Human structures may be occupied, at least temporarily, by insects and other arthropods. Some are perennial pests, while others occasionally invade structures. A field study in 2008 2009 determined that occasional invader species in natural and artificially-created harborages adjacent to buildings were primarily isopods (Malacostraca: Isopoda) (37%), spiders (Arachnida: Araneae) (28%), earwigs (Insecta: Dermaptera) (13%), and adult crickets (Insecta: Orthoptera) (8%). Pitfall trapping demonstrated seasonal differences in activity of occasional invaders with the number of harborages observed. In laboratory assays, five plant-extracted essential oils (spearmint, peppermint, wintergreen, cinnamon and clove) were repellent to Argentine ant, Linepithema humile (Mayr), workers at concentrations from 0.10 to 10% (v/v) at 0 and 7 days after application. Essential oils are viable alternatives to conventional chemical repellents of the Argentine ant and occasional invader species.