Do Nearctic hover flies (Diptera: Syrphidae) engage in long‐distance migration? An assessment of evidence and mechanisms

Abstract

Long‐distance insect migration is poorly understood despite its tremendous ecological and economic importance. As a group, Nearctic hover flies (Diptera: Syrphidae: Syrphinae), which are crucial pollinators as adults and biological control agents as larvae, are almost entirely unrecognized as migratory despite examples of highly migratory behavior among several Palearctic species. Here, we examined evidence and mechanisms of migration for four hover fly species (Allograpta obliqua, Eupeodes americanus, Syrphus rectus, and Syrphus ribesii) common throughout eastern North America using stable hydrogen isotope (δ2H) measurements of chitinous tissue, morphological assessments, abundance estimations, and cold‐tolerance assays. Although further studies are needed, nonlocal isotopic values obtained from hover fly specimens collected in central Illinois support the existence of long‐distance fall migratory behavior in Eu. americanus, and to a lesser extent S. ribesii and S. rectus. Elevated abundance of Eu. americanus during the expected autumn migratory period further supports the existence of such behavior. Moreover, high phenotypic plasticity of morphology associated with dispersal coupled with significant differences between local and nonlocal specimens suggest that Eu. americanus exhibits a unique suite of morphological traits that decrease costs associated with long‐distance flight. Finally, compared with the ostensibly nonmigratory A. obliqua, Eu. americanus was less cold tolerant, a factor that may be associated with migratory behavior. Collectively, our findings imply that fall migration occurs in Nearctic hover flies, but we consider the methodological limitations of our study in addition to potential ecological and economic consequences of these novel findings.

Insights into natal origins of migratory Nearctic hover flies (Diptera: Syrphidae): new evidence from stable isotope (δ2H) assignment analyses

“…Laboratory-derived isotopic calibration algorithms may differ from field conditions (Hobson et al 1999), so exploring these effects would be a useful avenue of future research. Interspecific differences in tissue δ²H may also be a useful avenue of further research, as it is interesting that δ²H values of A. obliqua were higher than those of E. americanus (Table 2), a pattern also apparent in a previous study (Clem et al 2022a). We suspect this difference may be related to body size given that A. obliqua is generally smaller than E. americanus (Clem et al 2022a), or there may be unknown differences in feeding behavior.…”

“…Interspecific differences in tissue δ²H may also be a useful avenue of further research, as it is interesting that δ²H values of A. obliqua were higher than those of E. americanus (Table 2), a pattern also apparent in a previous study (Clem et al 2022a). We suspect this difference may be related to body size given that A. obliqua is generally smaller than E. americanus (Clem et al 2022a), or there may be unknown differences in feeding behavior. Intriguingly, the calibration equation for E. americanus is remarkably similar to one developed for the similarly sized Episyrphus balteatus (Raymond et al 2014).…”

“…In a previous study, the migratory capacity of four common Syrphinae hover fly species in the midwestern United States was examined using, among other methods, stable hydrogen isotope analysis (δ 2 H) (Clem et al 2022a). Strong continental patterns in amount-weighted mean annual or growing season average δ 2 H in precipitation in North America exist, and can be visualized using isoscapes (Bowen and West 2019).…”

“…Here, we used δ 2 H from fixed (metabolically inactive) tissues to estimate natal origins of E. americanus and A. obliqua specimens captured in the southeastern United States during autumn. Our hypothesis was that because some E. americanus in the Midwest are apparently southward-moving autumn migrants (Clem et al 2022a), several individuals from the southeastern United States will have originated at a much more northern latitude. Furthermore, because A. obliqua are apparently less migratory, these specimens were expected to be assigned local origins.…”

“…In the Palearctic, hover fly species like Episyrphus balteatus exhibit partial seasonal migration where some individuals within the population overwinter locally while others migrate south in response to encroaching winter conditions (Aubert et al 1976, Menz et al 2019b, Wotton et al 2019, Jia et al 2022), followed by a multi‐generational return migration in the spring (Hondelmann et al 2005, Wotton et al 2019). Migration among Nearctic taxa, however, has received considerably less attention and until recently was practically unrecognized (but see Shannon 1926, Menz et al 2019a, Clem et al 2022a).…”