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.
Honey Bee Suite