Mechanisms for locating resources in space and time: Impacts on the abundance of insect herbivores

Herbivorous insects have the problem both of locating appropriate host plan ts and ensuring that the plant-feeding stages of their life cycles are sync hronized with the times when those hosts provide a high-quality food resour ce. Because the taxonomic range of potential hosts is generally narrow, and the temporal window when those hosts are suitable is often relatively shor t, developmental (especially diapause) and dispersal mechanisms may be crit ical factors in determining whether or not a species population is successf ul in a particular plant community. The present paper considers the impact of diapause and dispersal mechanisms on the ability of insect herbivores to cope with two attributes of their host plants: (i) the diversity of the pl ant community within which the hosts are located; and (ii) the seasonal pre dictability of host suitability. Some common dispersal mechanisms used by i nsect herbivores are much more appropriate to low-diversity than to high-di versity plant communities and, similarly, some diapause cues are appropriat e only to highly predictable plant phenology. Both agriculture and silvicul ture characteristically manipulate both these attributes of plant communiti es, that is, in order to make the human use of plants more efficient, culti vation strategies normally both reduce plant species diversity (often to a condition approaching monoculture) and increase the predictability of plant developmental patterns. Consequently, major pest species in managed system s may not be those that are most common in natural systems, and may be diff icult to predict in advance.