Honey bee guards recognise allospecific intruders via "different odours" not "harmful-intruder odours"

Honey bee (Apis mellifera) guards recognise intruders primarily by odour. This study tested two competing hypotheses underlying the rejection of allospecific intruders: "different odour", whereby intruders are recognised by guards because their odours do not match those of honey bees in general, versus the "harmful intruder odour", whereby guards specifically recognise and reject intruders of harmful species. We presented eight species of harmless arthropod and three harmful insects (worker common wasps, Vespula vulgaris; adult wax moths, Galleria mellonella; and wood ants, Formica lugubris) to entrance guards. Guards rejected all but woodlice (Porcellio scaber) more than non-nestmate bees and at rates similar to or greater than common wasps. This is not as predicted by the harmful odour hypothesis, and thereby strengthens the different odour hypothesis. Woodlice were accepted at rates similar to non-nestmate bees, possibly because they have few cuticular hydrocarbons and may smell mainly of their environment.