Janet J. Knodel
Extension Entomologist
Several field reports of low numbers of wheat stem sawfly adults flying in wheat have been received from NC (Ward-Mountrail Counties between Max and Parshall), NW (McKenzie County) and SW (Sioux County) areas of North Dakota (IPM survey data). It is important to note its presence and numbers; however, insecticide will NOT control wheat stem sawfly. Research (see Table 1) conducted in 2008-2009 during high populations of wheat stem sawfly found that a foliar-applied insecticide (Warrior®, lambda-cyhalothrin, pyrethroid insecticide) at the 4-6 leaf or the flag leaf stages were not effective in reducing the percent of damaged stems from wheat stem sawfly. Insecticide seed treatments (Cruiser 5FS®, thiamethoxam, neonicotinoid insecticide) applied at a low or high rate also were not effective in reducing the percent of damaged stems. Furthermore, the treatment (low rate of seed treatment plus a foliar-applied at the 4-6 leaf stage) was not effective either. In fact, application of an insecticide negatively impacted the parasitoids that attack wheat stem sawfly and resulted in higher numbers of wheat stem sawflies the following year. Reasons why insecticides are not effective against wheat stem sawfly include:
- Other life stages (egg, larval and pupal stages) are well-protected inside the plant stem and not affected by foliar-applied insecticides or systemic insecticide seed treatments.
- Spraying for adults has not been successful because newly emerged adults can migrate into a field that was sprayed, the sawfly emergence window is long (1 month) and adults that emerge after spraying have reduced exposure to insecticide.
- The adult is heavily sclerotized and does not feed or drink water, which minimizes exposure to insecticides.
- Economics do not show any significant yield gain from insecticides and actually resulted in a net loss due to the cost of the insecticide + application.
The best strategies to manage wheat stem sawfly continue to be early swathing if 15% of the stems are infested (swath as soon as kernel moisture drops below 40 percent to prevent infested stems from lodging), biological control with parasitoids (averaged 35 percent parasitism rate in western ND during 2000-2003 survey) and the use of solid-stemmed wheat varieties (host plant resistance), such as Mott HRSW.