Ecological Entomology (2015), DOI: 10.1111/een.12191 Does host-plant diversity explain species richness in insects? A test using Coccidae (Hemiptera) YEN-PO L I N, 1,2 DIANNE H. COOK, 3 PENNY J. GULLAN 4 and L Y N G. COOK 2 1 College of Life Science, Shanxi University, Taiyuan, China, 2 School of Biological Sciences, The University of Queensland, Brisbane, Australia, 3 Department of Statistics, Iowa State University, Ames, Iowa, U.S.A. and 4 Division of Evolution, Ecology and Genetics, Research School of Biology, The Australian National University, Canberra, Australia Abstract. 1. The megadiverse herbivores and their host plants are a major component of biodiversity, and their interactions have been hypothesised to drive the diversiication of both. 2. If plant diversity inluences 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. Signiicant 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. Key words. Adaptive radiation, coccids, herbivory, host breadth, host-plant diversity, insect-plant interactions, phytophagous insects, scale insects. Introduction Seed plants and the herbivorous insects that feed upon them are spectacularly species-rich and abundant, constituting the largest directly interacting component of terrestrial biodiversity (Lewinsohn et al., 2005). Many phytophagous insect lineages are more diverse than their non-plant-feeding relatives (e.g. Farrell, 1998; Strauss & Zangerl, 2002), and this has con- tributed to the idea that host-plant specialisation has driven the diversiication of insects (e.g. Ehrlich & Raven, 1964). In support of this idea, most insect groups have been found Correspondence: Yen-Po Lin, College of Life Science, Shanxi University, 92 Wucheng Road, Taiyuan 030006, China. E-mail: yenpo.lin@uqconnect.edu.au to be relatively host-speciic, with a majority of species each restricted to a single host-plant family (Strong et al., 1984; van Nieukerken, 1986; Schoonhoven et al., 2005; Lin et al., 2010). If host-plant diversity is a causative or enabling factor in insect diversiication, it would be expected that there would be more herbivores on more diverse clades of plants, and this is generally the case (e.g. Wright & Samways, 1998; Gonçalves-Alvim & Fernandes, 2001; Cuevas-Reyes et al., 2004; Proche¸ s et al., 2009; Lin et al., 2010). Thus, phytophagous insects and their hosts have been the focus of hypotheses about how interactions among organisms might drive diversiication of one or both part- ners, and provide many empirical examples used to test these hypotheses (Futuyma & Agrawal, 2009; Proche¸ s et al., 2009). There are several broad hypotheses about how plant diver- sity might drive insect species richness and all predict that the © 2015 The Royal Entomological Society 1