From Scientific American:
In 1907 Argentine ants (Linepithema humile) arrived in Los Angeles via a cargo ship. Within just a few years of their arrival the six-legged stowaways formed a single, massive colony — known as a supercolony — that stretched through California from south of the Mexico border to San Francisco
A liberal spraying of pesticides in the past century has done nothing to diminish the ants' numbers — L. humile infestations are the most common cause of pest control calls in southern California. The Argentine ant's takeover of coastal California is marked by small shifts in the local, native ecosystem. Populations of the coast horned lizard (Phrynosoma coronatum) have declined sharply in recent years due to the displacement of native ants the lizard depends on for food. Citrus farmers have required increasing quantities of pesticides to cope with rising numbers of aphids, scale insects and other pests that the Argentines actively protect in exchange for the sweet honeydew they produce.