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Caste specialization in behavioral defenses against fungus garden parasites in Acromyrmex octospinosus leaf-cutting ants

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Abstract

Division of labor and caste specialization plays an important role in many aspects of social insect colony organization, including parasite defense. Within leaf-cutting ant colonies, worker caste specialization permeates colony tasks ranging from foraging, substrate incorporation, brood care, and chemical defenses via glandular secretions and mutualistic bacteria. Leaf-cutting ants rely on protecting a mutualistic fungus they grow for food from microfungi in the genus Escovopsis that parasitizes the ant–fungus relationship. Here, we examine whether Acromyrmex octospinosus leaf-cutter ant castes (minors and majors) display task specialization in two behavioral defenses against Escovopsis: fungus grooming (the removal of Escovopsis spores) and weeding (the removal of large pieces of Escovopsis-infected fungus garden). Using behavioral observations, we show that minors are the primary caste that performs fungus grooming, while weeding is almost exclusively performed by majors. In addition, using a sub-colony infection experimental setup, we show that at the early stages of infection, minors more efficiently remove Escovopsis spores from the fungus garden, thereby restricting Escovopsis spore germination and growth. At later stages of infection, after Escovopsis spore germination, we find that major workers are as efficient as minors in defending the fungus garden, likely due to the increased importance of weeding. Finally, we show, using SEM imaging, that the number of sensory structures is similar between minor and major workers. If these structures are invoked in recognition of the parasites, this finding suggests that both castes are able to sense Escovopsis. Our findings support that leaf-cutter ant behavioral defense tasks against Escovopsis are subject to caste specialization, likely facilitated by worker sizes being optimal for grooming and weeding by minors and majors, respectively, with important consequences for cultivar defense.

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Acknowledgments

We thank J. Heinz and the BBPIC Laboratory at the University of Wisconsin, Madison, for assistance with SEM imaging, and S. Adams, S. Marsh, G. Suen, and J.M. Thomas for comments on a draft of this manuscript. We acknowledge the Smithsonian Tropical Research Institute for providing logistical help during fieldwork in Panama, and the Autoridad Nacional del Ambiente of the Republic of Panama for facilitating the research and granting collecting permits. This work was supported by the University of Wisconsin-Madison Hilldale Research Grant to DA, The Carlsberg Foundation to MP, and NSF CAREER grant DEB-747002 to CRC.

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  1. Department of Bacteriology, University of Wisconsin, Madison, 4325 Microbial Sciences Building, 1550 Linden Dr., Madison, WI, 53706, USA

    D. Abramowski, C. R. Currie & M. Poulsen

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  1. D. Abramowski
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  2. C. R. Currie
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  3. M. Poulsen
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Correspondence to M. Poulsen.

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Abramowski, D., Currie, C.R. & Poulsen, M. Caste specialization in behavioral defenses against fungus garden parasites in Acromyrmex octospinosus leaf-cutting ants. Insect. Soc. 58, 65–75 (2011). https://doi.org/10.1007/s00040-010-0117-y

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  • DOI: https://doi.org/10.1007/s00040-010-0117-y

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