Park records revealed that in the mid-1950s, biologists mixed DDT powder with diesel fuel and unloaded the combination from airplanes. Their target was the spruce budworm moth, a species whose larvae feed on spruce needles and kill the trees. Operating out of the Gardiner, Mont., airport, biologists sprayed 140,000 acres in 1953, 55,000 acres in 1955, and 80,000 acres in 1957, says park biologist Roy Renkin.
In 1957, tracer cards left out during the spraying showed that only .18 pounds per acre hit the ground, Renkin says, while .82 pounds of the chemical blew out of the area or into the Yellowstone River, where hundreds of dead fish drifted downstream following a 1955 spraying.
Scientists say the small amount of DDT detected in fish today is no longer dangerous, but they're also not surprised to find it.
"DDT is phenomenal," says Renkin. "Had foresters realized at the time how dangerous it was, I think they would have looked a little further into what they were doing."