Stingless bees: Some colonies have to kill thousands of wannabe queens

About one-fifth of all Melipona beecheii stingless bee larvae develop as queens, but the colony accepts only one – the rest are executed by worker guards

By Christa Lesté-Lasserre

Some stingless bees seem to be able to choose whether to become a worker or the queen – but thousands of contenders for the throne are executed in each hive.

About one-fifth of all Melipona beecheii stingless bee larvae start to develop as queens, but the colony accepts only one. The rest are attacked by strong-jawed workers within seconds of emerging from their cells.

Biologists suspected that colonies of this species overproduce queens as a clever evolutionary strategy to take over other hives. But it now seems that it is simply a result of the “selfishness” of individual larvae – to the detriment of the whole colony, says Ricardo Caliari Oliveira at KU Leuven in Belgium.

“This is a really far from perfect world,” he says. “People think there is a design in evolution, but in this case the bees are just making the best of a bad situation. The colony is spending a lot of resources to produce new individuals, and then all they can do afterwards is waste those resources and kill the queens.”

In most bee colonies, including those of around 500 species of stingless bees, workers select one larva to become the sole queen by placing it in a larger cell and feeding it a special diet. But larvae of the Melipona genus all live in cells of the same size and receive the same food.

In 2010, researchers suggested Melipona beecheii workers were feeding a chemical called geraniol to the future queens. Caliari Oliveira and his colleagues wondered if they chose so many queens in order to spread the colony’s DNA into other hives through parasitism.

To investigate further, they took genetic samples of queens and workers from 25 free-ranging M. beecheii colonies at or near the Autonomous University of Yucatan in Mexico.

In the lab, they also gave higher doses of geraniol or saline to more than 600 larvae. To their surprise, they discovered that extra geraniol didn’t affect the development of the larvae, meaning workers weren’t controlling queen production after all.

Genetic sequencing revealed that the DNA of each hive remained 100 per cent consistent, showing no evidence of parasitism – even though two other species of Melipona bees are known to invade other nests, says Caliari Oliveira.

In M. beecheii colonies, virgin queens run around the hive in desperate and usually unsuccessful efforts to evade the worker guards, he says. Survivors might try to enter a different hive, only to be killed by that colony’s guards.

“This is a very good example of the tragedy of the commons,” says Caliari Oliveira.

The study “settles a bit of a controversy” about Melipona bees, says Christoph Grueter at the University of Bristol, UK. “It excluded one of the explanations, which didn’t entirely make sense in my view,” he says.

But it doesn’t offer a new explanation for how the larvae “decide” to become queens. “For me, this is still one of the biggest puzzles in biology,” he says. “Is it a gene? A particular combination of genes? An interaction between certain nutritional factors and genetic elements? We just don’t know.”

While scientists talk about the larvae choosing to become a queen or being selfish, they emphasise that these are unlikely to be conscious desires of the individual larvae. Rather, they are biologically driven preferences. “There is genetic selfishness, but that doesn’t mean that the individuals have any kind of thoughts about it,” says Grueter.

Journal reference: Biology Letters, DOI: 10.1098/rsbl.2021.0498

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