With respect to the final hypothesis, bees, like many social insects, are relentlessly hygienic, removing alien organisms from their nests, and secreting antimicrobial substances that can reduce the viability and growth of pathogens in the colonies. Bees also raise their young in individual cells using, as a food source, substances with strongly antimicrobial properties (e.g. royal jelly; Albert & Klaudiny, 2004) . A testament to this hygiene is the fact that, even when facing severe colony-level infections by bacterial pathogens such as Paenibacillus larvae (for which < 10 spores are normally fatal to young larvae (Brodsgaard et al., 1998) , the vast majority of larvae show no signs of exposure (Evans & Pettis, 2005) . 'Social' barriers might also reduce exposure to minor, opportunistic, pathogens or saprophytes that have been proposed as generalized targets of insect immune defences (Hultmark, 2003) . While bees do carry an assemblage of microbes, and bacteria in particular (Gilliam, 1997) , exposure to these microbes is arguably lower than in free-living Drosophila (decaying plant material as larvae and adults) or Anopheles (septic aqueous environments as larvae). Further, bacteria found in bee colonies have only rarely been associated with disease pathologies, despite extensive study. In fact, some resident bacteria in colonies appear to add to external defences through their inhibition of bee pathogens (Evans & Armstrong, 2006) .