VSH Selection and Breeding in a Northern Climate

We are taking time in this article to explain and guide beekeepers on the differences of Northern VSH selection and queen breeding in the United States. We will also be adressing some commonly asked breeder selection questions we have received from everyone.

This AI breeder colony is a winter survivor and ready to receive the Harbo VSH assay.

Imagine you have the perfect colony. The colony has no mites, produces twice the amount of honey as every colony in the yard and is so gentle that you can kick the box and not receive a sting. You select this perfect colony during its first season and graft queens. The perfect colony arrives at it’s first winter and dies before the first snow fall. Most of the daughters of this perfect colony also collapse during the winter. What went wrong here? I can say from personal experience that the mistake here is not initially selecting for survival of a Northern winter. No matter how great the colony’s traits are, nothing is more important than being able to survive the genetic bottleneck of winter without treatment.

Overwintered colonies can be split to create nucs for sale. They will also produce more honey than a colony started the same season. Winter in the Northern United States is a major test, and takes more planning than in other parts of the US. One could even argue that selected Northern stock is more robust and able to overcome environmental adversity than colonies selected in a more forgiving winter environment. For this article, we are defining a Northern winter climate as a latitude greater than 42 degrees. We want to add to this by stating that this area cannot have the added benefit of residing on or near the ocean. Oceans provide too much of a heat reservoir to coastal areas that moderate and stabilize winter temperatures.

Northern stock is more robust and able to overcome enviromental adversity than colonies selected in a more forgiving winter enviroment.

The documented research into VSH (varroa sensitive hygiene) breeding in the United States has been done in a Southern climate. Winters are non-existent compared to our Northern environment. This Southern environment offers the ability to select and graft both earlier and later in the year while also performing the Harbo/VSH assay late into the season when the new colonies have had more time to develop. So, does this selection process translate into quality Northern VSH selective breeding? We don’t think so.

The Benefits of Doing a Spring Assay

You can tell it's still spring here in WI when you see a flannel and insulated vest being worn.

If we wait till the following spring to evaluate our potential breeder colonies in the North, what’s the advantage? Simply put, we get to see who survives the bottleneck of the Northern winter. This is the big test for all of our breeder colonies and provides a simple binary answer. There is no guess work involved and queen breeders of all skill levels can perform this test. The other important factor is making sure you are not applying any varroa treatments to your potential breeder colonies. Applying varroa treatment any time during the season to a breeder colony will create the issue of a false negative varroa presence when assaying for VSH behavior.

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If a breeder colony was started in the summer of the previous season, after winter all workers will be the offspring of the breeder queen. Colonies that have survived winter are a complete balanced unit at this point since enough time has passed in the re-queening cycle. A thriving overwintered colony has a robust population of all bees of the appropriate age cohorts to complete every task in the hive to the best of the colony’s ability. Most importantly, this includes the VSH behavior. There is a big difference between a colony that comes out of winter thriving, as opposed to just surviving. We want to select from colonies that come through winter with enough food stores and a large enough population to have a dynamic expansion during the spring. If we start selecting from weak colonies that limped across the finish line in spring, we will be ignoring the benefits of the genetic selection of the bottleneck of a Northern winter. Often times we hear about folks jumping the gun and trying to Harbo assay their breeding colony 60 days post queen introduction. In our opinion, you need to wait longer. Here are a few reasons:

• Seasonality makes a difference. If you installed a breeder queen in August, performing a Harbo assay in October will be rather challenging with the low levels of brood remaining and the physiology of the bees changing too. How does the different physiology of winter bees affect the expression of VSH? We don’t know because what we have learned about VSH expression has all taken place in the south where winter isn’t a factor.

• You need to factor in the time it takes for a queen to be introduced, begin laying at a normal rate again, the time for the population to turn over, the time for these new bees to age up to properly express VSH behavior and then the time for this behavior to impact the colony. This process takes over 60 days to occur easily. If we wait till after winter, the guess work of the colony’s population distribution has been removed.

After surviving winter, the colony can be assayed in the early spring before grafting begins. We now know that the colony has the ability to survive winter treatment free and has been assigned a Harbo score. But some may ask, aren’t mite levels lower in the spring? We can assay colonies in the spring, but then use late summer assays when mite levels may have increased again. For those that have doubts, this can confirm the colonies VSH score if you continue to let the colony expand all season. This is something we have done previously to confirm the quality of the breeders we produce and safeguard the production of virgin queens and drone stock.

Harbo Assay

Consistency is key when performing an assay.

