Continuity mistake: When James Whitmore is attacked by the ant in the Los Angeles sewer system, James Arness arrives, firing an M-1 Garand rifle. In the next scene, he's armed with a Thompson submachine gun.
Continuity mistake: When the girl wakes up in the ambulance, she has much less dirt on her forehead for one shot. It reappears as she lays down. (00:09:00)
Continuity mistake: When Patricia is pouring formic acid in the glass, the nurse's hands change position on the girl's chair. (00:23:35)
Continuity mistake: When they go to visit the trailer, Pat is surprised by an ant on a bluff. When they shoot it, it falls with it's head just over the edge and two bushes in front of it. They switch to a camera angle from behind the ant and now its head is further back and there is a large white rock in front of the ant's head.
Continuity mistake: When the police went into the trailer her did not have a pencil in his pocket, in the trailer bedroom he had a pencil in his shirt pocket to pick up the gun. Then in the next scene it is gone.
Continuity mistake: When the first officer is left alone at the store and hears the weird noise he shuts off the light. There is a different length of string than what was on it a second before.
Continuity mistake: When the two officers drive up to the first destroyed trailer the awning at the rear of the trailer is very visible. When the officer walks to the back of the trailer the awning is gone.
Continuity mistake: Near the beginning, the two cops leave the trailer to go to Gramps' store. When they leave, the shadows are low but there should be plenty of daylight left. They arrive at the store and it's pitch black out, but it was only a twelve mile drive.
Answer: Like the General said, the less people that know about the ants the better. They were a special task force assigned with the equipment and deputized to use any and all measures to stop the ants. Since the movies was set in the 1950's, James Arness and James Whitmores were most likely veterans of WW II and Korea.