Mycopathologia 150: 53–60, 2000. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. 53 In vitro evaluation of Trichoderma and Gliocladium antagonism against the symbiotic fungus of the leaf-cutting ant Atta cephalotes Adriana Ortiz & Sergio Orduz Biotechnology and Biological Control Unit, Corporaci´ on para Investigaciones Biol´ ogicas – CIB, Medellın, Colombia Received 11 April 2000; accepted 23 January 2001 Abstract The antagonistic activity of Trichoderma and Gliocladium isolates against Attamyces sp., a symbiotic fungus of the leaf-cutting ant Atta cephalotes, was investigated. A. cephalotes cultures this fungus as the primary food source. Most of the Trichodema and Gliocladium isolates tested in vitro (82.6%) inhibited the Attamyces sp. mycelial growth, which was probably due to their colonization ability and competition for nutrients, both of them known mechanisms of some species of these genera. T. lignorum strain T-26 was the strongest inhibitor achieving a colonization of 23%. Microscopical observations indicate that the inhibitory effect was caused by an interaction that took place in close contact with the host hypha, causing wall deformation that led to the collapse of the turgor pressure. Key words: antagonism, Atta cephalotes, Attamyces sp., Gliocladium sp., Trichoderma sp. Introduction Leaf cutting ants (Hymenoptera: Formicidae) are gen- eralist herbivores causing great losses in Neotropical crops, and are considered to be one of the 5 most im- portant pests in Latin America. The latest current con- trol measure against leaf-cutting ants has been the use of chemical insecticides, with the ensuing problems of toxicity on beneficial insect fauna, development of resistance in the target insect, and environmental contamination [1]. Fungus gardens of the Attini tribe are pure cultures of an obliged symbiont not clearly identified yet; nev- ertheless, biochemical and microbiological characters in the fungus’ mycelium indicate it to be a basidio- mycete [2]. These fungi produce hyphae with the gongylidiae, which constitute the larvae and queen’s food [3]. The fungus had been classified as Rozites gongylophora, but other names have been also as- signed, Leucocoprinus gongylophorus, Leucoagaricus gongylophora, and Agaricus gongylophora. Although its taxonomic status is still contradictory, it has been proposed the use of the name, Attamyces sp. [4, 5]. Fisher et al. [6] reported a basidiomycete in a 13 year old Atta cephalotes laboratory colony, and later identified it as Leucoagaricus gongylophorus. After a basidiome was sectioned and the tissue cultured, sim- ilar gongylidiae to which are found in the ants’ garden were observed. The complex social organization of the leaf-cutting cutting ants and the different enzymes and compounds they use to keep gardens pure, has generated alternat- ive research lines on control methods through inhibi- tion of the fungus’ growth. In natural ecosystems, ants prefer some plants, while others are not even tasted, possibly due to the plant defense mechanisms that can cause the ants’ death directly when ingesting sap, or indirectly by stunting the symbiont fungus’ growth [1, 7, 8]. Literature reports mycoparasitic fungi displaying lethal mechanisms against phytopathogenic fungi that could be of importance for plant disease biological control [9, 10]. Isolates of the genera Trichoderma and Gliocladium, have been the focal point of a number of studies concerning their ability to control plant patho- gens. Strains of the species T. harzianum has shown effectiveness when used in disease control caused by several fungi, including Sclerotium rolfsii, a widely