Varroa Mites


Anybody considering beekeeping should be aware of the importance of keeping them healthy. What most new beekeepers might not know is that there are many different diseases that can potentially affect the honeybee. Some of these are mild and will cause nothing more than a reduction in honey production, but others can be extremely serious and can wipe out entire colonies in short order. It is therefore essential that, as a beekeeper, you are aware of the diseases and can spot them as early as possible.

In this article I will focus on one of the biggest threats to the honeybee – varroa mites.

What are Varroa Mites?

Varroa mites are external parasites that feed on both adult and developing honeybee body fluids. These are the most serious threat to bees due to the fact that they can cause physical damage that weakens the bee. Without early intervention and an effective treatment, a varroa mite infestation can result in the destruction of an entire colony.

Varroa mites are part of the arachnid species, a species that also includes spiders and ticks as well as other mites. Most of the varroa mites that beekeepers encounter are mature females. Originally discovered in Asia during the early twentieth century, the varroa mite has spread to all parts of the world apart from Australia and is typically present in every bee colony. Unless continuously treated by the beekeeper, the infestation of these mites will increase exponentially.

Varroa destructor Bee-Mite
Photo by Pavel Klimov, Bee Mite ID (idtools.org/id/mites/beemites) unless otherwise stated in description. / Public domain

Varroa mites are visible to the naked eye and appear as small brown or red spots. They are typically visible on the thorax of a bee. These mites carry viruses and other pathogens that can damage the bee. Developing bees that have been affected by the varroa will often suffer physical damage such as deformed wings.

What Do Varroa Mites Look Like?

As mentioned above, the varroa mite is brown or red in color and is typically 1- 1.5mm in size. Although this may seem very small in human terms, when compared to the size of the bee these parasites are relatively large. As arachnids, varroa mites have the typical eight legs but theirs are found at the front of the body, and they have jaw-like structures that are used to puncture the body of their host (this is how they attach themselves to the bee). The mites are covered in fine hairs and relatively speaking they can move very fast, which allows them to move easily from bee to bee. Although not possessing eyes, these creatures use their sense of smell to detect honeybees.

How Do Varroa Mites Affect Developing Bees?

The survival of the varroa mite is completely dependent on the honeybee host as they cannot live separately from the bee for longer than a few days. The mites feed on the blood of adult bees but use pupae for reproduction purposes.

The adult mite enters the honeybee brood cell just before it is capped. Upon the cell being capped, the mite lays its eggs and pierces the brood cell to leave an opening. Once the eggs have hatched, the new mites may mate with each other, and they and the parent will feed on the developing brood.

As the mite reproduction cycle occurs within the capped brood cell, it can be difficult for untrained beekeepers to notice what is happening, which can lead to things getting out of control without intervention.

Varroa mites on pupae
By Waugsberg – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4622590

Varroa mites tend to seek drone broods because their cells tend to remain capped for longer than other types of bees. Basically, the longer the cell is capped, the more varroa offspring will be generated. Once the new adult bee leaves the brood cell, the parent mite and its offspring will be released. However, bees emerging from the infected brood will be weakened and ultimately have a shorter than average lifespan.

It is essential, then, that beekeepers are able to identify the signs of disease or they risk a mite infestation overtaking the bees, particularly during the late summer forage or the preparation for winter when the hive population has reduced. If this does occur, the colony might not be able to survive the winter even if there is sufficient food supply.

How Can Beekeepers Treat Varroa Infestations?

Knowing the damage that can be caused by the varroa mite, it is vital that a beekeeper can identify their presence and deal with them effectively. To ensure the health and survival of your bee colony, you must continuously monitor the levels of varroa mite. You should also be aware that it is virtually impossible to completely eradicate varroa mites from a colony, even with treatment.

What you should know is that varroa mite numbers can increase dramatically if left untreated. This is why controlling the varroa infestation is an absolutely essential component of effective beekeeping.

To manage the level of infestation then, it is necessary to use cultural, mechanical, or chemical approaches.

Varroa mites on pupa-small
Public Domain, https://commons.wikimedia.org/w/index.php?curid=140970

Cultural Methods

Cultural approaches involve the purchase of mite-resistant bees. The benefit of this is that you can reduce the need for chemical treatments. Such bees include the varroa-sensitive hygiene bee, ankle biters, and Russian bees.

Varroa-sensitive hygiene bees are able to spot the pupae that have been infested with varroa mites. They can remove dead brood as well as the mites.

Ankle biters – or leg chewers as they are also known as – physically damage mites by biting at their legs.

Russian bees are able to suppress the reproduction of mites, thus leading to a lower mite population increase.

Other cultural approaches include using smaller cell combs. Research has shown that although modern beekeeping has led to hexagons measuring around 5.4mm, feral bees tend to build comb from hexagons measuring 4.9mm. With smaller cells, there is a shorter period after capping in which fewer numbers of varroa mites are produced.

