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Psyche
Volume 2012 (2012), Article ID 389806, 6 pages
http://dx.doi.org/10.1155/2012/389806
Research Article

Diversity of Fungi Associated with Atta bisphaerica (Hymenoptera: Formicidae): The Activity of Aspergillus ochraceus and Beauveria bassiana

1Departamento de Biologia Animal, Universidade Federal de Viçosa, 36570-000 Viçosa, MG, Brazil
2Departamento de Microbiologia, Universidade Federal de Viçosa, 36570-000 Viçosa, MG, Brazil
3Curso de Engenharia Florestal, Universidade Federal do Tocantins, 77402-970 Gurupi, TO, Brazil

Received 10 August 2011; Revised 15 October 2011; Accepted 24 October 2011

Academic Editor: Alain Lenoir

Copyright © 2012 Myriam M. R. Ribeiro et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The grass-cutting ant Atta bisphaerica is one of the most serious pests in several pastures and crops in Brazil. Fungal diseases are a constant threat to these large societies composed of millions of closely related individuals. We investigated the occurrence of filamentous fungi associated with the ant A. bisphaerica in a pasture area of Viçosa, Minas Gerais State, Brazil. Several fungi species were isolated from forager ants, and two of them, known as entomopathogenic, Beauveria bassiana and Aspergillus ochraceus, were tested against worker ants in the laboratory. The two species were highly virulent, achieving 50 percent worker mortality within 4-5 days. It is the first time A. ochraceus, a commonly found fungal species, is reported to infect Atta species at a high prevalence. Possible uses for the fungus within biological control are discussed.

1. Introduction

Leaf-cutting ants (Atta and Acromyrmex genera) are considered dominant herbivores in the Neotropics [1] and also a severe pest when attacking crops. The leaf-cutting ants include a number of species which cut predominantly monocotyledonous plants and are known as grass-cutting ants. Knowledge of the morphological, biological, behavioural, and breeding characteristics of these species is still lacking, and are important by the difficulties in controlling their colonies. Among the nine Atta species occurring in Brazil, Atta bisphaerica Forel, Atta capiguara Gonçalves, and Atta vollenweideri Forel cut mainly monocots, and one, Atta laevigata (Smith), cuts both monocots and dicots.

Atta bisphaerica, or “saúva mata-pasto” as it is commonly known in Brazil, stands out as a major pest of pastures and sugar cane [2], currently occupying a leading position as pest species in São Paulo sugar cane plantations and pastures of southeastern Brazil. The control of this species through granular baits is not appropriate as the baits, which are available in the market, attract with the citrus pulp, which is unattractive to grass-cutting ants. This fact has stimulated the search for new materials for a bait production which is more attractive to these grass cutters [3, 4] and also the demand for natural control agents for leaf-cutting ants in general, including fungi [5] and parasitoids [6, 7].

The main advantage of biological control would be the reduction of the use of insecticides that pollute the environment and may constitute a barrier in the export of Brazilian products. Furthermore, in November 2009, the Forest Stewardship Council (FSC) approved the use of the sulfluramid, fipronil, and deltamethrin active ingredients of insecticide baits for another five years in the Brazilian eucalyptus plantations [8], which provides some time for the forestry companies to find alternatives for pest control.

The potential of using fungi to control leaf-cutting ants is large, considering that several entomopathogenic species have been found in worker ants, nearby or within the nests of the ants [913] and may cause the death of the worker ants or of the fungus garden. While leaf-cutting ants are constantly in contact with a diversity of entomopathogenic fungi, they do not appear to be normally infected by them. This supports the view that they are effectively protected against these parasites by behavioural mechanisms such as intensive self-grooming or allogrooming [14, 15] and utilization of antibiotics derived from glands or symbiotic bacteria. Knowing the diversity of these fungi and the ants’ resistance mechanisms is essential if one wants to implement safe and efficient microbial control methods for the pest control.

The purpose of this study was first to isolate and identify the filamentous fungal species associated with A. bisphaerica. Then, we tested the entomopathogenic activity of two species found in the first part of our study presented here: an Aspergillus ochraceus isolate, which was the most observed, and an isolate of Beauveria bassiana, a species widely used in microbial control of insects and also found in worker ants of A. bisphaerica, were tested in the laboratory.

