From the excellent 1,000 Misspent Hours:
Ants are truly amazing creatures. An individual ant, so far as anyone can determine, is entirely mindless, yet the complexity of their societies rivals anything that we humans have devised. Ants are master builders. They cultivate food crops. They raise livestock. They wage war and practice slavery. They are able to communicate elaborate instructions to one another through a combination of gestures and scent chemicals. In short, nearly every practical feature of human societies finds its analog in that of at least one species of ant. But whereas our social achievements are mainly the product of culture, consciously or unconsciously taught by one generation to the next, ants do it all by instinct alone. It is this awesome fact, I believe, that has led so many authors and filmmakers over the years to speculate about what might happen if some environmental change were to occur so as to put humans and ants into direct competition. Most of the time, this means little more than making the ants big enough to pose a direct threat to human life, as in Them!. Occasionally, though, someone will try the more thoughtful approach of leaving the ants at their natural size and giving them intelligence instead. Phase IV, one of my longtime favorite 70’s sci-fi movies, is probably the best example of this latter strain. Those with short attention spans will find it rough going, but anyone who enjoyed The Andromeda Strain or Colossus: The Forbin Project owes it to themselves to have a look ....
- and here's (naturally) the pages for it on Wikipedia and the IMDB
... A lot of people lump Phase IV in with the Mother Nature’s Revenge movies that were being made at about the same time, but I don't really think that's quite appropriate. There’s no revenge going on here, nor anything that would allow a person to say that we brought it all on ourselves— hell, there isn’t even any toxic waste lying around for the ants to eat! What we’re dealing with here is just the emergence of a new species capable of out-competing us in the great struggle for survival. For that very reason, Phase IV seems like much more serious and intelligent a movie than Frogs or Prophecy. The absence of much in the way of showy special effects is another big point in this movie’s favor, indicating as it does director Saul Bass’s confidence that the screenplay he was working from was strong enough to stand scrutiny without such things to distract audience attention. Or at any rate, that’s how I score it; it’s also a big part of the reason the short attention span crowd are going to be squirming uncomfortably in their seats before the first hour has passed. If you’re asking me, though (and if you aren’t, then why the fuck are you still reading this?), Phase IV gets along just fine with barely any action. The entire point here, after all, is that the challenge posed by the intelligent ants is one that can’t really be met with firepower. This isn’t war we're talking about, but evolutionary change. Modern man is so far removed from the days in which he existed at the same level as the rest of the biosphere that most of us have never stopped to consider what it would be like if we had meaningful ecological competition. By positing a rival for humanity that is too small to be hunted, too adaptable to be poisoned out of existence, and at least potentially too ubiquitous to be quarantined, Phase IV forces just such a consideration. Meanwhile, the fact that it ends before the two species have come to grips with each other on a large scale leaves open all of the questions that it raises. We never get more than the vaguest hints as to what the ants’ real agenda might be regarding humankind, nor is any conclusive answer forthcoming to the question of whether meaningful communication is possible between them and us. Though the ants are apparently comparable to humans in terms of intelligence, the vast social and biological differences between the species might preclude communication on any but the most concrete and tangible subjects. It isn’t even clear whether all the ants in the changed colonies are intelligent, or whether sapient queens are directing swarms of workers that are just as mindless as ever (although there are a few vague clues suggesting that the ants may have developed a new caste in their society that is as specialized for thinking as the soldier caste is for fighting). A lot of filmmakers seem to have lost track of this in recent decades, but there’s a reason why its thinner-skinned fans prefer to call science fiction by the rather more pretentious name of “speculative fiction”— the shit’s supposed to make you think! Phase IV does, and that’s a hell of a lot more than you can say for most movies about killer bugs.