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Why so many dead bumble bees?

Several of you have reported seeing many dead bumble bees on flowers, patios, or lying on the ground. This type of observation pleases me no end. It gives me hope to see so many people noticing wild bees and wondering about them.

As it happens, it is completely normal to see lots of dead bumble bees at this time of year. Unlike honey bees, most bumble bee colonies die at the onset of winter. Some colonies in warm climates may survive the winter, but as a general rule, a bumble bee colony lasts for only one season.

A colony starts with one bee

A bumble bee colony starts in the spring with a single mated queen. This queen hibernated all winter in a cozy protected spot, most often a narrow hole in the ground not much bigger than the bee herself. She survived on fat stored in her body, much like a bear or a hedgehog. You can often see these bumbles in the spring, remarkable for their ungainly size.

When the weather warms in spring, the queen feeds on the nectar of early flowers and cruises the countryside looking for a place to build her nest. The queens of most species choose an underground cavity—perhaps an abandoned rodent hole—as a shelter. In that hole, she begins the process of storing a little nectar, and building a small wax nest for her first batch of young workers.

Once the initial nest is fashioned from secreted wax, and tiny nectar pots are built and filled, the queen sits on her eggs very much like a hen. She keeps the brood warm with her body and drinks from her nectar pots until the young worker bees emerge from their cells.

The chores are turned over to workers

The queen works hard in the early spring, doing all the chores by herself. However, once the first workers emerge, she forages less and less. The labor-intensive jobs of brood rearing, foraging, and colony defense are turned over to her offspring, and she gets down to the business of egg laying. For most of the spring and summer this queen will lay eggs that produce nothing but more female workers.

Because of this system, nearly all the bumble bees you see in the spring and summer are workers. Like honey bees, they ply the flowers for both nectar and pollen, transporting the pollen back to their nests in baskets built into their rear legs.

In general, bumble bees have more size variability than honey bees. While all the honey bee workers in a colony are pretty much the same size, worker bumble bees come in a range of sizes. However, the smallest bumbles spend their entire lives in the nest, acting as nurses. It is only the larger workers that go out and do the foraging. Most of us never see the tiniest house-bound bumble bees.

Queens and males come later

Late in the summer the queen adds to the colony by producing both male bees and virgin queens. If you are observant, you can easily see the changes in your garden. The first thing you might notice is some huge bumble bees and an assortment of very small bumble bees, sometimes on the same flower. As you might guess, the large ones are queens. The small ones are males.

But wait. You never see honey bee queens and drones perusing the blooms, do you? So what are these bumbles doing out on the flowers?

Male bumbles have no home

The males spend their time trying to mate. Many males compete for this honor, but only a few get lucky. The rest of the time, they can be seen in the flowers, drinking nectar for quick energy. As it happens, they also sleep in the flowers because once they leave the nest they are not allowed back in.

Some people mention that these autumn bumble bees often look wet, lethargic, or dead. All of these may be true. The males don’t live very long, and after mating—or attempting to mate—they spend most of their time eating and sleeping. During the cool nights they get wet with dew and stiff with cold. Sometimes they warm up the next morning and live another day, or sometimes they get eaten, or simply die.

Queens fatten up for winter

The queens though, lead a different kind of life. During the fall months the newly-mated queens gorge on nectar and pollen, fattening up for winter. The more fat they can store, the better chance they have of making it through the cold months ahead. Once she is ready, a queen will go and find a nice protected hole where she can hibernate until spring.

Sometimes in the fall you will see a fat queen next to tiny male. Although they forage side-by-side, the difference in size is striking. By evening, however, you might notice dead males adhering to flowers, or dropping into the grass where they are quickly eaten by something else. In contrast, the queens are nowhere in sight, having flown back to the nest for protection.

The colony dies as winter sets in

Back in the colony, life is winding down. One by one, the newly-mated queens leave the nest to begin hibernation. The old queen slows her egg production and eventually dies. The workers die as well. The first hard freeze finishes off all the remaining bees except for the hibernating queens. As we go into winter, the once busy colony is but a memory.

Rusty
Honey Bee Suite

two-bumble-bees
Here two bumble bees, most likely Bombus vosnesenskii, share a flower. The large one on the left is probably a queen. © Rusty Burlew.
eight-bees
Seven honey bees and a bumble bee share a dahlia. © Rusty Burlew.

Comments

Andrea Balding
Reply

I love this. I have so many bumbles in my yard and had been wondering. Thank you!

Ellen
Reply

Rusty,

You have beaten my record of seven bees on a dahlia blossom! I wish I knew what the different bumble bees are that I see on my dahlias. The only one I am fairly sure of is the B. vosnesenskii. This is the first year I have paid attention to the non-bee pollinators on the dahlias and have seen three species of butterflies, various wasps and hover flies. What a delight!

Rusty
Reply

Ellen,

That’s ironic, seeing as how you gave me the dahlia in the first place! But thanks.

