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Killer magnolias

by William (Ned) Friedman, Director of the Arnold Arboretum
July 20, 2018


Killer magnolias

Everyone has heard of killer bees. But what about killer magnolias? That kill bees? Such is the case with the wonderful bigleaf magnolia (Magnolia macrophylla). For the past few years, I have been tracking this phenomenon at the Arnold Arboretum in the specimens growing amidst the hickory collection. So, how do I know that bigleaf magnolias kill bees?


Every June, I have a look inside the huge flowers (almost a foot in diameter when the tepals are fully reflexed). Lo and behold, there are frequently bees either in a state of stupefaction, flying erratically, barely moving, or not moving at all. Look closely at the left image, a seemingly perfect picture of natural domesticity. A honey bee (blue arrow) is just about to alight on the reproductive parts of the flower to collect and be covered in pollen. Now look closely at the lower tepal (petal-like structure) and you will see a very dead bee (yellow arrow) surrounded by stamens (pollen producing organs) that have abscised. The bottom right image is a close up of this scene, with ants (alive) going about their business. In the upper right image, a scene of carnage might not be obvious in the still life of a photograph, but the bee at the top of the cone-like female parts of the flower is paralyzed or dead, as are the two bees at left, and the bee at right (yellow dots). The bee on the bottom is very much alive, for now (blue dot). The appropriately named long-horned beetle (green dot) is moving along just fine.

How to make sense of a flowering plant that kills pollinating visitors? First, bees are probably not a major pollinator of bigleaf magnolias. I have never seen other kinds of pollinating insects killed by bigleaf magnolias – only bees. So, it is unlikely that these trees are killing their partners in reproduction (such as beetles). What we do know, so far, is that during the first phase of flowering, the female parts are covered by a liquid secretion for a matter of hours (this fluid helps the pollen germinate). The dead bees are always wet, and this secretion seems likely to be the toxic potion. Stay tuned. We have collected this secretion and a chemical analysis will soon reveal the key to the killer bigleaf magnolias (which is the only species of magnolia that appears to kill bees).

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