Modeling spatial establishment patterns of exotic forest insects in urban areas in relation to tree cover and propagule pressure

Cities serve as gateways for the introduction of invasive exotic pests, and their role in this regard is only expected to grow as international trade increases. Monitoring efforts could be enhanced if urban hot spots for invasions (i.e., sites where establishment of pests is likely) could be identified. This study sought to better understand how propagule pressure (high likelihood that exotic species would be introduced in cargo; two indicators are population and commercial/industrial land use) and tree cover (which reflects the availability of host species) interact to create hot spots for invasion. First, a model featuring a hypothetical generalist insect and the propagule-pressure and tree-cover indicators was created to delimit establishment hot spots. Then the model was tested against actual detection patterns for Asian longhorned beetle (Anoplophora glabripennis) and gypsy moth (Lymantria dispar) in six U.S. cities. The two propagule pressure indicators (population and land use) were better predictors than tree cover. In the Los Angeles and New York areas, the land use indicator performed better at identifying actual hot spots than the population indicator. In Chicago, in contrast, there was no difference in the two propagule-pressure indicators. Limitations of the study and implications for monitoring and management are described.