Hover flies (Diptera: Syrphidae) are an important group of insects that provide a multitude of key ecosystem services including pollination and biological control, yet many of their major life history traits are not understood. Some Palearctic hover fly species are known to migrate in response to changing seasonal conditions, yet this behavior is almost entirely unrecognized in Nearctic species. At least one species, Eupeodes americanus (Wiedemann 1830), is partially migratory during autumn while Allograpta obliqua may be non-migratory, but it is unknown where these insects originate and how far they may travel. We examined natal origins of two Nearctic hover fly species, Allograpta obliqua and Eupeodes americanus, using stable hydrogen isotope (δ²H) measurements of metabolically inactive tissues (wings and legs) to derive a hover fly δ²H isoscape. While Allograpta obliqua was mostly of local origin, several Eupeodes americanus were sourced from northern latitudes in the Midwestern United States and Canada, representing travel distances of up to 3,000 km likely using seasonally favorable air currents. This phenomenon is expected to have major ecological and economic ramifications, especially in the realm of plant pollination ecology and biological control.

Allograpta obliqua and Eupeodes americanus were hand-netted from fall-blooming goldenrod (Solidago sp.) and frost aster (Symphyotrichum pilosum) during September and October 2019 in North Alabama (Limestone County), USA, and areas of southern Alabama in November 2019. Specimens were also captured in Illinois and their offspring were used in a laboratory experiment to produce a δ²H water to hover fly calibration equation. δ²H isotopes in precipitation were obtained from the GNIP database (https://nucleus.iaea.org/wiser) and processed using the IsoriX protocol (found here: https://bookdown.org/content/782/introduction.html). GNIP data were used to produce geostatistical models which were then used to build precipitation-weighted isoscapes. Isoscapes were then calibrated using the hover fly calibration experiment data and used to assign natal origins of specimens captured in Alabama.

R Studio Version 4.1.1

'IsoriX' package, version 0.9.0

'dplyr' package, version 1.0.7

'ggplot2' package, version 3.3.5

'Cairo' package, 1.6-0

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