Genotype and environment interact to influence acceptability and suitability of white spruce for a specialist herbivore, Zeiraphera canadensis

. 1 The independent and interacting effects of plant genotype and site (i.e. environment) on the acceptability of white spruce, Picea gluaca (Moench) Voss, to the spruce bud moth, Zeiraphera canadensis Mut. & Free. and on plant suitability for egg development, were studied at four sites in New Brunswick, Canada. 2 A greater proportion of shoots on trees in two half-sib families, previously designated as highly susceptible, were partially eaten by spruce bud moths than shoots on trees in two half-sib families with low susceptibility. 3 At the site with the highest bud moth population, oviposition was highest on trees in susceptible families and on branches damaged by bud moth larvae. Oviposition was not higher on trees in susceptible families at the other three sites, resulting in a strong tree genotype × site interaction for oviposition. 4 Although there was a significant tree genotype × site interaction for egg predation, egg survival was higher on trees in susceptible families at all sites, due to lower levels of egg parasitism and predation. 5 Egg densities were positively but weakly correlated to shoot length and diameter. There were no consistent relationships between shoot length, shoot diameter, needle length or needle density and per cent egg survival, parasitism or predation. 6 Because egg survival was higher on trees in susceptible families at all sites but egg densities were only higher on trees in susceptible families at one site, host plant acceptability and suitability were positively related at only one site. We speculate that Z.canadensis does not distinguish between hosts of different suitability until trees are heavily damaged.