Can deuterium stable isotope values be used to assign the geographic origin of an auxiliary hoverfly in south-western France?

Deuterium δD isotopic analysis is increasingly being used to trace wildlife movement, and undoubtedly has much to offer in this respect, but questions still remain as to the feasibility and practicality of the method in ecology. Here we report our attempt to determine the geographic origin of an auxiliary hoverfly, Episyrphus balteatus, in south-western France. We used quantile regression to calculate the minimum separation distance, based on the International Atomic Energy Agency/World Meteorological Organization (IAEA/WMO) data, at which two insects could be said to originate from different latitudes with a given degree of confidence. We collected larvae in spring 2007 and 2009 to obtain the δD signal of indigenous hoverflies and we trapped adults during one complete year (from Dec. 2006 to Nov. 2007). The smallest separation distance calculated was about 1400 km in western Europe. Our results revealed greater variability in δD of adults in autumn than in spring. From this we infer an autumnal migration. Because of the presence of mountains and the Mediterranean Sea, the δD gradient in precipitation in western Europe is less clear than on the American continent, where it has been used successfully to infer geographical origins of animals under certain conditions. Despite the complications encountered in Europe, the minimum separation distance model proved a useful first step to obtain a first range of possible origins of E. balteatus and the application of the model to other arthropod species in Europe warrants investigation.