Host-plant use by two Orthomeria (Phasmida: Aschiphasmatini) species feeding on Macaranga myrmecophytes

Myrmecophytes depend on symbiotic ants (plant-ants) to defend against herbivores. Although these defensive mechanisms are highly effective, some herbivorous insects can use myrmecophytes as their host-plants. The feeding habits of these phytophages on myrmecophytes and the impacts of the plant-ants on their feeding behavior have been poorly studied. We examined two phasmid species, Orthomeria alexis and O.cuprinus, which are known to feed on Macaranga (Euphorbiaceae) myrmecophytes in a Bornean primary forest. Our observations revealed that: (i) each phasmid species relied on two closely-related myrmecophytic Macaranga species for its host-plants in spite of their normal plant-ant symbioses; and (ii) there was little overlap between their host-plant preferences. More O.cuprinus adults and nymphs were found on new leaves, which were attended by more plant-ants than mature leaves, while most adults and nymphs of O.alexis tended to avoid new leaves. In a feeding choice experiment under ant-excluded conditions, O.alexis adults chose a non-host Macaranga myrmecophyte that was more intensively defended by plant-ants and was more palatable than their usual host-plants almost as frequently as their usual host-plant, suggesting that the host-plant range of O.alexis was restricted by the presence of plant-ants on non-host-plants. Phasmid behavior that appeared to minimize plant-ant attacks is described.