2. Material and Methods

2.1. Collection of Ants in the Field and Research of Pathogenic Fungi

The ants were collected in a pasture in Viçosa, MG, Brazil (20° 44′S, 42° 50′W) on May 27, 2010, between 2:00 and 4:00 PM during their foraging time. We collected 100 foraging worker ants, which were on their return to the nest with grass fragments, each of three colonies (I, II, and III), which demonstrated foraging activity, totaling 300 workers ants. The ants were taken to the laboratory and isolated in Petri dishes supplied with a honey-water solution (1 : 1) and sterile distilled water, being monitored daily until death. The dead worker ants were washed in series with a solution of 70% ethanol, 4% sodium hypochlorite, and distilled water. After washing, they were transferred to sterile Eppendorf tubes, containing moist cotton and kept in the incubator at a temperature of °C, a relative humidity of %, and a 12-hour photoperiod, until the external appearance of fungal hyphae. Then the isolation of the fungi was performed in a vertical laminar flux hood. The fungi were inoculated in Petri dishes (9 cm diameter) with a potato dextrose agar (PDA) culture containing antibiotics (penicillin, 0.5 g/L, streptomycin, 0.5 g/L). After obtaining pure cultures, they were subjected to microscopic analysis by performing the identification and classification of the fungi with the aid of taxonomic keys [1618] and then confirmed at the generic and species level by Dr. Harry Evans.

2.2. Infection of the Ants in the Laboratory

To assess the potential pathogenic fungi B. bassiana and A. ochraceus, isolated from the workers collected in the field as described in Section 2.1, we tested the mortality of the A. bisphaerica worker ants taken from colonies maintained in the laboratory and inoculated with a spore suspension. The colonies were collected 10 days before at Viçosa, Minas Gerais State, Brazil, and maintained in the laboratory according to the methodology developed by Della Lucia [19]. They were maintained at °C, relative humidity of %, and a 12 : 12 light : darkness regime. On a daily basis, they received cut leaves of Hyparrhenia rufa (Poales: Poaceae) and Zea mays (Poales: Poaceae), in addition to clean water. Five hundred and fifty workers (head width ~3.2 mm) were collected in roughly equal proportions from the three colonies (A, B, and C, maintained under laboratory conditions) in the fungus garden, which probably were less likely to be contaminated. Conidial suspensions of the two fungi species were prepared from a Tween 80 solution of 0.05% at concentrations of 105, 106, 107, 108, and 109 conidia/mL. For each concentration, 50 worker ants received 1 μL of the suspensions on the pronotum. One μL of a 0.05% Tween 80 solution was applied to the control ants. This control group was used in the comparison with the two isolates since the experiments were conducted simultaneously. After receiving the treatments, the insects were held individually in Petri dishes supplied with a honey-water solution (1 : 1) and sterile distilled water renewed every two days. The mortality of the worker ants was daily monitored. The dead worker ants were washed as in the previous experiment and transferred to sterile Eppendorf tubes, containing moist cotton and kept in the incubator at temperature of °C, a relative humidity of %, and a 12-hour photoperiod, until the external appearance of fungal hyphae. Then we proceeded with the isolation of the fungi, which were randomly selected in all the used concentrations with 15 isolate plates for each individual concentration. This was done to confirm whether the death was due to the applied fungi.

2.3. Survival Analysis

Survival curves were generated as a function of the observation time through the Kaplan-Meier method. Firstly, we performed a comparison of multiple groups, and thereafter we compared the survival of two groups applying the nonparametric Log-rank test at 5% significance level with the aid of the Statistica software, version 7.0.

3. Results

The survival of the A. bisphaerica worker ants, which were brought from the field, did not vary among the three analyzed colonies ( ), and the maximum survival was 25 days (Figure 1). It was possible to isolate and identify 10 fungi species that may have deleterious effects on the worker ants. Among them, three Aspergillus, three Penicillium, one Cladosporium, two Mucor, and one Beauveria species were identified (Table 1). The most frequent fungus found in the worker ants was A. ochraceus, with 44 occurrences. It was found in individuals who had died about 5 days after the beginning of the analyses. The fungi that had not sporulated were classified as Mycelia sterilia (Table 1). The percentage of worker ants, which were infected with known entomopathogenic fungi, was 15.33%, namely, A. ochraceus (14.67%), A. niger (0.33%), and B. bassiana (0.33%).

tab1
Table 1: Species and frequencies of the fungi isolated from 300 foragers of Atta bisphaerica.
389806.fig.001
Figure 1: Cumulative time-related survival proportion (Kaplan-Meier curves) of the Atta bisphaerica forage worker ants from three different colonies.

