Effect of Alfalfa–Grass Intercrops on Insect Populations

Abstract

Alfalfa, Medicago sativa L., and 2 alfalfa–grass intercrops were sampled in 1995 and 1996 to determine the effect of intercrops on alfalfa insect pests and associated insect predator populations. The 2 grasses intercropped with alfalfa were smooth bromegrass, Bromus inermis Leyss., and orchardgrass, Dactyli glomerata L. In the 1st forage crops, potato leafhopper, Empoasca fabae (Harris), numbers were significantly greater in the alfalfa monocultures than the 2 alfalfa–forage grass intercrops. Significantly greater numbers of alfalfa weevil, Hypera postica (Gyllenhal), larvae were collected from alfalfa monocultures than alfalfa–orchardgrass intercrops, but monocultures were not significantly different from alfalfa–bromegrass intercrops. In the 2nd alfalfa crops, significantly greater numbers of potato leafhoppers were collected in the alfalfa monocultures compared with the alfalfa–orchardgrass intercrops. Significantly greater numbers of common damsel bugs, Nabis americoferus Carayon, were collected on alfalfa monocultures compared with the intercrops, but no consistent significant differences were observed between alfalfa–grass intercrops and the monoculture. Alfalfa stand densities and biomass estimates were significantly greater in alfalfa monocultures than alfalfa–forage grass intercrops. In the 1st and 2nd crops, orchardgrass densities and biomass were consistently larger than bromegrass. Few differences in total biomass were found among the treatments in either year. No significant differences in potato leafhopper and alfalfa weevil intensities on alfalfa plants were found among the treatments. The results of this study suggest that alfalfa–forage grass intercrops reduce insect pest populations compared with monocultures, but additional management tactics may be needed to reduce insect pest levels below economic thresholds.