The Pixar Theory

Bug's Life

That's right. The reason no humans show up in A Bug's Life is because there aren't a lot left. We know because of the cockroach that some of the insects survived, meaning they would have rebounded a bit faster, though the movie had to be far enough in the timeline for birds to have returned as well, though they're noticeably less intelligent than the bugs.

[I'll admit, the trees looking similar isn't enough to support the idea that A Bug's Life takes place after Wall-E, but there's definitely more reasons for why it's likely. Also, I'll bring the tree up again later because it appears in Up as well.]

There's something strikingly different about A Bug's Life when compared to other Pixar portrayals of animals, which leads me to believe it takes place in the future. Unlike Ratatouille, Up, and Finding Nemo, the bugs have many human activities similar to what the rats in Ratatouille were merely experimenting with.

The bugs have cities, bars, advertisements, their own machines, know what a bloody mary is and even have a traveling circus. This all assumes that the movie is in a different time period.

The other factor that sets A Bug's Life apart from other Pixar movies is the fact that it is the only one, besides Cars and Cars 2, that doesn't revolve (or even include) humans.

[Okay there is a a lot of contention over the idea that A Bug's Life takes place post-apocalypse, but hear me out. The reason I am so inclined to push the idea is because of how different the bug world is from the "animal" movies. No other Pixar movie has animals wearing clothing, wild inventions, animals creating machines, or so much human influence like bars and cities. In Finding Nemo, the most human thing we see is a school, and even that is pretty stripped down. But in A Bug's Life, we have a world where humans are barely even implied. At one point, one of the ants tells Flik not to leave the island because there are "snakes, birds, and bigger bugs out there." He doesn't even bring up humans. Yes, there are some humans, like the kid who allegedly picked the wings off of the homeless bug, but that still fits in a post Wall-E world. Also, the bugs have to be irradiated for them to live such long lifespans. The average lifespan of an ant is just 3 months, but these ants all survive an entire summer and allude to being around for quite some time by saying things like "this happens every year." One of the ants even says he "feels 90 again." That works if you accept that the ants are sturdier due to the extreme growth and mutated genes.]