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Varroa mites are external parasitic mites that are also known by their scientific name Varroa destructor. The parasite is present in all honeybee colonies across the U.S. and beyond. It is the only parasitic mite known for targeting both the young and adult host. Additionally, Varroa mites are a vector to several viruses, posing a serious risk to honeybee colonies. Statistics show that colonies that do not properly manage the Varroa mite pandemic stand a 90 to 95% chance of collapsing. Therefore, proper management of the mite can help save millions of bees from this destructive parasite.
What is a Varroa Mite?
The Varroa destructor is a mite that is easily distinguishable. It has a characteristic brownish-to-red coloration with a sesame seed size that makes it conspicuous. The mites, as tiny as they appear, measuring only 1.5mm in width, are one of the largest known parasitic mites when evaluated against their target host. To the honeybee, the mite would be likened to a whole plate-sized tick attached to a human body.
Varroa mites are native to Asia. The first recorded presence of the mites in regions such as North America, Hawaii, the UK, and New Zealand was in the early 20th Century. They are a parasite to multiple honeybee species but in particular the European honeybee Apis mellifera.
Effects of Varroa Mites on Honeybees
The mite affects honeybee colonies in various ways. A heavy infestation of the mites will take 3 to 4 years to build up and its ultimate effect will be clear. Common signs of an infestation include impaired flight performance, scattered brood, low forager worker bee return to the hive, crawling honeybees, and emaciated honeybees. Additionally, the mites cause the deformation and weakening of the honeybee brood.
1. Weaken Adult Bees and Larvae
The mites directly target the fatty cells of the insect tissue of the bees and larvae or which they gorge on and if untreated this will eventually lead to colony collapse.
The mites weaken the bees over years and reduce their lifespan substantially. Varroa mites target adult bees and young bees including the developing larvae and pupae. The females lay their eggs on the brood, and the adult mites feed and live on the adult honeybee.
The mite attack on honeybees occurs in two life stages. During its first stage of life referred to as the phoretic stage, the adult mite attaches itself to the adult bee and will rely on the bee hemolymph, a fluid within the honeybee’s circulatory system. At this point, mites will change hosts often and, in the process, transmit viruses.
Mites pick up a virus from one honeybee and transmit it to another bee through injection while feeding. These phoretic mites remain hooked to adult bees until their death due to old age. They can also be eliminated when the bees bite them off while grooming. These are the mites beekeepers find on the bottom board, commonly referred to as “natural mite drop”. Unfortunately, the mites that fall off the honeybee are insignificant when compared to the mite population hidden within capped cells.
The attack on the honeybee larvae marks the reproductive stage of a female Varroa mite. The mite that is ready to lay eggs will move out of the adult bee’s body and move into a cell of a developing larva.
The adult mite begins to feed inside the cell when the larva has started to pupate and this occurs immediately when the cell is capped. It will then lay its eggs, one unfertilized egg, and about four to six unfertilized eggs. The unfertilized mite egg becomes a male whereas the fertilized eggs become females.
Mite eggs that hatch inside honeybee cells begin to feed on the honeybee pupa and will mate with the one male inside the cell. Eventually, a sexually mature female mite will remain hooked to the host bee that emerges as an adult from the cell.
Female mites take 6 to 7 days to mature from the egg to adult and will live for between 3 to 8 months depending on the season. They tend to live longer during the fall. The mature mites can survive outside of the brood cell, with the survival highest survival rate of their offspring noted in a drone cell.
2. Increased Susceptibility to Pests and Diseases
Varroa mites lay the foundation for pests and diseases to decimate the honeybee colony. The weakened immunity of the adult bees and larvae makes it possible for other pests and diseases to creep in and wipe out a robust honeybee colony.
Varroa mites are a vector and incubator of more than 20 known honeybee viruses, including the deadly ones that completely devastate entire colonies. Most honeybees will not die from the mite itself but rather due to the viruses transmitted by the mites. These viruses are a leading cause of deadly diseases in honeybees.
Scientists have studied honeybee viruses for more than 50 years and considered them harmless until the prevalence of the Varroa mites in the 1980s. The majority of the 20 viruses that have been discovered in honeybees have been linked directly to Varroa mites. They are the physical and biological transmitters of these viruses.
An example of a deadly virus transmitted by the mite is the deformed wing virus. The control of Varroa mites will help counter the associated viruses that target honeybees. The symptoms of viral diseases in the bees also serve as an indicator of the presence of Varroa mites.
The viruses affecting honeybees target the pupal or the larval stage in bees. However, symptoms of viruses are apparent in adult bees. The bees acquire the viruses through pollen or the royal jelly that worker bees produce for feeding the brood.
Varroa mites also transmit viruses directly to honeybees. This occurs when the mites feed on the hemolymph and in the process transmit viruses directly into the honeybee’s open circulatory system. Ultimately, viruses end up in every cell inside the bee’s body.