During experimental infection, we found a significant difference in survival time of the worker ants subjected to different concentrations of both suspensions A. ochraceus ( ; see Table 2 for comparing two survival curves with a Log-rank test) (Figure 2) and B. bassiana ( ; see Table 3 for comparing two survival curves with a Log-rank test) (Figure 3). The shortest survival time was observed at a concentration of 109 conidia/mL in both treatments, with an LT50 of four days at a 109 concentration of A. ochraceus (Figure 2) and an LT50 of four days for concentrations of 108 and 109 of B. bassiana (Figure 3). The LT50 of the control group resulted in 10.5 days.

tab2
Table 2: values of Log-rank test to compare two Kaplan-Meier survival curves for Atta bisphaerica workers treated with different conidial concentrations of Aspergillus ochraceus. values less than 0.05 were considered significant.
tab3
Table 3: values of Log-rank test to compare two Kaplan-Meier survival curves for Atta bisphaerica workers treated with different conidial concentrations of Beauveria bassiana. values less than 0.05 were considered significant.
389806.fig.002
Figure 2: Cumulative time-related survival proportion (Kaplan-Meier curves) of the Atta bisphaerica worker ants after being treated with different suspension concentrations of the Aspergillus ochraceus fungus.
389806.fig.003
Figure 3: Cumulative time-related survival proportion (Kaplan-Meier curves) of the Atta bisphaerica worker ants after being treated with different suspension concentrations of the Beauveria bassiana fungus.

4. Discussion

Since certain entomopathogenic fungi present a high potential to control the ants, pathogenicity tests have been performed with fungi widely used within the microbial control of insects, such as Metarhizium anisopliae (Metsch.) and Beauveria bassiana (Bals.). They have proved to be highly virulent in the laboratory, although field tests do not reproduce the same results [2022]. In our study, M. anisopliae was not found and B. bassiana was obtained only once. The Aspergillus ochraceus was the most frequent and, for the first time, reported to infect ants frequently. The mortality test results of ants infected with A. ochraceus and B. bassiana are similar to those obtained in other pathogenicity tests of B. bassiana or M. anisopliae against leaf-cutting ants [2224]. Although the high control mortality indicates that the ants were stressed in some way (e.g., social isolation), and thus probably more susceptible to diseases, the parasite treatments caused significantly increased mortality. This allows us to say that A. ochraceus could be also a promising biological control agent of ants. Surely, more research should be undertaken before significant field use of the pathogen. Even though that Aspergillus are unusual pathogens of most insects, these fungi produce a diverse range of compounds that can be potent insect toxins and potentially useful as pesticidal [25]. However, the safety of these compounds is a major concern and more studies are needed to evaluate potential risks to humans and nontarget species.

Cladosporium fungi are phytopathogens [26] and may be transported by various insects causing their death. In a study of leaf-cutting queen ants, the Cladosporium fungus was prevalent in Atta laevigata and in A. capiguara [27]. It is believed that these fungi are frequently found due to their cosmopolitan distribution, being acquired from the environment and dispersed during the founding of a colony by queens or forage worker ants carrying leaves. The role of the Cladosporium species inside the ant colonies is not known for sure, but they have been considered potential antagonists of the fungus garden [28]. The same kind of reasoning can be adopted for the Mucor fungi species, which have also been found in the nests of leaf-cutting ants, however, without knowing exactly how they interact with these ants or the fungus garden. The fungi of this genus are considered invertebrate pathogens. For example, Mucor hiemalis can cause the mortality of Cupiennius and Ischnothele spiders [29]. Toxic metabolites of these organisms present an insecticidal activity against the adults of Bactrocera oleae and Ceratitis capitata (Diptera: Tephritidae) [30].

Considering the diversity of fungi found in this study, the importance of new surveys and tests of entomopathogenic fungi isolates with the potential of being used in microbial control of these ants becomes evident. Beauveria and Metarhizium are the species which have been most studied and which have been evaluated on tests on their pathogenic potential. It is evident now that A. ochraceus should be further investigated for its pathogenic potential for leaf-cutting ants as well as other fungi genera found here.

Acknowledgment

The authors would like to thank Dr. Harry Evans of CAB International for fungi identification.

