Reevaluating the economic threshold of the potato leafhopper (Empoasca fabae) in alfalfa

Potato leafhopper (PLH) (Empoasca fabae, Hemiptera: Cicadellidae) is one of the most injurious pests of alfalfa (Medicago sativa) in Wisconsin and the upper Midwest. Even at very small numbers, PLH feeding can cause significant damage to alfalfa yield and quality resulting in economic losses for farmers. Recent increases in alfalfa market value have raised grower interest in applying insecticides below the current economic threshold recommendations. Glandular haired (GH) alfalfa and alfalfa-grass intercrops have been previously shown to increase alfalfa tolerance and/or suppress PLH populations. The purpose of this experiment is to examine combinations of integrated pest management (IPM) practices and determine how economic thresholds vary in alfalfa cropping systems that utilize these IPM combinations. Two large multi year research plots located at Arlington, WI and Lancaster, WI Agricultural Research Stations have been established in 2010 and 2011 respectively to assess PLH response to GH alfalfa and orchard grass intercrop, and alfalfa yield and forage quality response to PLH pressure. Each trial consists of a completely randomized design with 8 replicates of a 2 x 2 whole plot factorial (two alfalfa varieties x grass presence or absence) and three split-plot insecticide treatments: current economic threshold, ½ current economic threshold, and a control with no insecticides. Weekly sweep net counts are used to enumerate PLH populations as well as to determine insecticide application timing. Linear regressions of alfalfa yield against potato leafhopper index (PLH/sweep/inch alfalfa height) are used to derive the economic thresholds.