Varroa Sensitive Hygiene (VSH) occurs when workers recognise that the brood has been infested with varroa and uncap the cell. They may then recap the cell or remove the parasitised brood from the cell. Uncapping and recapping can be enough to disrupt the varroa brood cycle, but removal of infected pupae will definitely reduce mite population growth. VSH is one trait that has been selected for in varroa resistance breeding programs.
Although breeding for VSH occurs around the world, there is yet to be a globally adopted protocol for selection. The selection assays are time consuming and laborious, hindering wide-scale uptake of this important trait. The trade-off between cost to perform the selection assay and commercial benefit of developing varroa resistance in breeding lines has not yet been balanced.
Several groups around the world are examining the chemical cues that initiate VSH behaviour. Researchers are exploring chemical cues released by unhealthy brood and immature varroa mites. The aim if the research is to provide a reliable, commercially available phenotyping tool that is easy to use in the field.
Unhealthy brood odour
One such tool developed by researchers in the United States, examines bees’ response to unhealthy brood odour (UBO). An odour can occur as a result of varroa infestation, viral infection, or other brood diseases. The test is for hygienic behaviour, encompassing VSH, and has applications to brood diseases.
A synthetic compound (UBeeO) made up of chemicals found in unhealthy brood is sprayed onto a known area of capped brood and after two hours, the number of uncapped cells is counted (Figure 1). Colonies with greater than 60% of the cells uncapped are considered hygienic, and may be able to maintain the mite population below 3 mites/100 bees and survive without treatment.
Response to the test has been linked with low varroa infestation levels, high varroa removal, overwintering in colonies not treated for varroa, and traditional freeze killed brood (FKB) hygienic behaviour tests. It is clear that high UBeeO response results in lower varroa infestations than high FKB responses (Figure 2).
A UBeeO breeding program has been initiated in Australia.
Figure 1. Unhealthy brood emit different hydrocarbons than healthy brood.#
#In the Unhealthy Brood Odour (UBeeO) assay, capped brood cells are treated with a mixture of pheromones and placed back in the colony. After two hours, the hygienic response of the colony is quantified. The response of the colony may be predictive of varroa infestation levels and colony survival.
Source: Wagoner, 2023.
Figure 2. High UBeeO response results in lower varroa infestations than high FKB responses.
Varroa parasitisation specific compounds
Another tool based on six varroa parasitisation specific (VPS) compounds is being developed in France. The test recognises parasitised brood and triggers VSH. Application is by injection of the compound mix into the brood cells and counting the number of uncapped cells 48 hours later. When the extracts are applied in combination in the field, they induce VSH behaviour similar to that found with natural VSH as measured by comparison of brood infestation levels on day 1 and 7 (Figure 3).
In their defence against varroa, a bees’ ability to differentiate healthy from unhealthy hive odours is critical. In vulnerable colonies, the majority of worker bees fail to show this ability. Researchers are working to develop new methods to enable large-scale phenotyping of traits that are associated with social immunity mechanisms in response to the varroa parasite.
Figure 3. Relationship between response to varroa parasitisation specific (VPS) compounds and traditional varroa sensitive hygiene (VSH) tests.
Can selecting for hygienic behaviour do the same thing?
Hygienic behaviour is selected by either freeze killing brood with liquid nitrogen or killing them with a. pin. Workers detect they are dead and remove them. Their deaths are not natural and the signals are different than that produced by unhealthy brood and varroa (Figure 4).
Hygienic behaviour reduces varroa infestations by disrupting their reproductive cycle, but not to levels where treatment becomes unnecessary. As hygienic behaviour does not adversely affect other traits such as honey production, temperament, or swarming, breeding for hygienic behaviour has been one strategy used to prepare Australia for varroa. It is something that beekeepers in varroa free areas may wish to begin or continue to select for, but it is not likely to be as effective as UBeeO, VPS, or other varroa resistance mechanisms.
Figure 4. Hygienic behaviour can be selected for by killing brood with liquid nitrogen (FKB) or a pin (PKB).
In FKB and PKB the dead brood emit signals (indicated in blue) which workers detect, resulting in their removal from the colony. Unhealthy brood emit a different signal (indicated in pink), while cells that have been infested with varroa emit different signals again (indicated in red). Hygienic behaviour can also be selected for by spraying unhealthy brood odours (UBeeO) onto brood and measuring the colony’s response. In this case you are selecting for a response to unhealthy brood odours, rather than to brood that have been killed by a situation not naturally encountered. Varroa specific hygiene is currently selected for by determining the number of varroa infested cells that have their contents removed. A new test is being developed that injects varroa parasitisation specific compounds into cells
More information
VIDEO 1: Inside the Hive TV: Dr. Kaira Wagoner on UBO Honey Bee assay- Testing bees for a Unhealthy Brood Odor Response https://www.youtube.com/watch?v=f9SeQYL0cAQ&t=433s
VIDEO 2: Bees get a fighting chance: Alison Mercer on VPS assay https://protect-au.mimecast.com/s/94eDCNLJyQUZEXKnnTm1kL2?domain=youtu.be
Acknowledgements:
- Holmes, Gerdts, Grassl, Mikeheyev, Roberts, Remnant, Chapman (2024) Resilient beekeeping in the face of Varroa. AgriFutures Australia.
- Plan Bee (National Honey Bee Genetic Improvement Program) is supported by funding from the Australian Government Department of Agriculture, Fisheries and Forestry as part of its Rural Research and Development for Profit program. The project is further supported by AgriFutures Australia, the Department of Regional NSW, University of Sydney, University of New England Animal Genetics and Breeding Unit, Better Bees WA Inc, Wheen Bee Foundation, Costa Group, Olam, Beechworth Honey, Monson’s Honey and Pollination, South Pacific Seeds, Australian Queen Bee Breeders Association, Australian Honey Bee Industry Council, and commercial beekeepers.
- https://honeybee.org.au/wp-content/uploads/2024/05/AGF575-Breeding-S3V2.pdf
- Mondet, Blanchard, Barthes, Beslay, Bordier, Costagliola, Herve, Lapeyre, Kim, Basso, Mercer, Le Conte (2021) Chemical detection triggers honey bee defense against destructive parasitic threat. Chemical Biology 17: 524-530
- Nazzi, Vedova, D’Agaro (2004) A semiochemical from brood cells infested by Varroa destructor triggers hygienic behaviour in Apis mellifera. Apidologie 35: 65-70
- Wagoner, Millar, Keller, Bello, Waiker, Schal, Spivak, Rueppell (2021) Hygiene-eliciting brood semiochemicals as a tool for assaying honey bee (Hymenoptera: Apidae) colony resistance to Varroa (Mesostigmata: Varroidae). Journal of Insect Science 21: 1-13
- Wagoner (2023) Helping bees help themselves. Science 380: 47C-D
- This article was peer-reviewed by Michael Holmes, Corinne Jordan and John Roberts.
