Gangs of beetle larvae lure fathers of their next meal into sex trap

(Source: arstechnica.com)

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Sometimes the Universe fills you with wonder. Other times, well, it leaves you filled with existential dread. This story falls into the latter category.

It’s about bees and the beetles that do horrifying things to them, like luring them into expecting sex, only to be swarmed by beetle larvae, who hop onto the bee to get a free ride to the bee’s nest, where they eat the next generation. New research shows that the beetles are not just kind of evil but phenomenally good at it, customizing a mix of pheromones to lure the local bee population and lurking where bees go looking for sex.

Dinner for dozens

The beetles in question are generically termed “blister beetles,” part of a large group of insects that you probably wouldn’t look twice at. The adults are pretty generically beetle-like, and generally flightless. Many adults are vegetarians. The problem is their kids.

The beetles lay hundreds of eggs in a group, and they all hatch nearly simultaneously. The resulting larvae stay together as a group and lure male bees to stop nearby. Once they do, the larvae hop onto the unlucky male, staying on board until he attempts to mate with a female. At that point the beetles switch allegiances and hop onto the female. Eventually, this allows them to be carried to somewhere that the female is laying eggs.

At this point, the beetle larvae hop off the bee and start eating everything in sight. This includes the food that the bees have left to feed their own larvae; the beetles eat the bee larvae, too, for good measure. Not just parasites and not just thieves, the behavioral suite has earned them the term cleptoparasite. And it serves the beetles well, allowing them to spread even though they themselves can’t fly and thus can’t cover much distance in their lifetime.

And you thought pesticides were the bees’ biggest problem.

How do the beetles manage to get the bees to cooperate in all of this? Based on earlier research, it’s all about sex. While bees that make large nests have no problems finding mates, there are many solitary species that rely on pheromones—small molecules that diffuse through the air and convey a specific message to their recipients. In the case of the bees that are the victims of these beetles, that message is “I’m female,” which is enough to draw males in, looking for a mate. The mass of larvae working together is essential here; individuals and small groups can’t produce enough signal to reliably draw in bees.

Better eating through chemistry

The new work looks a bit more closely at the production of these pheromones and how they are used as part of an attack on a bee. It turns out that a species of beetle (Meloe franciscanus) preys specifically on two species of bees along the Oregon coast. So the researchers looked at the chemical mix produced by the female bees, as well as by groups of beetle larvae. They found that the beetles actually produce a specific subset of the chemicals made by the female bees, including some that are not made by male bees. A related bee species from the Mojave desert produced a distinct set of chemicals and wasn’t attracted to the Oregon beetle larvae.

But the Mojave species is victimized by a local beetle. And those beetle larvae did draw in the Mojave bees, while the Oregon bees ignored them. A check of the chemicals produced by the insects showed that the beetles had evolved to make signals tailored to the local bees.

But it’s not only the chemical signaling that has been fine-tuned. The researchers noted that bees of the two different species went looking for mates at different heights—about 35cm above the ground in the desert, only 10cm along the beach. This appeared to be genetically programmed into the bees, as rearing Oregon bees in the desert had no effect on their cruising altitude, and vice versa.

And the beetles seem to have responded to this tendency. The larvae tended to climb something before settling down to produce pheromones, and the average height they climbed nearly perfectly matched that of the typical cruising altitude of their local bees. Again, rearing beetles in the wrong environment didn’t alter their behavior, suggesting it’s genetically controlled, rather than in response to some local environmental condition.

The extreme specialization undoubtedly helps the beetles to find food efficiently, raising the chances that they’ll send a next generation successfully off to do horrifying things to bees. But this level of specialization also comes with risks. If the bee species ever go extinct, it may be difficult for the beetles to adapt to new victims before they, too, exit the Earth.

PNAS, 2017. DOI: 10.1073/pnas.1718682115  (About DOIs).

More Info: arstechnica.com

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