page_title: Yellow rain
section_title: Honeybee hypothesis
hierarchical_section_title: Yellow rain / Explanation / Honeybee hypothesis
caption_attribution_description: English: Bee droppings on leaves, resembling "yellow rain"
context_page_description: Yellow rain was the subject of a 1981 political incident in which the United States Secretary of State Alexander Haig accused the Soviet Union of supplying T-2 mycotoxin to the Communist states in Vietnam, Laos and Cambodia for use in counterinsurgency warfare. Refugees described many different forms of attacks, including a sticky yellow liquid falling from planes or helicopters, which was dubbed "yellow rain". The U.S. government alleged that over ten thousand people had been killed in attacks using these chemical weapons.
  The Soviets denied these claims and an initial United Nations investigation was inconclusive. Some samples of the supposed chemical agent that were supplied to a group of independent scientists turned out to be honeybee feces, suggesting that the "yellow rain" was due to mass defecation of digested pollen grains from large swarms of bees. Other scientists questioned the accuracy of the refugee accounts and the reliability of the chemical analyses presented by the U.S. government.
  The majority of the scientific literature on this topic now regards the hypothesis that yellow rain was a Soviet chemical weapon as disproved.
context_section_description: In 1983, these charges were disputed by Harvard biologist and biological weapons opponent Matthew Meselson and his team, who traveled to Laos and conducted a separate investigation. Meselson's team noted that trichothecene mycotoxins occur naturally in the region and questioned the witness testimony. He suggested an alternate hypothesis that the yellow rain was the harmless fecal matter of honeybees. The Meselson team offered the following as evidence: separate "yellow rain drops" which occurred on the same leaf, and which were "accepted as authentic", consisted largely of pollen; each drop contained a different mix of pollen grains, as one would expect if they came from different bees, and the grains showed properties characteristic of pollen digested by bees (the protein inside the pollen grain was gone, while the outer indigestible shell remained). Further, the pollen mix came from plant species typical of the area where a drop was collected.
  The US government responded to these findings by arguing that the pollen was added deliberately, in order to make a substance that could be easily inhaled and "ensure the retention of toxins in the human body". Meselson responded to this idea by stating that it was rather far-fetched to imagine that somebody would produce a chemical weapon by "gathering pollen predigested by honeybees." The fact that the pollen originated in Southeast Asia meant that the Soviet Union could not have manufactured the substance domestically, and would have had to import tons of pollen from Vietnam.  Meselson's work was described in an independent medical review as providing "compelling evidence that yellow rain might have a benign natural explanation".
  After the honeybee hypothesis was made public, a literature search turned up an earlier Chinese paper on the phenomenon of yellow droppings in Jiangsu Province in September 1976.  Strikingly, the Chinese villagers had also used the term "yellow rain" to describe this phenomenon.  Many villagers believed that the yellow droppings were portents of imminent earthquake activity.  Others believed that the droppings were chemical weapons sprayed by the Soviet Union or Taiwan.  However, the Chinese scientists also concluded that the droppings came from bees.
caption: Bee droppings resembling "yellow rain"
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