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Hiking in the Sierra Nevada foothills, I saw the tree first, then its flowers, and then the pile of dead bees beneath the tree. The tree was a California buckeye, Aesculus californica, and the flowers had killed the bees.
Buckeye is a small group of North American trees. In Europe, related trees are known in Europe as horsechestnut, in the genus Aesculus, family Hippocastanaceae. The family name literally means horse chestnut (hippos – horse, castanea – chestnut). Trees in the genus Aesculus are sometimes called chestnut, causing confusion with ‘real’ chestnuts in the genus Castanea
What sets these trees apart from most temperate trees is that all parts of the plant are extremely toxic.
Honeybees are not native to North America, and when they get nectar from California buckeye, they die. Sometimes they die so quickly that they fall dead under the tree, as I observed that day . I have seen a pile of dead bees under a buckeye only one other time, under a horsechestnut in Rochester, NY.
As far as I know, our eastern buckeyes are not fatal to honeybees. However, all the other parts of the tree are filled with saponins, soap-like chemicals that are quite toxic. American Indians ate the nutritious seeds of horsechestnuts, but only after soaking the cracked seeds in water to leach out the toxins.
Indians also had another use for buckeye seeds. They would dam up a section of creek and cast broken fruits – seeds and husks – on the water surface. Fish, poisoned by the saponins, would float to the surface, dead or stunned. Cooking the fish made them safe to eat.
Buckeyes can be a problem for livestock. Cattle and horses will eat the fruits and seeds that fall into a pasture. Horses will get a stomach ache and vomit up the buckeyes, usually without further illness. Cattle are more likely to show neurological symptoms like muscle twitching, and may slip into a coma and die.
Buckeyes and horsechestnuts are gorgeous trees. In Kentucky, where I live, yellow and Ohio buckeye are abundant trees all over the state. They are often found in the understory of forests, leafing out very early and flowering beautifully. By midsummer, they seem to blend into the forest and may not be noticed as easily.