Contrasting leaf age preferences of specialist and generalist stick insects (Phasmida)

Abstract

Specialist and generalist herbivores may select for different types of plant defences or for different distribution of defences within a plant: e.g. between early and late stages of leaf maturation. The differentiation of age‐specific defences is particularly pronounced in tropical rain forests where young leaves are often produced year‐round, but effects on feeding choices of tropical herbivores are largely unknown. We compared feeding preferences of four species of tropical stick insects (Phasmida) between young or old leaves in dual choice experiments. Two phasmid species (Haaniella echinata, Lonchodes cultratolobatus) were highly polyphagous generalists. The other two species were classified as specialists, with Asceles margaritatus feeding mainly on Mallotus floribundus and M. miquelianus (Euphorbiaceae) and Dinophasma ruficornis mainly on Leea indica (Leeaceae) at the study site. Both specialists significantly preferred young leaves over old leaves of their respective host plants. In contrast, both generalists significantly preferred old leaves of the hosts of the specialist A. margaritatus. To reveal whether differential feeding choices were triggered by foliar chemistry, extracts (water, acetone, and hexane) of young leaves were applied to discs from old leaves and vice versa, and subjected to similar choice tests. For both Mallotus species, experimental results suggest that four chemical functions act in concert: (1) young leaves contain deterrents against generalists and (2) stimulants for specialists. Moreover, (3) old leaves contain deterrents against specialists and (4) stimulants for generalists. Deterrent compounds in young and old leaves, respectively, appeared in extracts using different solvents, suggesting the activity of multiple classes of secondary metabolites. Our study thus reveals that plant defences and herbivore offences are partly structured by leaf ontogeny and herbivore specialisation in a tropical plant‐herbivore system.