Abnormal diapause dynamics and, to a lesser extent, reduced efficiency of host utilization by hybrid larvae constitute potential post-zygotic barriers to gene flow between a multivoltine legume-feeder, Colias philodice (Lepidoptera: Pieridae) and a univoltine Vaccinium-feeder, C. interior. At the time when C. interior larvae enter diapause, approximately 50% of hybrid larvae ceased feeding but did not enter diapause, and subsequently starved. Hybrid larvae readily accepted the host plants of both parental species. However, relative to C. philodice, hybrid larvae displayed a signficantly reduced fifth instar relative growth rate (RGR) when fed the primary legume host, Medicago sativa. Reduced growth of hybrid larvae was attributable to both reduced relative consumption rate (RCR) and reduced efficiency of conversion of digested food (ECD), but not to reduced digestive efficiency (AD). Relative to C. interior, hybrid larvae fed Vaccinium myrtilloides displayed reduced pupal weight and reduced ECD. These results may explain in part why all known naturally-occurring hybrids among North American Colias arise from parental species with similar host plant ranges and diapause strategies.