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ALFALFA

Alfalfa Stand Tolerance to Potato Leafhopper and Its Effect on the Economic Injury Level

^a Monsanto Company, 3100 Sycamore Rd., DeKalb, IL 60115 USA
^b Dep. of Entomology, Iowa State University, Insectary Bldg., Ames, IA 50011-3140 USA

stephen.a.lefko{at}monsanto.com

In 1997, several seed companies released alfalfa (Medicago sativa L.) cultivars 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 determine if potato leafhopper-resistant alfalfa cultivars would require revised pest management guidelines. Two field experiments were planted in Ames, IA. Four resistant cultivars (initial release) were compared with a susceptible cultivar planted in 1996. Another experiment was planted in 1998 to compare the same susceptible control with three other resistant cultivars (secondary release). Cages were used to create four levels of leafhopper stress, and nymphs were collected from inside cages when the alfalfa was harvested. Estimates of alfalfa dry weight were used to calculate linear yield-loss models, and model coefficients were used to calculate economic injury levels and economic thresholds. Trials were run on seedling, second-cutting seeding-year, second-cutting second-year, and second-cutting third-year alfalfa growth. There were no measurable differences in nymph production on resistant or susceptible cultivars in any trial, indicating that an antibiotic resistance mechanism was unimportant under production conditions. The potential for resistant alfalfa to outperform susceptible alfalfa under leafhopper 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.

Abbreviations: EIL, economic injury level • ET, economic threshold

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