Wheat stem sawfly, Cephus cinctus Norton
The wheat stem sawfly is a prolific pest to wheat in Colorado, causing extensive damage in the eastern parts of the Great Plains. Adult wheat stem sawfly measures about 1.9 cm (3/4 inch) long with smoky-brown wings. It is wasp–like in appearance, with a shiny black body with three yellow bands around the abdomen. Sawfly larvae are cream colored, have a broad head, and are 1.3-1.9 cm (1/2-3/4 inch) long when fully grown.
- The wheat stem sawfly is a native grass-feeding insect that emerged as a significant pest of winter wheat in Colorado in 2010.
- Adults emerge in late May or early June, as the wheat is jointing, and are generally active when winds are calm and field temperatures are above 50° F.
- Solid stem varieties of wheat have been shown to be effective in reducing damage caused by the wheat stem sawfly.
- Several parasitic wasps attack wheat stem sawfly but the presence and effectiveness of natural enemies in Colorado has not been determined.
- The are currently no available insecticides that are effective at controlling wheat stem sawfly.
Wheat stem sawfly larva next to a ruler. Image credit: Daren Cockrell, Colorado State University
Adult wheat stem sawfly. Image credit: Pest and Diseases Image Library, Bugwood.org
What stem sawfly larvae in stub. Image credit: Frank Peairs, Colorado State University, Bugwood.org
Life history and habits
The wheat stem sawfly produces one generation per year. The larvae develop entirely inside wheat stems and overwinter as larvae in the stem stubs left behind after wheat is harvested. Adults emerge in late May or early June and are generally active when winds are calm and field temperatures are above 50° F. Females lay eggs immediately upon emergence and typically live about one week. The adult emergence and flight period continues for 3-6 weeks. Females lay an average of 30-50 eggs, depending on the size of available host stems.
The most visible sawfly injury is stem breakage or lodging. When wheat stem sawfly larvae mature, they move down towards soil level and cut a V-shaped notch around the interior of the stem. This often causes breaks at this weakened notch just prior to harvest. Some yield loss is caused by the larvae feeding on the internal stem tissue, but the majority of losses are from lodging, which can result in yield loss of five to ten percent due to unrecoverable wheat heads that the combine cannot pick up. Wheat stem sawfly are primarily pests of winter and spring wheat, but can be found in triticale, barley, wheat grasses, and many native grass species found in Colorado grasslands. They cannot complete their development in rye, corn, or millet.
To verify presence of the sawfly in a suspected plant, split the stem from top to bottom; the larva will most likely be in a chamber within the stem, just above the crown. Solid stem varieties of wheat have been shown to be effective in reducing damage caused by the wheat stem sawfly. Several solid stem varieties that are adapted to Colorado are commercially available including Fortify SF and Amplify SF. There are two economically important parasitic wasps of wheat stem sawfly that have been found to significantly reduce populations in other states, however their populations in Colorado in recent years (2017-2021) have been relatively small and of low economic relevance. There are currently no insecticides that can penetrate the stems of wheat and suppress wheat stem sawfly, however this is an active area of research.
Colorado State University. 2014. Wheat Stem Sawfly: A New Pest of Colorado Wheat – 5.612. Colorado State University – Extension. Available https://extension.colostate.edu/topic-areas/insects/wheat-stem-sawfly-a-new-pest-of-colorado-wheat-5-612/
Varenhorst, A., P. Rozeboom, P. Wagner, and A. Bachmann. 2022. Wheat Stem Sawfly: An Overview. South Dakota State University – Extension. Available https://extension.sdstate.edu/wheat-stem-sawfly-overview
Washington State University. (n.d.). Wheat Stem Sawfly. Washington State University – Department of Entomology. Available https://entomology.wsu.edu/outreach/bug-info/wheat-stem-sawfly/