Forage Yield and Nutritive Value Responses to Insecticide and Host Resistance in Alfalfa

There are conflicting reports on effectiveness of glandular-haired alfalfa (Medicago sativa L.) cultivars for reducing plant injury caused by potato leafhopper (PLH), Empoasca fabae Harris (Hemiptera: Cicadellidae). We measured forage yield and nutritive value in a glandular-haired cultivar rated as highly resistant to PLH (54H91) and a nonglandular-haired susceptible cultivar (54V54), both grown with and without an insecticide applied either early or late in each summer growth cycle across 3 yr. Cultivar. insecticide treatment interactions (P <= 0.01 and 0.14) were found for cumulative yield across years in two experiments. With the early insecticide treatment on 54H91, forage yield was up to 6% greater (P = 0.03) than without insecticide, while the susceptible 54V54 yielded 17 to 37% more (P < 0.01) with the early treatment than without insecticide. Crude protein (CP) of 54V54 in summer averaged 10 to 22 g kg(-1) lower (P < 0.05) for the untreated than with early insecticide treatment, but consistent differences in forage fiber content or fiber digestibility were not observed. Cumulative forage yield across all harvests or across summer harvests in the untreated 54H91 were similar to yield of the late insecticide-treated susceptible 54V54. The glandular-haired alfalfa cultivar suppressed PLH abundance and reduced yield losses caused by this pest in the absence of insecticide applications. Glandular-haired cultivars with high PLH resistance provide alfalfa growers opportunities to reduce insecticide use within an integrated pest management strategy in regions where the PLH causes significant yield losses in susceptible cultivars.