Invasion of the Leafcutter Ants

Invasion of the Leafcutter Ants

Tara A. Spears

Undulating like a soundless parade in the night, the legions of Leafcutter ants wave their pilfered leaves like colorful banners. Without interference, the marauding hordes can strip a two-meter tall shrub to bare stems in a matter of hours. For the first time in 15 years, my beloved Jaltemba Bay garden is under siege by the Leafcutter ants. 

The name, Leafcutter Ant, is actually an umbrella term for dozens of species belonging to the two genera Atta and Acromyrmex. Characterized by their spiny, reddish-brown bodies and long legs, leafcutter ants — also called parasol ants for the way they carry their leaves like parasols above their heads — are incredibly hardworking fungus farmers, and fascinating creatures all around. These species are known for their sprawling, complex colonies and their exceptional physical strength, but they can do a lot of damage to forests or gardens. 

Weirdly, the Leafcutter ants don’t eat the leaves they harvest. The sight of these insects, marching en masse with leafy greens held overhead, would naturally lead one to think they’re preparing a salad bar of epic proportions. However, the ants don’t eat the leaves; they feed them to their crops instead. A University of Montana study says the ants put them into “colony dumps,” similar to a landfill or compost pile, and that those dumps “create ideal conditions for the bacteria that make nitrous oxide,” a greenhouse gas. The decomposing leaves then help to fertilize the fungus gardens on which the ants sustain. 

While there are natural Leafcutter ant predators, such as Army ants, the population of Leafcutter ants outpaces them.  Birds, bats, and ground mammals feed on Atta queens searching for a nest site after mating as well as eating the ant brood. But, because the Leafcutter ant’s nest is inground, and consists of many individuals, a colony is difficult to eradicate. 

    The Leafcutter queen is the reproductive center of the colony and lives in chambers below the ground. Many colonies have five or more fertilized queens. Most eggs laid by the queens develop into sterile female workers. The workers (2 million or more) vary considerably in form and size; from 1/16 to ½ inch. Generally, large workers (soldiers) serve to protect the nest while medium-sized workers forage for plant material and construct tunnels and chambers, and the small workers maintain fungal gardens and care for brood ants. 

During the spring months, some immature ants will develop into winged males and others into winged females. Workers will emerge at this time to widen entrance holes and remove all vegetation hanging over these holes. Mating flights then occur on clear, moonless nights following a rain of at least ¼ inch, primarily in May and June. Mated females disperse, land, and then congregate with other females to establish a new colony to continue the life cycle. Each female carries with her a plug of fungus to begin a new fungal garden.

The activity of the ants above ground depends largely on temperature, as foraging ants are highly sensitive to temperatures encountered along feeding trails. During the summer, the ants remain underground during the heat of the day, often plugging the central nest holes with soil and vegetative debris, apparently to regulate temperature and/or humidity within the nest. At dusk, when temperatures drop below 86 F, ants emerge to forage throughout the night.

In the winter, ants forage during the day primarily from entrance holes above the central nest, but sometimes remain underground for extended periods when temperatures remain below 50 degrees F. Early spring and late fall are transitional periods when ants may be active both during the day and night. 

Treatment methods:  I try to avoid using strong poisons in my house and yard but the Leafcutters are tough. I poured very hot water on the nest- stopped the ants for only one day. I poured vinegar down the entrance- two days relief. Next, I sprinkled uncooked white rice which only deterred the ants for two days. At this point, I’m going to use poison because this colony is very established and quite large. The ants are winning the battle but hopefully not the war- I want my flowering plants back!

There are commercial insect poisons that are available at the local hardware stores: Koranay Ferreteria and Pina Ferreteria, carry powder/ pellets, like Trompa, that will do the job. The Agricultural supply store just 2 blocks south of the main La Penita Avenida, also has several treatments. Ask for “control de hormigas cortadoras de hojas.”

Amdro® was originally designed for fire ants, but with the addition of sugars, has become somewhat attractive to leaf-cutting ants. Applications can be made in all seasons, but treatments should be postponed during periods of prolonged rain. Once the bait is applied, foraging ants search out the pellets and carry them underground. The first signs of control will be a reduction in foraging and excavation activity, usually within 5 – 7 days, after bait application. These activities will gradually stop and the colony will become inactive within 2 to 3 weeks.

According to the U.S. Fish & Wildlife Service, studies of leafcutter ants have contributed to scientific advancements in pharmaceuticals and clean energy alternatives, due to their intake of cellulose, which they can’t themselves digest but their fungus crops help to break down. Recent discoveries of a sort of antibiotic-producing bacteria that they coat their bodies in has played an integral role in research on human antibiotics as well.

That is all well and good but my garden is being bombarded by these nocturnal marauders. It is so frustrating to come to sit on the patio with morning coffee and enjoy my tropical paradise…to find holes in the landscape instead. The ants can have all the jungle they want but I am drawing the ‘do not pass’ line before my yard.