Visible crew/equipment: When the state policemen meet for the first time, the shadow of some equipment is seen on the wall behind them. It falls and you can even hear it. (00:17:30)
Visible crew/equipment: When they find the giant nest in the desert, several parked cars and trucks are visible in the background. It is supposed to be an isolated place. (00:34:00)
Visible crew/equipment: When the policemen leaves with the girl, a crewmember's reflection is visible on their car. (00:04:15)
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.