Some of the viruses that are prevalent in honeybees include:
This was discovered in the 20th century and is presently recognized as the most prevalent in the European honeybee. Sacbrood affects the queen, workers, larva, and eggs. It is also present in varroa mites. It can be detected and eliminated on infected larvae by adult bees.
Deformed Wing Virus (DWV)
This is a common and widely distributed virus affecting honeybee colonies. It is present in 100% of all varroa mite-infested colonies and has been linked to winter colony mortality. Varroa mite treatment has been proven to substantially reduce DWV. The virus is transmitted by the Varroa mites and can also be passed from drone to queen, through food, from queen to egg, and through feces.
Black Queen Cell Virus
This is another honeybee virus that affects worker bees and broods. It leads to disjointed wing pairs in adult bees. Affected pupae exhibit pale-yellow skin that looks like a sac which later darkens. The black queen cell virus is linked to Nosema disease which affects the midgut in bees.
Chronic Bee Paralysis Virus
A virus that leads to chronic paralysis as suggested by its name. The affected bees exhibit the symptoms after 5 days and a few days later die. It is a major cause of colony collapse. It affects both adult bees and those bees in their developmental stages. However, developing bees have lower viral loads compared to adult bees.
Other common viruses associated with the varroa mite include Kashmir Bee Virus, Acute Bee Paralysis Virus, Israeli Acute Paralysis Virus, and the Slow Bee Paralysis Virus.
3. Effect on the Honeybee Population
Varroa mites reduce the honeybee population as the adult bees and young brood die out in the colony. The ultimate effect of that is that future bees are killed, and existing ones are reduced in number. Over time this has had a huge impact on the colony numbers.
Colonies rely on population for food collection, defense, nursing, cleaning, and regulation of hive conditions such as temperature and humidity. A lower population will mean a colony is not sufficiently equipped to counter its many challenges. Food such as nectar and pollen ends up being insufficient since foragers have fallen in number.
The colony’s ability to defend itself from various predators including robber bees is jeopardized when the colony population fall. The Varroa mite attack eventually leads to dwindled populations to a point of no reverse and it becomes almost impossible to save affected honeybee colonies.
How to Check for Varroa Mites
The majority of beekeepers do not test for Varroa mites since the task is time intensive. They should, however, make it a part of their usual inspection routine. Various methods can be used to check for Varroa mites, these include:
1. Powdered Sugar Test Method
This method of checking for Varroa mites is the most popular, as it is easy, fast, and gives fairly accurate results.
The recommended regularity of this test is monthly and should be done in all seasons. It is one of the best methods of ascertaining whether Varroa mite treatment will be necessary and which one to use.
The beekeeper will need the following:
- Paper towels
- A timer
- Dry powdered sugar
- A water spritzer
- A measuring scoop
- A cat litter box
- and a jar with lead.
You can purchase these as a package online or use available materials in your household.
How this is done:
Choose the frame to test, preferably the first one outside the brood area since this contains the nurse bees. These are the usually bees with the greatest number of mites. Ensure the queen bee is not among the bees on the selected frame.
Transfer the bees into the plastic tub by shaking the frame. The bees should be cautiously dropped into the tub without harming them. You can then return the frame.
Tilt the plastic tub to ensure the bees are collected at one corner then scoop the bees into the sampling container. Close the sampling jar. You can also use a jar with a one-cup amount marked. Set it on the top of the frames and pour the bees into it.
Use a hive tool to scoop the powdered sugar and gently pour it into the sample jar. Roll the jar smoothly and slowly with your hands to ensure the sugar is uniformly smeared upon the bees.
Allow the jar to rest. You can then spread the white paper towels on the plastic tub. You can then invert the jar and shake it to allow the mites to fall through the screen lid into the white paper towels. You can then open the jar and pour the bees into the hive.
Use the water spritzer to dissolve the sugar that is in the plastic tub. This will melt the sugar and reveal the mites. You can use a magnifying glass to count the mites you can see.
It is now time to evaluate the results. Multiply the number of what has been sighted by 3 to get the hive infestation percentage.
2. Alcohol Wash Test Method
This method is also known by the name of the alcohol roll method. It is a simple and quick method. It is an ideal method of determining the infestation rate that can help decide if treatment for the mites will be necessary.
The test can be done monthly before treatment. It should also be done within 10 to 14 days after treatment. The beekeeper will be more efficient if the test is done regularly.
Isopropyl alcohol is required for this method of Varroa mite testing and you will also need a Varroa EasyCheck or Varroa Mite Test Bottle. A sample of 300 bees will be required for the test and unfortunately, these will never be returned to the hive. It is disliked by some beekeepers since the bee mortality rate from the test is high.
