The rapid spread of Leptoglossus occidentalis in Europe: A bridgehead invasion

Retracing the routes of invasions and determining the origins of invading species is often critical in understanding biological invasions. The Western conifer seed bug, Leptoglossus occidentalis, an insect native of western North America, was first accidentally introduced to eastern North America and then to Europe. The colonization of the entire European continent occurred in ca. 10–15 years, probably promoted by independent introductions in different parts of Europe. A multi-marker approach (mtDNA and microsatellites) combined with approximate Bayesian computation analyses was used to track the origin of European populations and to determine whether this rapid invasion was caused by multiple introductions. Our results show that at least two independent introductions of L. occidentalis have occurred in Europe. Moreover, the analyses showed a stronger genetic similarity of European invasive populations with the eastern North American populations than with those of the native range, suggesting that invasive North American population acted as a bridgehead for European invasion. The results also revealed that natural dispersal as well as human-mediated transportations as hitchhikers probably enhanced the rapid spread of this invasive pest across Europe. This study illustrates the complexity of a rapid invasion and confirms that bridgehead and multiple introductions have serious implications for the success of invasion.