In the late 1970s after the end of the Vietnam War, many Vietnamese and Laotian people began noticing that a sticky yellow liquid periodically rained down from otherwise sunny skies. Witnesses claimed the strange substance killed plants and sickened people.
One ethnic group seemed especially affected by the substance: the Hmong, who had fought with France against Communists in Southeast Asia since the 1950s in the sparsely developed mountains in northern Vietnam and Laos. The CIA later recruited and mobilized Hmong soldiers, making them the target of sectarian violence when U.S. troops left the region. The newly empowered Communist regimes attacked the remaining Hmong, forcing them to flee to refugee camps in Thailand or face labor and reeducation camps in their home countries.
After a lengthy investigation U.S. analysts concluded in 1981 that the substance, dubbed “yellow rain,” was a chemical weapon made of fungus toxins and created by the Soviet Union. The U.S. secretary of state at the time, Alexander Haig, Jr., announced that the Soviet Union gave the weapon to the Viet Cong and Pathet Lao, who dropped it on the Hmong (and other ethnic groups) in their villages and while they fled to refugee camps.
If the Soviet Union was really stockpiling and distributing chemical and biological weapons, it would have been breaking a century of international laws and treaties. The Soviets denied the accusations vehemently and soon found an unlikely ally: Matt Meselson, a Harvard University biologist.
Meselson was skeptical of the claims made by Haig and in 1983 acquired samples of yellow rain from government officials in order to analyze them at Cambridge. Meselson found that the substance included large amounts of hollowed-out pollen indigenous to Southeast Asia. This alone made Haig’s claims seem dubious: pollen would be an extremely ineffective dispersal method for poison. Meselson’s team realized the pollen was the same pollen eaten by giant Asian honey bees, which digest the protein inside pollen grains but not the outer shell, hollowing it out. Furthermore the team found that the concentration of mycotoxins was not significantly greater in samples of yellow rain–covered leaves than on plants anywhere else in Southeast Asia.
If U.S. officials were correct, the Soviet Union would be importing tons of predigested pollen from Southeast Asia, only to turn around and send that same pollen back as a difficult-to-disperse chemical weapon. Meselson proposed a simpler explanation: yellow rain was no weapon at all—it was harmless bee feces. The health problems being reported were the result of poor sanitation and lack of food among people being bombed and raided by Communist soldiers. It’s still a sore subject among the Hmong, but Meselson theorized that the connection between yellow rain and sickness was part hyperbole, part imagination.
After reviewing the original study done by U.S. investigators, Meselson’s team found inconsistencies and contradictions in the testimony of people who witnessed yellow rain. The liquid sometimes rained down when there wasn’t a plane in sight. Most of the health problems could be explained by dysentery and nutritional deficiencies in refugee camps. It also became clear that the original interviewers had asked leading questions and unconsciously manipulated testimonies.
Meselson’s findings were initially met with skepticism by other scientists and the U.S. government. It would take thousands of bees to make enough waste to look like rain. Why would so many bees be in such specific locations and why would they be pooping at the same time?
In 1989 a Canadian biologist teamed up with a Malaysian scientist to solve the puzzle. The most revealing detail of yellow rain was when it tended to fall: hot, sunny days. The scientists measured the body mass of hundreds of bees before they left their hives and after they came back, finding that the insects lost 20% of their weight on the return flight. The bees would usually leave together in a giant swarm, defecate, and come back to care for their larvae. The scientists confirmed that the phenomenon occurred most frequently on hot days.
Why were they excreting more on hot days? Asian honey bee larvae are sensitive to high temperatures and become deformed if they overheat. The adult bees needed to reduce their mass so they could keep their larvae cool inside the hive. This and later research led to the general acceptance of Meselson’s explanation, although the U.S. government never retracted the original accusations against the Soviet Union.
By Jacob Roberts
Image: A 1678 plate depicting a beekeeper’s hive from Vinetum britannicum by John Worlidge. (Othmer Library)