Effect of alfalfa-grass intercrops on insect populations

Alfalfa, Medicago sativa L., and 2 alfalfa-grass intercrops were sampled in 1995 and 1996 to determine the effect of intercrops on alfalfa insect pest s and associated insect predator populations. The 2 grasses intercropped wi th alfalfa were smooth bromegrass, Bromus inermis Leyss., and orchardgrass, Dactyli glomerata L. In the Ist forage crops, potato leafhopper, Empoasca fabae (Harris), numbers were significantly greater in the alfalfa monocultu res than the 2 alfalfa-forage grass intercrops. Significantly greater numbe rs of alfalfa weevil, Hypera postica (Gyllenhal), larvae were collected fro m alfalfa monocultures than alfalfa-orchardgrass intercrops, but monocultur es 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-orchar dgrass intercrops. Significantly greater numbers of common damsel bugs, Nab is americoferus Carayon, were collected on alfalfa monocultures compared wi th the intercrops, but no consistent significant differences were observed between alfalfa-grass intercrops and the monoculture, Alfalfa stand densiti es and biomass estimates were significantly greater in alfalfa monocultures than alfalfa-forage grass intercrops. In the Ist and 2nd crops, orchardgra ss densities and biomass were consistently larger than bromegrass. Few diff erences in total biomass were found among the treatments in either year. No significant differences in potato leafhopper: and alfalfa weevil intensiti es on alfalfa plants were found among the treatments. The results of this s tudy suggest that alfalfa-forage grass intercrops reduce insect pest popula tions compared with monocultures, but additional management tactics may be needed to reduce insect pest levels below economic thresholds.