Survey results from July 5 through 19.
Similar to the regional scouting results from June 28 through July 12, the scouts tasked with carrying out the soybean aphid survey described last week in northwest and west-central Minnesota found no aphids through last week Friday, July 19 (Figure 1).
When a treatment is warranted.
After their arrival in North America almost two decades ago, university entomologists worked hard to closely study soybean aphid to determine at what density and for what duration infestations must be to result in yield loss and at what point these yield losses will become large enough to justify treatment.This work led to the development of the research-based economic threshold. This threshold is designed to allow time for a treatment to occur before aphid feeding results in economic injury.
Now and until soybeans reach the R5 (beginning seed, but before seeds completely fill the pod), economic thresholds suggest that one should treat soybean fields with an effective insecticide if the following three criteria are met:
- more than 80% of plants are infested with aphids
- there is an average of 250 aphids per plant,
- the aphid population is growing.
There is still time for aphid populations to reach economic thresholds.
Crop producers, advisors and scouts have likely noticed that soybean aphid populations do not reach economic thresholds every year. Spring and summer weather conditions can impact planting date and both aphid population and soybean growth and development.Weather conditions can affect soybean aphid survival, growth and development. Unusually cold winter temperatures can influence survival of eggs not insulated by snow cover. Soybean aphid can suffer a higher rate of mortality when temperatures are consistently hot and conversely reproduce at faster rates when temperatures are a mild 77 to 82 degrees (Tilmon and O’Neal, 2015). Should temperatures stay mild after aphids arrive in a field this year, there is still ample opportunity for densities to grow enough to reach economic thresholds.
In 2019 wet weather conditions throughout the spring in west-central Minnesota delayed soybean planting which, along with the cooler conditions that followed, delayed growth and development. As of last Friday a scout surveying fields throughout most of this region found soybeans that had not yet reached reproductive growth stages (Figure 2), meaning that there is still quite a bit of time for aphids to move in and population densities to increase to economic thresholds. It was much rarer to find fields that had not reach reproductive growth stages further north in west-central and northwest Minnesota. Fields in this region had largely progressed to either beginning or full bloom (Figure 2). Regardless of how far along your soybean crop happens to be, continue scouting as there is still time this year for economic thresholds to be reached.
Risks of treating aphid populations that have not yet reached economic thresholds.
Treating before economic thresholds have been reached carries with it several risks. One risks also controlling many of the naturally occurring insect enemies that are working free-of-charge to help keep the soybean aphid population in check including lacewing larvae, Asian lady beetle larvae and adults and parasitic wasps. Treating aphid populations that have not reached the economic threshold can result in having to make multiple applications in a single growing season if populations eventually rebound to reach threshold, costing more than a single better-timed treatment. Lastly, insecticide-resistant aphid populations were identified by failing to manage aphids with the pyrethroid class of insecticide. This has taught us that soybean aphids follow the basic biological principals of selection and evolution that and that we must preserve other insecticides labeled for soybean aphid management so that we have management tools that are still effective when population densities DO reach economic thresholds.References.
Potter, B. and Koch, R. 2018. Soybean Pest Management: Leaf feeders and Defoliators: Soybean Aphid. Online. University of Minnesota Extension.
K.J. Tilmon and M.E. O’Neal. 2015. Soybean Aphid, pp. 144-146. In Hartman, G.L., Rupe, J.C., Sikora, E.J., Domier, L.L., Davis, J.A., and Steffey, K.L. (eds.) Compendium of soybean diseases and pests, 5th ed. APS Press, St. Paul, MN.
