Reducing Damage from Potato Leafhoppers on Alfalfa in New York through Cultivar Selection: A Comparison of Resistant vs. Susceptible Cultivars under Insecticide Treatment and No Treatment 2000
Project Leaders: Julie Hansen, Jill Miller-Garvin, Keith Waldron, and Don Viands
Advanced cultivars and experimentals of alfalfa, Medicago sativa L., resistant to potato leafhopper (PLH), Empoasca fabae (Harris), were released in 1998 and planted at Ithaca that year. Cultivars released in 1999 were planted in Ithaca in a separate trial in spring 1999. In these experiments, we wanted to compare the PLH-resistant cultivars and experimentals with susceptible ones for PLH damage, yield, agronomic characteristics, insect counts, and forage quality under both natural populations of PLH (unsprayed) and insect-free conditions (sprayed). It was a particularly wet growing season in Ithaca in 2000, and insect populations were generally light, going over threshold at only one sampling date, 26 July, 10 days before second harvest. The trial planted in 1999 showed "carryover" yield decreases in 2000 in plots which had not been sprayed and thus were not protected from PLH damage in 1999 (the seeding year of the trial). Furthermore, unsprayed susceptible plots, which suffered more PLH damage than resistant ones in 1999, also showed lower yields in 2000. The trial planted in 1998 generally showed that the resistant cultivars and experimentals often yielded slightly less than susceptible entries, but the resistant entries showed lower PLH damage. We also found that the PLH-resistant cultivars showed slightly reduced percent stand and less fall growth than the susceptibles. After considering four years of research comparing PLH-resistant vs. susceptible cultivars, we can make several generalized conclusions: a grower may have to spray a resistant cultivar in the seeding year if PLH numbers are very heavy, and/or soil moisture is limiting; the PLH-resistant cultivars had consistently higher percent crude protein and were earlier to mature than the susceptible cultivars; under moderate to heavy PLH pressure (unsprayed), the PLH-resistant cultivars had lower PLH damage and lower nymph counts than susceptibles, though the resistant cultivars do still support populations of both adult PLH and nymphs; and yield of advanced PLH-resistant cultivars has improved relative to earlier-generation cultivars, and is often comparable to yield of susceptible cultivars. If a producer does not want to spray, does not want to invest in chemicals or hire commercial sprayers, or cannot spray all acreage in a timely manner, then planting PLH-resistant cultivars is an alternative that allows the producer to reduce, but not eliminate, the risk of yield loss due to PLH damage.