Adaptation to different host plant ages facilitates insect divergence without a host shift

Host shifts and subsequent adaption to novel host plants are importantdrivers of speciation among phytophagous insects. However, there is considerablyless evidence for host plant-mediated speciation in the absenceof a host shift. Here, we investigated divergence of two sympatric sisterelm leaf beetles, Pyrrhalta maculicollis and P. aenescens, which feed on differentage classes of the elm Ulmus pumila L. (seedling versus adult trees).Using a field survey coupled with preference and performance trials, weshow that these beetle species are highly divergent in both feeding and ovipositionpreference and specialize on either seedling or adult stages of theirhost plant. An experiment using artificial leaf discs painted with leaf surfacewax extracts showed that host plant chemistry is a critical element thatshapes preference. Specialization appears to be driven by adaptive divergenceas there was also evidence of divergent selection; beetles hadsignificantly higher survival and fecundity when reared on their natal hostplant age class. Together, the results identify the first probable example ofdivergence induced by host plant age, thus extending how phytophagousinsects might diversify in the absence of host shifts.