Host specialization of leaf-chewing insects in a New Guinea rainforest

1. Data on host use by herbivorous insects in the tropics cannot be cross-referenced between the studies as only a fraction of the species can be formally identified. It is thus imperative for each study to include a wide range of plant and insect taxa, but this requirement has rarely been met because of logistical difficulties. 2. A novel approach using mass insect collecting and rearing by parataxonomists was applied to study the use of 59 species, 39 genera and 18 families of woody plants by 58 588 individuals and 1010 species of leaf-chewing insects in a lowland rainforest. 3. Most species had wide host plant ranges with reference to congeneric plants. The modal host range for a herbivore feeding on a particular genus (Ficus, Macaranga or Psychotria) included >90% of congeneric species studied. Only 3.7% of species feeding on these genera were monophagous. 4. Most herbivores were specialized with respect to confamilial plant genera, with modal host range of 1 genus from 9 studied in Euphorbiaceae and 13 in Rubiaceae. This pattern was corroborated by modal host range of 1 plant family from 18 studied. 5. Because of the overlap among the herbivore communities on congeneric plants, the total number of herbivores on speciose plant genera was relatively small. For example, although 336 species of leaf-chewers used the 13 study species of Ficus, the 35 additional Ficus species present locally would support only estimated 163 additional species. 6. Since large genera constitute a significant proportion of tropical floras, these results have implications for regional estimates of herbivore species richness. Our estimate of 10.6-24.1 leaf-chewing species effectively specialized to a rainforest tree species is an order of magnitude lower than previously suggested. 7. The number of new herbivore species (y) resulting from the addition of the xth plant species to the compound community (x = 1, 2, 3,..., n where n is the total number of plants studied) can be described as y = cx(k), where c and k are constants. k is a useful descriptor of similarity among herbivore communities from different hosts.