Wheat Stem Sawfly

Cephus cinctus — the wheat stem sawfly — is one of the most economically important PESTS OF SPRING WHEAT in the northern Great Plains of North America and one of about 100 species in family Cephidae (the stem sawflies — distinct from the more familiar Tenthredinidae sawflies which feed on plant leaves). The species is widespread across the northern Great Plains of North America (especially Montana, North Dakota, southern Saskatchewan, Alberta) where it is a major pest of SPRING WHEAT, a foundational crop of the region. Adults are 8-12 mm long, with the species' diagnostic features: black body with bright yellow bands on the abdomen (looking like a small wasp — sawflies are in order Hymenoptera but lack the narrow 'wasp waist' of stinging Hymenoptera), four membranous wings, and the typical sawfly female ovipositor (used for inserting eggs into wheat stems rather than for stinging). The species' biology is unusual: female sawflies use the ovipositor to insert single eggs INSIDE WHEAT STEMS during spring (typically in the lower internodes of young wheat stems). Larvae hatch and DEVELOP INSIDE THE WHEAT STEM throughout the growing season, feeding on stem tissue and tunneling through the inner stem cavity. As the wheat plant matures and approaches harvest, the mature larva (now near the bottom of the stem) PERFORMS THE CRITICAL DAMAGING BEHAVIOR: the larva CHEWS A NOTCH AROUND THE INSIDE OF THE WHEAT STEM JUST ABOVE GROUND LEVEL, weakening the stem at that point so the upper portion BREAKS AND FALLS OVER (called 'LODGING') in wind or rain. The larva then plugs the cut stem with a 'wad' of plant tissue and overwinters inside the stub at ground level. The lodging damage is the species' major economic impact: lodged wheat stems with the grain head lying on the ground are INACCESSIBLE TO COMBINE HARVESTERS, and the unharvested grain is lost from the harvest. Annual NA wheat losses to wheat stem sawfly total $25-100 MILLION+ per year, with major outbreak years causing $200M+ in losses across affected regions. The species is the focus of major USDA Agricultural Research Service and Canadian agricultural research programs because of the dramatic regional economic impact. Modern control approaches include: SOLID-STEM WHEAT VARIETIES (wheat cultivars bred to have solid pith inside the stem rather than the usual hollow center — the solid stem prevents larval development inside the stem; Solid-stem wheats like 'Conan', 'Choteau', 'Bearpaw' provide partial control of wheat stem sawfly), trap-cropping with non-resistant wheat varieties to attract females away from main fields, parasitoid wasp biological control (with native Bracon braconid wasps that parasitize sawfly larvae), and crop rotation to non-wheat crops in heavily-infested fields. The species is harmless to humans and is a flagship example of stem-boring sawfly biology.