Sex, Lies, and Grappling Hooks: How Parasitic Beetles Trick Bees

The life cycle of a blister beetle is not only fascinating, it may be shedding new light on how new species form.

By Jason Bittel
Published 14 Sept 2018, 09:49 BST
A male burrowing bee covered in parasitic blister beetle larvae, which make pheromones that mimic those ...
A male burrowing bee covered in parasitic blister beetle larvae, which make pheromones that mimic those of a female bee ready to mate.
Photograph by Leslie Saul-Gershenz, PhD

Imagine going on a first date with someone whose perfume drives you wild. But when you lean in for that first kiss, you realise your suitor is actually nothing more than a writhing mass of parasitic blister beetle larvae.

This is the plight of the burrowing bee.

You see, sometimes when a male bee is buzzing along the sand dunes, he smells what appears to be a female’s pheromones. Mating is highly competitive in these species, so it pays for the male to buzz in and have a look.

Unfortunately for him, blister beetle larvae have evolved the ability to create chemicals that make them smell like a female burrowing bee. The critters even boost the profile of their scent by crawling up a strand of grass and forming a bee-sized ball of baby beetles. These larvae are known as triungulins, for their feet, which have three claws that resemble grappling hooks.

When the male bee attempts to mate with this decoy, the triungulins latch onto him with their hook-like claws and tackle him to the dunes below. Eventually, when the male flies off in search of a real female, he does so with a horde of hitchhikers attached to his fuzzy body. The larvae then latch onto the female and ride her to a burrow.

There, she lays a single egg and deposits a ton of pollen and nectar. But those nutrients may not make it to the baby bee, because the triungulins gobble them up first before transforming into adults.

But that isn’t even the most interesting part.

This week, a study published in Proceedings of the National Academy of Sciences reported that depending on where they are in the world, blister beetles of the same species are able to tweak their chemical cocktails to target different types of bees in different environments.

That means the beetles literally smell different in California than they do in Oregon. This extreme divergence has led researchers to suspect the beetles could be on their way to becoming different species.

Leslie Saul-Gershenz, a chemical ecologist at the University of California, Davis, and lead author of the study, says this speciation is probably happening “but it takes a long time.”

The Beetle Nose Knows

The blister beetle larvae hitch a ride back to the burrow of the female and eat the nutrition intended for the baby bee.
Photograph by Leslie Saul-Gershenz, PhD

The human nose can’t detect beetle or bee pheromones, but there is yet another aspect to the new study that can be seen with the eye.

When blister beetle larvae do their little kids-in-a-trench-coat ruse on the sands of California’s Mojave Desert, they always crawl up at least 11 inches on their grass stalks. However, triungulins of the very same species never ascend higher than 4 inches on the coastal dunes of Oregon.

There could be two reasons for this.

“In the Mojave, the sand is really hot,” says Saul-Gershenz, who notes that temperatures can exceed 120 degrees Fahrenheit.

But it’s not just that the beetles dislike hot sand. Indeed, the triungulins’ perch heights also reflect the preferred patrolling heights of the species of burrowing bee they parasitise.

What’s more, when Saul-Gershenz and her colleagues took some blister beetle eggs from Oregon and let them hatch in the Mojave, the beetle larvae stayed low on the grass. Likewise, beetle larvae from California climbed way up on the grass when placed in the Pacific Northwest.

So not only are the two populations of beetles evolving different scents, but they are also developing different behaviours to boot.

At this point, Saul-Gershenz says it might be interesting to next look at the pheromones the beetles produce to attract each other to see if these scents have also changed. If females from Oregon would no longer mate with males from California, or vice versa, it would be yet more evidence that the species is splitting.

“The history of natural selection and adaptation in this species of beetle is written in their chemistry,” says Gwen Pearson, an entomologist at Purdue University who wasn’t involved in the study.

“Tiny Chemical Factories”

The study is yet another example of “how insects are really tiny chemical factories,” says Pearson.

In fact, it’s worth noting that blister beetle chemicals have probably graced your newsfeed before, but under another name—Spanish fly.

Blister beetles produce a defensive secretion called cantharidin, which has long been rumoured to be an aphrodisiac for humans. In reality, it’s more like a poison, triggering vomiting and diarrhoea when consumed. Even a dab on human skin can cause blisters, which is how the beetle got its common name.

“Don’t lick them or rub them,” says Pearson. “You will really, really regret it.”


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