We prefer to perform the Harbo assay via examining 200 cells. We prefer this because it provides a more complete picture than should we stop at 100 cells. Could you continue and do 300 cells? Sure, you can do as many cells past 200 as you wish. At the end of the day, the Harbo assay is just a tool that we can use to make an educated guess of the colony’s fitness. We can never know if pulling 201 cells instead of 200 cells would make a difference by finding that one reproductive varroa cell. Or perhaps you picked the cell to the left instead of to the right that contained a reproductive mite. That one reproductive cell is the difference between a perfect scoring 4 on the assay or the breeder colony being downgraded to a score of 3. The key is that you must be consistent with all your assays. You cannot flip flop between cell counts or alter the protocol if you want to have quality data to document and share with other beekeepers. We cannot stress this fact enough when it comes to data collection and processing. The more variables that the beekeeper places in the data, the weaker the impact. It’s also important that you have a good sample size before drawing any conclusions from your data.

A current copy of Dr. Harbo’s VSH protocol can be viewed using this link. We consider this method the gold standard when it comes to selecting for VSH in our population. Remember that we use behavioral assays like this as a shortcut in our breeding selection. The Harbo Assay serves as a proxy for the bees' ability to survive varroa, but we are not aware of a proxy for a long cold winter.  Wintering bees confined in a propolis sealed cavity do not easily lend themselves to study.  Much of what goes on in the dark over winter remains a secret.  The Southern selected bees may have the traits needed for Northern survival in their genome, but we know that ours do because those that don't have been weeded out.  The best way to select breeder colonies is by being patient for years and seeing if the breeder colony and daughter colonies can survive multiple seasons on their own while we perform behavioral assays like the Harbo assay to identify the mechanism of resistance.

You cannot flip flop between cell counts or alter the protocol if you want to have quality data to document and share with other beekeepers.

The Pitfalls of Summer Assays

We want to do our best to answer some additional common questions we receive from folks as well and a few we thought about when we designed our breeding protocol.

Can I treat my breeder colonies the previous summer/fall and then assay them in the spring?

Any treatments applied will inhibit the accuracy of your results selecting for varroa resistance. If you have to treat a potential breeder colony, then it wasn’t worth selecting from in the first place. The only exception to this is creating clean colonies to install artificially inseminated queens into. We call this giving them a “clean start” and have written about it in our care instructions for our AI queens. The basic idea is that you need to select brood from colonies that already have low varroa or use a treatment to knockdown the varroa load before installing an AI breeder queen. Installing your expensive AI breeder queen into a colony overrun with varroa and then expecting it to perform well is foolish, a waste of your money and a waste of the queen producer’s time.

Given that spring is a very busy time of year for beekeepers, what are the disadvantages of waiting till summer to perform selective breeding for mite resistance?

If you are using the potential breeder colony for grafting in the spring without assaying it first, you will go in blind. It’s important to verify your level of resistance before taking the leap of faith of grafting daughters from a perspective breeder colony.

Spring and summer are typically a time beekeepers prefer to split our overwintered colonies. If we are splitting the breeder colony, does this disrupt the population distribution and prevent efficient VSH behavior?

This could be an issue because there may not be enough bees of the right age to complete the tasks effectively. We can also ask ourselves, how long after splitting do we wait for the colony to balance itself before assaying? While we don’t have the answers to these questions yet, we can speculate that altering the population dynamic will affect our assays on mite resistance. We suggest Harbo assaying a colony before splitting them in the spring. We would also advise not continuing to alter the colony for the rest of the season if you wish to Harbo assay the colony in the late summer.

Does a strong nectar flow affect the colonies behavior in terms of VSH? Is nectar collection more important than stopping varroa reproduction?

Some studies have shown that a heavy nectar flow will reduce the expression of the VSH trait (Bigio et al., 2013). The hypothesis is that nectar collection and storage is considered greater on the colony’s hierarchy of needs than inhibiting varroa reproduction. As a result, we suggest performing the Harbo assay on potential breeder colonies in the early spring before the first major flow begins and also in August when the last major flow has concluded.

If drone brood is present, does it matter?

We have received many questions about performing the Harbo assay while large amounts of drone brood are present. At this point, we are unsure. But our thought is that it does not. Even if varroa is only allowed to reproduce in drone brood, varroa will never reach as high of an infestation rate as utilizing both drone and worker brood to reproduce. Early spring assays allow the user to perform the assay when little drone brood or drones are present in the colony if drone brood is a concern to the producer.

Spring Is King In the North

Different climates call for modifications to breeding protocols. The timing of seasonality and physiological differences can have a large impact on the selection method and success. For queen breeders like ourselves in the far North of the United States, winter survivors remain the king for selecting potential breeder colonies. We hope that you reflect on what we have written and consider what improvements you can make to your own protocols to best suit your region. Feel free to reach out and comment. You may have other considerations. We are all in this together and you may have points we never considered before. 

Adrian and Trevor on the roof of the Marquette honey bee lab in Milwaukee Wisconsin.

We want to take a moment to end this article by thanking Dr. Harbo for all his work on VSH research over the decades. The selective breeding and artificial insemination work we do at Lloyd Street Bees would not be possible without the foundational research and techniques he has provided. We also thank Adrian Quiney for reviewing these thoughts and techniques.