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Odair Correa Bueno

Professor of General and Applied Biology at Unesp-Rio Claro


Sauvas: from the laboratory to the field

There are about 14,000 species of ants cataloged across the planet. It is estimated that the abundance of ants in the terrestrial environment is 20 times 10 to the 15th of individuals, of which 3 times 10 to the 15th are epigeic (forage on the ground). Thus, ant biomass is equivalent to 12 megatons (12 times 10 to the power of six tons) of carbon, that is, 20% of human biomass.

Among the importance of the presence of ants on the earth's surface, in addition to the accumulation of biomass, they stand out as the main predator of the environment (live or dead animals), defoliating plants, collecting pollen and nectar (mainly extrafloral nectaries) from plants and in nutrient cycling. In ecological standards, they act in seed dispersal.

All ant species are eusocial (highly social) and belong to a single family of Hymenoptera, Fomicidae. The ants are included in the subfamily Myrmicinae, tribe Attini and subtribe Attina , with about 140 species that cultivate mutualistic fungus and occur exclusively in the Americas. Three genera (Atta, Acromyrmex and Amoimyrmex) are known as leaf cutters, among them are the ants. In the forest sector, two species stand out: Atta sexdens (lemon ants) and Atta laevigata (glass-head ants).

The eating habits of the first Hymenoptera are basically the same as those of other insects: phytophagous, that is, primary consumers of terrestrial plants. The emergence of holometaboly allowed juveniles and adults of the same species to use different food resources, avoiding competition between them, and began to exercise larval herbivory and larval zoophagy. In the latter, he developed the ovipositor, which, with adaptations, gave rise to the stinger.

Part of these insects returned to the phytophagous condition , such as bees, wasps and ants, but not directly using plants in food, but exploring, in various ways, already prepared plant products as a wide way of feeding. Afterwards, a group of these ants began to use the plant material to grow their own food, with the help of a fungus, considered the main food source of the colony. Among them, the most derived are leaf-cutting ants.

These insects, originally with stingers, have in common life in society. Different from bees, whose command is centered on the queen, ants do not have a central command, but of the entire society. The mutualistic fungus of leaf-cutting ants is a Basideomycota (Basidiomycete), within the Order Agaricales, species Leucoagaricus gongylophorus, which rarely appears in reproductive (mushroom) form and is found only inside nests; therefore it is transferred by the içás during the nuptial flight when establishing a new sauveiro.

The first workers, when they emerge, organize a structure with plant fragments, forming a structure similar to a sponge, the fungus garden. It basically consists of long hyphae and, at its end, forms a rounded structure called gongylid, rich in nutrients: sugars, proteins and lipids. The group of gongylids is called staphyla , the main structure that is provided to the larvae during their feeding.

The digestive tract of adult ants has, before the mouth itself, a cavity where they store food before going to the stomach. This infraoral cavity works as a filter, limiting the passage of material. In ants, it is responsible for retaining solid particles, so that nothing larger than a micrometer passes into the stomach. This implies that adult ants only ingest liquid food. On the other hand, ant larvae have a mouthparts typical of chewing insects and therefore ingest both liquids and solids (adults suck cane and larvae eat cane).

Another interesting aspect is related to the unique structures that occur in adult ants. First, at the end of the thorax, on each side, there are two openings of the metapleural glands, responsible for the production of antimicrobial compounds, which prevent contamination of the nest by microorganisms. The second is the presence of a pair of glands of the salivary system that open at the end of the pharynx, they are the postpharyngeal glands or diverticulum of the foregut.

They were considered exclusive to ants, but recently they have been observed in some wasps. The function of these glands has always been debatable, initially related to the production or mixture of compounds (hydrocarbons) responsible for the odor of the colony, but, recently, it was verified that they are responsible for the metabolism of lipids ingested during feeding. To understand the difficulties encountered in controlling these ants, it is necessary to understand the complexity of a sauveiro.

The main food comes from attacking vegetables, especially leaves. While cutting the leaves on the plants and then cutting these leaves on the ground or inside the nest, the adult workers ingest sap and prepare the small cut pieces to form and maintain a garden of their mutualistic fungus. It is responsible for the degradation of plant material and for transforming it into simpler compounds for the ants' nutrition, especially for the larvae, which receive them directly in the mouth by the workers.

Also due to the extracellular digestion of the fungus, the garden is full of sugars, mainly the glucose that can be ingested in the nutrition of the larger workers. The queen of the colony also receives pieces of fungus provided by the workers directly in her mouth. Inside the nest, protective bacteria occur together with the secretions of the metapleural glands, whose products prevent contamination of the fungus garden. In addition, there is a vast amount of associated microorganisms that help in the degradation of plant material and in the protection of adult and immature ants against entomopathogenic organisms.

