While searching for Potassium (K) deficient soybean images online, I found an article from the Iowa State University IPM program on the topic. It stated there has been a link established between aphid populations and K deficient soils: lower soil K levels favor aphid reproduction.
Research at the University of Wisconsin and Michigan State University indicates K deficiency in soils puts soybeans at a greater risk of aphid outbreaks. When the outbreaks occurred in Iowa, it was in beans following alfalfa, which can lead to K deficiency in soybeans. Sandy soils in Michigan have also been an issue with K deficiency.
Host specific insect: Soybean aphids feed entirely on soybeans, obtaining their nourishment from the plants’ phloem. Also stated in the article, nutrient concentrations can vary throughout the growing season. The most movement of nitrogen occurs during high amounts of vegetative (upward movement) and during senescence (downward movement). At other times, concentrations of N may be low. Could this difference lead to the “white dwarfs” spotted on soybeans? These are soybean aphids that lack the Mountain Dew color and are smaller.
Over winter on buckthorn: This does not explain why some regions get high populations and other areas have none. Buckthorn, a common brush species in NY, is the overwintering host of aphids and provides the initial infestations on soybeans in spring. Mike Stanyard has found high populations of aphids on early planted V2 or V3 beans. Are these populations exacerbated by K deficiency?
Soil test: This is another reason for routinely soil testing your fields! Keep track of your rotation. Do beans follow alfalfa, a crop that removes excess potassium from the soil? Do some beans follow a different crop? Try some of your own on-farm research.
Aphid estimator: I also found this nifty tool from the University of Minnesota: Soybean aphid growth estimator. It is a spreadsheet where high and low temperatures are entered, initial aphid populations, and days to threshold are estimated. The estimator does not factor in effects of predators. Follow the link halfway down the page: http://www.soybeans.umn.edu/crop/insects/aphid/aphid_sagemodel.htm.
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