Does host-plant diversity explain species richness in insects? A test using Coccidae (Hemiptera)

1. The megadiverse herbivores and their host plants are a major component of biodiversity, and their interactions have been hypothesised to drive the diversification of both. 2. If plant diversity influences the diversity of insects, there is an expectation that insect species richness will be strongly correlated with host-plant species richness. This should be observable at two levels (i) more diverse host-plant groups should harbour more species of insects, and (ii) the species richness of a group of insects should correlate with the richness of the host groups it uses. However, such a correlation is also consistent with a hypothesis of random host use, in which insects encounter and use hosts in proportion to the diversity of host plants. Neither of these expectations has been widely tested. 3. These expectations were tested using data from a species-rich group of insects-the Coccidae (Hemiptera). 4. Significant positive correlations were found between the species richness of coccid clades (genera) and the species richness of the host-plant family or families upon which the clades occur. On a global scale, more closely related plant families have more similar communities of coccid genera but the correlation is weak. 5. Random host use could not be rejected for many coccids but randomisation tests and similarity of coccid communities on closely related plant families show that there is non-random host use in some taxa. Overall, our results support the idea that plant diversity is a driver of species richness of herbivorous insects, probably via escape-and-radiate or oscillation-type processes.