Reconstruction of the historical biogeography of aphids is aided not only by fossils and phylogenies, but also by the extreme host specificity of many aphids. Host associations are most specific in aphids with complex (host-alternating) life cycles. Specificity to plant families, and even
genera, maps generally to aphid tribes, which appear to have diverged rapidly in the Upper Cretaceous. Because host associations are presumably old, aphid biogeography can be interpreted in the light of the historical biogeography of their host plants (for which more extensive fossil
evidence exists). Evidence suggests that, despite their current overwhelming greater diversity in the northern hemisphere, aphids may have originated in the southern hemisphere-or at least aphids now confined to the southern hemisphere may constitute some of the most ancient lineages.
Host alternation may have evolved independently in several lineages in the Tertiary, along with seasonality and the development of grasslands.
Intercontinental disjunct distributions within several aphid tribes and genera were formed also in the Tertiary, possibly as a consequence of the periodically available floristic migration routes such as the Bering and North Atlantic land bridges.
Keywords: Aphids Biogeography
The ESA 2001 Annual Meeting - 2001: An Entomological Odyssey of ESA