Prevalence and drivers of a tree-killing bark beetle, Ips typographus (Coleoptera, Scolytinae), in international invasion pathways into the USA

The unintentional transport of insects beyond their native ranges has greatly increased with globalization over the past century, leading to higher propagule pressure in non-native ranges of many species. Knowledge about the prevalence of a species in international invasion pathways is important for predicting invasions and taking appropriate biosecurity measures. We investigated the spatiotemporal patterns and drivers of interceptions—detections of at least one individual with imported goods that potentially serve as a proxy for arrival rates—for a tree-killing bark beetle, the European spruce bark beetle (Ips typographus L.; Coleoptera: Curculionidae: Scolytinae), in the USA from 1914 to 2008. Across the study period, there were 505 interceptions of I. typographus with shipments originating from > 25 countries at ports in 22 US states. Interceptions first occurred in 1938, peaked at 33 and 25 in 1984 and 1996, respectively, and declined after the mid-1990s. Interceptions of I. typographus did not have a statistically detectable relationship with outbreak levels in the native range, were inversely related to annual import volume (an artifact likely driven by changes in inspection policies), and were more frequent during the winter. Thus, while interceptions of I. typographus are challenging to predict, we found evidence that (i) biosecurity practices against this beetle could be increased during winter but not in response to outbreaks in source regions and (ii) the overall abundance of this beetle in invasion pathways has recently decreased, probably because strengthened phytosanitary protocols have reduced contamination levels and/or decreased the perceived need for inspections.