Symbionts of societies that fission: mites as guests or parasites of army ants

Recently, Hughes et al. (Trends in Ecology & Evolution, 23, 672-677, 2008) have theorised that symbionts of large, long-lived, homeostatic, and well defended social insect colonies should mostly be of low virulence. If the symbionts are rare, i.e. few workers are co-infected, competition between symbionts should be minimal and they should be selected to avoid over-exploiting their hosts. Here we analyse the mites that occur on Eciton burchellii army ant workers and note that our findings are consistent with the predictions from evolutionary theory. The mites were species diverse but rare; only 5% of the 3146 workers we examined from 20 army ant colonies had mites. Only one worker was co-infected by mites of different species and the one relatively common parasitic mite (Rettenmeyerius carli) was limited to only two individuals per ant. We also showed that certain mites are more common on workers in nomadic rather than statary army ant colonies and that different worker castes differed in their infestation patterns. We suggest that the three traits E. burchellii and honey bees (Apis mellifera) have in common (queens with very high mating frequencies, propagation by colony fission, and low number of parasites among the mite species they host) are associated with one another. Colonies that fission are likely to inherit symbionts and multiple mating will promote genetic diversity within colonies, which may help to limit the abundance of deleterious mites. We conclude that most of the symbiotic mites found on workers of the army ant E. burchellii are probably relatively harmless guests, exploiting their hosts for phoresis or, for example, to use their waste deposits.