Biogeochemical markers of plant pest geographic origin: Informing invasion processes and pathways

Abstract

Knowledge of where an individual of an invasive insect species has immediately anived from can provide important information about the establishment status of a newly discovered pest. This can translate into many millions of dollars saved through mobilisation of the appropriate level ofresponse. Understanding origins can also better inform about source of the risk contributing to pathway leakage and the continuing management of local re-invasions. DNA-based methods can provide information about potential source populations through recent evolutionary associations. However such approaches are imprecise unless every possible geographic source population can be referenced and consideration is made for the fact that the genetic pool will change over time with on-going invasion processes. A novel approach to determine natal geographic origin is to utilise the you are what you eat principle, based on the premise that plant uptake of biogeochemical markers, such as hydrogen and strontium isotope ratios, will be reflected through the food chain. This theoretically becomes a powerful indicator of a pest's geographic origin, given the relatively predictable presence of these markers in the physical environment from which they are sourced. The method has proven invaluable in tracing the origins of food products and items of forensic interest. However, biogeochemical provenancing of insects is in its infancy, and the interpretation of such signatures to predict origins of polyphagous biosecurity specimens presents some unique challenges. Technological difficulties such as reliable extraction of the target biogeochemical markers, locating fixed signal in biological tissues and use of very small tissue samples add to the potential complexity. Nevertheless, with judicious choice of markers and analytical models, measuring natural abundance trace element concentrations and isotope ratios shows considerable promise, and region of origin discrimination has been de.monstrated for Helicoverpa armigera Hubner(Lepidoptera: Noctuidae). Progress in this cross-disciplinary research program to develop appropriate geographical 'isoscapes' and understand the biological influences on the system will be described.