Alfalfa stand tolerance to potato leafhopper and its effect on the economic injury level

In 1997, several seed companies released alfalfa (Medicago sativa L.) culti vars that were marketed as resistant to potato leafhopper, Empoasca fabae ( Harris), the key pest of this crop in the Midwest and northeastern USA. Our objectives were to investigate the mechanism of insect resistance and dete rmine if potato leafhopper-resistant alfalfa cultivars would require revise d pest management guidelines. Two field experiments were planted in Ames, I A. Four resistant cultivars (initial release) were compared with a suscepti ble cultivar planted in 1996. Another experiment was planted in 1998 to com pare the same susceptible control with three other resistant cultivars (sec ondary release). Cages were used to create four levels of leafhopper stress , and nymphs were collected from inside cages when the alfalfa was harveste d. Estimates of alfalfa dry weight were used to calculate linear yield-loss models, and model coefficients were used to calculate economic injury leve ls and economic thresholds. Trials were run on seedling, second-cutting see ding-year, second-cutting second-year, and second-rutting third-year alfalf a growth. There were no measurable differences in nymph production on resis tant or susceptible cultivars in any trial, indicating that an antibiotic r esistance mechanism was unimportant under production renditions. The potent ial for resistant alfalfa to outperform susceptible alfalfa under leafhoppe r stress began after initial seedling growth and continued through Year 3. The mechanism was described as stand tolerance, and appeared to increase as the alfalfa stand matured. The onset of stand tolerance after the initial growth interval of the seeding year raised the economic threshold from 8 to 80 leafhoppers per 10 sweeps.