Soybean aphid populations were found in North Dakota during the 2017 growing season with reduced pyrethroid effectiveness from nine North Dakota counties, including Cass and Barnes. This is a concern as insecticide options for soybean aphid treatment are limited. Fortunately, during the 2018 and 2019 growing seasons, soybean aphid populations were low which meant there were not as many insecticide applications. However, soybean aphids were collected in 2019 from a Grand Forks County field that were fairly resistant to bifenthrin (Brigade 2EC and generics) with 60-70% of the aphids surviving and resistance to lambda-cyhalothrin (Warrior II and generics) with up to a 50% survival rate. Pyrethroid resistant soybean aphid populations are most likely established in North Dakota or are migrating into North Dakota from other resistant areas.
The economic threshold for soybean aphids is 250 aphids per plant with increasing populations in 80% of the field. Waiting to treat for soybean aphids until the population reaches economic threshold avoids unnecessary insecticide applications which is important for several reasons. There is a temptation to make earlier insecticide applications as “cheap insurance” but oftentimes this results in the need for a second insecticide application, adding to the input costs. Early insecticide applications kill beneficial insects that serve as natural enemies against soybean aphids and allows for soybean aphids to re-establish and/or allow secondary pests such as spider mites to move in. Insecticide resistance is also another major concern when multiple insecticide applications are used repeatedly from the same mode of action group.
To slow insecticide resistance, follow these recommendations:
Do not use reduced insecticide rates.
Use appropriate spray pressure and spray nozzle to treat aphids.
Do not skimp on water. Spray at least 15-20 GPA in ground applications and 2-5 GPA in air applications..
Do not apply insecticide applications during windy conditions, a temperature inversion or very hot weather as those application conditions can reduce control .
Scout fields 3-5 days after application to check insecticide performance.
Do not retreat a field with the same insecticide group for consecutive applications.
Insecticide premixes usually are not recommended from a resistance management standpoint because they usually contain a reduced rate of at least one active ingredient. However, tank mixes might need to be used in situations where a second insecticide application is needed. Any potential tank mixes should be tested for mixing compatibility with a jar test before applying. Always read and follow the pesticide label. For more information, contact the Stutsman County Extension office at 701-252-9030.