From the ‘in case you wondered too’ file: The bumblebee die-off

(Photos by Kersti Muul)

We’ve received multiple inquiries about dead/dying bumblebees, particularly along California SW from The Junction to Morgan Junction. One of the people who noticed was local naturalist (and more) Kersti Muul, who provided the photos. We investigated a similar phenomenon a couple years ago, and recalled it wasn’t a case of spraying – as most assume when they happen onto the bees – but was traced to a particular kind of tree. Kersti, also an arborist, subsequently noted the trees in question are lindens:

Other parts of the nation/world have noted this phenomenon. Last fall, the New York Times wrote about researchers looking into it; they hadn’t entirely solved the mystery.

Not all bees are susceptible to this, as noted in this one-sheet from Oregon. The dead bees Kersti saw/collected are almost all yellow-faced bumblebees, of the species Bombus vosnesenskii. That Oregon document also reminds us that lindens are the trees that draw so many aphids, their secretions drip onto cars parked beneath them (it’s not the sap making those little spots, it’s the bugs). But you don’t want to spray those trees to kill the aphids – because that will kill other insects, such as bees, too.

24 Replies to "From the 'in case you wondered too' file: The bumblebee die-off"

  • newnative July 2, 2018 (4:17 pm)

    Ladybugs eat aphids. I was surprised to see in Indiana that they sold ladybugs by the gallon.

    • Kersti E. Muul July 2, 2018 (5:28 pm)

      They sell them here at true value!

  • KM July 2, 2018 (4:45 pm)

    Kersti, you are always a great resource about thing going on in our natural world. I was beginning to wonder what was going on as I found a few casualties in my yard yesterday and on my walk today.

    • Kersti E. Muul July 2, 2018 (5:31 pm)

      Thank you KM…. Awesome initials too ha ha.I am curious about everything around me, and this one was particularly sad. I never noticed before but I recently moved and these trees are now on my daily path.I’ve literally seen about 100 dead bees this weekend.

    • heartless July 2, 2018 (6:02 pm)

      Yes, I’d like to second the thanks.We were walking along 44th today and saw 4 or 5 dead on the sidewalk–an unusual enough number that I was curious.Lo and behold ye olde interwebs deliver yet again.  The information is much appreciated, thank you WSB and Kersti.

  • Joan July 2, 2018 (5:14 pm)

    I have found a couple small dead bumbles in my yard, but I usually do find some every year, so it didn’t seem  unusual. I don’t have any lindens in  my yard. I assumed they just reach the end of their days.

    • Kersti E. Muul July 2, 2018 (9:10 pm)

      Could be completely unrelated. Some males die after mating, as well as other possibilities; age etc.I have seen dead ones in the surrounding areas… several blocks away. Not sure how immobilizing it is initially but some get away from the trees and die elsewhere.

  • Soup Ninja July 2, 2018 (6:09 pm)

    I have been wondering about this for years. I didn’t notice the connection between the types of trees and where I’ve been seeing the dead bees on my walks . Thanks for the information. 

  • Beckyjo July 2, 2018 (7:57 pm)

    Why would they plant harmful trees?

    • Kersti E. Muul July 2, 2018 (9:07 pm)

      I imagine this was unbeknownst to them at the time.Like many readers here, there was a lot of curiosity so I would guess the same applies for the city… Hindsight..

  • dsa July 2, 2018 (8:15 pm)

    Is it the tree flower or the spraying for aphids causing harm to the bumbles?

    • Kersti E. Muul July 2, 2018 (9:03 pm)

      It is the flower.They are not spraying the trees.

  • Thorly July 2, 2018 (8:35 pm)

    Are these street trees? Can we petition the city/county to replace them with bee-safe, if not native trees?

    • Kersti E. Muul July 2, 2018 (9:05 pm)

      Yes, street trees and many of them. They line California Ave, as well as a few side streets.They are mature and it would be a giant undertaking as well as a loss of canopy cover which provides habitat, shade, ersoion and runoff control among other things. Might be a trade off here, not sure what the city would have to say. We should ask!

      • CA Pack July 3, 2018 (9:36 am)

        Will bumblebees eat nectar from feeders, like hummingbird feeders? If so, maybe enzymes could be added to feeders in the linden trees. Seems like an elaborate solution, but if it worked, maybe it would save some of the bees and keep the trees. 

  • dsa July 2, 2018 (10:34 pm)

    If the tree is killing off the bumble bees it should be declared a noxious tree by the state.  Then the city would have to remove them.

  • John July 3, 2018 (10:09 am)

    Although ladybugs do eat aphids (and other insects) there is little science based support for buying and releasing them for local control of aphids.   This gardening myth is exposed  by studies showing  the ladybugs just fly away.   The ladybug breeders, garden shops that sell them and good intentioned gardeners that believe in ladybug intervention are welcome to dispute the science.

  • Krista July 4, 2018 (9:57 pm)

    Glad I saw this post. This afternoon, I saw a bumble bee in the middle of 42nd Ave SW, between Admiral Metropolitan Market and Bartells, trying to cross the street. Found it strange that a bee was walking for an extended period of time, instead of flying. Then, I saw a dead bumble bee at California and College, and another bumble bee about 3 feet away from the dead bee, walking on the ground.

  • Kelly July 8, 2018 (4:58 pm)

    I did some more research and it looks like the tree itself may not be to blame, but possibly starvation associated with a flowering tree that has reduced nutrition late in the blooming season. In any case, keep planting your urban bee gardens.

  • waikikgirl July 8, 2018 (6:56 pm)

    Our bee’s are going strong in our yard on our 3 HUGE lavender plants but not to say we haven’t seen some dead and I too thought maybe they lived their life out as Joan 

  • Heather Hanson July 9, 2018 (4:27 pm)

    So the pollen from the flowers on the tree is killing the bees?  I always thought the bees were overloaded and died because they couldn’t fly.  My office is on California Ave and I’ve picked up lots of crawling bees and placed them near flowers and bushes just so they don’t get crushed.  This makes me sad…..

    • WSB July 9, 2018 (4:59 pm)

      No, if you follow the link, researchers say it could be a variety of things – it’s not as simple as “the trees are toxic.”

  • Patricia Davis July 14, 2018 (11:29 am)

    Last night I also (near Metropolitan Market) found a bee that was ‘stunned’ and not moving properly.     Has anyone contact the City of Seattle Arborist?  Also please – all of us – PLEASE  DO  NOT  USE  ANY  PESTICIDES,  LAWN FERTILIZERS,  MOSS  KILLERS  AND   OTHER  CHEMICALS  IN   YOUR  YARD.  Those chemicals are toxic to the environment and certainly to bees .  Chemicals are poison and bees are struggling for their existence. We all need to help them survive by not putting toxins in the world. 

  • anonyme July 14, 2018 (12:44 pm)

    John, thanks for the bringing up the issue of releasing lady beetles and other ‘beneficial’ insects into the landscape.  It should also be noted that the lady beetles sold for this purpose are not native species, so not only is the existing problem of aphids not being addressed – a new one is being created.  The practice of releasing non-native species into an environment in order to control a pest is one that is centuries old, has never worked, and yet persists to this day.

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