References

  1. B. Hölldobler and E. O. Wilson, The Ants, Harvard University Press, Cambridge, Mass, USA, 1990.
  2. F. A. M. Mariconi, As Saúvas, Agronômica Ceres, São Paulo, Brazil, 1970.
  3. C. A. Lima, T. M. C. della Lucia, R. N. C. Guedes, and C. E. da Veiga, “Development of granulated baits with alternative attractants to Atta bisphaerica forel (Hymenoptera: Formicidae) and their acceptance by workers,” Neotropical Entomology, vol. 32, no. 3, pp. 497–501, 2003. View at Scopus
  4. M. C. Oliveira, T. M. C. Della Lucia, D. Nascimento Junior, and C. A. Lima, “Espécies forrageiras preferidas para o corte por Atta bisphaerica Forel, 1908 (Hymenoptera: Formicidae),” Revista Ceres, vol. 49, pp. 321–328, 2002.
  5. E. Diehl-Fleig, M. E. Silva, A. Specht, and M. E. Valim-Labres, “Efficiency of Beauveriana bassiana for Acromyrmex spp. control (Hymenoptera: Formicidae),” Anais da Sociedade Entomológica do Brasil, vol. 22, pp. 281–286, 1993.
  6. M. A. L. Bragança, A. Tonhasca, and T. M. C. Delia Lucia, “Reduction in the foraging activity of the leaf-cutting ant Atta sexdens caused by the phorid Neodohrniphora sp,” Entomologia Experimentalis et Applicata, vol. 89, no. 3, pp. 305–311, 1998. View at Publisher · View at Google Scholar · View at Scopus
  7. A. Tonhasca Jr., M. A. Braganca, and T. M. C. Della Lucia, “Delayed suppression of the foraging activity of the leaf-cutting ant Atta sexdens rubropilosa by a phorid parasitoid,” Bulletin of the Ecological Society of America, vol. 78, p. 322, 1997.
  8. R. Isenring and L. Neumeister, “Recommendations regarding derogations to use alphacypermethrin, deltamethrin, fenitrothion, fipronil and sulfluramid in FSC certified forests in Brazil,” FSC Editor, p. 99, 2010.
  9. W. O. H. Hughes, L. Thomsen, J. Eilenberg, and J. J. Boomsma, “Diversity of entomopathogenic fungi near leaf-cutting ant nests in a neotropical forest, with particular reference to Metarhizium anisopliae var. anisopliae,” Journal of Invertebrate Pathology, vol. 85, no. 1, pp. 46–53, 2004. View at Publisher · View at Google Scholar · View at PubMed · View at Scopus
  10. E. Lopez and S. Orduz, “Metarhizium anisopliae and Trichoderma viride for control of nests of the fungus-growing ant, Atta cephalotes,” Biological Control, vol. 27, no. 2, pp. 194–200, 2003. View at Publisher · View at Google Scholar · View at Scopus
  11. A. Silva, A. Rodrigues, M. Bacci, F. C. Pagnocca, and O. C. Bueno, “Susceptibility of the ant-cultivated fungus Leucoagaricus gongylophorus (Agaricales: Basidiomycota) towards microfungi,” Mycopathologia, vol. 162, no. 2, pp. 115–119, 2006. View at Publisher · View at Google Scholar · View at PubMed · View at Scopus
  12. E. H. Varón Devia, Distribution and Foraging by the Leaf-Cutting ant, Atta cephalotes, in Coffee Plantations with Different Types of Management and Landscape Contexts, and Alternatives to Insecticides for Its Control, University of Idaho, Moscow, Idhao, USA, 2007.
  13. E. Diehl and L. K. Junqueira, “Seasonal variations of metapleural secretion in the leaf-cutting ant Atta sexdens piriventris santschi (Myrmicinae: Attini), and lack of fungicide effect on Beauveria bassiana (Bals.) vuillemin,” Neotropical Entomology, vol. 30, no. 4, pp. 517–522, 2001. View at Scopus
  14. A. Kermarrec and M. Decharme, “Ecopathological aspects in the control of Acromyrmex octospinosus Reich (Form., Attini) by entomophagous fungi,” in The Biology of Social Insects, M. D. Breed, C. D. Michener, and H. E. Evans, Eds., p. 148, Westview Press, Boulder, Colo, USA, 1982.
  15. A. Kermarrec, G. Febvay, and M. Decharme, “Protection of leaf-cutting ants from biohazards: is there a future for microbiological control?” in Fire Ants and Leaf Cutting Ants : Biology and Management, C. S. Lofgren and R. K. Vander Meer, Eds., pp. 339–356, Westview Press, Boulder, Colo, USA, 1986.
