Abstract

During colony relocation, the selection of a new nest involves exploration and assessment of potential sites followed by colony movement on the basis of a collective decision making process. Hygiene and pathogen load of the potential nest sites are factors worker scouts might evaluate, given the high risk of epidemics in group-living animals. Choosing nest sites free of pathogens is hypothesized to be highly efficient in invasive ants as each of their introduced populations is often an open network of nests exchanging individuals (unicolonial) with frequent relocation into new nest sites and low genetic diversity, likely making these species particularly vulnerable to parasites and diseases. We investigated the nest site preference of the invasive pharaoh ant, Monomorium pharaonis, through binary choice tests between three nest types: nests containing dead nestmates overgrown with sporulating mycelium of the entomopathogenic fungus Metarhizium brunneum (infected nests), nests containing nestmates killed by freezing (uninfected nests), and empty nests. In contrast to the expectation pharaoh ant colonies preferentially (84%) moved into the infected nest when presented with the choice of an infected and an uninfected nest. The ants had an intermediate preference for empty nests. Pharaoh ants display an overall preference for infected nests during colony relocation. While we cannot rule out that the ants are actually manipulated by the pathogen, we propose that this preference might be an adaptive strategy by the host to “immunize” the colony against future exposure to the same pathogenic fungus.

Highlights

  • In social insects, the selection of a suitable nest site during colony establishment and relocation is critical for colony success

  • The selection of a new nest site is critical during colony relocation, a common phenomenon in eusocial insects [2], as migrating between nests is risky and energetically costly, and the colony usually has to select the best site among several candidates

  • We found that emigrating Monomorium pharaonis colonies were able to discriminate between infected and uninfected nests, but surprisingly, they displayed a clear preference for the infected nests (Fig. 2)

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Summary

Introduction

The selection of a suitable nest site during colony establishment and relocation is critical for colony success. Intra- and inter-specific competition level, as well as abiotic factors characterizing the new site can strongly affect colony fitness [1]. The selection of a new nest site is critical during colony relocation, a common phenomenon in eusocial insects [2], as migrating between nests is risky and energetically costly, and the colony usually has to select the best site among several candidates. Studies conducted in honeybees [3] and Temnothorax ants [4] have shown that individual scouts explore and assess the overall quality of potential nest sites on the basis of several factors. Scouts appear able to evaluate the cleanliness of potential sites, as sites with corpses are avoided, presumably due to disease risk [5]

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