Insecticide Now Available to Fight Wheat Stem Sawfly in Montana

BOZEMAN – Montanans who battle wheat stem sawfly now have a new weapon to consider using.

Over the next four years and possibly longer, wheat producers will be able to use a granular insecticide against the No. 1 small grain insect pest in Montana. The insecticide, Thimet 20-G, must be incorporated into the soil at least 85 days before harvest.

“This is big news because it’s something that works,” said Montana State University Cropland Entomologist Kevin Wanner. “For the first time, producers will have a pesticide as an option.”

MSU Pesticide Education Specialist Cecil Tharp said Montanans may use Thimet 20-G on other crops, but the Montana Department of Agriculture recently issued a 24 (C) special location need registration that allows the insecticide to be used in wheat against the wheat stem sawfly. The department may renew or end this special registration on Dec. 31, 2019.

If producers want to continue using the insecticide, they will have to apply it properly, Tharp said. Doing so will not only keep themselves and the environment safe, but it will help ensure that this insecticide special registration will continue to be available for other Montanans.

“If it’s misused, we can expect to lose this tool quickly,” Tharp said. “It could be as simple as not cleaning up granular spills and killing a population of birds.”

This insecticide is extremely toxic to humans, other mammals, fish and birds, Tharp said. Therefore, it must be incorporated into the soil by applicators who are licensed for restricted-use products. They must plant it at least one inch deep between the crop rows, using an enclosed loading system that is rated for dermal protection according to worker protection standards. The enclosed tractor cab must also provide respiratory protection equivalent to a dust/mist filtering respirator, or the applicator must wear a NIOSH-approved respirator with any N, R, P or HE filter (prefix TC-21C) while within the cab. In addition, the applicators must wear a long-sleeve shirt, long pants, shoes and socks while inside the cab. When they leave the enclosed cab, they must wear protective eyewear, coveralls, chemical-resistant gloves and footwear, and a NIOSH-approved dust/mist filtering respirator.

Applicators must have both the supplemental 24c label and section 3 product label when using Thimet for managing sawflies. For more details about application and instructions about spills, the 24c label and section 3 label, go to and find “Wheat Stem Sawfly and Thimet.”

Wanner said the wheat stem sawfly is active earlier than normal this year, so producers could wait until next year to apply the insecticide and continue using the methods they’ve used in the past against wheat stem sawfly. One such method is planting solid stem wheat, which makes it more difficult for wheat stem sawfly to feed inside the stem. Another is planting varieties that are more attractive to wheat stem sawfly around the outsides of a field, so producers can harvest those areas first.

Research has proven the effectiveness of Thimet 20-G against wheat stem sawfly, Wanner said. It is a systemic insecticide, taken up by the roots and moved up the wheat stem where it kills the wheat stem sawfly larvae. Applications must be completed before the insect begins to fly and lay eggs, which in Montana can begin at the end of May or sometime during June.

“Efficacy is achieved by applying Thimet 20-G after the crop emerges, closer in time to when wheat stem sawfly larvae are feeding within the stem,” Wanner said. “Trials in Montana have demonstrated a 64-100 percent reduction in damage measured as stem cutting by the larvae. Yield increases of 5.4-6.1 bushels per acre were achieved in spring wheat and winter wheat crops, respectively.”

Tharp said wheat and wheat products are safe to eat according to EPA tolerance thresholds if producers wait at least 85 days between planting the insecticide and harvesting their wheat.

For more information, contact local MSU Extension agents. For safety-related questions, contact Tharp at (406) 994-5067. For information about managing insects, contact, Wanner at (406) 994-5663. For more information on Thimet 20-G, lid adaptors for the Lock’N’Load system or efficacy, contact Amvac at 1-888-462-6822.


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