DIAPAUSE DYNAMICS AMD HOST-PLANT UTILIZATION OF COLIAS-PHILODICE, COLIAS-INTERIOR AND THEIR HYBRIDS (LEPIDOPTERA, PIERIDAE)

Abnormal diapause dynamics and, to a lesser extent, reduced efficiency of host utilization by hybrid larvae constitute potential post-zygoti c barriers to gene flow between a multivoltine legume-feeder, Colias p hilodice (Lepidoptera: Pieridae) and a univoltine Vaccinium-feeder, C. interior. At the time when C. interior larvae enter diapause, approxi mately 50% of hybrid larvae ceased feeding but did not enter diapause, and subsequently starved. Hybrid larvae readily accepted the host pla nts of both parental species. However, relative to C. philodice, hybri d larvae displayed a significantly reduced fifth instar relative growt h rate (RGR) when fed the primary legume host, Medicago sativa. Reduce d growth of hybrid larvae was attributable to both reduced relative co nsumption 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 reduce d pupal weight and reduced ECD. These results may explain in part why all known naturally-occurring hybrids among North American Colias aris e from parental species with similar host plant ranges and diapause st rategies.