Tracking invasions of a destructive defoliator, the gypsy moth (Erebidae: Lymantria dispar): population structure, origin of intercepted specimens, and Asian introgression into North America

Genetic data can help elucidate the dynamics of biological invasions,
which are fueled by the constant expansion of international trade. The
introduction of European gypsy moth (Lymantria dispar dispar) into North
America is a classic example of human-aided invasion that has caused
tremendous damage to North American temperate forests. Recently, the even
more destructive Asian gypsy moth (mainly L. d. asiatica and L. d.
japonica) has been intercepted in North America, mostly transported by
cargo ships. To track invasion pathways, we developed a diagnostic panel
of 60 DNA loci (55 nuclear and 5 mitochondrial) to characterize worldwide
genetic differentiation within L. dispar and its sister species L.
umbrosa. Hierarchical analyses supported strong differentiation and
recovered five geographic groups that correspond to 1) North America, 2)
Europe plus North Africa and Middle East, 3) the Urals, Central Asia, and
Russian Siberia, 4) continental East Asia, and 5) the Japanese islands.
Interestingly, L. umbrosa was grouped with L. d. japonica, and the
introduced North American population exhibits remarkable distinctiveness
from contemporary European counterparts. Each geographic group, with the
exception of North America, shows additional lower-level structures when
analyzed individually, which provided the basis for inference of the
origin of invasive specimens. Two assignment approaches consistently
identified a coastal area of continental East Asia as the major source for
Asian invasion during 2014–2015, with Japan being another source. By
analyzing simulation and laboratory crosses, we further provided evidence
for the occurrence of natural Asian-North American hybrids in the Pacific
Northwest, raising concerns for introgression of Asian alleles that may
accelerate range expansion of gypsy moth in North America. Our study
demonstrates how genetic data contribute to bio-surveillance of invasive
species with results that can inform regulatory management and reduce the
frequency of trade-associated invasions.