Influence of cultivars and pesticides on population trends and interrelationships of pea aphid,Acyrthosiphon pisum(Harris), spotted alfalfa aphid,Therioaphis maculata(Buckton), and associated arthropod taxa were studied over 3 growing seasons at Rose-mount, Minn. Greatest cultivar differences in resistance were for pea aphid and spotted alfalfa aphid. Cultivar differences were less for other pest species, but significant some sampling periods. Pesticides increased forage yields, the benefit varying with severity of pest pressure and cultivar. Fungicidal sprays resulted in greater yield increases than did insecticidal sprays; combinations were additive in effect. Populations of both aphid species and of associated entomophagous taxa were consistently higher in subplots treated with fungicides. Fungicidal sprays enhanced aphid populations by suppressing entomogenous fungi.Weekly correlations between populations of aphids and associated arthropod taxa revealed consistent patterns of significant positive correlation. Taxa tending to strong association with pea aphid included: tarnished plant bug,Lygus lineolaris(Palisot de Beauvois); alfalfa plant bug,Adelphocoris lineolatus(Goeze); the coccinellids,Hippodamia convergensGuérin-Méneville,H. tredecimpunctata tibialis(Say), andColeomegilla maculata lengiTimberlake;Orius insidiosus(Say);Chrysopaspp.; hemerobiids; syrphids; spiders; nabids; and parasitic wasps. Taxa tending to strong association with spotted alfalfa aphid included potato leafhopper,Empoasca fabae(Harris); hemerobiids; nabids;Orius insidiosus; spiders; and parasitic wasps. Strong positive correlations between either aphid species and any entomophagous taxon were interpreted as evidence of a density-dependent relationship.