Leafcutter ant

Leafcutter ants are remarkable insects that form some of the most complex animal societies on Earth. They belong to the tribe Attini and include at least 55 different species. These ants are found in tropical regions of South and Central America, Mexico, and parts of the southern United States. Unlike many other insects, leafcutter ants are famous for their advanced agriculture. They do not eat the vegetation they harvest. Instead, they use leaves, flowers, and grasses as a substrate, which is the material used to grow a specific type of fungus.

This relationship is known as an obligatory mutualism. This means the ants and the fungus depend on each other to survive. The ants provide the fungus with fresh plant material and protect it from pests. In return, the fungus produces nutritious, swollen tips called gongylidia. These structures grow in bundles called staphylae to feed the ant larvae. The adult ants actually feed on leaf sap. This farming system is so successful that these ants fully domesticated their fungal partners 15 million years ago.

The ants are highly organized into different groups called castes. These castes are determined by their physical size, a trait known as polymorphism. Minims are the smallest workers, often with a head width of less than 1 mm. They stay in the nest to care for the brood and the fungus gardens. Minors are slightly larger, with head widths between 1.8 and 2.2 mm. They act as a first line of defense by patrolling foraging columns. Mediae are the general foragers that cut and carry leaf fragments back to the nest. Finally, majors, also called soldiers or dinergates, are the largest members. Some Atta laevigata soldiers can reach a body length of 16 mm.

Starting a new colony is a difficult and dangerous process. It begins with a nuptial flight, or revoada, where winged males and females leave their nests en masse. A female mates with multiple males to collect approximately 300 million sperm. After mating, the female finds an underground lair and sheds her wings. She must carry a small piece of the parental fungus mycelium in her infrabuccal pocket, which is a part of her oral cavity. Most queens fail at this stage; only about 2.5% successfully establish a long-lived colony. While most colonies are founded by a single queen, a process called haplometrosis, some may have multiple founders, known as pleometrosis.

Maintaining the health of the fungus garden requires constant work and waste management. The ants must protect their food from a parasitic fungus called Escovopsis. This parasite can be very dangerous, appearing in 6.6% of new colonies and up to 60% of colonies that are one or two years old. To fight this, ants perform "weeding" by removing infected pieces of the garden. They also use a symbiotic bacterium called Actinomycetota, specifically from the genus Pseudonocardia. This bacterium lives in the ants' metapleural glands and produces antibiotic chemicals to kill harmful microbes.

Leafcutter ants also use sophisticated communication to keep the colony running. They use chemical signals and stridulation, which are vibrations sent through the ground, to talk to one another. They are even sensitive enough to detect chemical signals from the fungus itself. If a specific plant is toxic to the fungus, the colony will detect this and stop collecting that type of leaf. This intelligence helps them manage their resources effectively. They often prefer disturbed habitats because the pioneer plants found there have higher nutrient concentrations and fewer secondary metabolites.

Because of their massive size and appetite, these ants can impact human environments. A single colony can be enormous, with a central mound and radiating mounds covering large areas. These nests can house as many as 3.55 million individuals. In some areas, they are considered agricultural pests because they can defoliate an entire citrus tree in less than 24 hours. Interestingly, researchers found that placing the refuse, or waste, from an ant nest around crops can act as a deterrent for 30 days. This shows how even the waste of their complex society can interact with the world around them.