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Triungulins: hitchhikers that ride bees

This nasty-looking piece of work is a triungulin, the first instar larva of a beetle in the family Meloidae. This particular one is riding an Andrena bee that is foraging on a willow blossom. At the time I photographed this creature, I was sitting at the edge of the forest near my home, trying to figure out the controls of a new flash. I was concentrating on the flash, not the bee, so the bee is barely in focus. I never even saw the hitchhiker until I was home, flagrantly deleting most of the experimental images.

I sent the photo off to BugGuide.net and to Glen Buschmann at OlyPollinators.com. “What is this?” said I. They both concluded it was a triungulin, something that, until now, I was happy not to know about.

The larvae of blister beetles

These larvae grow into adults called oil beetles or blister beetles. The first instar larva shown below is actually the hitchhiker stage which doesn’t harm the bee. Their name is based on the three claws they have on each foot: apparently, “ungula” is Latin for claw. Based on what I’ve learned so far, this little Andrena is lucky to have just one guest. Sometimes an entire platoon will jump on at once, dozens and dozens all worming around each other.

The lifestyle of these beetles is the stuff of fiction/horror/fantasy. According to BugGuide, masses of larvae climb a plant and congregate at the very top. Once there, they cluster into a shape that resembles a female ground-nesting bee, such as the Andrena shown below. Then, improbably as it may seem, they emit a scent that mimics a female bee’s pheromones.

Getting into the bee nest

The scent attracts male bees who come along and try mating with the clump. (I will defer all unseemly comments about male mating behavior for another day.) During this futile attempt, the triungulins attach to the male. They ride around on the back of the male bee until he finds a mate—the real thing—at which time the triungulins transfer to the female bee.

They are still just hitchhiking at this point. But once on the female, the phoretic larvae are transported to the nest she is building in the ground. There, they dismount and begin to feed on the pollen balls the bee has prepared for her own young, along with the eggs. Now the hitchhiker has become a true parasite, living off the work of the ground-nesting host. Very rude, indeed.

Variations on a theme

This entire sequence is detailed for one particular species of blister beetle in a paper by Saul-Gershenz & Millar (2006) titled “Phoretic nest parasites use sexual deception to obtain transport to their host’s nest.” It sounds like there are probably many variations from species to species, but the basic mechanism is widespread throughout the Meloidae family.

Dana at AbundantNature.com has some good photos. He found bees laden with triungulins in a willow, so if you want to see these things, a willow may be a good place to start. Most willows are very attractive to bees, so it makes sense.

I’m always amazed at the number and type of parasites that can be found on bees and the bizarre lifestyles they have developed. I’m going back to the willow today, just to see what else I can find.

Rusty
Honey Bee Suite

The hitchhiker shown here is a triungulin, the first instar larva of a blister beetle or oil beetle. This one is riding an Andrena bee, hoping to be taken into the bee's nest where it can plunder the food stores.
The hitchhiker shown here is a triungulin, the first instar larva of a blister beetle or oil beetle. This one is riding an Andrena bee, hoping to be taken into a bee’s nest where it can plunder the food stores. © Rusty Burlew.

Comments

Alice
Reply

Deceptive little larvae aren’t they? They obviously know their trick will work on the, um…, less than observant male Andrena!

I know little about the Andrena bee or the blister beetle and its triungulin, but I want to be encouraged by them in thinking about the long-term relationship between the honey bee (feral and/or managed) and SHB and Varroa. A search for how long have triungulin been using these techniques to live and reproduce and has it affected the overall population of the Andrena bee just might be in my future.

Found and enjoyed your post on the Andrena bee from 2013. We don’t have willows, but we do have wild blackberries. I’ll be watching them while they’re blooming this year. Maybe I’ll get to see a orange-legged Andrena. Not sure if I’d be as excited to see a triungulin, though.

Kathleen A.
Reply

Well, Rusty. I have Andrenas on my baked clay, south-facing Piedmont Virginia hillside. They are my delight in a harsh location; gentle, busy bees. I will look for this nasty creature as I watch these natives this spring.

Blaine Nay
Reply

Blister beetles can be useful critters. One of my sons had stubborn warts that resisted the typical treatments. But the warts were destroyed when a dermatologist applied juice from blister beetles.

Rusty
Reply

Whoa. That is so cool!

Glen Buschmann
Reply

Thanks Rusty –

You query caught me in a weak moment, but when it comes to some of this stuff I’m an easy mark. I saw the hitchhiker on the bee and thought that it looked like a small rove beetle, and went from there. The more I read about these characters, well … .

As you noted, triungulins are immature “Oil Beetles”, or “Blister Beetles”. Blister beetles emit — sometimes forcefully — some potent oils. Get this: some other beetles are attracted to the oils of these blister beetles. The males lick up the oils and lure females with Cantharidin — the oil. The male beetle, adequately filled with its procured oil, in a love tryst coats its sperm package with that oil, which in turn coats the eggs and protects the eggs from predation.

There is more. Cantharidin has also been used “for millennia” by people, as an aphrodisiac called “Spanish Fly”. Fatalities and other consequences — as well as modern pharmaceuticals — have fortunately reduced this product’s popularity. But it would not surprise me if it first came into use because of some long-ago observant and enterprising insect watcher: if one male species can be suckered, why not another?

Ray
Reply

Yuk! I thought this was an April Fool’s joke until I realised it was the 2nd not the 1st of April. So I say again, Yuk!

Bill Abell
Reply

Rusty,

Pheromones seem to drive so much positive and negative reactions in the insect world I am wondering if that is where we might find a solution to the Varroa and SHB of the honey bee.

The title of your monthly column “The Curious Beekeeper” hits the nail on the head. Another great column in the April issue.

Thanks.

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