What is Varroa Sensitive Hygiene in Honeybee Colonies?

Scientists have put in the work when it comes to efforts to counter the many threats that threaten honeybee colonies. The unleashing of Varroa Sensitive Hygiene (VSH) stock of bees was a game changer in beekeeping. Varroa Sensitive Hygiene is a trait that describes the ability of bees to recognize, open, and eliminate capped broods that contain reproducing Varroa mites. This special behavior is not common with all honeybees but can be seen in colonies that demonstrate strong resistance to Varroa infestations. This has made a tremendous difference in apiaries and made the work of beekeepers much easier. This minimizes the need to use potentially harmful chemicals to treat the deadly Varroa mites.

Origin of Varroa Sensitive Hygiene

Varroa Sensitive Hygiene is attributed to a team of scientists based in the USDA Bee Breeding Lab in Baton Rouge Louisiana that were able to come up with the idea through careful observation and experimentation. They painstakingly studied honeybees for long periods and were able to pinpoint the natural defenses that honeybees have been using to defend their colony from mite infestations.

Initially, Varroa Sensitive Hygiene was referred to as Suppressed Mite Reproduction (SMR). The name was coined with reference to the way the colonies with the traits eliminated the Varroa mites. The term was later changed since it was established that the colonies with hygienic behavior exhibited a high sensitivity to the presence of reproducing Varroa mites. Therefore SMR was renamed to VSH.

The selection of the superior traits in bees over many generations has made it possible to not only defend colonies from mites but has also boosted resistance to honeybee diseases such as chalkbrood and the American foulbrood. It has also helped colonies resist small hive beetles and the wax moth.

Queen rearing has made it possible to pass Varroa Sensitive Hygiene traits to future honeybees. Proper selection of breeding stocks in the bees at the beginning will help transform the entire local population of honeybees into an army that will destroy Varroa mites. This will help eliminate most of the problems caused by Varroa mites. The development and use of Varroa Sensitive Hygiene bees have demonstrated that humans and nature can collaborate and benefit mutually.

Further research was conducted by scientists where susceptible and resistant queens were exchanged between colonies. The results from the study showed a tremendous improvement in colonies where a resistant queen was introduced. In such colonies, the Varroa mite populations went down. Conversely, colonies, where a susceptible queen was introduced exhibited negative results. Instead, the mite populations increased in such colonies.

Varroa Sensitive Hygiene bees were picked from a number of domestic honeybee colonies in Michigan and Louisiana. These comprised honeybees that portrayed some mite resistance. The bees were all American bees and the study began the year 1995. Initial studies focused on determining mite and bee populations and measuring traits that were associated with mite resistance. Subsequently, a study on traits related to the growth of mite populations was conducted. Further analysis was done to statistically determine the level of heritability of such traits.

About the Varroa Mite

The Varroa mite is also referred to as Varroa destructor and is a parasite that targets honeybee colonies. It only reproduces in honeybee colonies. It is regarded as the greatest threat to honeybee colonies worldwide. It feeds on honeybee haemolymph and acts as a vector for deadly honeybee viruses and diseases. Colonies that are infested are prone to collapse if the mites are left untreated.

The Varroa mite might have proven to devastate honeybee populations in most parts of the world. However, some stock of bees has proven to successfully survive the mite infestation. This clearly demonstrates the fact that these bees have developed natural defense mechanisms against the mites. This is what is termed Varroa Sensitive Hygiene which has helped colonies to counter the mite infestation. The trait involves the worker bees detecting and removing paralyzed broods.

Varroa Mite Behavior

The female Varroa mite will enter a worker brood cell at the time the larva is 5 days old, and just before the cell is capped. It will remain hidden behind the larva as she awaits the cell to be capped. Once the cell is capped, it will begin to lay its first egg 3 days after entering the cell. This first egg will be a male then followed by a female egg at intervals of 30 hours.

The male mite will mate with all the sexually mature female mites before the worker bee comes out of its cell. In total, it takes only 7 days for a female mite to be sexually mature and fertile. This translates to about 1 male, 3 immature female mites, and 2 mature females by the time the worker bee hatches inside the capped cell.

