No. 6 Article 6/May 5, 2006

Soybean Aphid and Potassium Fertility: Update on Continued Research

Research on the influence of potassium deficiency in soybean plants and the relationship with soybean aphid population dynamics continues. We have reported on this area of research in a previous issue of the Bulletin (issue no. 5, April 22, 2005).

In a recently published article, University of Wisconsin entomologists indicated that soil and soybean leaf potassium levels "may play an important role in soybean aphid population dynamics." The article (Environmental Entomology, 35: 219-227, April 2006), published by Scott W. Myers and Claudio Gratton, is titled "Influence of Potassium Fertility on Soybean Aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), Population Dynamics at a Field and Regional Scale." The authors found that the population growth rate for soybean aphids was approximately 15% greater on potassium-deficient plants.

During the summer of 2004, the investigators measured potassium levels in soil and soybean leaf samples from 34 fields across Wisconsin. Soybean aphid densities were determined weekly from July 20 through August 26 in each field by estimating the total number of aphids from 50 soybean plants that were randomly selected. On July 20, soil samples were removed from each field to estimate the availability of potassium and phosphorus. On August 9, additional soil samples were removed in each field to estimate "plant available nitrogen." In a separate study, the University of Wisconsin scientists used clip-cages to confine two neonate soybean aphids on soybean leaves when they reached the V5 stage. If both aphids reached the adult stage, one of the survivors was removed from the cage. Two plants in each plot were infested in this manner. The three potassium treatments were 0 kg/ha KCL fertilizer added; 56 kg/ha KCL; and 112 kg/ha KCL. Phosphorus was applied throughout the experiment at 112 kg/ha. Each treatment was replicated 14 times. The number of offspring produced from each aphid (observed daily) within a clip-cage was recorded for 35 days. Each day during the 5 weeks, new offspring were removed from each cage. This was a truly labor-intensive project!

The authors of the article offered the following remarks in their discussion section: "The results of this study provide strong evidence that variation in K plays an important role in influencing soybean aphid population dynamics at both the field and regional scale. In general, fields or small plots that exhibited low K levels tended to have the highest rates of soybean aphid population growth.

"Results from the clip-cage experiment showed aphid performance improved and natural aphid populations grew more rapidly on K-deficient soybean plants. This response was principally caused by the marked increase in aphid fecundity in the low K treatment over the medium and high K treatments."

Myers and Gratton pointed out that soybean aphid densities in Wisconsin during the 2004 season were at their lowest levels since first reported in that state in 2000. However, soybean aphid densities did exceed the economic threshold (250 aphids per plant) in the low potassium treatment on August 26. Additional research over multiple years will be required to further elucidate the yield relationship between soybean aphid densities and potassium levels in outbreak and non-outbreak years.--Mike Gray

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