Accumulation of native parasitoid species on introduced herbivores: a comparison of hosts as natives and hosts as invaders.

Herbivore species newly introduced into foreign locations (hosts as invaders) are often attacked by native parasitoid species. Here we compare the structure and diversity of 87 such parasitoid complexes with those on the same herbivore species in their native regions (hosts as natives). Overall parasitoid attack rates are generally lower on hosts as invaders than on hosts as natives. Also, parasitoid complexes on hosts as invaders are generally less rich and contain a higher proportion of generalists than those on hosts as natives. Overall richness shows a weak tendency to increase with duration in the region of introduction over the first 150 yr, but the ratio of generalists to specialists does not change over this time period. These results, in part, parallel those for herbivore complexes on introduced host plants and suggest that common theoretical principles may apply to both trophic levels. The herbivores were also categorized by level of concealment and taxon (order) to determine whether life-style or phylogeny influenced parasitoid richness in native or foreign locations. No strong influences emerged. Our most novel result is a vulnerability-to-parasitism regression; the numbers of parasitoids attacking host species in invaded regions are correlated with the numbers in native regions. The biological characteristics of the herbivore as well as extrinsic region-specific factors may play important roles in setting parasitoid richness levels on hosts as natives and on hosts as invaders.