Hygiene Defense Behaviors Used by a Fungus-Growing Ant Depend on the Fungal Pathogen Stages

Abstract

Parasites and their hosts use different strategies to overcome the defenses of the other, often resulting in an evolutionary arms race. Limited animal studies have explored the differential responses of hosts when challenged by differential parasite loads and different developmental stages of a parasite. The fungus-growing ant Trachymyrmex sp. 10 employs three different hygienic strategies to control fungal pathogens: Grooming the antibiotic-producing metapleural glands (MGs) and planting or weeding their mutualistic fungal crop. By inoculating Trachymyrmex colonies with different parasite concentrations (Metarhizium) or stages (germinated conidia or ungermianted conidia of Metarhizium and Escovopsis), we tested whether ants modulate and change hygienic strategies depending on the nature of the parasite challenge. There was no effect of the concentration of parasite on the frequencies of the defensive behaviors, indicating that the ants did not change defensive strategy according to the level of threat. However, when challenged with conidia of Escovopsis sp. and Metarhizium brunneum that were germinated or not-germinated, the ants adjusted their thygienic behavior to fungal planting and MG grooming behaviors using strategies depending on the conidia germination status. Our study suggests that fungus-growing ants can adjust the use of hygienic strategies based on the nature of the parasites.

Keywords: Attini; Escovopsis; Metarhizium; Trachymyrmex; immune system; parasites; social insects.

Figure 1

Ant workers’ behavioral response when exposed to three different Metarhizium brunneum conidia amounts. Error bars represent standard errors.

Figure 2

Mean ± standard errors frequencies of the hygienic behaviors metapleural gland (MG) grooming (MG: Grey dots) and fungal mutualist planting (black dots) exhibited by Trachymyrmex sp. 10 fungus-growing ants, in response to the germinated conidia or ungerminated conidia of the Escovopis and Metarhizium fungal parasites. Control treatments were talcum powder (for ungerminated conidia) or talcum powder contaminated by a fungal mutualist (for germinated conidia).