Enemy-free space via host plant chemistry and dispersion: assessing the influence of tri-trophic interactions

It has been argued that generalist natural enemies of insect herbivores provide a major selection pressure for restricted host plant range. This idea is a subset of the enemy-free space (EFS) hypothesis, whereby insect herbivores escape their enemies by being scarce in space and time and/or chemically defended via containing plant allelochemicals. To date, there are only two complete tests of EFS via host plant chemistry and two via host plant dispersion, and only two of these tests support the EFS hypothesis. However, three corollaries to existing views on EFS are sufficiently supported by data to warrant direct testing of the view that EFS is obtained via host plant chemistry's effects on enemies of insect herbivores. So the issue remains. Resolution will require a more collaborative, methodological approach to examine the relative importance of the major multiple factors that shape patterns of feeding specialization of insect herbivores. Predation is certainly one of these factors, but its role is still not clear.