The immature females and the male are unable to survive outside the cell. The other two mature females will move out of the cell in search of uncapped cells where they begin their reproductive cycle. Each of these will carry out several reproductive cycles over its lifetime.

Varroa mites that develop inside a drone cell enjoy an extra time of 3 days that makes it possible for the 3 immature female mites to complete their development and invade the nearby uncapped cells where they can begin their reproductive cycles. That can explain why the mites have a preference for drone cells. Here they can have enough time for all their offspring to mature and propagate.

Sensitivity to Varroa Mites

Varroa Sensitive Hygiene worker bees have demonstrated the ability to easily identify Varroa mites within the pupae. This is believed to be an inheritable trait allowing the bees to detect the mites either through their movement or odors.

There is also the possibility of the pupae giving a chemical cue that signals the responsible worker bees of the Varroa incursion. The Varroa Sensitive Hygiene bees will then respond to the situation by uncapping the affected cells and removing the pupae with the varroa mites.

Varroa Sensitive Hygiene honeybees have also been found to uncap and later recap pupae that are infested with Varroa mites. This helps eliminate the invading mites. This uncapping response is carried out by the hygienic bees when the pupa is at the purple, white, and pink-eyed stage. At this time, the female Varroa mite has two to three offspring and the uncapping will help remove the mother alongside its offspring.

Displacing Varroa mites after uncapping makes it possible for adult worker bees to kill them. If it is lucky to escape then it will move into a new larval cell where the mite is no longer able to reproduce a normal offspring. The non-reproducing mites are common in colonies with high levels of Varroa Sensitive Hygiene tendencies.

The fact that Varroa Sensitive Hygiene bees are able to interrupt the life cycle of the Varroa mites helps keep the mite population in low numbers. The trait can be inherited and breeders are able to combine this with their integrated pest management programs.

Breeding for Varroa Sensitive Hygiene Traits

Any stock or race of bees can be bred for hygienic traits. Beekeepers should choose from their best breeder colonies when selecting stock for hygienic breeding. Examples of these colonies that exhibit the best traits will include those that have proven to winter well, produce enough honey, are friendly, and exhibit the desired behavior. The queen producer will get a head start when they choose hygienic behavior by selecting queens from chalkbrood-free colonies.

Colony screening for hygienic behavior can help when considering queens to be used to propagate Varroa Sensitive Hygiene genes. Those colonies that are quick to remove the testing brood within 24 hours make the best and should be considered for rearing queens. These queens will produce drones for the first generation and these carry good hygiene genes. They will exhibit the behavior of quickly removing infected broods.

Another way to pass on hygienic genes is through the artificial insemination of hygienic queens using semen acquired from drones originating from hygienic colonies. These queens can also be mated naturally in an isolated area within a locality where the available drones are from hygienic colonies. These strategies will help fix the desired trait on the line of bees. If many bee breeders collectively raise bees for Varroa Sensitive Hygiene traits then over the long haul the trait will be prominent in the existing population of bees. This will mean it becomes easier for future queens to encounter drones that carry the traits.

If you are buying a hygienic queen, then it is imperative to ensure that the majority of the drones she mated with originated from hygienic colonies. A queen that has mated with drones from unhygienic colonies will give birth to worker bees that will not exhibit the Varroa Sensitive Hygiene traits and the colony will not be hygienic. To make it possible for hygienic queens to mate with hygienic drones, the beekeeper should ensure the beehive is positioned in close proximity to apiaries with hygienic behavior. Therefore, it will be wise when buying a hygienic queen to ask the breeder about the drone-producing colonies that mated with the queen. Otherwise, you risk introducing a queen that will give birth to unhygienic worker bees.

In addition to keeping mites populations lower, selecting hygienic behavior on the line of bees will help curb some common honeybee diseases such as chalkbrood and American foulbrood. There exists a close link between the ability of a colony to remove infected brood and disease and mite resistance.

Since there are no negative characteristics recorded on hygienic lines of bees, it will be of great benefit to the beekeeper and the entire beekeeping industry to pass over these genes to future-generation bees. These hygienic colonies might be infected with diseases but they are better placed to deal with these diseases.  They, therefore, require no treatment for diseases such as chalkbrood. Instead, occasional treatments for mites will be required. Consequently, continued use of this hygienic stock will give rise to mite-resistant stock that needs fewer treatments.

