Fitness consequences of nest infiltration by the mutualist‐exploiter Megalomyrmex adamsae

Abstract

1. Fungus-growing ants are obligate mutualists. Their nutrient-rich fungus garden provides a valuable food store that sustains the ant hosts, but can also attract social parasites.2. The 'guest ant' Megalomyrmex adamsae Longino parasitises the fungus-growing Trachymyrmex zeteki Weber queen just after nest founding. The parasitic queen infiltrates the incipient nest, builds a cavity in the fungal garden, and lays eggs that develop into workers and reproductive males and females.3. This study compared young parasitised and non-parasitised laboratory colonies by measuring garden growth and biomass, and the number of host workers and reproductives. Host queen survival and parasite colony growth were also monitored.4. Parasitised Trachymyrmex colonies had reduced host worker and alate numbers, as well as lower garden biomass, compared with non-parasitised control colonies, confirming that M. adamsae is a xenobiotic social parasite. Host queen survival was not significantly different between parasitised and control colonies.5. This is the first study that experimentally infects host colonies with a xenobiotic social parasite to measure fitness cost to the host. The natural history of M. adamsae and the fungus-growing ant mutualism are evaluated in the context of three general predictions of (Bronstein, Ecology Letters, 4, 277-287, 2001a) regarding the cost of mutualism exploiters.

Chemically armed mercenary ants protect fungus-farming societies

“…Several species of Megalomyrmex (Solenopsidini) belong to this category, associating in varying capacities with fungus-growing ant hosts (Attini), a clade of the same ant subfamily (8)(9)(10)(11)(12) (Fig. 1).…”

“…1). Most details about the diversity and life history adaptations of these ants have been clarified only recently (11), and much of their biology remains to be discovered (SI Text: Study System and Tables S1 and S2). Free-living predatory Megalomyrmex are known to biosynthesize venom alkaloids that are used in defense (13,14) whereas the fungus-growing ant associates seem to aggressively dispense these venoms when they attack host colonies (15).…”

“…Behaviorally derived lineages of Megalomyrmex have become guest ant parasites of the higher attine genera Trachymyrmex and Sericomyrmex (11,16,17) (Fig. 1A).…”

“…1A). The Trachymyrmex specialist, Megalomyrmex adamsae, appears to have remained as equally rare as the agro-predators and thief ants (11), but the Sericomyrmex specialist Megalomyrmex symmetochus (Fig. 1B) is surprisingly common, with a prevalence of >80% in some host populations (16).…”

“…1B) is surprisingly common, with a prevalence of >80% in some host populations (16). Newly mated guest ant queens of both species likely enter host colonies by stealth and establish themselves in the fungus garden, where their developing colony will consume host brood and fungus garden for years (11). Their presence slows host colony growth and also prevents or reduces host reproduction, because both guest ant species clip the wings of host gynes (virgin queens), but not males (11).…”