Chemical signatures affecting host choice in the Eucalyptus herbivore, Gonipterus sp. (Curculionidae: Coleoptera)
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It is well known that herbivorous insects respond to host plant volatiles. Yet details of how these insects perceive the complex profile of volatiles from different potential host plants have not been studied for most insects. Gonipterus spp. are important pests of Eucalyptus worldwide, but differ in their preference for different species of this host. In this study, we consider whether host volatiles affect the host choice for a Gonipterus sp. and we characterize the response of the female insect to the volatile profiles from these hosts in an electro-antennographic experiment. We sampled volatiles from freshly damaged leaves of three Eucalyptus species and analysed the profiles by gas chromatography coupled to electro-antennography (GC-EAD) and gas chromatography coupled to mass spectrometry. Female weevils gave a mixed range of electro-physiological responses to volatile puffs from leaves of different tree species. This suggests that differences in volatile profiles of different trees play a role in how these beetles discriminate between potential hosts. GC-EAD analysis showed that responses were as complex as the volatile chemical compositions of the leaves. A number of these chemicals were identified, and responses were mostly due to general green leaf volatiles. This was also evident from the fact that the insects showed a markedly greater response to the total volatile profile from freshly damaged leaves for all species. The females of the Gonipterus sp. can therefore detect damaged leaves, which may indicate host quality. Host specificity information is further expected to lie in the relative differences in emission ratios and synergism between different host chemical compounds, rather than specific individual compounds.
KeywordsGas chromatography Electro-antennography Mass spectrometry Gonipterus scutellatus Eucalyptus Volatile compounds
We are grateful to the members of the Tree Protection Co-operative Programme (TPCP), the THRIP initiative of the Department of Science and Technology (DST), South Africa and the National Research Foundation (NRF) for providing financial support for this study. We also thank Dr. Jeff Garnas for assistance with some of the statistical tests and for providing unpublished data pertaining to the identification of the Gonipterus sp. used in this study.
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