Honey bee hygienic behaviour does not incur a cost via removal of healthy brood

In the honey bee, hygienic behaviour, the removal of dead or diseased brood from capped cells by workers, is a heritable trait that confers colony-level resistance against brood diseases. This behaviour is quite rare. Only c. 10% of unselected colonies show high levels of hygiene. Previous studies suggested that hygiene might be rare because it also results in the removal of healthy brood, thereby imposing an ongoing cost even when brood diseases are absent. We tested this hypothesis by quantifying hygienic behaviour in 10 colonies using a standard technique, the freeze-killed brood (FKB) bioassay. At the same time, we also quantified the removal of untreated brood. The study colonies showed a wide range in hygienic behaviour, removing 19.7-100% of the FKB. The removal of untreated brood ranged from 2% to 44.4%. However, there was no correlation between the two removal rates for any of the four age groups of untreated brood studied (eggs, young larvae, older larvae from uncapped cells and larvae/pupae from capped cells). These results do not support the cost-to-healthy-brood hypothesis for the rarity of hygienic behaviour.