Phenological Disruption and Economic Consequence of Injury to Alfalfa Induced by Potato Leafhopper (Homoptera: Cicadellidae)

Abstract

A field study to characterize the effect of potato leafhopper, Empoasca fabae (Harris), feeding on alfalfa developmental physiology was begun in 1984 (one trial) and continued in 1985 (two trials). The rate of crop development was significantly reduced, with heavily infested plots (200 potato leafhoppers/m2) maturing about 30% later than uninfested plots. This phenological delay resulted in a concomitant reduction in daily accumulation of dry matter, crude protein, and digestible energy per hectare, but much of the loss was compensated for with additional regrowth time. To account for the disparity in maturity among plots, estimated nutrient yields at the predicted date of first bloom were compared with calendar date nutrient yields (all plots harvested when uninfested plots reach first bloom). These comparisons indicate that the economic consequence of potato leafhopper feeding was more severe with alfalfa managed on a calendar-date basis because time for compensatory regrowth was not available. Values for injury per insect were estimated from linear equations that predicted crop phonological delay based on potato leafhopper density. Economic losses associated with delays were based on slopes of nutrient yield accumulations per day of regrowth. Separate economic injury levels were calculated for potato leafhopper based on losses in digestible energy, crude protein, and dry matter.