Them! (1954)

"Them! is a 1954 American black-and-white science fiction monster movie from Warner Bros. Pictures, produced by David Weisbart, directed by Gordon Douglas, that stars James Whitmore, Edmund Gwenn, Joan Weldon and James Arness.[2] The film is based on an original story treatment by George Worthing Yates, which was then developed into a screenplay by Ted Sherdeman and Russell Hughes.

Them! is one of the first of the 1950s "nuclear monster" films, and the first "big bug" feature.

A nest of gigantic irradiated ants is discovered in the New Mexico desert; they quickly become a national threat when it is discovered that two young queen ants and their consorts have escaped to establish new nests. The national search that follows finally culminates in a battle with Them in the concrete spillways and sewers of Los Angeles."
https://en.wikipedia.org/wiki/Them!