The area infested by European L. dispar is confined to the northeastern United States and the eastern maritime provinces of Canada, with an advancing front slowly moving in a southwesterly direction (Epanchin‐Niell & Liebhold, 2015). Consequently, the moth has been able to gradually expand its distribution. Additionally, the moth faces no major natural enemies, competitors, or diseases in North America that would control populations like in their native habitat (Liebhold et al., 1995). In the United States, there are 400 different tree species European L. dispar larvae feed on (US Department of Agriculture et al., 1981). The European spongy moth Lymantria dispar dispar (Lepidoptera: Erebidae, Linnaeus 1758) (subsequently referred to European L. dispar and formerly known as the gypsy moth) is native to all of temperate Eurasia and northern Africa, and was brought to the United States between 18 near Boston, Massachusetts, from where it escaped and spread shortly after (Fernald & Forbush, 1896). On the other hand, stable isotope analysis (SIA), particularly of hydrogen, nitrogen, sulfur, and oxygen, is a useful tool for tracking an individual's recent whereabouts (Bowen & West, 2008 Hobson, 1999). While genetic analyses can yield insight into a population's geographic origin, it does not provide the nuanced data often required for effective management of a newly arrived invasive species, such as whether the individual was born on site (domestic/local) or transported in (exotic). Therefore, the determination of invasive insects' natal origins has been a topic of investigation for many years, but has largely focused on genetic analysis (Picq et al., 2018 Wu et al., 2015, 2020). The ability to identify the natal origin of a pest insect captured on non‐native territory would significantly support the implementation of pest management strategies. The global economic and ecological burden posed by invasive insect species is high, with costs rising to $70 billion dollars per year (Bradshaw et al., 2016).
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. Furthermore, our data suggested that eggs found on cargo ships in the United States harbors in Alaska, California, and Louisiana most probably originated from Asian L. dispar in East Russia. East Asia was the most frequently identified location of probable origin. Our models suggested that around 25% of the field‐captured spongy moths worldwide were not native in the investigated capture sites. To enable the application of these methods on eggs, we established an egg‐to‐adult fraction factor for L. dispar (Δegg‐adult = 16.3 ± 4.3‰). With our regression models, we were able to isolate potentially invasive individuals and give estimations of their geographic origin.
We confirmed that the natal δ 2H and δ 15N values of our specimens are related to the environmental signatures at their geographic origins. We established the percentage of hydrogen–deuterium exchange for L. dispar tissue (P ex = 8.2%) using the comparative equilibration method and two‐source mixing models, which allowed the extraction of the moth's natal δ 2H value. dispar specimens for their natal isotopic hydrogen and nitrogen signatures imprinted in their biological tissues (δ 2H and δ 15N) and compared these values to the long‐term mean δ 2H of regional precipitation (Global Network of Isotopes in Precipitation) and δ 15N of regional plants at the capture site. We analyzed field‐captured (Europe, Asia, United States) and laboratory‐reared L. The spongy moth ( Lymantria dispar, Linnaeus 1758) is a persistent invasive pest in the Northeastern United States and periodically causes major defoliations in temperate forests. A reliable method that tracks back an invaded insect's origin would be of great use to entomologists, phytopathologists, and pest managers. The spread of invasive insect species causes enormous ecological damage and economic losses worldwide.
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