Food plants and chemical ecology of sympatric species of endemic New Zealand alpine grasshoppers : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Ecology at Massey University, Palmerston North, New Zealand
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2023-12-20
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Massey University
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Abstract
In some habitats, multiple related species coexist without competitive exclusion. This is possibly because sympatric species have adapted to use different resources in their habitat, such as shelter and food. Sympatric species may also have sophisticated mate recognition to avoid hybridization and maximises reproductive fitness. Therefore, exploring resources, feeding and sensory structures and sexual communication systems would allow us to understand how species coexist and maintain diversity.
In central South Island, three endemic species of New Zealand alpine grasshoppers Brachaspis nivalis, Sigaus australis and Paprides nitidus (Orthoptera: Acrididae) occur in sympatry at high elevation (>1200 m above sea level). Past studies showed that these species are closely related and each have a preference for a particular vegetation type (rock/scree or vegetated), but their communication systems have never been explored. Insects rely on chemical cues to locate and recognize their food and mates but the majority of chemical communication systems in acridid grasshoppers are focused on economically important species (locusts). Thus, this is the first study to explore chemoreception in the New Zealand endemic grasshopper radiation.
In this thesis, I first reviewed current knowledge of chemical ecology and olfaction in acridid grasshoppers to understand the tools and techniques used to investigate chemoreception and chemical ecology in insects (Chapter 2). Then, I investigated mechanisms of the coexistence of B. nivalis, S. australis and P. nitidus by examining their diet, mandible morphology (Chapter 3), antennal sensory organs i.e., sensilla (Chapter 4), olfactory perception of plant volatiles (Chapter 5) and chemical profiles (Chapter 6).
The three grasshopper species were found to have broadly similar diets, sensory organs and olfactory responses to plants. Microhistological epidermal and DNA analyses (Chapter 3) showed the three species had similar food plants in their gut, with shrubs and herbs detected more often than grasses. Despite significant differences in mandible morphology, males and females eat similar plant species. Females of all species had larger mandibles than males, suggesting that they may be adapted to eating thicker plant tissues than males, and S. australis mandibles of both sexes were more heavily melanized than the other species indicating adaption to eating tougher plant tissues.
Five morphological types of sensilla (one taste and four olfactory receptors) were observed on the grasshopper antennae, but none were specific to a particular species or sex (Chapter 4). Brachaspis nivalis, however, had the fewest taste sensilla but the most olfactory sensilla compared to the other two species and showed olfactory responses to plant-derived smells even at the lowest concentration. This may be related to their rock/scree habitat where the food resources are scattered and thus require higher olfactory sensitivity (long-distance cues) than taste reception (short-distance cues) compared to more vegetated habitats of S. australis and P. nitidus. Olfactory response recordings to plant volatiles (Chapter 5) showed all three species responded more strongly to green leaf volatiles than to terpenoids, which could indicate sensitivity to plant damage rather than to plant-specific smells.
A higher abundance of olfactory sensilla was observed in male S. australis compared to conspecific females but no sex differences were observed in B. nivalis or P. nitidus (Chapter 4). However, female-specific compounds (oleamide and octadecanamide) were detected in cuticular hydrocarbons (CHCs) of all three species and some compounds were more abundant in particular species than in others (Chapter 6). This shows CHC composition has potential information for mate recognition in these grasshoppers. Altogether, males and females of three New Zealand alpine grasshopper species showed similarity in their food plants, sensory organs and sensitivity to plant smells, but sexual and species differences in mandible morphology and CHC composition allow one to infer specific adaptation to food plants and sophisticated mate recognition system that explain how these three alpine grasshoppers can co-occur.
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sympatry, Acrididae, chemical ecology