Hygienic behaviour is a collective defence against brood diseases. Hygienic workers uncap sealed brood cells containing dead and diseased larvae and remove the contents. In this way they reduce the spread of pests and diseases within the colony. For example, if a diseased larva or pupa is removed quickly the disease may not yet be infective. In the case of varroa mites, female offspring will not yet be mature and so will die, thereby reducing varroa increase in the colony.
Hygienic behaviour was first studied before WW2 in the USA to control American foulbrood (AFB), a serious brood disease caused by a particular species of bacteria. More recent research by Professor Marla Spivak of the University of Minnesota has confirmed that hygienic behaviour can be highly effective in AFB control.
In June 1998 and 1999 in Minnesota, colonies were challenged with AFB by inserting a 15 x 15cm patch of cells containing hundreds of AFB scales (larvae killed by AFB). Of the 18 hygienic colonies tested (these removed more than 95% of freeze-killed brood, FKB, within 2 days), 7 showed symptoms of AFB of which 5 recovered by mid-August. Of the 18 non-hygienic hives tested (which removed only 32-73% FKB) all 18 showed AFB symptoms of which only 1 recovered. The same experiment also monitored chalkbrood occurring naturally in the same hives. None of the hygienic colonies versus all but one of the non-hygienic colonies had symptoms (mummies) at the final hive inspection in August.
LASI has studied the effect of hygienic behaviour on varroa and deformed wing virus, DWV. Over one year, varroa numbers in highly hygienic colonies (more than 95% FKB removal in 2 days) increased by less than half that of non-hygienic colonies. Highly hygienic colonies also had 10,000 times less DWV. When a colony has a high level of DWV it will have some workers with shrivelled wings (Fig 6). This is usually a sign that the colony will soon die. However, if the colony is given a hygienic queen, this can save its life.
Hygienic behaviour helps control pests and diseases in sealed brood cells. Research results show that this behaviour is highly effective against AFB, chalkbrood, and DWV, and slows down varroa population growth. Although chemicals can be used against AFB and varroa, none are effective against chalkbrood or DWV.
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F. Ratnieks, February 2017.