Breeding of honeybee colonies with Varroa Sensitive Hygiene traits presents a long-term solution to the fight against Varroa mites. It is the best alternative to chemical treatments such as the use of acaricides. It should therefore be a part of the beekeeper’s integrated pest management strategies since it contributes to supporting strategies that support a stable host and parasite relationship.  

How Diseased Brood is Detected

Scientists have not been able to pint-point how exactly worker bees from hygienic colonies are able to recognize the presence of reproducing mites in affected capped cells. However, it is believed that the smell from the wounded or diseased pupa and the mite’s odor could be the potential triggers. The honeybees are unable to discover and remove mites that do not begin reproducing.

Additionally, various tests carried out showed that hygienic bees are more sensitive to odors when compared to non-hygienic bees. They target reproducing mites and ignore non-reproductive or sterile mites. This trait to detect and remove reproducing mites has been proven to be additive, meaning that colonies that express the trait more strongly are those with more Varroa Sensitive Hygiene stock of bees. The traits can also be passed on naturally by queen bees that are bred from pure Varroa Sensitive Hygiene breeders.

Advantages of Varroa Sensitive Hygiene Genetics

The ability of honeybees to detect and remove Varroa mites from the colony is an important one, since Varroa mites pose a significant threat to honeybee colonies globally. They not only affect the honeybee’s health and productivity but also cause huge losses to the beekeeper.  Therefore, the beekeeper will benefit from Varroa Sensitive Hygiene genetics as per below:

1. Natural defense

It is the most effective and natural defense against Varroa mites eliminating the need to use chemical treatments as an option.

2. Boost productivity

Beekeepers that breed bees with Varroa Sensitive Hygiene traits are able to minimize the impact of Varroa mites on their apiaries leading to healthier and more productive honeybees.

3. Mite resistance eliminated

The Varroa Sensitive Hygiene trait is a natural behavior in bees that is passed over to future generations of bees through genes. It is therefore not possible to mites to develop some level of resistance to the Varroa Sensitive Hygiene line of bees as they may develop under chemical treatments. For the latter, mites have been reported to develop some level of resistance after being subjected to a chemical treatment continuously.

4. Sustainable solution

It is an important and sustainable tool for beekeepers to manage Varroa mites without impacting the environment negatively.

5. Economical solution

This is an economical option for the beekeeper over the long haul. The initial cost might be high but subsequent expenses on mite treatments become substantially lower. As a matter of fact, the possibility of colony collapse is greatly minimized when the beekeeper raises a healthy and productive honeybee colony.

6. Diseases prevention

The hygienic behavior in honeybees helps limit pests and diseases in honeybee colonies. This line of honeybees demonstrates a strong ability to resist pests and diseases. They are also more productive when compared to colonies with lower Varroa Sensitive Hygiene tendencies. Some of the common viruses that Varroa Sensitive Hygiene honeybees have proven to exhibit high resistance to include Chronic Bee Paralysis Virus and the Deformed Wing Virus. This can be attributed to the fact that the mites that act as transmitters of these viruses are eliminated making it difficult to spread the viruses.

7. Better wintering ability

The Varroa Sensitive Hygiene bees exhibit a greater survival rate over winter when compared to standard honeybees that are not mite-resistant. This was recorded at 65% survival for Varroa Sensitive Hygiene bees compared to the 25% survival rate for standard honeybees. Additionally, the non-hygienic bees require extensive chemical treatments for the mites making it more costly to manage the mites.

Disadvantages of Varroa Sensitive Hygiene Genetics

Varroa-sensitive hygiene bees have not demonstrated any negative traits in all research and studies that have been undertaken so far.

Testing for Varroa Sensitive Hygiene Traits

There are two recommended methods that can help determine a colony’s hygienic behavior. These methods help ascertain if a colony has hygienic tendencies or not. They include the freeze killed brood method and the liquid nitrogen method.

1. Freeze Killed Brood

This method involves the use of a comb section of sealed brood that has approximately 100 cells on each of its sides, that is, measures 2 x 2.5 inches or 5 x 6cm. This is cut from the frame and frozen at -10°F or -20°C for at least 24 hours.

