Economic Thresholds For Potato Leafhopper In Susceptible and Resistant Alfalfa In An Era Of Rising Alfalfa Hay Market Value.

Economic thresholds are foundational to an integrated pest management program. Economic injury level and threshold decision rules take into account how many insects cause crop injury, at what point this injury becomes economic, and when to treat with an insecticide. Economic thresholds for potato leafhopper in alfalfa were established over 30 years ago. Since then, alfalfa hay market value has increased significantly and new alfalfa varieties are available, including potato leafhopper resistant varieties. In response to rising market value of alfalfa hay and farmer interest in treating below current economic thresholds, experiments were conducted to determine how both susceptible and resistant alfalfa varieties tolerate feeding damage. This was determined by infesting cages in alfalfa fields with varying densities of potato leafhoppers in order to derive a regression equation for each alfalfa variety relating yield loss to insect density. Cages placed in 2013 on susceptible and resistant alfalfa stands just after harvest were infested with 10 densities of potato leafhoppers for two separate growth cycles. The regression equation calculated from this data was used to compute the economic injury level; the threshold was then set at 75% of the injury level. Absolute densities of potato leafhoppers in cages were converted to leafhoppers per sweep through a linear regression developed by Degooyer et al. (1998) in order to give alfalfa farmers practical recommendations to support insecticide application decisions.   Yield loss estimates will be discussed as well as implications for current potato leafhopper management in alfalfa.