Dead ant walking: a myrmecophilous beetle predator uses parasitoid host location cues to selectively prey on parasitized ants

Abstract

Myrmecophiles (i.e. organisms that associate with ants) use a variety of ecological niches and employ different strategies to survive encounters with ants. Because ants are typically excellent defenders, myrmecophiles may choose moments of weakness to take advantage of their ant associates. This hypothesis was studied in the rove beetle, Myrmedonota xipe, which associates with Azteca sericeasur ants in the presence of parasitoid flies. A combination of laboratory and field experiments show that M. xipe beetles selectively locate and prey upon parasitized ants. These parasitized ants are less aggressive towards beetles than healthy ants, allowing beetles to eat the parasitized ants alive without interruption. Moreover, behavioural assays and chemical analysis reveal that M. xipe are attracted to the ant's alarm pheromone, the same secretion used by the phorid fly parasitoids in host location. This strategy allows beetles access to an abundant but otherwise inaccessible resource, as A. sericeasur ants are typically highly aggressive. These results are the first, to our knowledge, to demonstrate a predator sharing cues with a parasitoid to gain access to an otherwise unavailable prey item. Furthermore, this work highlights the importance of studying ant -myrmecophile interactions beyond just their pairwise context.

Adaptive suicide: is a kin-selected driver of fatal behaviours likely?

“…Rove beetles were found to selectively locate and prey upon ants parasitized by phorid parasitoid flies. Parasitized ants acted less aggressively towards the beetles than healthy ants, meaning that rove beetles can eat them alive without interruption [46]. Unable to access the aggressive, unparasitized ants as a food resource, M. xipe appeared to almost exclusively prey on parasitized ants, but this could also benefit the infected ants as being consumed would reduce the phorid fly population free to infect their kin.…”

“…As in aphids [27][28][29], however, adaptive suicide in eusocial insects may not always involve spatial separation of a host from its kin; selective predation on parasitized hosts could also help hosts altruistically protect their unparasitized kin from a parasitoid. Mathis & Tsutsui [46] studied the rove beetle Myrmedonota xipe, which associates with-typically highly aggressive-Azteca sericeasur ants. Rove beetles were found to selectively locate and prey upon ants parasitized by phorid parasitoid flies.…”

“…Unable to access the aggressive, unparasitized ants as a food resource, M. xipe appeared to almost exclusively prey on parasitized ants, but this could also benefit the infected ants as being consumed would reduce the phorid fly population free to infect their kin. On the one hand, this system seems a good candidate to meet the criteria for the host-suicide hypothesis, as A. sericeasur is a polygynous and polydomous social insect that forms wide-spanning territories and so emerging mature parasitoids are far more likely to encounter their host's kin than non-kin [46]. On the other hand, it may be that not all phorid fly larvae successfully mature, and so selective predation of ants that would survive parasitism would ultimately cost the colony as a whole [46].…”

“…On the one hand, this system seems a good candidate to meet the criteria for the host-suicide hypothesis, as A. sericeasur is a polygynous and polydomous social insect that forms wide-spanning territories and so emerging mature parasitoids are far more likely to encounter their host's kin than non-kin [46]. On the other hand, it may be that not all phorid fly larvae successfully mature, and so selective predation of ants that would survive parasitism would ultimately cost the colony as a whole [46]. Parasitized workers may also be active colony members during the development of the parasitoid, and in these cases the benefit of eliminating the larvae via predation may be offset by the costs to the colony incurred from losing productive parasitized workers [46].…”

“…On the other hand, it may be that not all phorid fly larvae successfully mature, and so selective predation of ants that would survive parasitism would ultimately cost the colony as a whole [46]. Parasitized workers may also be active colony members during the development of the parasitoid, and in these cases the benefit of eliminating the larvae via predation may be offset by the costs to the colony incurred from losing productive parasitized workers [46]. Further work exploring the true costs and benefits of selective rove beetle predation to parasitized ants will certainly shed more light on the evolution of this system but, as Mathis & Tsutsui conclude, beetle predation may indirectly benefit ants where parasitized ants can reduce the numbers of developing parasitoids by increasing their appeal as prey.…”