Impact of Alfalfa Harvest on Potato Leafhopper Populations with Emphasis on Nymphal Survival12

Egg, nymphal, and adult populations of Empoasca fabae (Harris) were monitored in 2 fields in Montgomery Co., Va., during 1976 and 1977. Centigrade degree day (COD) accumulations for egg and nymphal development were related to alfalfa growth and harvest date. Effect of harvest on nymphal survival and subsequent buildup of potato leafhopper populations also were investigated. In general, development from egg to adult, using 136 CDD (base = 7.6°C) for egg hatch and 153 CDD (base = 9.4°C) for nymphal development, was longer than the period from adult immigration and egg laying in the field to harvest. Samples collected indicated that within 7–10 days after harvest the nymphal population was reduced by ca. 95% when alfalfa was cut to a 2–5 cm stubble height. However, when poor harvest practices or lodged alfalfa allowed alfalfa stubble containing leafy material or uncut stems to remain in the field survival of nymphs was much higher. Also, apparent in poorly harvested fields was an increase in the number of nymphs found on early regrowth alfalfa which is highly susceptible to potato leafhopper damage. Information is still needed that will relate population levels of potato leafhopper nymphs during the post harvest to regrowth period to action thresholds for insecticide application.