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Dryad

Tracking Global Invasion Pathways of the Spongy Moth (Lepidoptera: Erebidae) to the U.S. using Stable Isotopes as Endogenous Biomarkers

Abstract

The spread of invasive insect species causes enormous ecological damage and economic losses worldwide. A reliable method that tracks back an invaded insect’s origin would be of great use to entomologists, phytopathologists and pest managers. The spongy moth (Lymantria dispar (L.d.), Linnaeus 1758) is a persistent invasive pest in the north-eastern United States (U.S.) and periodically causes major defoliations in temperate forests. We analysed field-captured (Europe, Asia, U.S.) and lab-reared L.d. specimens for their natal isotopic hydrogen and nitrogen signatures imprinted in their biological tissues (δ2H and δ15N) and compared these values to the long-term mean δ2H of regional precipitation (Global Network of Isotopes in Precipitation) and δ15N of regional plants at the capture site. We established the percentage of hydrogen-deuterium exchange for L.d. tissue (Pex=8.2%) using the comparative equilibration method and two-source-mixing-models, which allowed the extraction of the moth’s natal δ2H value. We confirmed that the natal δ2H and δ15N values of our specimens are related to the environmental signatures at their geographic origins. With our regression models, we were able to isolate potentially invasive individuals and give estimations of their geographic origin. To enable the application of these methods on eggs, we established an egg-to-adult fraction factor for L.d. (Δegg-adult = 16.3‰ ± 4.3‰). Our models suggested that around 25% of the field-captured spongy moths worldwide were not native in the investigated capture sites. East Asia was the most frequently identified location of probable origin. Furthermore, our data suggested that eggs found on cargo ships in U.S. harbors in Alaska, California and Louisiana most probably originated from Asian L.d. in East Russia. These findings show that stable isotope biomarkers give a unique insight into invasive insect species pathways and thus, can be an effective tool to monitor the spread of insect pest epidemics.