Many herbivorous insects have generalized diets over the species' entire geographical ranges but they function as specialists with restricted diets in local communities. Local feeding specialization can be produced by biochemical, behavioral, ecological, and evolutionary processes. Much evidence is incompatible with the widely held assumptions that diet breadth is a species characteristic and that specialization among herbivorous insects implies greater efficiency and less niche overlap.

We found the hybrid zone between Eucalyptus amygdalina and Eucalyptus risdonii to be a center of insect and fungal species richness and abundance. Of 40 taxa examined, 73% were significantly more abundant in the hybrid zone than in pure zones, 25% showed on significant differences, and 2% were most abundant on a pure host species. The average hybrid tree supported 53% more insect and fungal species, and relative abundances were, on average, 4 times greater on hybrids than on either eucalypt species growing in pure stands. Hybrids may act as refugia for rare species: 5 of 40 species were largely restricted to the hybrid zone. Also, 50% of the species coexisted only in the hybrid zone, making for mique species assemblages. Although hybrids support more species and greater abundances, all hybrids are not equal: 68% of the 40 taxa examined were significantly more abundant on one hybrid phenotype than another. While herbivore concentrations on F1 type intermediates were rare, concentrations were common on phenotypes resembling backcrosses either to E. amygdalina or E. risdonii. For specialist herbivores, the hybrid phenotype most heavily utilized appears to be determined by its phenotypic affinity to its host species. Generalists exhibit an overall greater abundance on hybrids, but are less likely to utilize one hybrid phenotype over another. Mechanistic explanations for these distributions are numerous and probably species specific, but are likely to include: increased genetic susceptibility of hybrids due to hybrid breakdown; increased stress in the hybrid zone resulting in greater plant susceptibility; and a greater diversity of resources in the hybrid zone which could support more species. Seed capsule production by hybrids and their parental species is negatively correlated with herbivory. However, it is difficult to determine whether herbivores cause this pattern as hybrids may have inherently lower sexual reproduction. Laws enacted to protect rare and endangered species do not include hybrids. We argue that a re-examination of our current "hybrid policy" is warranted. Plant hybrid zones are centers of plant evolution and speciation, sources of economically important plants and potential biocontrol agents, and, as our study suggests, also provide essential habitats for phytophagous communities.

Experimental reduction of insect grazing pressures suggests that longterm, continuous feeding by phytophagous insects has severely depressed growth and productivity of subalpine Eucalyptus trees (measured by annual ring growth) for at least 25 years.

In a subalpine association of three Eucalyptus species, phytophagous insects showed strong host preferences even among very closely related Eucalyptus species. Sixty-three per cent of insect species attacked only one eucalypt species, 30% attacked two species and only 8% fed on all three. Ninety-seven per cent of individuals belonged to species using one or two hosts. The results are discussed in relation to the degree of host specificity that might be expected when most potential hosts are members of a single genus, the way in which host specificity is determined, and the potential of insects with host preferences to affect the dynamics of plant communities.

Levels of insect attack and yields of leaf essential oils in Eucalyptus vary widely within and among species. We tested the hypotheses that 1) metabolic cost of oil detoxification increases with increasing oil yield, resulting in lower herbivore growth rates and, consequently, 2) in lower herbivore damage to plants. Distribution of insect damage, eggs, immature insects and adults and feeding rates, growth and survivorship of insects do not support the hypotheses, although a threshold level of oil may be necessary to influence herbivorous insects. Herbivorous beetles tested do not detoxify essential oils. Levels of leaf nitrogen, rather than oil content, explained differences in insect feeding and growth.

High levels of insect damage on Eucalyptus have been noted but not quantified in previous literature. We present estimates of leaf damage for 44 Eucalyptus species from a variety-of habitats. Overall, an average of 15% of expanded leaf area was missing. While some species were generally lightly grazed, others, especially those in more mesic, higher altitude communities, usually sustained much heavier leaf damage. In the age and size classes, communities and years that we sampled, the overall levels of damage to eucalypts were higher than chronic damage levels reported for north temperate communities. We also suggest a simple method for rapidly estimating the proportion of leaf area missing from trees.

Many long‐lived plants are known to prolong dormancy in response to abiotic stresses such as drought. We are unaware, however, of any reports of plants prolonging dormancy in response to biotic stresses such as herbivory. We monitored 140 putative Solidago missouriensis clones (hereafter ‘clones’) ≥ 13 years before, during and after intense defoliation by the specialist herbivore Trirhabda canadensis. Eight of the clones produced no above‐ground growth in the season following defoliation. Though apparently killed, these clones reappeared 1–10 years after they disappeared, with six of them robustly recovering in a single season. We used 38 RAPD markers to test the hypotheses (denoted by H and numbered with subscripts) that territories were recovered by (H1) seedling establishment or (H2) rhizomes. We compared pre‐defoliation and post‐recovery genotypes in two clones using the same 38 markers. Our data document the existence of very large clones (60–350 m2 with c. 700–20 000 ramets), and support the hypothesis that recovery is from rhizomes. Within‐clone diversity is low, and the pre‐defoliation and post‐recovery genotypes match. We consider mechanisms that could enable plants entering dormancy with depleted resources to robustly recover, and the implications of dormancy for avoiding biotic stress such as that induced by T. canadensis.

Sawfly larvae of the subfamily Perginae have a diverticular pouch of the foregut in which they store an oily fluid that they regurgitate when attacked. The fluid, which is an effective deterrent to ants, birds, and mice, is essentially identical chromatographically to the oils of the Eucalyptus leaves on which the larvae feed. The significance of the defensive use by animals of the "secondary substances" of their food plants is discussed.