1. The consequences of hybridization between two related birch species, the mountain and the dwarf birch (Betula pubescens ssp. tortuosa and B. nana), on the performance of insect herbivores were studied in Finnish Lapland. Leaf quality of both arboretum-grown birches and birches of a natural stand was bioassayed with 14 insect species (one geometrid lepidopteran, one argid, one cimbicid, and 11 tenthredinid sawflies). All the herbivore species belonged to the local birch leafchewer guild.
2. There were considerable differences among the herbivore species in the relative suitability of the two parent birch species. Larvae of different species grew equally well on the pure mountain birches and on the hybrids, irrespective of the mother tree. In none of the cases were the hybrid birches superior as an insect diet. Dwarf birches were the least suitable for most of the insect species.
3. A pronounced seasonal decline in leaf quality was observed in the mountain birches and in hybrids but not in the dwarf birches.
4. All the insect species were collected from mountain birches, and they differed in their responses to dwarf birches. Three kinds of responses were observed: two species totally refused to feed on dwarf birch, three species were indifferent, and nine species had their slowest growth on the dwarf birch.
5. Two sawfly species, Arge fuscinervis and Nematus viridis, were reared in short- and long-term growth trials on leaves from arboretum and natural stand birches. Arboretum and natural birches did not differ as a diet for the larvae, and short- and long-term growth trials produced consistent results.
6. When fed dwarf birch foliage, individuals of A. fuscinervis had slower growth, lower survival and delayed adult emergence in the next season than individuals fed on mountain birch or hybrids. There were no such effects on N. viridis.
7. The results indicate that hybridization and introgression between northern birch species enhances variation in the leaf quality for insect herbivores. Accordingly, hybridization between host-plants may diversify solutions whereby herbivore species adapt to their hosts.