Sweeper Ants of the Selva Seca

By Julie Etra

I got introduced, so to speak, to hormigas barrenderas through my friends and neighbors Doreen and Larry. They had described a somewhat terrifying episode of a home invasion of these carnivorous ‘sweeper’ ants native to the Selva Seca or Selva Baja (dry tropical forest) found along the Oaxacan Riviera. They are a type of army ant and although there are over 400 species and sub species of ants found in the soils of Mexico this species is unique in its social make up and behavior. They are carnivorous predators and consume only live prey. Other common names are legionarias (soldiers) and marabunta (crowd). 

They appear as a black crawling a mass or living carpet moving ceaselessly across the forest floor, always in motion with seemingly nothing to impede their progress. They are extremely aggressive and nothing edible in their path including cockroaches, crickets, scorpions, salamanders, geckos, and lizards is spared. The good news is that since they eat everything in their path, with creatures fleeing as they march on, they leave a house remarkably free of ‘pests’, hence the sweeper nickname. One observer witnessed the sweeper ants totally overwhelming a very aggressive species of wasps, taking out an entire hive, including the larvae, in less than an hour.

In general, army ants have huge colonies. Only the males and female queen have eyes; the rest including the workers and soldiers are blind and follow chemical trails which they exude, which is why they are so hard to disrupt when en route. The soldier ants, with huge jaws (so large they can’t maneuver them to feed themselves) watch over the workers. They have basically two strategies of attack, in columns or in swarms, and they usually raid at dawn. The ones we have around our place are swarmers, with stronger bites and more potent stings than column raiders.

The ‘nests’ of army ants are not nests at all but are comprised of the ants themselves and referred to as ‘bivouacs’ where the ants rest, clustered together to form walls or balls and fastened onto each other using their mandibles and leg claws. Work by Dr. Couzin, a mathematical biologist at Princeton University, has extensively studied the swarming behavior of army ants. Dr. Couzin wanted to know why army ants move to and from their colony in an organized manner. They form a collective brain able to make decisions and move like a single organism by following chemical markers. They also sweep the air with antennae to avoid colliding with other ants.

Just after dawn one morning in March, I looked out of our second story window to the forest behind the house and saw one flank of sweepers headed towards the house. Instead of following our friend’s advice to flee for a half day we decided to ‘fight’ back and turn back the siege. Or at least try. This had happened before while I was out of town, and my husband had hosed them back from his retreat in the pool. This time the siege lasted almost 12 hrs. We used concentrated chlorine powder along multiple trails along with high-pressure water, producing chlorine gas, no doubt, in an effort to minimize the chemical trail, while we periodically retreated to the pool to avoid their nasty bite. Although it worked, next time we will grab the dog and leave.

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