Local adaptation of monophagous insects on wild host plants at a regional scale

Local adaptation leading to higher performance on local hosts is a well known pattern that occurs in many host-parasite systems. This pattern has been observed at individual, population and regional scales; and is demonstrated by herbivorous insects performing better on local than on foreign host plants. It is expected that by specialising on local hosts insects increase their mean fitness, and therefore experience a trade-off leading to reduced performance on foreign hosts. We found distinct regional patterns of adaptation by insect monophages of the endemic New Zealand cabbage tree, Cordyline australis (Agavaceae). The phylogenetic structure of C. australis populations has three genetically distinct groups following a latitudinal gradient; Northern, Mid-latitude, and Southern. The Mid and Southern groups are thought to have separated following the last Ice Age, ending around 10,000 years ago. We sampled specialist and generalist insects from 13 provenances of C. australis trees sampled from across these genetic groups, all of which were grown under uniform garden conditions at a site located within the Southern and Northern phylogenetic regions. In the Southern region we found evidence for adaptation by several monophagous herbivores onto regionally local trees, with a trend of decreasing abundance on trees from Mid to Northern latitudes. In contrast, abundance of generalist herbivores did not differ significantly among host provenances. We are currently analysing the analogous Northern region data, where we predict stronger patterns of local adaptation since these areas are not glaciated during glacial maxima. Our results to date are confirmation of the pervasive nature of local host adaptation in insect monophages on wild hosts. They suggest that only a few thousand years are sufficient to create local host adaptations strong enough to result in large differences in herbivore abundance.