Why Are There So Many Dead Bumble Bees Around San Francisco This Summer?

SF residents have noticed more dead and dying bumble bees than usual. Why? It’s a layered answer.

Bumble bees are like the black labs of the aerial insect world. They’re bigger than their honey bee cousins, boasting immense, furry thoraxes and generally more robust, they-hit-the-inverbate-gym-five-times-a-week body types. Unlike honey bees, bumble bees — which San Francisco is home to five resident species of, with at least another four being infrequently documented — aren’t great at producing honey. 

What they lack in honey yields they more than make up for in their ecological importance as keystone pollinators; bumble bees, on average, are three to four times better pollinators than any species of honey bees.

Bumble bees are social creatures; they’re surprisingly intelligent; they appear to grasp the concept of “play”; they elicit senses of both nostalgia and biophilic whim whenever we see them.

However, per some anecdotal observations over the past few weeks, many of these beloved, bulbous insects have turned up dead or dying across San Francisco.

Has anyone else noticed an alarming number of dead or dying bumblebees around San Francisco recently?

We’ve seen no less than six today.

— Underscore_SF (@Underscore_SF) June 14, 2023

“Has anyone else noticed an alarming number of dead or dying bumble bees around San Francisco recently,” we tweeted on June 14th after coming across several listless examples of the flying insects on a walk around Nob Hill. “We’ve seen no less than six today.”

The replies to that inquiry confirmed our suspicions: Lifeless bumble bees were, in fact, being noticed across San Francisco by other locals.

“Yes, I have been seeing them in bursts of a few on the ground a day (around our place) and then I don’t notice them,” wrote one Twitter user. “I have been seeing this for many months though, it could be their population cycles up and down. Still sad, I love those lil critters.”

Others were less descriptive about their observations, but they all noted that they had seen dead bumbles as well… and that it was “sad.”

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A post shared by San Francisco Beekeepers Assoc (@sfbeekeepers)

Like honey bees, bumble bees have incredibly short lifespans — the exception to this rule being queens, which are capable of living for years, as opposed to days, as is the case for nonreproductive females and male drones. Bumble bees grow more active earlier in the spring and summer seasons than their smaller relatives, making them more visible than honey bees, according to Marc Johnson, a multi-hyphenate member of the San Francisco Beekeepers Association (SFBA).

“Bumble bees get a gift early in the season because their colonies, albeit small, are more active earlier in the year than when honey bees start ramping up their colonies that have overwintered,” Johnson tells Underscore in an email. “In my community garden (in the Sunset), we saw lots more bumbles than honey bees early. Since they are both ‘general pollinators’ it stands to reason that we should see each competing for the same flowers.”

Johnson, who serves as an instructor and apiary (read: a collection of beehives) manager for SFBA, noted that he was at a garden earlier this month in Nob Hill and saw “much more bumble bees” than honey bees, despite a honey bee hive on site — “I also saw lots of dragonflies, amazingly, since there was no close large water source.”

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A post shared by San Francisco Beekeepers Assoc (@sfbeekeepers)

SFBA’s “Swarm Line” has also taken many bumble bee calls this spring and summer, so the organization has quantifiable data that there are plenty of these pollinators out there.

What gives, then? If populations of these inquisitive winged insects seem healthy, why does it appear that an exceptional number of them have been spotted lifeless around San Francisco? It boils down to the weather as of late, especially when one considers that bumble bees forage more than honey bees, leaving them increasingly susceptible to the elements.

In fact, the meteorological spring and summer months (observed, thus far) are among the coldest ever recorded in San Francisco.

San Francisco's avg temp 54.9F meteorological spring (Mar-May) & met summer (Jun-Aug) to date is tied 8th coolest on record, 53.6F 1999 1st. Max 74F Apr 27th & May 13th, zero days = or > 75F not seen at least since 1911 (1891-1910 June data gap, each missing 22 days data). #CAwx pic.twitter.com/Shpu5j795Q

— NWS Bay Area 🌉 (@NWSBayArea) June 23, 2023

“San Francisco’s avg temp 54.9F meteorological spring (Mar-May) & [net] summer (Jun-Aug) to date is tied 8th coldest on record, 53.6F 1999 1st,” reads a tweet from the SF Bay Area chapter of the National Weather Service, adding insight to how abnormally cold these past few months. “Max 74F Apr 27th & May 13th, zero days = or > 75F not seen at least since 1911 (1891-1910 June data gap, each missing 22 days data).”

Bumble bees, though more cold-tolerant than honey bees, are incapable of sustaining flight when temperatures drop below 52 degrees Fahrenheit; when body temperatures dip below 41 degrees Fahrenheit, both honey bees and bumble bees perish.

The single most deadly factor for bees of any kind in cold climates? Windchill. Couple the aforementioned cold spell with the fact that SF’s windiest months all fall during spring and summer —  June and July are regularly the gustiest months each year — and it becomes clear why this perfect marrying of climate characteristics has led to an increased number of bumble bee bees falling dead.

Johnson later confirmed our hunch.

“The cold weather this Spring has been tough on all bees, not just bumble bees,” Johnson writes in a follow-up email. “Our colony losses are higher this year than normal. It’s also a fact that we delayed our bulk bee purchase at least three weeks since the breeders in Northern California did not have well-mated queens.”

Bumblebees buzz-pollinate over 20,000 species of plant in the world 🌍

This includes garden favourites, wildflowers and crops like apples, tomatoes, peppers, kiwis—the list goes on 🍓🌺

But bumblebees need our help. We can #BeeTheChange 👉🏽 https://t.co/8McJueJy5K#InsectWeek23 pic.twitter.com/XQuz6wFDIR

— Bumblebee Conservation Trust (@BumblebeeTrust) June 24, 2023

Even though this unusually cold weather has exacerbated bumble bee deaths recently, we’re witnessing a part of the natural life cycles of these crucially important creatures.

“It’s getting near the end of bumble bee season,” Philip Gerrie, the owner of Noe Valley Bees, tells us in an email.  “They are leaving their nests. Each nest creates several queens that will go and hibernate solo until the next season. I find their nests full of wax moths. They will nest in one spot for only one season, not to return.”

As of publishing, Gerrie currently has four bumble bee nests in my apiary in my nucs; it’s “sort of like a bee orphanage,” he writes

Thankfully, spring’s bounty — a seasonal haul executed by the amount of record-breaking rain we received earlier this year, which has produced massive super blooms and still is creating lots of hyperlocal flower growth — is helping turn the corner.

“This might be a good honey harvest this year,” Johnson waxes in closing. We’ll gladly take that honey-drizzled slice of optimism with us into the coming months.

For information on what you can do to help bolster local bee populations, including how you can report a “swarm” so that those bees can be safely relocated, visit www.sfbee.org; much like SFBA, Noe Valley Bees also offers hive removal services and sells regionally-sourced honey, purchasable at a number of local specialty stores and cafes — visit noevalleybees.com to learn more. 

Matt Charnock