The frozen comb section is then placed inside a frame of sealed brood within the colony that is being tested. This frozen section can be sourced either from the colony under testing or from a different colony. The frame with freeze-killed brood should be placed at the center of the brood nest and observations made on how the colony will respond to it.

After a 24-hour time-lapse, the test frame is removed from the beehive, and the total number of sealed cells is identified and recorded. The results will be as follows:

• The Varroa sensitive hygiene colonies will uncap and remove 95% of the frozen brood within 24 hours.
• Non-hygienic colonies will take up to 6 days to completely eliminate the frozen brood.

2. Liquid Nitrogen Method

This second method for testing for Varroa Sensitive Hygiene traits in honeybee colonies is more efficient and less destructive when compared to the initial method. Liquid nitrogen is easily available and within reach for most beekeepers. The use of industrial-grade liquid nitrogen is permitted in most states and all safety protocols should be observed during transportation and use.

There are a number of precautions that need to be observed when handling liquid nitrogen. It is extremely cold with a boiling point of -195°C or -320°F and will cause severe frostbite when it comes into contact with your skin.

A hollow cylinder is required for pouring the liquid nitrogen into the chosen section of the sealed brood. This could be at minimum 4 inches long since the liquid nitrogen will boil once it comes into contact with the brood. A total of 160 cells will be required for the test and you will need about 10 ounces of liquid nitrogen for it. An amount that is too little will not deliver the required results. The 10 ounces or 300MM will be sufficient.

Choose a frame that has at least a 3-inch diameter of sealed brood and less than 30 unsealed cells. Pour 50 to 60 mm of the liquid nitrogen into these cells and wait for the nitrogen to freeze on the edges or evaporate. The remaining liquid nitrogen can then be poured into the cylinder. Allow the cylinder about 10 minutes to thaw.

Mark the frames where the test has been carried since some highly hygienic colonies can clean and repair these combs immediately and it might be difficult to locate the test area when you come back.  You can place the test frame in the middle of the brood nest.

Allow the frozen brood 24 hours and remove it to find out the results. Check the number of sealed cells that have remained within the circle. The test results will be as follows:

• A colony that can be considered hygienic will remove 95% of the frozen brood on both test methods within 24 hours.
• A colony that removes 95% of the frozen brood within 24 hours on the first test and only 50% on the second test is considered unhygienic.
• For more reliable results, allow 6 to 8 weeks for colonies that have been recently requeened before doing the test, since after this time the existing queens in the colony will be daughters of the new queen.

Conclusion

Varroa Sensitive Hygiene honeybees have shown a high-level and vigorous response to highly infested broods. They uncap and remove any pupae that are infested with the varroa mites. In the case of the least-infested broods, these hygienic honeybees show less intensity of uncapping and removing these less affected pupae. It has also been noted that the Varroa Sensitive Hygiene line of bees targets fertile mites. They reduce the fertility of the mites within a few weeks of being introduced to a colony with non-Varroa Sensitive Hygiene bees. This trait led to the introduction of the term Suppressed Mite Reproduction (SMR). Varroa Sensitive Hygiene honeybees provide a more effective, economical, and sustainable solution to combatting Varroa mites.

References

• https://www.ars.usda.gov/Services/docs.htm?docid=2744&page=13
• https://wildflowermeadows.com/2017/12/vsh-varroa-sensitive-hygiene-bees/
• https://www.sare.org/publications/a-sustainable-approach-to-controlling-honey-bee-diseases-and-varroa-mites/breeding-for-resistance/the-difference-between-hygienic-behavior-and-varroa-sensitive-hygiene-vsh/
• https://www.sare.org/publications/a-sustainable-approach-to-controlling-honey-bee-diseases-and-varroa-mites/breeding-for-hygienic-behavior/
• http://hdl.handle.net/10113/43776
•  http://www.extension.umn.edu/honeybees/components/pdfs/Apidologie_37_2006.pdf
• http://www.extension.org/pages/Varroa_Sensitive_Hygiene_and_Mite_Reproduction
• http://www.glenn-apiaries.com/vsh.html#anchor417460
• http://ars.usda.gov/pandp/people/people.htm?personid=2313

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