When I was a kid, there were bees everywhere. They’d be on the clover, the dandelions, and any other flowers that happened to be around. I must have been stung a dozen times. But now that I think of it, I don’t see a heck of a lot of bees anymore. Where’d they all go?
Over the past 20 years, bumblebees in the U.S. have seen their population drop by an astonishing 90%, lost entirely from at least eight states in their former range. We can’t afford to lose the bumblebee because 71 out of the top 100 food crops are pollinated by it and other bees. This insect is in such trouble that it is under review by the U.S. Fish and Wildlife Service for inclusion on the endangered species list. Another major pollinator, the honeybee, is also facing sharp declines.
The American bumblebee has disappeared entirely from Maine, Rhode Island, New Hampshire, Vermont, Idaho, North Dakota, Wyoming, and Oregon, and it’s down to 1% remaining in New York
There are a few main reasons for this, among them rampant pesticide use, climate change, and the spread of pathogens. Pesticide use not only directly harms bees but also makes it easier for pathogens to do harm.
Farmers aren’t going to give up pesticides anytime soon, because they need to achieve good yields on their land in order to sustain themselves. This is also a big reason you will see reluctance to put bees on any endangered list, because then you’d have to start banning pesticides, and subsidizing farmers. But many crops require pollination by bees, so overdependence on pesticides could wind up killing the golden goose. Then what? A landscape littered with the husks of pollinator drones? Yikes.
We’ll clearly have a lot of work to do to return bees to anywhere near their previous levels, but we are at least beginning to understand better how some pathogens can be kept at bay. One common bee parasite called Crithidia bombi can reduce queen bee fitness by 40%, threatening the stability of an entire colony. It has other weird cognitive effects on bees, too, such as preventing them from learning colors of productive flowers. It’s been observed that sunflower pollen can sharply reduce C. bombi infection in many kinds of bees, but up to now it hasn’t been clear why.
The trypanosome parasite Crithidia bombi. The flagellum sticking out of this parasite turns out to be key for its ability to colonize the bee gut
In companion April 5 papers in the journals Functional Ecology and Proceedings of the Royal Society B: Biological Sciences, researchers at the University of Massachusetts Amherst find that the key to the effectiveness of sunflower pollen isn’t what’s in it, but what’s on it. Wildflower pollen does not have any ability to fight off C. bombi in bees, but they found that it suddenly gains this ability if it’s mixed with sunflower pollen, and more specifically and curiously, it also gains this ability if it’s mixed with only the exines, or spikes, found on the surface of sunflower pollen particles. They also found that in the wild, the more sunflowers you have planted in an area, the better free-foraging bumblebees do. They’ll have less C. bombi in their guts, and they’ll produce more and healthier queens.
This is a really peculiar result because of its physical rather than chemical nature. It’s no surprise that there are chemical compounds out in the wild that fight off pathogens. It had been found in 2019, for example, that a chemical called callunene found in heather nectar prevents C. bombi infection in bees by knocking off their flagella, which they need to anchor themselves in the bee gut in order to take on their parasitic role:
Top: Callunene removes the flagella of newly-ingested C. bombi, so it cannot establish itself in the ileum. Bottom: Once the parasite does establish itself, callunene is not very effective because most of it gets digested and degraded in the midgut.
This effect was found to hold true in live bees in the wild, and not just laboratory-bound ones. So we could then argue that preserving heathlands will benefit bees in a natural way.
Around 85% of heathland in the UK has been lost over the past 150 years through agricultural development and the planting of conifers
But a physical mechanism like this to keep bee pathogens under control had never been observed before. How could simply feeding pollen spikes to bees prevent C. bombi infection? It’s true that the pollen of the sunflower, compared to the rest of its family, the Asteraceae, is just about the spikiest:
Pollen grains of the common sunflower, Helianthus annuus
But they also found that pollen from four other members of the Asteraceae family with less-spiky pollen — cocklebur, sagebrush, dog fennel, and dandelion — was also very effective, and the best was actually dandelion, at 92% C. bombi reduction!
All these types of pollen do have spikes to some degree, but it’s interesting that the length of the spikes didn’t correlate at all with the effectiveness of the pollen in preventing infection. You can see that dandelion pollen is spiky too, but its spikes aren’t as long as those on sunflower pollen:
Pollen grain of the dandelion, Taraxacum officionale
So what is it about these spikes, if not their length, that tamps down C. bombi so effectively? It’s possible that the abrasive nature of the spike tips irritates the digestive tract of the bee enough to cause inflammation, which might lead to a heightened immune response or might change the surface characteristics of the bee gut such that C. bombi can’t take hold.
It’s also possible there’s something structural going on. We know of both natural and synthetic surfaces with nanostructures that are lethal to microorganisms and viruses. A cool example is the cicada wing. It has bumps uniformly distributed on it that trap and impale pathogenic Pseudomonas bacteria. Check this out:
Bactericidal effect of cicada wing surface on Pseudomonas aeruginosa. a) P. aeruginosa cells penetrated by its nanopillar structures. Scale bar = 1 μm. b) A P. aeruginosa cell sinking between the nanopillars on the cicada wing surface. Scale bar = 200 nm.
It could also be that some molecule on the spike surface just so happens to block a receptor on the parasite surface that signals it to proliferate.
Could the Asteraceae have evolved their spiky pollen because it promotes the health of their pollinators by interfering with pathogens that might harm them? Wouldn’t surprise me even a little.
In any case, our UMass researchers have opened up a series of questions that should make for productive and fascinating research. Truly understanding how these spikes work will allow us to attack the problem most intelligently and translate that into helping bees regain their footing.
And on a practical level, we’ve got to do more research to find out the range of pollinators and pathogens this applies to. The authors point out that there are some indications that sunflower pollen may be effective against pathogens beyond C. bombi, yet it also appears that sunflower pollen doesn’t affect all bee species equally.
Sunflower pollen is actually not terribly nutritious to bees, because of its low protein content and lesser digestibility, but that’s OK because bees generally forage on multiple types of flower. A diet of half sunflower pollen and half more-nutritious pollen (like broad bean, canola, or wildflower) has been shown to keep down C. bombi very well and yet promote colony health about as well as a diet of optimally nutritious pollen alone. All signs point to the prevalence of sunflowers being beneficial for bees.
Who doesn’t love sunflowers?
There’s no single panacea that will suddenly bring all the bees back. But right now, it seems the most expedient thing we can do is to promote plant species that are beneficial to bees. Plant sunflowers! Let some dandelions grow on your property! Plant some heather as ground cover!
I must admit I’ve tried to grow sunflowers, and every time I do, right after they emerge, some critter eats the stem and leaves a severed plant lying prostrate. If you can do better, then please do so! But I’m sure I can handle leaving some dandelions out in my lawn. I’m pretty good at procrastinating on yardwork. And I do like the idea of my own mini-heathland. I’m going to take a serious look at that this spring, before the bumblebee is gone from Massachusetts, too.
