POTATO LEAFHOPPER MANAGEMENT - EFFECTS ON PRODUCTION AND QUALITY OF ALFALFA-TIMOTHY MIXTURES

Alfalfa (Medicago sativa L.) mixtures with grasses, especially timothy (Phleum pratense L.), are widely planted in the Great Lakes Region, b ut most information on potato leafhopper [Empoasca fabae (Harris)] (PL H) management for new stands is based on alfalfa monocultures. Field r esearch was conducted in 1991 and 1992 to evaluate the ability of alfa lfa to recover from PLH injury following insecticide application and t o compare the effect of injury on yield and quality of first-year mono cultures or mixtures. Four replicates of alfalfa monoculture and alfal fa-timothy mixtures were established each year. One week after the fir st harvest in early July, eight split-plot treatments were established in each main plot: an uncaged check, a caged cheek, and six combinati ons of two PLH infestation densities (25 or 50 PLH adults m(-2)) and t hree insecticide application dates (1 wk after infesting plots, 3 wk a fter infesting plots, and unsprayed). Crop mixture had no effect on PL H nymphal populations on a per stem basis. Although differences betwee n years were noted, delaying insecticide application generally reduced forage yield and quality, delayed crop maturity, and increased weed c ontent, particularly in forage mixtures. There was little evidence tha t seedling timothy could compensate for PLH injury to alfalfa. Data on stem elongation and dry matter accumulation indicated that alfalfa ca n recover from leafhopper injury; however, dry matter production did n ot increase for at least 2 to 3 wk following a spray application. Redu ctions in forage crude protein due to PLH injury were a result of both reduced crude protein content in the alfalfa and an increase in the p roportion of weed biomass in the forage. Early detection and rapid res ponse to economically significant populations are necessary to realize the greatest benefit from treatment. The presence of timothy in the m ixture does not alter PLH management recommendations for new stands.