Combination of morphometric and isotopic tools for studying spring migration dynamics in Episyrphus balteatus

Migration and population dynamics are important in organisms that provide ecosystem services as they determine the occurrence of individuals in a given place at a given time, which, in turn, determines the efficiency of the service provided. To design appropriate landscape management strategies to improve the efficiency of biological control, it is essential to know the origin of beneficial insects that colonize agricultural fields. However, studying migration dynamics in insects is complicated by their small size and short life span which largely prevent the use of conventional techniques based on capture-markrecapture, or remote sensing. [br/]Stable isotopes and morphological characters are intrinsic markers that can be used to infer the geographical origin of many organisms including insects. In this study, we first determined whether the hydrogen isotopic ratio and the wing morphometrics are appropriate markers to study the spring migration dynamics of a major aphidophagous hoverfly species (Episyrphus balteatus). To this end, we assessed the magnitude of variation of these indicators in seven populations along a north-south gradient in Western Europe. Second, we used the two markers in E. balteatus individuals collected in a French agricultural landscape over the course of the spring to assess the proportion of immigrants from southern regions and the period of immigration. [br/]Our results revealed uncertainties associated with the use of the hydrogen isotopic ratio and wing morphometrics, but showed that the spring population of E. balteatus in Western France very likely comprises mainly local individuals, with the probable arrival of some immigrants from nearby regions located further south. The low proportion of immigrants in the spring population has important implications for the biological control provided by E. balteatus in terms of landscape management strategies.