Breeding varroa-tolerant honey bee stock
Figure 1: Adult varroa mite
- By Michelle Taylor and Harlan Cox
The Horticulture and Food Research Institute of New Zealand Limited
Uncontrolled, varroa (Varroa destructor) will usually kill bee colonies within a year. Varroa is controlled mostly through the use of synthetic miticides, but this approach is not sustainable in the long term. Overseas, varroa has quickly built up resistance to these chemicals, resulting in large hive losses.
Varroa-resistant bees bred
Over the past three years, the honey bee research unit at HortResearch, among other bee- and pollination-related projects, has been breeding honey bee stock that displays resistance to varroa (Figures 1 and 2), to provide a more sustainable control option. The project is part of a programme funded by the Sustainable Farming Fund, HortResearch, National Beekeepers' Association, ZESPRI and individual beekeepers.
The aim of the breeding project has been to select stock that display a genetically inherited trait called delayed suppressed mite reproduction (SMRD). Varroa reproduce inside the brood cells, feeding on developing honey bees. Varroa in hives with high levels of the SMRD are, however, not able to reproduce.
Figure 2: Adult varroa mite on a
newly emerged honey bee
The initial honey bee stock was sourced from beekeepers throughout the North and South Islands and assessed for the ability to suppress varroa reproduction. The best ten queens were selected for breeding. Each successive year the best queens were inbred using artificial insemination to increase the percentage of SMRD expressed. This was done using a closed mating population, where virgin queens were artificially inseminated with semen from drones (male honey bees) produced by their mother (Figure 3).
Impact on varroa
Each year, an increase in percentage of suppressed mite reproduction has been observed, with the latest results revealing that 80 percent of varroa in one of the lines do not reproduce. This is an important step forward. However, we have only been selecting for a trait, so although we have culled any lines that have not bred well, have been overly aggressive or have had high levels of disease, we do not know how these bees will perform in relation to honey, propolis or pollen production.
High SMRD queens cannot simply be introduced to hives around New Zealand, because with every generation of queens that mate with drones that do not carry the SMRD gene, the tolerance of these colonies to varroa will reduce. The only way to maintain and improve this stock on the mainland is with a breeding programme utilising artificial insemination. The reliance on artificial insemination makes such a system expensive to operate, and continued funding is required to maintain the gains made.
Figure 3: Artificial insemination
of a virgin queen bee
Isolated colony to maintain resistant stock
Because of this, we have been searching for a place to maintain the stock as a closed population with minimal management. This requires an area where the bees will be isolated from all other managed and feral colonies, to ensure that the lines maintain their genetic resistance to varroa.
To this end, we are moving the most resistant stock onto Great Mercury Island, off the coast of the Coromandel peninsula, where there are currently no other honey bees. It is hoped that the bees will form a self-sustaining population with high levels of the SMRD gene. This population will be used to provide stock to beekeepers on the mainland which, over time, hopefully will become one useful tool in the fight against varroa.
Acknowledgement:
Sir Michael Fay and David Richwhite for permission to move our honey bee colonies to Great Mercury Island and providing staff and transport to assist with the trial.
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Page last updated: 30 April 2008