Food plant odor perception in three sympatric alpine grasshopper species (Orthoptera: Acrididae: Catantopinae) in Aotearoa New Zealand

Abstract

The alpine grasshoppers Sigaus nivalis, Sigaus australis and Sigaus nitidus are sympatric in the central mountains of South Island, Aotearoa New Zealand. These grasshoppers feed on a range of alpine plants but show preference towards dicots over monocots. Because herbivorous insects often use smell and taste to locate and recognize food plants it was expected that these grasshoppers would show sensitivity to their favorite foods and potential sensitivity to nonhost plants. Here, we determined feeding preference in captivity allowing each of these three sympatric grasshoppers the same choice of six native alpine plant species. We analyzed the chemical compositions of the plants used in these experiments using gas-chromatograph coupled with mass-spectrometry (GC-MS) and then recorded olfactory responses in the grasshoppers to plant-derived smells (with synthetic compounds) using electroantennogram (EAG). The grasshoppers were able to distinguish between the potential food plants and ate the shrub Coriaria sarmentosa but not the grass Chionochloa pallens, however, the chemicals we detected in the six plant species were very similar. High sensitivity to fatty acid derived aldehydes (decanal, (E,Z)-2,6-nonadienal, hexanal) and a 6-carbon alcohol ((Z)-2-hexen-1-ol) compared to terpenoids (α-phellandrene, β-myrcene, β-ocimene, eucalyptol, (S)-(-)-limonene, (1S)-(-)-α-pinene) or an aromatic compound (2-phenylethanol) was recorded in the antennae of all three grasshopper species and no species- or sex-specific sensitivity to particular compounds was observed. As aldehydes and alcohols are emitted upon plant damage, it is possible that these generalist grasshoppers are sensitive to the smells of damaged plants rather than species-specific plant smells.