Abstract • To determine how the presence of non-host plants impact populations of lepidopteran pests and yield of broccoli, Brassica oleracea var. italica L., broccoli was intercropped with tomato, Lycopersicon esculentum Miller, or yellow sweetclover (YSC), Melilotus officinalis L., and compared with broccoli monoculture grown at two densities (high and low). • During a 1997 study, numbers of early instar imported cabbageworm, Artogeia rapae L., were greater in broccoli monoculture compared with intercropped plantings. In contrast, late instar A. rapae numbers were significantly higher in intercropped broccoli during most of the season, indicating greater survival of early instar stages in intercropped habitats. • Cabbage looper, Trichoplusia ni Hübner, eggs and larvae were more abundant in broccoli monoculture than intercropped broccoli in 1997. In addition, T. ni populations were found at significantly lower levels in broccoli-YSC compared with broccoli-tomato habitat throughout the season. • During a 1998 study, numbers of early and late A. rapae instars were greater in intercropped broccoli on most sampling dates. • The mean percentages of harvested broccoli heads infested with insects and associated frass were higher in monocultures than intercropped treatments in 1997. Additionally, a significantly lower number of contaminants were found in the crowns of broccoli harvested from broccoli-YSC (YSC) compared with broccoli-tomato (tomato) habitat. However, broccoli heads were significantly smaller in intercropped habitats. • Variations in insect responses and challenges to using crop diversification as a management tool are discussed.