Behind Laos's yellow rain and tears

A controversial Radiolab episode opens old wounds and raises countless questions for Minnesota's Hmong

Seeley told Meselson how, after hibernating, North American bees take "cleansing flights," where they swarm from the hive and defecate en masse. But Seeley wasn't sure if Southeast Asian honeybees did the same thing. So in March 1984, Meselson and Seeley took a trip to Thailand to try to investigate the phenomenon.

Fred, the public health guy from Ban Vinai, joined them for part of the trip as a guide and translator. One day, he remembers, the team spread out sheets of paper in concentric circles around a hive in Khao Yai National Park, a forest north of Bangkok.

"Sure enough, in the morning, the bees pooped en masse, and I watched it land on the sheets," Fred says. "I stood there and watched yellow rain."

Eng Yang and his niece, Kao Kalia Yang, at St. Paul's Hmongtown Marketplace
Mark N. Kartarik
Eng Yang and his niece, Kao Kalia Yang, at St. Paul's Hmongtown Marketplace
This Hmong boy reported that his stomach rash was a result of yellow rain
courtesy of Fred
This Hmong boy reported that his stomach rash was a result of yellow rain

But then he catches himself. "At least what they were saying was yellow rain."

Meselson recalls two other incidents during the trip when the team got caught in feces showers. During one, they were driving when a hive in front of them "kind of changed color," Meselson says. Seeley had his bee gear on and watched, but Meselson took cover in the car.

"We could actually hear these spots hitting the metal roof of the Land Rover," Meselson says.

The scientists had proven that Southeast Asian bees also take cleansing flights. They didn't think the bees were pooping out toxins, though. They thought that Mirocha had been wrong.

Meselson speculated that Mirocha could have easily contaminated the samples. His lab at the U trafficked in high quantities of these same toxins, and he was testing for tiny amounts of it.

"To do that work you have to be sure that your lab is very, very clean," Meselson says. "And preferably that it's a lab that has never seen those very same substances."

As yellow rain became an increasingly bigger issue, more and more labs around the world began testing samples. And none of them found any poison.

Other pieces of the case had started breaking down too. Deeper analyses of the pollen showed that it was from plants native to Southeast Asia, and from a wide variety of them — meaning that it was almost certainly natural in origin.

As Meselson looked into it more, he discovered that a similar scenario had unfolded before, in a province in China. In 1974, the area had suffered a large earthquake, and in its wake, Chinese villagers reported mysterious yellow spots, which they thought were poisonous. But when a professor took samples of these spots, he discovered that they were full of pollen.

"They were frightened by this and simply didn't recognize what it was, even though they had lived there for generations," Meselson says. "If you have a population under stress, you can understand how they can attribute their illness to something like this happening that they don't understand."

Meselson believes that's what happened to the Hmong.

"There's not a single shred of objective evidence," Meselson says today. "Deep inside the United States government I think they knew it was a mistake, but unfortunately they have never admitted it."


Mirocha knows Meselson and the honeybee theory well. But he disagrees that it debunks the stories of toxic yellow rain.

"The honeybee fecal matter explanation is a red herring," Mirocha says. "If the Hmong people were told that they were not really the victims of chemical warfare, then that is nonsense."

Mirocha stands by his initial findings, and insists that his lab procedures were excellent. Two years after the first analysis, his lab participated in a trichothecene testing trial, and was one of the few labs involved in the experiment that produced fully accurate results.

Furthermore, recently declassified government data confirms that all of the environmental controls Mirocha tested came back negative — in other words, he didn't find false positives in the toxin-free samples.

Mirocha's analyses and methodology had been "put through the wringer" and come out intact, said Gary Crocker, the State Department's senior intelligence officer at the time, in a 1991 New Yorker article on yellow rain. Crocker added, "One thing I can say about Dr. Mirocha is that he is not involved in politics. He's a pure scientist."

Crocker also readily admitted that "an awful lot of our environmental samples were nothing but bee spots," which is in fact what Meselson showed.

But not all of them were. Recent research by professors from Princeton and George Washington University, who concluded that chemical weapons were likely used against the Hmong, found that some of the government's environmental samples didn't contain any pollen — meaning they weren't bee droppings.

Mirocha sees an additional oversight. Meselson and Seeley "did very good work" with their honeybee research, he says. "That article came out as the first account of mass honeybee defecation."

Then Mirocha shakes his head. "But the natives had known this for hundreds of years!"

From Mirocha's perspective, this lack of consideration of local knowledge was just one way the original controversy was unfair to the Hmong.

"I believed what was going on over there. It would be stupid for me not to, because it's just a no-brainer," Mirocha says. "People are being killed. People are dying. And they're not lying."

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