Ants that grow their food have to weed, too. Now, the first detailed study of ants tending fungus gardens shows that whether the gardener has two legs or six, the chore looks much the same.
Like the best human gardeners, ants try to stop a weed invasion in its early stages. Ants settle in one spot and weed, then move to the next spot. They are amazing weeders, and today about 200 ant species cultivate a spongy lump of fungus inside their nests. Leaf cutter ants have elaborate gardens which use bits of foliage they collect to nourish their fungi.
15 colonies of Atta colombica ants from Panama were brought into a lab and each colony had its own soup-can-size fungal mass which workers cultivated with brute force.
Into one garden the scientists put a nugget of an invasive fungus, and in about 10 seconds a worker ant would come along, pick up the nugget, and carry it off to the dump pile.
The scientists then presented a bigger challenge by spraying the top of the fungus gardens with plain water or a solution of spores from one of two weed fungi. The tiniest workers, which do most of the gardening, quickly assembled.
At first they would lap up the droplets, within an hour the surface was bristling with ants but they didn't seem to be moving much. What they were doing was a largely stationary behavior called fungus grooming. Ants settled in a spot, patted the nearby garden with their antennae, and combed nearby fungal filaments with their mouthparts. The ants were scraping up invader spores.
The ants then sacrificed parts of the crop, as if containing a plague. Ants will actually straddle a leaf snippet, rocking and tugging until it comes loose. Bigger ants lugged the chunks to the dump.
The ants responded mildly to the water, but more vigorously to spore solutions. A spray of Escovopsis, a dangerous pathogen specializing in ant gardens mustered the most workers. They groomed the most intensely and hauled out the most trash. Despite the extra effort, however, Escovopsis persisted.
In recent studies of ant gardening, the power of pathogens to drive ant evolution has been surprising, and let us know that ants, like humans, will always have a weed battle on their hands that they may or may not be able to win.