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Keep Them Aphids Rollin': Ant-Aphid Symbiosis in West Texas

“Myrmica puceron”. Licensed under CC BY-SA 3.0 via Wikimedia Commons

Predators and fierce competitors are facts of life in nature. But for many creatures, mutually beneficial relationships are just as important.

In West Texas, we can look to the insect world for proof. On trees and shrubs across the region, ants and tiny aphids live in a mutually beneficial symbiosis.

Aphids are soft-bodied, sap-sucking insects. They've earned the anger of farmers and gardeners. They use piercing mouth parts to tap into plants and pull out nutrients and water. They're destructive agricultural pests.

But for ants and other insects, this activity has an upshot. As they feed, aphids secrete a sugary liquid called “honeydew.”

It's a nutrient-rich liquid that ants love. Worker ants will even herd groups of aphids for this honeydew supply.
It's called aphid farming. But to a West Texan, it looks more like ranching.

Michael Nickell is museum scientist at the Sibley Nature Center.

“They will move these aphids from pasture to pasture on a plant, like a mesquite or something like that,” Nickell said. “If it looks better over here, then they will pick up their aphid 'cows' and move them to a different pasture. It's not like they drive them up there, but they will pick them up and transport them.”

Aphid farming is common among ants around the world. In West Texas, it's practiced by native army and carpenter ants and non-native fire ants, among other species.

The ants will go out of their way to maintain the aphid herd.

“There are some ants that will take those eggs for wintertime, take them underground into their burrows and take care of the eggs,” Nickell said, “and when they hatch, they will take the newly hatched aphids out to pasture, on the plants.”

Pheromones are the key to communication in the ant's sophisticated social world. With these chemical cues, ants can alert to a threat, leave a trail for their companions or identify their role in the colony. And they use pheromones to shape aphid behavior as well.

“What keeps the aphids from running all amok up and down the plant? Well, it's pheromones from the feet of the ants,” Nickell said. “It basically keeps the aphids in place. It reduces their will, shall I say, to go wandering off.”

With the aphid herd effectively “fenced in,” individual ants will “milk” aphids for honeydew.

“When they feel like, 'I need a little pick-me-up,' so to speak, they will actually stroke the aphids with their antennae,” Nickell said. “Then the aphids will produce drops of honeydew, and they get a quick meal.”

A good deal for the ants. But what's in it for the aphids?

Protection. Ants guard their aphids from predators, which can include ladybugs, lacewings and certain wasps and spiders. With stings or bites, the ants will protect their aphid stock – which is something to watch for, Nickell said.

“If you see this type of phenomenon going on on a tree or mesquite bush, and you want to explore it a little bit, don't use your finger,” Nickell said. “Pick up a little twig or something, because the ants will take care of their possessions.”

Aphids can reproduce both sexually and through “parthenogenesis” – in which females clone themselves. When the conditions are right, aphids produce a generation of winged offspring. The ants, of course, aren't interested in aphids that can fly away.

“Now here's where it becomes really interesting,” Nickell said. “Ants know about this. And so those aphids that are producing a generation of winged individuals, the ants will sometimes clip the wings, or produce a chemical that will cause a stunting of the growth of the wings.”

For the ants, symbiosis has its limits.

“So, yes – it is a mutualistic type of situation,” Nickell said. “The aphids do benefit. Still, the ants have the trump card. They're running the show.”

It's been said that the state of nature is a “war of all against all.” Looking at the relationship of ants and aphids, we can see that's not the full story.