Plant Allelochemicals, Tritrophic Interactions and the Anomalous Diversity of Tropical Parasitoids: The "Nasty" Host Hypothesis

The number of species in many groups of parasitoid Hymenoptera does not increase rapidly with decreasing latitude. Discussion of the processes generating this pattern has focussed upon changes in host demographic traits and upon the effect of seasonality. Here we advance a further and compatible hypothesis, that tropical parasitoid hosts are less available to parasitoids than are extra-tropical hosts because their tissues are, on average, more chemically toxic than are the tissues of extra-tropical hosts. There is some evidence that suggests that tropical woody plants are, in general, richer in toxic secondary compounds than are extra-tropical species, and evidence exists to demonstrate that these allelochemicals may have adverse effects on parasitoids attacking phytophagous insects feeding on such plants.