In October, the 12th Air Commando Squadron, on Ranch Hand defoliation and crop destruction missions in South Vietnam, assumed a secondary mission of destroying malaria-carrying mosquitoes with insecticide. The first UC–123 aircraft diverted for the new mission, nicknamed Patches, was stripped of its camouflage paint and coated with an alodine compound to retard the corrosive effects of the poison. On October 17, it began malaria-control spraying in South Vietnam in an operation called Fly Swatter. Patches originally flew from Tan Son Nhut, but in 1967 it moved to Bien Hoa AB, where the 12th Air Commando Squadron was based.
Insecticide spraying differed greatly from herbicide spraying. Each mosquito control mission covered more territory because the aircraft delivered only about eight ounces of insecticide per acre, covering 15,000 acres in one sortie. Insecticide spraying also required less precise navigation than defoliation missions.
By mid-1967, the 12th Air Commando Squadron was flying 20 insecticide sorties per month. It delivered 118,985 gallons of insecticide over South Vietnam in 1967 and 111,998 gallons in 1968. The squadron devoted only one of its 19 spray aircraft for the mosquito-control mission.
Questions about the safety, practicality, and morality of herbicide spraying in South Vietnam convinced Ranch Hand administrators to decrease defoliation operations. By July 1970, when the 310th Tactical Airlift Squadron took over spray operations from the inactivated 12th Air Commando Squadron, insecticide sorties outnumbered herbicide sorties. The 310th Tactical Airlift Squadron used two UC–123s against malaria, regularly spraying 15 sites across South Vietnam from its home base in Phan Rang.
The insecticide operation cost several lives. On February 11, 1971, an insecticide plane crashed after going out of control near Phan Rang AB, killing five crew members. On June 1, the second plane suffered minor damage after it was hit with small arms fire.
The 310th Tactical Airlift Squadron continued insecticide spraying in South Vietnam after herbicide missions ended in January 1971. In June 1972, the squadron stopped flying the UC–123 spray aircraft, which were transferred to the South Vietnamese air force as part of President Nixon’s Vietnamization program. Operation Fly Swatter helped to lower disease rates in populated areas, benefiting both sides of the battlefield.