On the other hand, antagonistic fungi to the mutualist also occur inside the colony whose development is impeded by the workers and protective bacteria. They only develop when the workers lose control of the maintenance of the sauveiro. Leaf-cutter ants, despite being cited since the discovery of Brazil, continue to occupy a prominent place in the agricultural scenario and in the establishment of forests in the country.

They are often referred to as key pests in reforestation, severe pests in agriculture and, in certain monocultures, such as sugar cane, citrus and pastures, cited in many scientific articles. It should be added that attempts to control these insects are as old as their pest status, but we are certainly losing a lot. It is not for lack of work with ants, as there are, in the world literature and especially in Brazil, thousands of articles on leafcutters; however, more than 80% refer directly to the control, many are out of date, and repeatability is very high. This type of publication is lost in time.

Two factors contribute to this: the lack of diversification of efficient active ingredients and the low investment in knowledge of leaf-cutting ants, including adequate monitoring. If the loss caused by these ants in forestry can reach millions of reais, why not increase investments in greater knowledge of this society and in the interaction with the environment and in the qualified training of the workforce that performs the monitoring and control of these insects? It is important to point out that there has been a drastic reduction in the number of researchers specifically involved in the area.

Several commercial products and control methods are indicated to combat leaf-cutting ants. This occurs despite the intense search for new products and control alternatives, which have not been found so far. Over that time, there has been a great evolution in the attitude of forestry organizations towards the control of cutters, such as the search for more selective chemical products with less impact on the environment, the reduction in the quantity of these products to be applied in the field and, above all, the development of an awareness of environmental preservation.

All this, naturally, leads to the integrated management of leaf-cutting ants, including monitoring, which is already part of the activities of most reforestation companies in Brazil. The world chemical industry is not investing in the discovery of new molecules, just remember the Nature article (Volume 543 of 2017) “ When pesticides run out ”. This is not what happens with other agricultural pests, where there are high investments in chemical pesticides, but also in knowledge of the pest and in the use of new methodologies.

Biological control, always cited as important, has shown little applicability with ants, but one should always hope for success. Few studies relate the factors involved in the defensive capacity of leaf-cutting ants, see the complexity of the sauveiro. The investigation of defense mechanisms, such as social behavior and the presence of symbionts, become relevant, as it makes it possible to clarify the ability of the workers to detect and remove pathogens and even detect the presence of undesirable chemical compounds.

Basic research on this subject may reveal the physiological characteristics of leaf-cutting ants and may contribute to rethinking the control methods currently used. The forms of chemical control of leaf-cutting ants are widely known, but the technology of toxic baits is the only one possible to measure so far. Currently, it is the most used form in forestry, exceeding the level of 80%. The novelties in the leaf-cutting ants control sector are the new way of applying liquid insecticide by spraying directly onto the soil, still very little evaluated, and the use of moisture-resistant toxic baits, which allow it to be used in rainier regions and with high soil moisture.

The major failures in the fight against leaf-cutting ants are related to the use of unsuitable active ingredients, which cause the ants to repel or act too quickly, which also repels the ants, and inadequate monitoring when establishing what a nest is, estimate of its size, calculation used to quantify the amount of bait to be applied, the places where they should be applied and the application methods (mechanized or manual). Even using the best market baits, monitoring and application techniques are responsible for the low level of success in controlling leaf-cutting ants.

In fact, implanted forests can be, for researchers, real open-air and large-scale laboratories, a process that is still in its infancy. Investment in the training of qualified human resources is also essential in order not to repeat the achievements of the past. The big dream: it would be to think about the logic of reverse engineering. In terms of control of leaf-cutting ants, the action's extras have to be involved: the plants, monitoring, including the application of baits and the society of ants.

For plants, it is possible to think about genetic improvement, molecular biology, including transgenics. In monitoring, invest in the training of qualified and competent personnel and in the forms of application and their equipment, such as the use of mechanics and drones. In terms of the ants' society, the most complex of ants, agricultural researchers (agronomists and forestry engineers) and biological researchers can contribute. This is in short supply, but when it is possible, let it be with boldness.

The Nature article signals investments in biocontrol , both classic and new technologies, such as omics (genomics and proteomics), genetic engineering, among others. This is happening with other pests, especially those with a large return on the world market. The costs of new technologies and of training high-level human resources are high, and Brazilian financing agencies (their managers and analysts) have little sensitivity in approving audacious and high-risk projects.

Therefore, it is high time that radical changes took place; not that this will immediately solve the problem, but it will start a new era, not only investing in new lines of research, but also in training qualified human resources. Actions must be joint, coordinated and centered on the final objective, to enhance efforts to control these insects. It does not mean preventing a researcher or someone from his group from carrying out a certain research, but rather interacting in discussions about what can be done and how to do it quickly and with practical answers. An essential question to be clarified is the following: has the fight against leaf-cutting ants achieved its objective, considering that it has been carried out for more than 50 years, mainly with ant baits? I remember that little man sitting in front of the garden; when asked about a huge ant nest he says: “Look, young man, that sauva has been there for over 60 years, it never ends”.