VALLEY CITY, N.D. (NewsDakota.com) – Last summer, resistance to pyrethroid insecticides was present in soybean aphid populations in eastern North Dakota. This year, there may be pressure to add an insecticide seed treatment to soybean seeds to combat soybean aphids. However, it may not pay to spend the extra dollars on this seed ‘extra’.

Research from several land-grant universities (including work from the NDSU Extension Service) does not show a soybean yield advantage from using an insecticide treatment.

This research project was conducted in North Dakota, Minnesota, Indiana, Iowa, Wisconsin and Kansas over two years (Krupke et al. 2017). The study was 2-pronged. How much thiamethoxam the active neonicotinoid chemical in CruiserMaxx®, was present in the plant as the plant grew? And secondly, how does crop yield respond to four treatments: soybean treated with CruiserMaxx ® (thiamethoxam plus 2 fungicides), soybeans treated with ApronMaxx® (2 fungicides), untreated seeds, and untreated seed followed by application of POST insecticide at the IPM threshold of 250 aphids/plant?

Soybean foliage was analyzed for tissue concentrations of thiamethoxam. By the V2 stage, thiamethoxam concentrations were statistically similar to untreated seed. Locally, we typically do not see soybean aphid infestations until mid-summer when plants are flowering and are physiologically far past the V2 stage.

All soybean plants can handle a few aphids. Both, the thiamethoxam treated soybean plants and those treated at IPM thresholds had less aphid-days than untreated seeds and seeds treated with fungicide-only in a combined location-state analysis. However, when it came to crop yield, only the IPM threshold treatment showed significantly higher yields.

You may have heard testimonials that insecticide-treated seed fields have less aphids. This research study relates to those testimonials that less aphid-days accumulated with fields that incorporated an insecticide seed treatment. The bottom line of this research is that the control provided by the insecticide treated seed does not impact aphid day feeding enough to provide an economic benefit. Also, soybean aphids are very mobile and can be patchy across our landscapes (including across the gravel road). It’s hard to make direct comparisons due to this patchiness without treated and non-treated seed being side-by-side.

Ultimately, farmers should continue to adhere the current IPM recommendations with the use of POST products. If a second application is warranted in a field season, switch to a new mode of action stand-alone product to help reduce insecticide resistance development.