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Watch Out Below: Wasps Battle Ants by Dropping Them

29 March 2011 7:01 pm
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As anyone who has been stung by one would know, wasps have anger management issues. The yellow jacket wasp is no different, but when it wants to bully something small, it eschews the stinger for something more creative. When a wasp comes upon a swarm of ants on food it wants, it will simply pick the pests up and fling them away: a previously undocumented way of dealing with a competitor that is reported for the first time in Biology Letters today.

This inventive wasp, Vespula vulgaris, is native to the Northern Hemisphere; when it invaded New Zealand 30 years ago, it saddled the country with the highest density of wasps in the world. "It's a big ecological problem here," says biologist Julien Grangier of Victoria University of Wellington in New Zealand. With these large numbers of wasps suddenly wanting access to limited protein sources, Grangier was curious whether they would compete directly with native species such as ants.

Face off. Although tiny by comparison, ants make a valiant effort to bite and spit acid at the wasp before ultimately being thrown.
Credit: Julien Grangier/Victoria University of Wellington

So he set up a cage match. He placed little piles of canned tuna fish on plates in beech forests where the ants and wasps live and trained cameras on the dishes. When a wasp approached a swarm of ants on the tuna bait, the ants would begin charging at it, spitting acid and trying to bite the insect, which is 200 times larger. Often this was enough to make the poor wasp flee in a panic, but in rare bursts of bravery, it would occasionally grab a Napoleonic ant in its mandibles, fly backward a few centimeters with the insect twitching in its jaws, open its mandibles, and let gravity do its work. The dazed ant generally didn't come back after that. But the wasp seemed to weigh its odds carefully and when it did approach a swarm, the more ants there were, the farther away they were flung. Grainger says the video evidence didn't show either insect winning in the end: both ultimately maintained a presence around the tuna.

Little buggers. The wasps picked off ants one by one, and the more ants on the food, the farther away the wasps would drop them.
Credit: Julien Grangier/Victoria University of Wellington

"It's very intriguing," says behavioral ecologist Monica Raveret-Richter of Skidmore College in Saratoga Springs, New York. "I didn't know who to root for." Richter says it's surprising that wasps would get so aggressive when the food was already swarming with ants, spending time and energy removing the food competitors rather than just abandoning the meal. Grangier says that under natural conditions, the wasps would more likely prefer to disable a single scout before it could recruit its friends.

Dropping ants seems an unusual way of accomplishing this. "Wasps are massive and could crunch them, but on the other hand, ants are walking chemical factories," says behavioral biologist Robert Jeanne of the University of Wisconsin, Madison. "I'd guess the wasps pick them up gingerly and drop them quickly because if they crush them, they'd get a mouthful of something pretty distasteful."

The violence of the competition surprised the researchers as well. "Despite the difference in size, ants are formidable predators," says Jeanne. Just a few ants clinging onto a wasp's wing or leg could bring the flier down. "It's not nice to have your legs bit by small animals," Grangier says.

The observation that the wasp relocates ants farther when the swarm is bigger is a "wonderful example of behavioral plasticity," he adds. Grangier plans to investigate in the future whether wasps in their native habitat, North America, exhibit the same behavior or whether it is something new that the invasives in New Zealand have taught themselves out of necessity.

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