Soybean Aphid Update: Insecticidal Seed Treatments and Resistant Varieties

In mid-June, Chris DiFonzo, Extension Entomologist at Michigan State University, reported that a soybean field (Saginaw Farm) was infested with soybean aphids (~44% of plants infested).  Chris indicated that none of the plants had received an insecticidal seed treatment. Reports from other entomologists (Iowa and Minnesota) indicate that soybean aphids are being found at more modest levels. At this point, it remains unclear what soybean aphid infestations will look like for much of the Corn Belt. A hot and dry summer would work against aphid establishment, mild temperatures throughout the summer would enhance aphid survival and increase their numbers.

A journal article was recently published by some entomologists (M.T. McCarville and M.E. O’Neal) at Iowa State University concerning the population of soybean aphids as influenced by resistant soybean varieties and an insecticidal seed treatment (thiamethoxam). The use of insecticidal seed treatments on soybeans is increasingly common. It is generally conceded that approximately 46 to 55 days after planting, much of the insecticidal benefit has been lost. So, depending upon planting date, by mid-July, insecticidal seed treatments offer very limited effectiveness in slowing down the late colonization of a soybean field. In contrast, soybean varieties that offer host plant resistance have the potential to provide more season-long soybean aphid population management. Some commercial soybean varieties may utilize the Rag1 gene by itself or, other varieties may utilize both the Rag1 and Rag2 genes. Previous research has shown soybean aphid control improves when both genes are pyramided within a soybean variety. Because resistant soybean aphid biotypes have emerged, there continues to be interest in using resistant soybean seed along with an insecticidal seed treatment. This combined approach may be particularly useful when large densities of aphids are present (especially a resistant biotype) and only a single gene soybean variety has been planted. Provided are key findings in quotes reported by McCarville and O’Neal.

McCarville, M.T., and M.E. O’Neal. 2013. Soybean aphid (Aphididae: Hemiptera) population growth as affected by host plant resistance and an insecticidal seed treatment. Journal of Economic Entomology, Volume 106, Number 3, pages 1302-1309. DOI: http://dx.doi.org/10.1603/EC12495

• “In our study, we found both insecticidal seed treatments and soybean aphid host plant resistance to significantly decrease soybean aphid populations at one or more times during the season.”
• “In examining the interaction between thiamethoxam and aphid-resistance we found evidence for unequal effects of the seed treatment across the four soybean lines. At 42 dap, when the seed treatment significantly reduced soybean aphid growth rates on the susceptible line, we were unable to measure any impact of the seed treatment on soybean aphid populations on the Rag1 + Rag2 pyramid. The lack of an effect of the seed treatment on the Rag1 + Rag2 line was likely because of the design of our assay and the already high efficacy of the pyramid line, which resulted in almost complete soybean aphid mortality from the pyramid line in the absence of a seed treatment.”
• “This result suggests that while insecticidal seed treatments may provide protection to soybean aphid-susceptible varieties, and also soybean aphid-resistant varieties carrying a single resistance gene, they are likely unnecessary at this time for varieties with multiple resistant genes.”
• “As a whole this study shows soybean aphid-resistance to provide greater and more consistent soybean aphid control throughout the growing season when compared with an insecticidal seed treatment.”

Some researchers have found that insecticidal seed treatments can have a negative impact on natural enemy densities within a field. It would seem that in many instances, the use of pyramided aphid resistant soybean varieties along with natural enemies could keep aphid populations in check. McCarville and O’Neal offered the following cautionary statement with respect to this issue: “Therefore, care should be taken when pairing insecticidal seed treatments with soybean aphid host plant resistance.”

I offer my thanks to these Iowa State University researchers for their work on this important topic.

Mike Gray