However, the self-medicating behaviour does have limits. Honey bee colonies infected with pathogenic bacteria did not bring in significantly more propolis – despite the fact that the propolis also has antibacterial properties. ''There was a slight increase, but it was not statistically significant,'' Simone-Finstrom says. ''That is something we plan to follow up on.''

There may be a lesson here for domestic beekeepers. ''Historically, US beekeepers preferred colonies that used less of this resin, because it is sticky and can be difficult to work with,'' Simone-Finstrom says. ''Now we know that this is a characteristic worth promoting, because it seems to offer the bees some natural defense.''

The paper, Increased resin collection after parasite challenge: a case of self-medication in honey bees?, was co-authored by Dr. Marla Spivak of the University of Minnesota and published March 29 in PLoS ONE. The research was funded by the National Science Foundation.

The constant pressure posed by parasites has caused species throughout the animal kingdom to evolve suites of mechanisms to resist infection. Individual barriers and physiological defenses are considered the main barriers against parasites in invertebrate species.

However, behavioural traits and other non-immunological defenses can also effectively reduce parasite transmission and infection intensity. In social insects, behaviours that reduce colony-level parasite loads are termed ''social immunity.''