Potassium-induced tolerance to potato leafhopper, Empoasca fabae (Harris) (Homoptera: Cicadellidae), in alfalfa, Medicago sativa

Integrated pest management relies primarily on preventive approaches to reduce pest injury below damaging levels. One such approach is environmental tolerance, in which the damage associated with a pest is reduced because of an interaction with some environmental factor, such as soil fertility. Reports have suggested that high levels of potassium fertility are associated with reduced susceptibility of alfalfa to potato leafhopper, the primary insect pest in the northeastern U.S.;To evaluate the response of potato leafhopper to alfalfa plants fertilized with varying levels of potassium, field and greenhouse experiments were conducted. Incremental rates of potassium influenced alfalfa density and dry weight, the relative density of alfalfa and weeds, and the within-field distribution of leafhopper adults and eggs, but not alfalfa developmental rate. Significantly more adults and eggs were found on elevated than on low fertility plots. These results do not support the hypothesis that elevated potassium rates are associated with reduced suitability of alfalfa as a host for potato leafhopper. Results from greenhouse tests demonstrate that neither adult mobility nor oviposition were affected by potassium treatments on well watered plants. However, under low water conditions, leafhopper nymph development was shorter, and adult size was larger on alfalfa grown on elevated potassium level.;To quantify a possible potassium by leafhopper interaction on alfalfa growth, field, greenhouse, and laboratory studies were conducted. In a field experiment, incidence of hopperburn was unaffected by potassium treatments. The effects of potassium and leafhopper treatments on stem height, density, dry weight and developmental rate were significant, but the two factors did not interact. In the greenhouse, tillering diminished with incremental rates of potassium for both injured and uninjured plants lacking nodulation. Laboratory bioassays combining varying levels of potassium and nitrogen show increased photosynthesis, leaf area, and rate of shoot elongation at elevated levels of potassium and nitrogen over higher potassium to nitrogen ratio.;In conclusion, elevated potassium levels neither reduce potato leafhopper densities on alfalfa nor change the magnitude of alfalfa response to leafhopper feeding. However, elevated rates of potassium and nitrogen enhanced growth in alfalfa regardless of leafhopper infestations.