These Parasitic Beetles Use Sex Pheromones To Trap Bees And Prey On Their Young

Close up of a blister beetle on a leaf.
kurt_G / Shutterstock

A new study into the behavior of blister beetles has found that the larvae of a particular species, scientifically known as Meloe franciscanus, have devised a cunning technique to lure bees and gain access to their nests, reports Ars Technica.

These insects belong to a family of beetles famous for their painful defense mechanism — a blistering agent that they secrete called cantharidin, which burns or poisons potential assailants.

This toxic compound is also found in the Spanish fly — an emerald-green beetle known as Lytta vesicatoria and belonging to the same family — and has historically been used as an aphrodisiac, the Inquisitr previously reported.

While adult Meloe franciscanus are nothing to worry about unless they channel their blister beetle powers, it turns out that the larvae of the species are ruthless killers. The discovery was made by entomologists from the University of California, who found that these baby beetles — which hatch in large groups of up to hundreds of individuals — use sex pheromones to attract male bees looking for a partner and then swarm them, latching onto their bodies.

Once the beetle larvae are nice and settled, they remain with the unlucky male until he attempts to mate — at which point the parasites hop onto the female bee and await to be carried inside the nest.

This technique helps the parasitic larvae to hitch a ride to the bee’s nursery and wreak havoc on its eggs, feeding on the next generation. The video below, uploaded on YouTube by Twin Cities PBS, explains the entire process using visuals created by illustrator David Gillette.

As if this wasn’t scary enough, the researchers uncovered that the parasitic larvae adapt their pheromone cocktail to entice male bees depending on where they live, with local nest parasites being “significantly more attractive to male bees than nonlocal parasites.”

“This study provides strong evidence for two different but complementary types of local adaptation in geographically isolated populations of a parasitic insect,” the authors wrote in their paper, published on Monday in the journal Proceedings of the National Academy of Sciences.

According to their findings, the larvae of the Meloe franciscanus beetle “locally adapt their deceptive chemical signals, which mimic the sex pheromones of their host bees, to the differing pheromone blends of their local host species.”

What’s more, these beetle larvae are not only parasites, but kleptoparasites, notes Ars Technica. These means that — once they get inside the bee nest — they go on a feeding frenzy, devouring not only the insects’ eggs but also the food that as supposed to feed their larvae.

This cruelly efficient mechanism also serves another purpose, that of helping these flightless beetles get around, spreading their influence to areas that otherwise would have been inaccessible.

The team also discovered that the reproductive technique employed by the Meloe franciscanus larvae has evolved to target two species of solitary bees belonging to the Habropoda genus, which live along the coast of Oregon.

As solitary bees, these insects rely on pheromones to find a mate, and the beetle larvae have found a way to use that against them. By mimicking the specific pheromones of these bee species, the parasitic beetles signal to the males that a female is nearby, trapping them into a horrific rideshare that spells doom for the bees’ offspring.

Parasitic beetle larvae
  Laura P. Zarcos / Wikimedia Commons/Resized (CC BY-SA 4.0)

One of the ways in which the larvae manage to pull this off is by working together as a large group, as “individuals and small groups can’t produce enough signal to reliably draw in bees,” explains Ars Technica. The other secret of their success is customizing the pheromone mix that they use to entrap their victims.

After comparing the pheromones secreted by the beetles to the ones produced by actual female bees of the targeted species, the entomologists uncovered that the parasitic insects “actually produce a specific subset of the chemicals made by the female bees.”

One final detail that renders the attack plan foolproof is the proliferation of the larvae in the exact same locations where these bees typically search for partners. According to study, the mating grounds of these bee species are located at specific heights, where the beetle larvae have learned to climb to in order to perfect their technique.

However, Ars Technica claims that this high level of specialization could mean that these beetles might face extinction if their host bees ever disappear.