Avoiding invasives: trade-related policies for controlling unintentional exotic species introductions

International commodity trade serves as the primary conduit for unintentional introductions of damaging exotic species. We use a simple model of contaminated goods trade to analyze the optimal mix of tariffs and inspections as means of controlling damage from this negative externality. Among other policy-relevant results, we find that (1) while it is always optimal to employ tariffs, there are non-trivial cases in which inspections should optimally be set to zero, (2) a higher infection rate requires a higher tariff, but beyond a point optimal inspections decrease in the infection rate, and finally (3) taking a dynamic view and considering future effects of current introductions leads unambiguously to more stringent inspections, but may give rise to higher or lower tariffs.