When ants battle bumble bees, nobody wins

When bumble bees fight invasive Argentine ants for food, bees may win an individual skirmish but end up with less to feed the hive.

Bumble bees are already under pressure from habitat loss, disease, and pesticides. Former UC Riverside entomology graduate student Michelle Miner wondered whether aggressive ants might be adding to that stress.

“With how important bumble bees are as pollinators, it made sense to try and understand more about what’s going on in these tiny nectar wars, because they could have a big impact,” Miner said.

Her research, published in the Journal of Insect Science, analyzed over 4,300 individual behaviors from more than 415 bumble bees.

Performed in UCR professor Erin Wilson Rankin’s lab, the experiments involved six separate bumble bee colonies foraging in a shared arena. The bees could choose feeders that either had ants nearby or were ant-free.

Argentine ants, sometimes called sugar ants, do not sting but they do bite and can overwhelm other insects with sheer numbers.

“They can dominate a food resource just by showing up en masse,” Wilson Rankin said.

Given the choice, bees avoid ants. The more ants at a feeder, the less likely bees were to attempt feeding there. Unsurprisingly, more ants also meant a higher chance of bees being bitten. Still, the ant bites were not fatal. And if provoked, some bees fought back.

“We do see the aggression being bi-directional,” Wilson Rankin said. “Sometimes you’ll see ant heads on the bee legs because they were feeding and in response to an attack, the bee bit the ant and decapitated it.”

Though bumble bees can sting without dying, unlike honeybees, Miner didn’t observe any stings during the study. Instead, battling bees typically used their mandibles to fight their ant adversaries.

“Mandibles are like teeth but not only used to chew,” Miner said. “They can flare open to handle flowers or crunch a foe.”

Thanks in part to their much larger size, bumble bees who responded aggressively to the ants were generally victorious in one-on-one fights in this lab study. But that likely doesn’t translate into a win for the entire bee colony.

Instead of returning to foraging after encountering an ant, bees often stayed locked in further confrontations.

“The ant presence induced prolonged aggressive exchanges,” Wilson Rankin said. “Even though that one bee might benefit from being aggressive in the short term, it may not be beneficial for the colony overall.”

When the bees go into attack mode, they aren’t feeding.

“They’re wasting energy, they could be getting harmed, and they’re not bringing food back,” Wilson Rankin said.

Whether the hive compensates for lost food is unknown.

“We do know that the youngest bees don’t leave the colony. Once they’re old enough to ‘get their licenses’ they go out and forage,” Wilson Rankin said. “We don’t know if the colony sends out additional foragers when one comes back short. That’s the next question it’s important to answer.”