Figure 2.

Some theoretical expectations for how phenological change in spring may affect synchrony between specialized insect herbivores and their host plants. (a) If a plant leafs out earlier with warming, and its native herbivores also emerge early as plant-eating larvae (caterpillars), herbivory rates may remain unchanged. (b) If a plant leafs out earlier owing to climate change and, as a consequence, is exposed to novel herbivores, herbivory may increase if the plant has no natural defences to the novel herbivore; however, herbivory may decrease if the plant is not recognized as a host by the novel herbivore. (c) If a plant leafs out earlier, and its key insect herbivore(s) do not emerge earlier, it may experience an ‘enemy-free’ period early in the year. Herbivory may then decrease if young leaves with fewer herbivore defences are able to develop rapidly and increase defensive compound concentrations prior to herbivore emergence. (d) If herbivores emerge before leaf-out, herbivore populations may crash, resulting in reduced herbivory (although there should be strong selection for host-tracking, and herbivory may be expected to return to historical levels within a relatively short time period). These hypotheses are testable with herbarium specimens, which provide data on plant phenology (timing of leaf-out and first flower) and insect damage (herbivory), and museum data for insects on time of year of first flight and, in cases where larval collections are available, data on phenological timing of larval stages. (Online version in colour.)