The beekeeper will require below supplies: protective clothing, measuring scoop, timer, fine mesh strainer, Varroa EasyCheck/Varroa Mite Test Bottle, plastic container, large plastic tub, Isopropyl alcohol, and a large leaded container.
To do this test, follow the below steps:
Collect the sample bees while ensuring the queen bee is not among them. You will need the plastic tub for collecting the bees. Pick a frame, preferably the one with nurse bees, and shake it over the plastic tub. Tilt the tub on one side to round up the bees on one side of the plastic tub. Scoop the bees and dump them inside the test bottle.
Shake the test container once to wet the bees. Open and add the fluid to the upper end of the outer container.
Shake the container vertically, horizontally, and in a circular motion for about 60 seconds to separate the bees from the mites.
Hold the container against light to find out the number of mites that are collected at the bottom of the test container.
Do the calculation to get the hive infestation percentage.
3. Sticky Board Test Method
This method is also referred to as Mite Monitoring boards and provides an easier way of monitoring Varroa mites infestation in the beehive. The method makes it possible to do so without opening the hive.
The sticky board test measures the number of mites that drop from the hive over a determined period. The helps is more effective when done regularly throughout the season. It helps detect whether the mite levels have increased or dropped over time and allows the beekeeper to respond accordingly.
The sticky board test is done with the use of:
- A screened bottom board
- A mite collection surface
- Cooking oil
- Paper towels
- Magnifying glass
To do the test follow the below steps:
The screened bottom board is smeared with cooking using paper towels. Two tablespoons of the oil are enough for the test.
The oiled board is then placed into the slot. The tape is used to seal it into place to keep off ants and bees.
The screened board is left for three days, pulled out from the slot, and taken for evaluation. A well-lit area is required for checking for mites.
The magnifying glass is used to find the mites. A toothpick with a wet tip can be used to pick up the mites and separate them on the board.
The test is done for three days and the mite numbers are evaluated to get an estimate of the average daily mite count.
The number of mites captured during the test should be written down and graphical representations done for a clearer interpretation. Numbers tend to be lower at the start of the season and will increase by midsummer. Samples should be disposed of at end of every test to avoid reintroducing the mites to the honeybee colony.
How to Get Rid of/Treat Varroa Mites
1. Brood Break
This is an effective method of controlling Varroa mites in honeybee colonies, helping minimize the number of brood cells that can be available for Varroa mites to reproduce.
A queen break of about 3 weeks means removing the queen from the hive to allow all available brood cells to hatch. The queen can also be caged for the same duration to keep her from laying eggs in existing cells. When the queen is restrained, the mites will be forced to come out alongside the hatched brood and they will have to remain hooked on adult bees. Other Varroa mite control methods can be applied thereafter to eliminate the mites.
The brood break will also help boost grooming in bees and this will contribute to keeping the mite numbers in check. This will be complemented using a screened bottom board.
2. Mechanical Control Measures
These are primarily methods that involve colony or hive manipulation to remove Varroa mites from the honeybee colony. It includes the use of powder sugar dusting, screened bottom boards, and drone brood removal.
A screened bottom board is used instead of a solid board to allow the mites to fall through to the ground. The method helps reduce the mite population in the brood.
Mechanical drone brood removal makes it possible to eliminate Varroa mites since they have a preference for these cells. By adding a drone comb to the colony, it will work as a trap for attracting varroa mites. The combs will then be removed before the drone bees emerge. These can be frozen and removed later. Alternatively, the infected brood cells can be scraped off and disposed of.
The use of powdered sugar helps encourage bees to engage in grooming behavior. This is an effective method of dusting off the mites that are attached to the adult bees.
3. Chemicals Methods of Varroa Control
This method takes two forms, soft and hard chemicals. They are best used before the production the onset of winter.
Some commonly used soft chemicals are those made from organic acids, hop beta acids, and essential oils. They are effective and never leave behind residues on bee products such as wax. Examples of these include formic acid, oxalic acid, and thymol.
Hard chemicals can also be used to control Varroa mites. These are acaricides or miticides that target the mites. They have an efficacy rate of 95% in killing the mite population. Some of the commonly used hard chemicals are coumaphos and fluvalinate. These chemicals are legal to use but highly discouraged considering the effect it has on honeybee colonies. For more Varroa mite treatment options, you can read our article on the best treatments for Varroa mites.
Varroa mites inflict serious damage on honeybee colonies. They are the main cause of colony collapse in most European honeybee colonies. The beekeeper should regularly monitor their apiaries to determine the presence of the mites. Consequently, the right Varroa mite control method should be applied to get rid of the pest. These mites not only target the adult bees but they indiscriminately suck the life out of the developing brood. They target drone cells since these cells give the Vvarroa mites sufficient time to develop and devastate honeybee colonies. Beekeepers are encouraged to apply rotation treatment to avoid chemical resistance and to effectively eliminate the mites.