Its natural, Jim. From an obituary of Julian Perry Robinson in Nature
In 1981, the US government publicly accused Soviet-backed forces in southeast Asia of waging toxin warfare and violating their legal obligations under the 1925 Geneva Protocol and 1972 Biological Weapons Convention. It alleged that aircraft dispersed ‘yellow rain’ containing mycotoxins that were “not indigenous to the region”. Julian Perry Robinson, working alongside biologist Matthew Meselson at Harvard University in Cambridge, Massachusetts, established that what actually fell was wild-honeybee faeces containing naturally occurring toxins. He died on 22 April, aged 78.