Fri. Jun 2nd, 2023
A desert (left) and beach (right) male, covered in beetle larvae that will eat his children.
enlarge / A desert (left) and beach (right) male, covered in beetle larvae that will eat his children.

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 who do horrific things to them, like tricking them into expecting sex, only to be overrun by beetle larvae, who jump on the bee to get a free ride to the bee nest, where they get the eat the next generation. New research shows that the beetles are not only a species of malignant, but pheromones are good at it, modifying a mix of pheromones to attract the local bee population and lurk where bees go looking for sex.

Dinner for tens

The beetles in question are commonly referred to as “blister beetles,” part of a large group of insects that you probably wouldn’t look at twice. The adults are quite commonly beetle-like and generally flightless. Many adults are vegetarians. The problem is their children.

The beetles lay hundreds of eggs in a group and they all hatch almost simultaneously. The resulting larvae stick together as a group, enticing male bees to stop nearby. Once they do, the larvae jump on the unlucky male and stay on board until he tries to mate with a female. At that point, the beetles switch loyalties and jump on the female. This allows them to eventually be carried to a place where the female lays eggs.

At this point, the beetle larvae jump off the bee and begin to eat everything in sight. This includes the food the bees left behind to feed their own larvae; the beetles also eat the bee larvae 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 disperse even though they can’t fly themselves and thus not much distance in their lives.

And you thought pesticides were the bees’ biggest problem.

How do the beetles get the bees to cooperate? Based on previous research, it’s all about sex. While bees that make large nests have no trouble finding a mate, 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 victimized by these beetles, that message is “I’m a female,” which is enough to lure males in, looking for a mate. The mass of larvae working together is essential here; individuals and small groups cannot produce enough signal to reliably attract bees.

Better food through chemistry

The new work takes a closer look at the production of these pheromones and how they are used as part of an attack on a bee. It turns out that some kind of beetle (meloe franciscan) specifically hunts two species of bees along the Oregon coast. So the researchers looked at the chemical mix produced by the female bees, as well as 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 array of chemicals and was not attracted to the Oregon beetle larvae.

But the Mojave species falls victim to a local beetle. And those beetle larvae attracted 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 create signals tuned to the local bees.

But not only the chemical signaling has been refined. The researchers noted that bees from the two different species searched for mates at different heights — about 35 cm above the ground in the desert, just 10 cm along the beach. This turned out to be genetically programmed into the bees, as growing Oregon bees in the desert had no effect on their cruising height and vice versa.

And the beetles seem to have responded to this tendency. The larvae tended to climb somewhere before settling to produce pheromones, and the average height they climbed almost perfectly matched that of the typical crotch height of their local bees. Again, breeding beetles in the wrong environment did not change their behavior, suggesting that it is genetically controlled, rather than in response to a local environmental condition.

The extreme specialization no doubt helps the beetles find food efficiently, increasing the likelihood that they will successfully send the next generation off to do horrific things to bees. But this level of specialization also carries risks. If the bee species ever goes extinct, it could be difficult for the beetles to adapt to new victims before they too leave Earth.

PNAS2017. DOI: 10.1073/pnas.1718682115 (About DOIs).

By akfire1

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