Some beekeepers prefer to manage varroa infestation using brood breaks. To accomplish a brood break, you need to remove the queen from the colony for a number of weeks. While the queen is away, the brood will hatch, and the mites will be released from the cells. With no new brood in which to reproduce, the mites will be forced on to the adult bees, which will in turn reduce the population growth.

Varroa destructor on drone
Waugsberg / CC BY-SA (http://creativecommons.org/licenses/by-sa/3.0/)

Mechanical Treatments

There are several mechanical methods that can be used to control mite infestation. One such method is utilizing a bottom board made of mesh rather than solid wood. This is known as a screen bottom board.

This method is simple in that it relies on mites falling off the bee through natural movement. When the mite falls to a solid board, there is a high probability that it will climb back on to the bee. However, with a screened bottom, the mite will fall through the holes in the screen and onto the ground, decreasing the likelihood of it getting back into the colony. This method can reduce the population of varroa mites in the colony but is most effective when used in combination with other methods of control.

Powdered sugar is often used in conjunction with screened bottom boards. When beekeepers sprinkle powdered sugar on their honeybees, it encourages grooming, which in turn results on more mites falling to the ground.

Another popular mechanical method is mite trapping. This works by relying on the varroa mites’ preference for drone broods over worker broods. As mentioned above, varroa mites can reproduce in higher numbers in a drone brood due to the longer post capping time.

By adding a drone comb to a colony, it can be used as a trap for the mites. The beekeeper can remove the drone colony before the drones emerge, which then means the reproducing mites in the drone brood are also removed. Once removed, the drone brood is either frozen and returned to the colony or scraped from the frame. As above, this method will help to reduce mite infestation but should be used in conjunction with other methods for effective mite control.

Chemical Treatments

Beekeepers also have the option to use either soft or hard chemicals to try to control and manage the infestation of varroa mites. Nevertheless, using chemicals is typically a method of last resort when all other methods have not worked effectively.

Soft Chemicals

Soft chemicals are considered as such because they are natural. These include essential oils, organic acids, and hop beta acids. Beekeepers are encouraged to try soft chemicals before resorting to hard chemicals.

Essential oils like thymol are extracted from plants. Thymol comes from the thyme plant. This is particularly useful in tackling varroa mite infestation. However, beekeepers should be aware that while thymol is effective at controlling mites on adult bees, it is not capable of penetrating capped cells. This means that it is ineffective when trying to deal with mites within the capped cells.

The effectiveness of thymol depends on external temperature. It is most effective at temperatures between 68oF and 75oF. You should be aware though that it can cause robbing behavior among bees as well as an increase in aggression. Most beekeepers using thymol use it in conjunction with other treatments.

Oxalic acid occurs naturally in certain plants and can also be used to control mites. For the treatment of mite infestation, two formulations are available: dribble and vapor. Like thymol, oxalic acid does not penetrate capped cells and therefore should be used with other treatments. It is also important to be aware that oxalic acid in high doses can harm bees while overuse can decrease worker bee activity and lifespan.

Honeybee coated with oxalic acid
By Chamblis – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=44483567

Formic acid is present in honeybee venom and honey. In high doses, it can penetrate capped cells and target the reproducing mites. One of the biggest drawbacks of using formic acid is that its efficacy is dependent on temperature. Temperatures lower than 50oF significantly reduce how effective it is but conversely temperatures above 85oF mean there is an increased risk of brood mortality, which could in turn cause the death of the queen.

Most soft chemicals should be used with other methods for increased efficacy.

Hard Chemicals

As you have probably guessed, hard chemicals are not natural, and most are synthetically made. While hard chemicals such as coumaphos and fluvalinate were once widely used due to their efficacy in treating mite infestations, they are less popular now as mites have developed a resistance to them. These chemicals can also leave residue in the hive, which can then become covered in wax. This wax-covered residue can harm bees and leave them at higher risk of Nosema disease. Another issue is that the wax-covered residues can make its way into bee products. As you can imagine, this is not desirable for customers.

When it comes to hard chemicals for mite infestation, amitraz (or apivar as it is sold) is the most common. The benefit of this chemical is that it does not contaminate wax or honey, but there are concerns about interactions between amitraz and some viruses that can result in a higher rate of bee death. Furthermore, research has uncovered some evidence of mite resistance to amitraz, which will inevitably affect its efficacy going forward.

Conclusion

As one of the most prevalent and serious threats to the honeybee, beekeepers must be aware of the presence of varroa mites at all times and take measures to keep infestations under control. As you can see from above, there are many different methods to help with this endeavor. While each has their pros and cons, it might be worthwhile experimenting and using more than one method to keep your bees healthy and productive.


Below are some interesting videos on the topic:


Top-of-page image credit: https://www.flickr.com/photos/sanmartin

Anthony

Anthony is a content creator by profession but beekeeping is one of his great passions.

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