Riders of the Whistling Pines (1949)

While tracking Forest Ranger Charles Carter, who is suspected of allowing lumberjack Henry Mitchell to harvest protected timber, Gene accidentally shoots and seemingly kills Carter while trying to fend off a menacing mountain lion. However, it turns out that Bill Wright, Mitchell's partner, is responsible for Carter's death, as the ranger had uncovered a tussock moth infestation that, if left unreported, would lead to the trees dying and benefiting Mitchell and Wright financially. To make amends, Gene sells his sportsman's camp and gives the proceeds to Carter's daughter, Helen. On his way to Texas, Gene encounters the infestation and is tasked by the Forest Department to oversee aerial spraying of DDT. After the first day of spraying, enraged ranchers blame the DDT for the death of numerous animals from poisoning.