Host shifting by Operophtera brumata into novel environments leads to population differentiation in life-history traits

1. Operophtera brumata L. (Lepidoptera: Geometridae), a polyphagous herbivore usually associated with deciduous trees such as oak Quercus robur L . , has expanded its host range to include the evergreen species heather Calluna vulgaris (L.) Hull and, most recently, Sitka spruce Picea sitchensis (Bong.) Carrière. 2. Phenology, morphology, and survival of O. brumata were measured at several life‐history stages in populations from the three different host plant communities sampled from a range of geographical locations. The data were used to test for population differences, reflecting the marked differences in host‐plant secondary chemistry, growth form, and site factors such as climate. The hypothesis that spruce‐feeding populations originated from populations feeding on moorland, commonly sites of coniferous afforestation, was also tested. 3. Altitude, not host plant species, was the major influence on the timing of adult emergence. An effect of insect population independent of altitude was found, implying that additional unidentified factors contribute to this phenological variation. Larval survival and adult size varied between populations reared on different host plant species. Survival of larvae was affected negatively when reared on the novel host plant, Sitka spruce, versus the natal plant (oak or heather) but oak and heather‐sourced insects did not differ in survivorship on Sitka spruce. 4. Host range extension into novel environments has resulted in population differentiation to the local climate, demonstrating that host shifts pose challenges to the herbivore population greater than those offered by the host plant alone. The hypothesis that Sitka spruce feeding populations have arisen predominantly from moorland feeding populations was not supported.