Cassava ants

General

• Globally, cassava ants include more than 40 species. Atta sp . (19 species) and Acromyrmex sp. (30 species) belong to the Attini tribe.

• Several species plague vegetables and fruit trees in Guadeloupe and Guyana.

• Observed in the open field and under shelters.

• Sensitive botanical family (s) * :

All botanical families are susceptible to the ravages of cassava ants. 

• Affected production areas :

• Organs attacked

Symptoms, damage

Even today, the cassava ant is considered one of the most formidable crop pests (if not the most feared) in French Guiana. Cassava ants are active throughout the year. The damage is therefore visible at all climatic periods.
All production can be compromised during an attack by cassava ants. Pastures are apparently less affected by the problem of cassava ants.
The damage is mainly carried out during the night. During harvest round trips, the workers form a veritable path that allows them to optimize movement.
To understand the extent of the damage that can be caused by cassava ants, let us quote the figure of 15%, which represents the total percentage of leaves from tropical forests that are eliminated by the action of these insects in South America.

• Symptoms :
  • The leaves of the plants are cut
  • In case of strong attack, one can observe a complete defoliation of the attacked plants

• The individuals that make up the colony

There are several castes (individuals of the same species whose morphology is different) within the same colony. For the Atta sexdens species, there are 5 castes which each have well-defined tasks:
- The queen ensures the production of ants in order to perpetuate the population. It is large: about 2.5 cm.
- The soldiers whose role is to protect the colony from external attacks. For this their powerful mandibles easily cut the skin of any offender. Their size is between 1 cm and 1.2 cm.
- The major workers locate the harvesting areas, collect the plants and transport them to the nest. They also have a role in the construction of the chambers and in the defense of the nest. They measure 0.6 to 1 cm.
- The media workers take care of cutting the fragments brought by the majors into smaller pieces. They also manage the colony's waste and take care of the queen.
- The minor workers or gardeners cultivate the mushroom by cutting and arranging the leaf fragments on the mushroom. They take care of the brood (eggs, larvae and pupae). Even smaller than the majors (2 to 4 mm) their life is mainly underground.

The workers are the most numerous: they can reach up to 8 million individuals in a colony of A. sexdens. The soldiers and workers are all sterile.
Fertile males and females are produced by the queen shortly before the swarming flight: period of colonization of a new area. They appear only after three years after the formation of a new colony.

• Colony life

The queen produces all the individuals of a colony of cassava ants. In an established colony, the life of the queen is dedicated to egg laying. The eggs give birth to larvae after 3-4 weeks.
The larvae are fed by the workers with their secretions and infertile eggs produced by the queen.
3 to 4 additional weeks are needed to form the pupa. Pupation also lasts 3–4 weeks and provides an adult ready to work for the colony.
Leaf harvesting takes place more than 60 m from the colony. Cutting foliage and transporting fragments are two tasks that are sometimes performed by ants of different sizes within the major worker caste. The larger ones cut the leaves and the smaller ones transport.
The workers (all castes combined) feed on 95% of the sap that flows from the leaves during the work of harvesting, cutting and grinding plants.
For the maintenance of the garden, the life of cassava ants breaks down into several main stages:
- Bring the piece of leaf to the nest,
- Examine, lick and clean the leaf fragments
- Cut the leaf into small pieces
- Prepare a porridge with the leaf, saliva and excrement,
- Place this mixture on the mushroom garden to serve as a substrate,
- Trim the garden,
- Harvest the mushroom from the garden

Waste management is handled by the media workers. This part is very important because the waste is a source likely to harbor parasites of the symbiotic fungus. The total mass of waste from an Atta colony can reach a few hundred kg. This waste is stored in specific rooms. Inside these chambers live the older individuals.

• The symbiosis with the fungus

Most studies tend to show that the relationship between the fungus and the Atta ants is obligatory: one cannot survive without the other.
From the leaf fragments taken by the workers, the garden ants cultivate a fungus. Only one species is cultivated, it is Leucocoprinus gongylophorus . The latter is grown from plant fragments (leaves, flowers, fruits, petioles) taken by the workers. The minor workers form a mush with the fragments of leaves and their saliva which will constitute the substrate of the fungus.
To develop, the fungus has specific enzymes that degrade lignocelluloses (molecules of which plants are made).
Ants are adept at consuming fungus protuberances called gongylophores thanks to their specific enzymes that allow them to break down fungal cell walls. If we visualize the colony as a living organism in its own right, we can consider that the fungus provides the first phase of plant digestion.
In order to prevent the development of other fungi in the colony, some workers harbor a procession of bacteria on their cuticle which produce antibiotic substances against these parasitic or antagonistic fungi. In addition, ants secrete substances which inhibit the germination of certain antagonistic fungi of the cultivated fungus.
This is how cassava ants achieve a pure culture of a single species of fungus.

• The architecture of the nest

The nest can reach a diameter of 20 m. According to the literature consulted, the depth of the nest is generally 6 to 8 meters in Atta sexdens .
There are several types of lodges called chambers within an anthill:
- the mushroom chambers containing the mushroom (also called millstone) whose size is about 20 cm in diameter
- rooms reserved for the development and care of young people
- the dump chambers (where corpses and other waste such as non-productive portions of mushrooms are stored)

The rooms are interconnected by galleries. Air vents allow optimal ventilation of the nest while limiting water ingress. Exit holes allow workers and soldiers to quickly access the outside of the nest. the In A. sexdens nests may contain more than 1000 chambers connected by thousands of underground tunnels.

• Colony formation

In colonies established for at least 3 years, the queen will produce fertile males and females during the first rains of the wet season. A queen can thus produce 3000 females / year for the swarming period.
These virgin females carry away during the swarming a small portion of the fungus in order to ensure the creation of the future nest. During the flight of sexual individuals, the female is fertilized by several males and retains a stock of spermatozoa for her entire life.
It lands several kilometers from the nest from which it originated and buries itself in the ground about 15-30 cm from the surface. She fertilizes her mushroom pellet with her excrement and her first eggs. Only 0.05% of females will produce a viable nest.
The female begins to lay eggs after 4 days at a rate of 50 eggs per day. Part of the eggs are consumed by the queen who continues to fertilize the mushroom, the other part of the eggs being destined to produce the future individuals of the colony.
The first larvae feed on the fungus. The first workers ensure the longevity of the fungus and help in the rearing of the larvae. Then come the first explorers and cutters. This first stage of life of a new colony lasts about 9 weeks.
The queen devotes the rest of her life (which can last more than 10 years) to laying eggs. Soldiers do not appear until 3 years after nest formation.
The lifespan of a colony of A. sexdens is limited to the lifespan of the queen, which is 10 to 20 years.