Stable isotopes and trace elements as geo-location markers for biosecurity

Background. Benefits of accurate point of origin discrimination in biosecurity include achieving appropriate operational responses in exotic pest eradication and post-border incursion campaigns, and identifying risk pathways. Stable isotope and trace element analyses are used to trace the origin of a wide range of materials. In an earlier attempt to determine the origin of important biosecurity pests, analyses of ²H and ¹³C were employed. However, the results were tenuous as the accuracy of the method in biosecurity applications is unknown, as the samples sizes are small and from an unknown and unpredictable place, time and host. Despite the potential of isotope tracing, the application of mass-spectrometry methods in insect geo-location has been limited to date and our understanding in this field is rudimentary.

Research aims. The internationally distributed Helicoverpa armigera [Noctuidae] is being used to examine the processes fundamental to the location-to-plant-to-insect elemental profile imprinting in phytophagous insects, including the turn over of elements in adult insects, the influence of polyphagy and local variation in precipitation ²H. This improved understanding is being applied to assess the validity of using stable isotope ratios and trace element profiles to differentiate insects of New Zealand natal origin from insects of exotic origin.

Results. An integrated method of TC-IRMS, ICP-MS and MC-ICP-MS analyses of natural abundance ²H, ⁸⁷Sr/⁸⁶Sr, ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb isotope ratios and trace element profiles from single insect specimens has been developed. No single geo-location marker has been found in a preliminary comparison of international moths, although the latitudinal cline of δ²H on a continental scale is confirmed. However, multivariate analysis revealed successful geo-location discrimination for some areas.

Conclusion. The promising but imperfect geo-location demonstrated here provides solid leads for further investigation. Any geo-location system developed is likely to be applicable to other disciplines, including forensics, ecological studies and pest management.