Microorganisms form associations with most animal species, and a fascinating
example is the multiple interactions in the colonies of leaf-cutting ants. The effects of
these interactions (positive and negative) are exhibited in the health and development
of the colonies. Therefore, the understanding of the interactions that occur among the
microorganisms into leaf-cutting ants colonies is important to support of biological
control of this pest. This work was divided into three chapters. The first chapter aimed
to compare the development of colonies of Atta sexdens (Linnaeus) in contact with two
types of soil: (i) from an area used for nesting and (ii) from an area not used for nesting
of leaf-cutting ants. Two experiments were conducted in the laboratory. In experiment
I, newly fertilized females founded the colony in a plastic pot with gypsum and, after
106 days were transferred to a plastic pot with soil. In experiment II, newly fertilized
females founded their colonies directly on the soil. Colony mortality rate 106 days after
nuptial flight and founding in a plastic pot with gypsum was 28.6%. When they
developed directly in contact with the soil, mortality rate increased to 67.2%. The
results support that incipient colonies of A. sexdens undergo strong selective pressure
from soil microorganisms at the time of foundation. However, after the emergence of
the worker force, social immune defense mechanisms likely guarantee the
development of the colony, despite the presence of pathogenic microorganisms in the
soil of the nests. The second chapter aimed to isolate and identify actinobacteria from
soils of fungi garden chambers of A. sexdens and to evaluate the inhibitory effect of
these isolates on fungi associated with leaf-cutting colonies. To identify the isolates,
the 16S rRNA gene was sequenced from nine actinobacteria: six of Streptomyces
genus, two of Nocardia genus and one of Kitasatospora genus. Two Streptomyces and
one Kitasatospora isolates inhibited not only the fungus Escovopsis sp., but also the
entomopathogenic fungus Metarhizium anisopliae and the antagonistic fungus of the
cultivar symbiont of leaf-cutting ant Trichoderma aff. strigosellum. Since there is no
evidence of cultivation of actinobacteria on the Atta worker cuticle, it is possible that
these workers establish temporary adaptive symbiosis with soil microorganisms
producing antifungal and antibiotic substances and living in some part of their nest or
even in the interior of their body. It can be hypothesized that pathogenic fungi present
in the soil adjacent to the leaf-cutting ant nest, despite the risk they represent, are
controlled by the secretions produced by the workers, as well as by the metabolites of some actinobacteria. The third chapter had the objective of verifying the acceptance
and incorporation of baits containing mycelium of Escovopsis sp. by young colonies of
A. sexdens. We verified the transport of baits in all tested colonies. There was a
reduction in the weight of the fungus garden of the colonies that received baits with
Escovopsis sp., and an increase in the weight of the fungus garden of colonies that
received control treatment. It is concluded that the use of baits with mycelium of
Escovopsis sp. was satisfactory to introduce the fungus parasite in the fungus garden
of A. sexdens colonies.
|
