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

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.

入侵昆虫物种的全球扩散已造成极为严重的生态破坏与经济损失。研发可溯源入侵昆虫起源的可靠方法，对昆虫学家、植物病理学家及害虫防控从业者均具有重要应用价值。舞毒蛾（Lymantria dispar，简称L.d.，Linnaeus 1758）是美国东北部持续成灾的入侵性害虫，定期在温带森林引发大规模落叶灾害。我们对采自欧洲、亚洲及美国的野外捕获样本与实验室饲养的L.d.标本进行分析，检测其生物组织中留存的出生地氢、氮同位素特征（δ²H与δ¹⁵N），并将这些数值与捕获点区域降水的长期平均δ²H值（取自全球降水同位素观测网，Global Network of Isotopes in Precipitation）以及区域植物的δ¹⁵N值进行比对。我们采用比较平衡法与双源混合模型，测定了L.d.组织的氢-氘交换百分比（Pex=8.2%），借此可推算出该蛾类的出生地δ²H值。我们证实，样本的出生地δ²H与δ¹⁵N值与其地理起源地的环境同位素特征存在相关性。借助所构建的回归模型，我们能够甄别出潜在的入侵个体，并对其地理起源进行推断。为了将该方法推广应用于虫卵样本，我们测定了L.d.从虫卵到成虫的同位素分馏系数（Δegg-adult = 16.3‰ ± 4.3‰）。模型结果显示，全球范围内采获的舞毒蛾中，约25%并非调查捕获点的本土种群。东亚是被推断出的最常见的潜在起源地。此外，我们的数据表明，在美国阿拉斯加、加利福尼亚及路易斯安那州港口的货轮上发现的虫卵，其最可能的来源为俄罗斯远东地区的亚洲型舞毒蛾。本研究结果证实，稳定同位素生物标志物可独特揭示入侵昆虫的扩散路径，因此可作为监测害虫疫情扩散的有效工具。