Naturally occurring movement of species into the United States is uncommon. Most invasive species arrive in association with human activities or transport. Species can be brought into the country and released intentionally, or their movement and release can be an unintentional byproduct of cultivation, commerce, tourism, or travel. Many species enter the United States each year as contaminants of commodities. Agricultural produce, nursery stock, cut flowers, and timber can harbor insects, plants pathogens, slugs, and snails.
Weeds continue to enter the United States as seed contaminants.
Plant pathogens sometimes arrive as unintended contaminants of plant materials. Fish and shrimp pathogens and parasites have been introduced into the United States on infected stock for aquaculture. Crates and containers can harbor snails, slugs, mollusks, beetles, and microorganisms. Military cargo transport also brings in harmful species, such as the Asian gypsy moth and brown tree snakes. Ballast water that is released from ships as cargo is loaded or unloaded has brought in several destructive aquatic species.