Pathogenicity of Syncephalastrum racemosum towards leafcutter ant nests

Abstract

Leaf-cutting ants use fresh leaves and flower parts as substrate to sustain the growth of a mutualistic fungus. Due to forage a large amount plant substrate, these insects are considered major agricultural pests. In Brazil, the control of leaf-cutting ants is accomplished with chemical insecticides, especially those that have the active ingredient sulfluramide, shown to have bioaccumulative effects and toxicity to non-target organisms. Previous studies by our research group (FAPESP grant # 06/58043-3 - Leaf-cutting ants control: integrated studies) showed that nests treated with sulfluramide are prone to development of the fungus Syncephalastrum racemosum. Few studies on the biology of S. racemosum suggested this fungus acts as an antagonist towards the fungus cultivated by leafcutter ants. However, information regarding the pathogenic potential of this microorganism is still incipient. This study aims to evaluate the pathogenicity of S. racemosum directly in the ant fungus gardens. Therefore, the first step is to check whether S. racemosum fulfill Koch's postulates. To accomplish this task spores of S. racemosum will be applied in subcolonies of the ant Atta sexdens rubropilosa. To evaluate whether S. racemosum acts as an opportunistic pathogen, bioassays using cross-inoculation of S. racemosum and Metarhizium anisopliae (one entomopathogenic fungus) spores will be carried out. This experiment will reveal if S. racemosum is capable to establish an infection or if is just an opportunistic fungus that receives benefits when fungus gardens are deprived of ants using an insect pathogenic fungus. Considering the need to reduce the use of chemical insecticides that have detrimental effects to the environment, the results of this proposal will contribute to find microorganisms with putative use as biological control agents of these pest insects.