Linepithema expert Alex Wild posted a succinct but thorough guide on how to identify the important Argentine ant:
The Argentine Ant (Linepithema humile), a small brown ant about 2-3mm long, is one of the world’s most damaging insects. This pernicious ant is spreading to warmer regions around the world from its natal habitat along South America’s Paraná River. Linepithema humile can drive native arthropods to extinction, instigating changes that ripple through ecosystems. In California, horned lizard populations plummet. In South Africa, plant reproduction is disrupted. Worldwide, the Argentine ant is a persistent house and crop pest. This is not a good ant.