When most people think of Ants, they think of them as pests. There are around 1000 different ant species here in the US (more than 13,000 species worldwide) but only around 25 of these species are pests. So why do they get such a bad reputation? My guess is because the ones that are pests tend to invade peoples homes and no one likes to share their kitchen with ants (not even an entomologist).
The Argentine Ants are one of the species of ants that are considered pests. They are commonly found all throughout California and live in something called a super colony. While most people are concerned with just getting this ant out of their kitchen, what most people don't know is that this little home invader has an interesting tale of global domination behind it.
Argentine ants are not good neighbors. When they meet ants from other colonies, even colonies of the same species, they fight to the death, and tear the other ants to pieces. While other kinds of ants are usually indifferent to other colonies (or sometimes fight like two puppies), the Argentine ants do not fool around. If you’re not part of the colony, you’re dead.
This aggressive behavior comes from the harsh environment of their home turf: the flood plains of northern Argentina. Frequent flooding there results in these ants having to flee their territory often, then once the flooding subsides, the ants all battle each other over new territory. This results in creating one mean territorial ant. This behavior is the driving reason behind why these Ants now dominate the planet (and your kitchen). In the early 1900's one of these ants made their way over to California and have been wrecking havoc ever since. They are much more aggressive than the endemic California Ants and have pushed many of these ants out of parts of California. This is a situation we find world wide from Europe to Australia. In fact, they are on every continent except Antarctica. And the this behavior is also the reason you always find them in your home right around a rainy season. They are experts at finding safe places to hide during rains. So the conclusion is they come to hide from the rain and stay for your food.
More info about the story of the Argentine Ant can be found here. Listen to this great podcast where they explore the story of the Argentine ants and talk to researchers studying this phenomena.