A dragonfly (d2H) isoscape for North America: a new tool for determining natal origins of migratory aquatic emergent insects

1. Tracking insect migration at continental scales is intractable using exogenous markers because of tiny body size and high improbability of recapture. Naturally occurring endogenous isotopic markers, such as tissue d2H and d18O, are a means of assigning origins to both vertebrate and invertebrate populations, but the success depends upon derivation of a robust algorithm linking measured tissue isotope values with large-scale geospatial isotopic patterns (isoscapes) in the terrestrial hydrosphere. 2. We derived a North American dragonfly wing d2H and d18O isoscape from known-origin dragonflies of three species (Aeshna interrupta, Aeshna umbrosa and Pachydiplax longipennis) obtained across North America. A strong relationship (r2 = 0.75) was found between wing d2H and hydrologic geospatial d2H patterns, and between wing d2H and d18O (r2 = 0.92). The strong coupling between emergent insect tissue and hydrologic spatial patterning suggested that this dragonfly isoscape may be applicable to other aquatic emergent migratory insects in North America and elsewhere. 3. As a proof of concept, we used the wing isoscape algorithm to map the probability of natal origin of Common Green Darners (Anax junius) migrating through southern Texas. Results showed that these Texan dragonflies were a mix of local and far-distant migrant (e.g. northern United States) individuals. We suggest that this isoscape algorithm opens new opportunities to quantify the migration and natal origins of dragonflies and other aquatic emergent insects where conventional methods have failed.