Alfalfa resistance to the potato leafhopper: deciphering the resistance mechanism and updating management guidelines

In 1997 several seed companies released alfalfa products that were marketed as resistant to the potato leafhopper, (Empoasca fabae Harris), the key pest of this crop in the Midwest and northeastern United States. The objectives of this research were to investigate the mechanism of insect resistance in leafhopper-resistant alfalfa and to determine if potato leafhopper-resistant alfalfa would require updated pest-management guidelines. It was determined that an antixenotic mechanism functioned at a plant or stem level, and without a choice, leafhoppers could feed as much on resistant alfalfa compared with susceptible alfalfa. Antixenosis was largely a function of insect behavior. The antixenotic mechanism detected in lab studies was not detected at the field-plot scale. The densities of potato leafhopper adults and nymphs were similar among plots of one susceptible alfalfa and four leafhopper-resistant alfalfas during three years of sampling. Cage studies were used to compare the potential for loss from potato leafhopper and leafhopper population growth potential on resistant and susceptible alfalfas. Resistant alfalfas had a lower potential for loss, compared with susceptible alfalfa, beginning with the second cutting of the seeding year. Moreover, similar numbers of potato leafhopper nymphs were produced on susceptible and resistant alfalfas. We used a stand tolerance concept to describe why these new alfalfas have a greater yield potential compared with susceptible alfalfa when the leafhopper number is high; the leafhopper population-damage-potential might be reduced if leafhoppers aggregate on a fraction of the stand. Stand tolerance implies the interplay of more than one resistance mechanism, but emphasizes the impact this tactic will have on pest management by raising the economic injury level. We calculated economic thresholds for both alfalfa types and found that the threshold was similar for both alfalfa types when the alfalfa was young. The threshold in susceptible alfalfa went from 8 adult leafhoppers, per 10 seeps during the seeding year up to 33 leafhoppers per 10 sweeps in subsequent years. The threshold in tolerant alfalfa went from 8 per 10 sweeps during the first cutting of the seeding year up to 80 per 10 sweeps in subsequent cuttings and years