Where I grew up, they were called “Foggers” and they sprayed some nasty chemical that ostensibly was effective against mosquitos. They came through every two weeks in the summer months. (which always stuck me as odd because the life cycle of a mosquito is far less than two weeks). I don’t remember them being particularly effective- I still got mosquito bites all the time. Then (probably late 1960s) they stopped using them. I guess either there was no money or they were so ineffective that there was no reason to do it any more. I never rode behind the fogger truck – seemed stupid.