There’s been a somewhat heated battle on BugGuide for the last couple years about identifying B. vosnesenskii vs B. caliginosus. It seems they are both in our area and they are extremely hard to tell apart. BugGuide says, “B. caliginosus is shaggier than B. vosnesenskii and has yellow hairs on the underside of the abdomen where B. vosnesenskii has only black hairs on the underside of the abdomen.”

So even though B. vosnesenskii is much more common in our area, I always hesitate to say for certain which it is. John Ascher of the American Museum of Natural History usually assigns them to the subgenus Pyrobombus (to which they both belong) rather than assigning a species name, especially when identifying from photographs.

I’m not doubting your i.d., I just find it interesting. To describe one as “shaggier” than the other suggests the need to see them side by side. And to see their undersides side by side, suggests they need to be dead. With that requirement, I am happy to stick with the subgenus or as I said in my post “probably.”

Of all the bee species I’ve tried to identify, I think bumbles are the hardest.

Ellen
Reply

That is all very interesting, and I think I will adopt your use of the “probably” in identifying B. vosnesenskii. I’m not even sure how I arrived at that Id, as when I now look at images of B. caliginosus they appear the same. Thanks for bringing that to my attention.

Nancy
Reply

How wonderful, Rusty, to learn these secrets of our most taken-for-granted bee.

Did I miss a step? How does the old queen produce a new one? Do the little indoor bees have their own equivalent to royal jelly, that allows a female egg to develop sexually?

We wouldn’t have eggplant without bumbles. That by itself endears them to me!

Nan
Northern Kentucky

Rusty
Reply

Nancy,

I checked in Bumble Bees of North America (2014) Princeton University Press. It reads on page 14: “How fertilized eggs become queens instead of workers is not fully understood, but it is probably associated with larval diet and possibly exposure to queen pheromones.”

Sorry I can’t tell you more.

Nancy
Reply

Something we have yet to learn! Now that is cool.

Nan

Emily
Reply

Occasionally large numbers of bumble bee workers are seen dead under lime trees. There are two main theories about this – either the bumbles are poisoned by the lime’s toxic nectar or they die of exhaustion/starvation after the lime nectar runs out.

Today I saw a huge queen bumblebee stocking up in ivy nectar. Lovely to see.

Rusty
Reply

Emily,

Interesting. If I understand correctly, some lime (linden) trees have this toxic effect and some don’t?

rpb
Reply

I believe this is true for the male homo sapien as well.

Bill
Reply

After reading your information last night about bumble bees I went out today and saw two bumble bees mating on some wax cappings I left for my bees to clean up. I tried to attach the pictures to this comment but they won’t paste. How can I send you a couple of these pictures?

Rusty
Reply

Bill,

I sent you an e-mail.

Jennifer Dixon
Reply

Wow Rusty, thanks for this info! Going to bookmark, love it. I love the bumbles, and this gives me a clearer perspective on their lives.

patsquared2
Reply

Thanks for this post. I always hated seeing dead or dying bumble bees. I had no idea they only lived for a year! I should have figured it out. I actually had a bumble bee nest tucked just under the roots of one of my blueberry bushes. That year, I couldn’t get within 3 feet of that bush – bumblebees get pretty “starchy” about anyone approaching. So I worried about having to deal with them again, the next year, but they were gone! Now, I also know why! Thanks Rusty!

Maryann
Reply

Hi, I searched out on the web why bumble bees are dropping out of my Rose of Sharon on my patio dead and found your blog. You mentioned in this blog that it is normal at the end of a season but, I live in California and it is only June 28th. I have collected 20 dead bumblers from just yesterday and today there were even more. Is this normal? If you know anyone that could analyze the dead bees, I could send the ones I have collected to them. I personally have never seen this before. On a personal note, I am also a beekeeper but my honey bees seem to be fine.

Rusty
Reply

Maryann,

The bumble bee “season” is not the same as the calendar season. For example, some species may be active only in March through May, others in May through July, or any other two- or three-month period. Still, to have a lot of bumbles drop out of one tree is unusual. It almost sounds like a pesticide was applied to the tree. Is that possible? Rose of Sharon often gets treated for beetles, but if it’s your tree and you’re a beekeeper, I doubt it was treated.

Are the bees you collected males or females or both? Are they all the same species? Are their tongues hanging out? Are the wings frayed? Do you know where their nest is/was?

Kimberly
Reply

Hi. I am in MN and have a sudden explosion of bumble bees in my garden. More than I have seen all summer, there are 100’s of them on my blooming succulents and 100’s more on the catmint.

They are behaving a bit differently than the ‘summer bees’ in that some of them seem to be ‘drunk’. They practically fly right into me where as I feel they usually avoid me. It has been unseasonably warm this fall. I am hoping that this is a good thing. I have worked hard to make my yard bee friendly but this sudden explosion is crazy!!

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