Aphids and Potassium Deficient Soil: Fact or Fiction, or, why do I have
soybean aphids and my neighbor doesn’t?
Nancy Glazier, NWNY Team Assistant
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?
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.
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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