Population development of the hop aphid,Phorodon humuli(Schrank), was studied on insecticide-free field-grown hopsHumulus lupulusL. in 1992. The influence of predators was assessed using large sleeve cages installed 2 July after aphid immigration ended. Aphid immigration numbers increased geometrically throughout July in cages that excluded predators but increased more slowly and then fell in cages with access by predators. Aphid numbers remained much lower on uncaged plants. Hop cone yields were 97, 381, and 598 g per plant from exclusion-caged, open-caged and uncaged plants, respectively. The main predators were Coccinellidae,Coccinella transversoguttataFaldermann andHippodamiaconvergens Guérin-Méneville; Chrysopidae,Chrysoperla carnea(Stephens),C. rufilabris(Burmeister) andChrysopaspp.; Hemerobiidae, Hemerobius sp.; Anthocoridae,Orius tristicolor(White); Geocoridae,Geocorissp.; Nabidae,Nabissp. Predatory Diptera were scarce as were hymenopterous parasitoids. Hops are dioecious, perennial, climbing plants that are grown on a 6-m trellis. They grow best where day length increases rapidly in the spring (45–51°N or S) and do not grow well where days are<14 h long. The hop cones (female flowers) contain pollen-like, lupulin glands. The commercial value of hops lies in the lupulin glands, which contain resins and oils. The resins are used for bittering beer, and the essential oils contribute to beer flavor.