Beginning of the End (1957) on Netflix

Reporter Audrey Ames is driving along a highway in Illinois when she is stopped by the military. She then finds out that a small town was destroyed and everyone has seemingly disappeared. She then goes to a lab run by the Department of Agriculture. While she is there she meets the lab's director, Dr. Ed Wainwright. Ed then tells her that strange things have been happening ever since he discovered that a bunch of grasshoppers managed to get into a silo containing a batch of radioactive wheat. They soon discover that the grasshoppers have grown to monstrous proportions and not only are devouring the local vegetation, but have developed a taste for human flesh as well. Now the locusts are marching towards Chicago and the military is threatening to destroy the city with the atom bomb. Can another solution to stop the monsters be found before it is too late?