PERFORMANCE OF INSECT HERBIVORES ON HYBRIDIZING TREES - THE CASE OF THE SUB-ARCTIC BIRCHES

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 Fin nish Lapland. Leaf quality of both arboretum-grown birches and birches of a natural stand was bioassayed with 14 insect species (one geometr id lepidopteran, one argid, one cimbicid, and 11 tenthredinid sawflies ). All the herbivore species belonged to the local birch leaf-chewer g uild. 2. There were considerable differences among the herbivore speci es in the relative suitability of the two parent birch species. Larvae of different species grew equally well on the pure mountain birches a nd on the hybrids, irrespective of the mother tree. In none of the cas es were the hybrid birches superior as an insect diet. Dwarf birches w ere 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 specie s were collected from mountain birches, and they differed in their res ponses to dwarf birches. Three kinds of responses were observed: two s pecies totally refused to feed on dwarf birch, three species were indi fferent, and nine species had their slowest growth on the dwarf birch. 5. Two sawfly species, Arge fuscinervis and Nematus viridis, were rea red in short- and long-term growth trials on leaves from arboretum and natural stand birches. Arboretum and natural birches did not differ a s a diet for the larvae, and short- and long-term growth trials produc ed consistent results. 6. When fed dwarf birch foliage, individuals of A. fuscinervis had slower growth, lower survival and delayed adult em ergence in the next season than individuals fed on mountain birch or h ybrids. There were no such effects on N. viridis. 7. The results indic ate that hybridization and introgression between northern birch specie s enhances variation in the leaf quality for insect herbivores. Accord ingly, hybridization between host-plants may diversify solutions where by herbivore species adapt to their hosts.