Monocultures do occur in natural ecosystems, e.g. bracken heather or natural forest. The establishment of species in monocrop plantations is certainly not a radical departure from this and should not automatically make them more vulnerable to pests(Speight and Wain house, 1989). In forest systems in particular it has been shown that cyclical pest outbreaks can occur as a consequence of natural changes in the physiological condition of the host, with weather often playing an important role