VSH is targeted specifically towards varroa mites reproducing inside wax-capped cells occupied by young developing bees (brood cells).
Worker bees uncapped the varroa-infested brood cells, killed the developing bee inside each cell, and removed the cell contents, including the mites.
Researchers found that this complex sequence of behaviours interrupted the reproductive cycle of the mite and enhanced the likelihood of colony survival.
Apis mellifera generally failed to show high enough levels of varroa-sensitive hygiene to defend their colonies adequately against the mite, and while bee breeding techniques had the potential to rectify this problem, progress in this area had been slow, researchers said.
This was due, in part, to a lack of clarity about which traits to select for, researchers said.
The identity of signals that triggered VSH behaviour had been unclear, and how worker bees selectively targeted brood cells containing varroa was unknown, researchers said.
During her doctoral studies, lead researcher Fanny Mondet set out to address this shortfall.
Dividing her time between the National Institute for Agricultural Research in Avignon, France and the Department of Zoology at the University of Otago in Dunedin, Mondet assembled a team of researchers to help resolve these important questions.
In a paper published recently in Nature Chemical Biology, Mondet and colleagues identified six varroa-related compounds (four ketones and two acetates) that triggered VSH behaviour.
The compounds were found in varroa-infested brood cells and their levels provided a reliable indicator of the numbers of juvenile mites present in the cells.
Detection of these compounds enabled worker bees to target selectively varroa-infested cells, Mondet said.
"If the varroa-related compounds alone are presented to bees, all bees appear able to detect the odours. However, not all bees are able to tell the difference between the smell of healthy brood odours and the smell of brood odours containing varroa-related compounds.
"In their defense against varroa, bees' ability to differentiate healthy from unhealthy hive odours is critical. In vulnerable colonies, a majority of worker bees fail to show this ability."
For breeding purposes, colonies needed to be identified that showed high levels of VSH activity. Using conventional techniques, this was difficult and time consuming, Mondet said.
"However, in field trials, we were able to show injection of the varroa-related compounds into brood cells induces responses in worker bees that mimic intrinsic VSH activity in bee colonies. A simple and reliable bioassay of this kind has the potential to assist greatly the breeding of bees with stronger resistance to the devastating impacts of the varroa mite."
Efforts to develop such a test are under way.