  16. S. B. Alves, “Fungos entomopatogênicos,” in Controle Microbiano de Insetos, S. B. Alves, Ed., pp. 289–381, FEALQ, Piracicaba, São Paulo, Brazil, 1998.
  17. G. L. Barron, The Genera of Hyphomycetes from Soil, Robert E. Krieger Publishing, New York, NY, USA, 1972.
  18. G. Barson, N. Renn, and A. F. Bywater, “Laboratory evaluation of six species of entomopathogenic fungi for the control of the house fly (Musca domestica L.), a pest of intensive animal units,” Journal of Invertebrate Pathology, vol. 64, no. 2, pp. 107–113, 1994. View at Publisher · View at Google Scholar · View at PubMed · View at Scopus
  19. T. M. C. Della Lucia, As Formigas Cortadeiras, Folha de Viçosa, Viçosa, MG, Brazil, 1993.
  20. E. Diehl-Fleig and M. E. Silva, “Patogenicidade de Beauveria bassiana e Metarhizium anisopliae à formiga saúva Atta sexdens piriventris,” Boletim do Grupo de Pesquisadores de Controle Biológico, vol. 6, p. 15, 1986.
  21. E. Diehl-Fleig, M. E. Silva, and M. M. Pacheco, “Testes de patogenicidade dos fungos entomopatogênicos Beauveria bassiana e Metarhizium anisopliae em Atta sexdens piriventris (Santschi, 1919) em diferentes temperaturas,” Ciência e Cultura, vol. 40, pp. 1103–1105, 1988.
  22. A. M. C. Castilho, M. E. Fraga, E. L. Aguiar-Meneze, and C. A. R. Rosa, “Selection of Metarhizium anisopliae and Beauveria bassiana isolates pathogenic to Atta bisphaerica and Atta sexdens rubropilosa soldiers under laboratory conditions,” Ciencia Rural, vol. 40, no. 6, pp. 1243–1249, 2010. View at Scopus
  23. E. S. Loureiro and A. C. Monteiro, “Patogenicidade de isolados de três fungos entomopatogênicos a soldados de Atta sexdens sexdens (Linnaeus, 1758) (Hymenoptera: Formicidae),” Revista Árvore, vol. 29, pp. 553–561, 2005.
  24. A. V. Santos, B. L. De Oliveira, and R. I. Samuels, “Selection of entomopathogenic fungi for use in combination with sub-lethal doses of imidacloprid: perspectives for the control of the leaf-cutting ant Atta sexdens rubropilosa Forel (Hymenoptera: Formicidae),” Mycopathologia, vol. 163, no. 4, pp. 233–240, 2007. View at Publisher · View at Google Scholar · View at PubMed · View at Scopus
  25. R. A. Humber, “Fungal pathogens and parasites of insects,” in Applied Microbial Systematics, F. G. Priest and M. Goodfellow, Eds., pp. 203–230, Kluwer Academic Publishers, Dordrecht, Netherlands, 2000.
  26. G. M. Richards and L. R. Beuchat, “Infection of cantaloupe rind with Cladosporium cladosporioides and Penicillium expansum, and associated migration of Salmonella poona into edible tissues,” International Journal of Food Microbiology, vol. 103, no. 1, pp. 1–10, 2005. View at Publisher · View at Google Scholar · View at PubMed · View at Scopus
  27. F. C. Pagnocca, A. Rodrigues, N. S. Nagamoto, and M. Bacci, “Yeasts and filamentous fungi carried by the gynes of leaf-cutting ants,” Antonie van Leeuwenhoek, vol. 94, no. 4, pp. 517–526, 2008. View at Publisher · View at Google Scholar · View at PubMed · View at Scopus
  28. A. Rodrigues, M. Bacci, U. G. Mueller, A. Ortiz, and F. C. Pagnocca, “Microfungal “weeds” in the leafcutter ant symbiosis,” Microbial Ecology, vol. 56, no. 4, pp. 604–614, 2008. View at Publisher · View at Google Scholar · View at PubMed · View at Scopus
  29. W. Nentwig, “A zygomycetous fungus as a mortality factor in a laboratory stock of spiders,” The Journal of Arachnology, vol. 18, pp. 118–121, 1990.
  30. M. A. Konstantopoulou, P. Milonas, and B. E. Mazomenos, “Partial purification and insecticidal activity of toxic metabolites secreted by a Mucor hiemalis strain (SMU-21) against adults of Bactrocera oleae and Ceratitis capitata (Diptera: Tephritidae),” Journal of Economic Entomology, vol. 99, no. 5, pp. 1657–1664, 2006. View at Scopus