Ants’ Fear of Turning into Zombies Help Protect Coffee Farms

There’s a delicate dance happening on the coffee farms in Central America between several interconnected insects. Central to this tango are the massive colonies of Azteca ants. These fiery red devils protect its friend the green coffee scale from outside attacks. While under protection, these little guys feast on the coffee beans and secrete a sugary substance that the ants in turn feed on themselves.

While this sounds like a great situation for both species, it’s a nightmare for coffee farmers. Needless to say, they don’t want these little buggers eating up all of their crops. One method of trying to control their populations without the use of abrasive chemicals is by killing off the surrounding shade trees that harbor the Azteca ants, thus removing the green coffee scale’s protectors.

But not so fast.

There are two other dancers in this tango – the predatory lady beetle and the parasitic phorid fly. The former is the dancer that eats the green coffee scale – the critter that the Azteca ants are protecting the scales from.

The latter turns ants into zombies.

The tiny humpbacked phorid fly has an eye for ants in motion. When it spots them, it attacks, laying its eggs inside the bodies of the ants. Once hatched, the larvae take control, making the ant wander aimlessly as they devour their prey from the inside out. Eventually, acidic secretions from the invaders dissolve the ants neck, causing its head to fall off and the larvae to emerge into the world.

Naturally, when ants come under attack from the phorid flies, it is in their best interest to drop everything that they’re doing and stand still in order to avoid detection. This is coordinated colony-wide by the ants secreting a chemical pheromone warning others of their impending doom. For hours after an attack, an ant colony’s hustle and bustle can decrease by as much as half.

An Azteca ant attacking the larvae of a lady beetle on a coffee plant in Mexico. Lady beetle larvae are covered with waxy white filaments that protect them from ant attacks. Photo by Ivette Perfecto.

And this time-out gives the predatory lady beetle the perfect opportunity to lay some eggs.

With the ants mostly motionless, it becomes safer for the beetle to approach. And approach they do, as the presence of ants naturally also means the presence of green coffee scales and food for their offspring when they hatch.

Now, in a recent paper, Ivette Perfecto from the University of Michigan and Heidi Liere from the University of Wisconsin have shown that predatory lady beetles can in fact detect the warning pheromones given off by ants. While male beetles don’t seem to pick up the scent, lady beetles – especially those read to lay eggs – can tell when ant colonies are under their “red light” stage and drop by to leave a little present.

The thing is, the beetles actually are helped by the ants presence, because it tells them where the green coffee scales are. And they’re helped by the phorid fly, who knocks out the ants activities so that they can get their eggs close to the food source.

So rather than destroying all of the Azteca ants in order to leave the green coffee scales more vulnerable, it might actually be best for coffee farmers to conserve the colonies, so that the predatory beetles know where to lay their eggs and eat the pests.

Funny how the world works.

The authors suggest that such complex interactions may be common in nature, “and their uncommon occurrence in the literature is the product of investigators failing to search for them in the first place.”

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About bigkingken

A science writer dedicated to proving that the Big Ten - or the Committee on Institutional Cooperation, if you will - is more than athletics.
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