains of North America (Criddle 1922; Weiss and Morrill 1992). Damage to wheat was first reported in Souris, Manitoba, Canada, and Indian Head, Saskatchewan, Canada in 1895 (Fletcher 1896). By 1923, Criddle (1923) reported that C. cinctus was found in over half of the wheat growing area of Manitoba, a wide range in Saskatchewan and present “over quite an extensive territory in Alberta” (Criddle 1923). This species is widely distributed in areas west of the Mississippi River in the United States of America and Ontario in Canada (Ivie 2001), as far south as the 36th parallel (Ainslie 1920, 1929) and as far north as the Peace Lowland Ecoregion (Ecological Framework of Canada 2017) of northern Alberta (Moorhouse 1914). Within this geographical region of Canada and the United States of America, the areas subjected to greatest attack are southern Alberta and Saskatchewan, southwestern Manitoba, eastern and northern Montana, North Dakota, northern South Dakota, and western Minnesota. Cumulative grain-yield losses and annual economic losses associated with outbreaks of this pest can exceed 30% and $350 million (CAD), respectively (Beres et al. 2011b).
Criddle (1923) described the life history of C. cinctus in western Canada. After completing obligatory diapause, larvae pupate in spring and adults emerge in June to July from overwintering sites in wheat stubble and migrate to infest adjacent wheat fields. Adults live for approximately one week, with females each laying up to 50 eggs into elongating grass stems, at a rate of one egg per stem per visit. Multiple eggs from different females are common at high densities, but only one larva develops to maturity (Holmes 1982). Eggs are deposited using a saw-like ovipositor, usually in the second last developing internode (Holmes 1979). The larva feed on parenchyma and vascular tissues, causing reduced kernel weight and seed set before moving down to the base of the stem and chewing...