Free amino acids in the phloem sap of host plants are the nitrogen source for members of the Aphididae, which are known to be N-limited. During 2000 and 2001, a relationship between high soybean aphid populations and potassium deficient soil was observed in the field. It is possible that soil potassium regulates the quantity or quality of amino acids translocated in the phloem sap of soybean. This study examined the relationship between potassium deficiency and soybean aphid populations through both laboratory and field studies. 206 m2 strips in a field with soil that originally contained 168 kg/ha (75 ppm) of potassium were fertilized with 0kg, 2.1 kg, or 4.2 kg of 62% potassium fertilizer at the beginning of the season. After soybean emergence, four m2 cages were placed in each strip. Two cages per strip were maintained as aphid-free controls and two cages were infested with soybean aphid. Aphid population, plant characteristics, and phloem sap amino acids were monitored weekly. In the laboratory, soybean plants were grown in the greenhouse in soil collected from the field site. Plants were subjected to the same potassium treatments that were used in the field study. Individual first instar aphid nymphs were clip-caged on the plants. Survival, development time, lifespan, reproductive lifespan, and number of offspring were monitored. The free amino acid content of the phloem sap of the experimental plants was also measured via sap exudation at the conclusion of the study.
Species 1: Homoptera Aphididae Aphis glycines (soybean aphid)
Keywords: soil fertility
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