Because we have been breeding mite and disease tolerant stock for over 22 years, our threshold for unmedicated mite-tolerant genetics is very strict. Late summer is a time we are especially careful to evaluate every hive, with particular attention to potential breeder colonies.
In the fall of 2017 we began to see the presence of some Deformed Wing Virus (DVW) in the apiary. We scrutinized, culled, and prepared our stock to overwinter in much the way we always do, but by early February it became obvious that something was seriously different.
As the populations rapidly dwindled we pulled and incubated queens from the collapsing colonies; then made an emergency March-madness run to California to purchase queenless packages to act as warm-bodies to save the queens.
Although we lost 70% of our colonies over the winter of 2017-2018, by rescuing all the unmedicated queens that had been heavily exposed to DWV, we chose to see this as a new opportunity in our breeding program.
So in June of 2018 when Randy Oliver’s 2016-2017 Overwinter Loss Survey results, from samples submitted in 2016, confirmed that our losses were caused by high infestations of the most lethal strain of DWV-type ”A”, we were not surprised.
We had been here before:
In 2008 we successfully recovered from 90% colony losses due to devastating exposure to n. ceranae. Subsequent lab reports verify that more than a decade later, our hardy survivor stock continues to remain healthy and productive in spite of carrying large n. ceranae spore loads.
…and we believed our proven stock could do it again.
2018 July samples submitted to the USDA Lab as part of the APHIS National Honey Bee Survey, confirmed the continued presence of DWV, but now without obvious symptoms. The unmedicated survivor colonies continued to build-up and thrive … leading us to seek the answer to an important question: Is it possible that survivors of DWV could develop immunity?
In response to that query Jessica Kevill, [Post Graduate researcher for the University of Salford, Manchester England,] replied the following:
It’s hard for me to tell, as DWV is a tricky one. Colonies die once the viral loads exceed the host threshold, whilst colonies which are just below the threshold survive until loads then become unsustainable and they die. Often there are no visible disease symptoms and bees with high loads may not be visibly different to bees with low loads.
If you are interested in resistance, breed from your survivor stock and don’t move bees from outside of the area into your apiary. You will either breed a resistant trait in the bees OR maintain a viral infection that the bees can cope with, we’ve seen this in the UK.
Since we are in a unique situation that allows us to open-mate our queens in relative isolation, the indisputable fact is that our unmedicated recovery stock is not only managing DWV viral loads, but also appear to have succeeded in passing this adaptive trait to their offspring.
To be fair, there is a difference of opinion in the scientific bee community as to whether or not viral exposures can result in “heritable resistance traits” that can be replicated in offspring. We don’t have all the answers. We will leave that debate to the “experts”.
We can only draw conclusions based on lab data results and what we see in the apiary…. and what we see is “survivability”. Our 2019 overwintered survival rate was the best ever, allowing us to sell a record number of spring nucs!
Is this resistance?…tolerance…? Only time will really tell.