Results of research conducted by University of Wisconsin, Madison entomologists and a soil scientist on the “Effect of soil potassium availability on soybean aphid population dynamics and soybean yield” were published in the February 2005 issue of the Journal of Economic Entomology.
Research Brief: Soybean Aphid and
Soil Potassium Availability
Eileen Cullen, Extension Entomologist
UW Entomology Department
Results of research conducted by University of Wisconsin, Madison entomologists and a soil scientist on the “Effect of soil potassium availability on soybean aphid population dynamics and soybean yield” were published in the February 2005 issue of the Journal of Economic Entomology. Full article citation and research group are listed at the end of this article. The study was initiated in response to on-farm visits and field observations of Extension entomologists and others during the 2000 growing season in Wisconsin, which indicated that many of the soybean fields most heavily infested with soybean aphids were also exhibiting symptoms of potassium (K) deficiency (e.g., leaf yellowing exhibited by K-stressed soybean). The research experiment was designed to examine the effect of potassium on soybean aphid population growth, both in the field during the 2001 and 2002 growing seasons, and in the laboratory in 2003
Field K Manipulation Experiment
Small plot field experiments were established in two fields at the University of Wisconsin ArlingtonAgricultural Research Station (ARS) during the 2001 and 2002 growing seasons. The soybean variety used in this experiment was Asgrow 2001 RR (glyphosate resistant), planted in 30-inch rows on 10 May 2001, and on 9 June 2002. Plot dimensions were 10 ft. (4 rows) wide by 22 ft. long. The experimental design consisted of three soil test K levels crossed with two insecticide treatments (sprayed and unsprayed).
Key to the field design was the fact that the fields used in this study contained plots with three, pre-existing potassium soil test K levels – High, Medium and Low. UW soil scientists had established the High, Medium and Low level treatments in the plots in a previous study using three rates of K fertilizer applied as a surface broadcast application of KCl. The potassium fertilizer applications made in the previous study from 1998 to 2000 consisted of a Low K treatment (no K application); a Medium K treatment (41 lbs/acre K applied annually equivalent to a 1X crop grain removal rate); and a High K treatment (82 lbs/acre K applied annually equivalent to a 2X crop grain removal rate). K fertilizer applications were not continued when the fields were used in the soybean aphid experiment, 20012002.
Insecticide spray treatments were also established across each soil test K level plot to test for K and soybean aphid effects on yields. Soybean aphid populations were allowed to develop undisturbed in the unsprayed treatments, whereas pyrethroid insecticide (Warrior) applications were made on sprayed treatments to minimize aphid pressure, maintaining as near an aphid-free control plot as possible. Soybean aphid counts were taken in plots during July and August of both years, and soybean yield estimates were taken, from the middle two rows of each plot. Soil and leaf tissue samples were taken on 10 July 2001 and 9 July 2002 to quantify the K soil test level in the field and plant tissue for the three K treatments (High, Medium and Low).
Laboratory Performance of Soybean Aphids Fed on K-deficient vs. K-normal Soybean Leaves
In 2003, laboratory experiments were conducted to evaluate the performance of soybean aphid on K-deficient and K-normal (non-deficient) soybean leaf material. Soybean leaves were collected from plants in fields at the UW Arlington ARS within two categories to be fed to soybean aphids in the lab: 1) plants from K-deficient fields and 2) plants from K-normal (nondeficient) fields showing no K-deficiency symptoms. Researchers collected soil samples and leaf material from the same fields on three dates in July for K content analysis confirmation.
Soybean aphid nymphs (< 24 hrs old) were placed individually on leaves of each of the two treatments (10 leaves per treatment) in Petri dishes for observation. At daily intervals, each dish was examined to record the aphid nymph growth state (development time), its survivorship, and the number of offspring produced by adult females.
Results
The laboratory feeding test found that soybean aphid populations developing on K-stressed soybean leaves grew significantly more rapidly than those feeding on normal (non-deficient) soybean. Aphids in both treatments reached adulthood at approximately 5 days, and no significant difference was observed in mean generation time between the two treatments. However, aphids fed on the K-deficient leaves produced significantly higher mean total number of nymphs than those on the control leaves. Aphids fed on K-deficient leaves in the laboratory had a substantially higher reproductive rate and greater survivorship over aphids feeding on normal (non-deficient) leaves.
In contrast to the laboratory findings, results of the field study did not produce greater soybean aphid populations on K-stressed soybean in the field plots 2001 or 2002. The authors note that small plot size may have hindered their ability to detect aphid population differences between soil and leaf K levels in the field. They observed that soybean aphid densities in both years of this experiment were unusually high compared with other fields surveyed in southern Wisconsin during the study years, particularly during 2002 a relatively low soybean aphid year in Wisconsin. It is possible that soybean aphids were attracted to the leaf yellowing in the Low K treatment plots, migrants initially colonized K-deficient plots and then spread throughout the plots, among all three K levels.
The laboratory study showed that there are likely nutritional differences between K-deficient and nondeficient soybean leaves that influence soybean aphid population growth. The physiological mechanism for this effect is not well understood, but important biochemical steps in protein synthesis are in part dependent on potassium, and deficiencies in K may lead to increased free-amino acid concentrations in soybean plant tissues. A high content of amino acids in plants are typical for either high nitrogen supply, or impaired protein synthesis due to certain deficiencies such as potassium (Marschner 1995). The UW laboratory study suggests that K deficiency in soybean may provide an advantage, perhaps a nutritional benefit (increased soluble N within the plant) to soybean aphid populations, allowing for increased reproductive rates.
Although it is clear from the field study that soybean aphid feeding and K stress both serve to impact soybean yield (separately, there was no interaction between K stress and aphid numbers), it remains unclear whether K-deficient soybean plants actually promote soybean aphid outbreaks in the field. The authors conclude that additional work is needed on the effects of K on soybean aphid reproduction, and the impact of K on aphid populations on a larger spatial scale. This new laboratory study and small plot field research have contributed important knowledge to improve our understanding of the relationship between soil fertility, plant nutrition, and insect pest management.
Research brief adapted from:
Myers, S.W., C. Gratton, R.P. Wolkowski, D.B. Hogg, and J.L. Wedberg. 2005. Effect of soil potassium availability on soybean aphid (Hemiptera:Aphididae) population dynamics and soybean yield. Journal of Economic Entomology 98(1): 113-120.
Additional Reference:
Marschner, H. 1995. Mineral nutrition of higher plants, 2nd Ed. Academic Press, London.