20 Oct Pest & Disease – Varroa Mite
Varroa mite has been prioritized by the Southern IPM Center as a #1 target to ensure honey bee survival.
Southern Integrated Pest Management Center http://www.sipmc.org/SIPMC/assets/File/SERAPriorities2017(1).pdf.
What are Varroa mites?
Varroa destructor (Varroa mite) is an external parasitic mite that feeds on and reproduces exclusively in honey bee colonies. These mites attach to and feed on the fat-body of the bee. This pest is a vector for a number of debilitating bee viruses, including RNA viruses such as the deformed wing virus (DWV). A seemingly small infestation can become a significant problem that can lead to the death of a colony in winter months. Currently, Varroa mite damage and management is among the greatest economic impacts stressing the beekeeping industry.
Recent surveys of beekeepers in North America suggest their #1 concern is Varroa mite.
Kulhanek, K. et. al. A national survey of managed honey bee 2015-2016 annual colony losses in the USA. Journal of Apicultural Research 56, 328-340 (2017).
Varroa mite – Life Cycle
Varroa mites reproduce on a 10-day cycle. The female mite enters an open honey bee brood cell that contains a young larvae and waits for the cell to be capped. As soon as the cell is capped, she lays her eggs on the larva. Typically, she will lay, on average, 1 male and 5 female eggs. It takes about 2 days for each egg to hatch. After hatching, the male will mature after about 5 days (132 hrs) and the female matures near day 7 (165 hrs). Male mites will mate with female siblings as soon as they reach maturity. The male will continue to mate with the most recently matured female until multiple mating with each female has occurred. After pupation, the young bee emerges from the cell and the Varroa mites also leave the cell and spread throughout the colony. This process can cause the spread of genetic defects throughout the colony through several viruses that the mite carries, as well as spreading any fungi and bacteria residing on the host bee.
The newly mated female Varroa mites will then preferentially choose to infest drone brood because the extra 3 days it takes for drones to develop and emerge from the cell allows the mite to reproduce an additional time, compared to the brood of worker bees. Social structure is intricately intertwined with honey bee biology in some surprising ways and this parasite primarily reproduces in drone pupae, while a rare genotype among workers gives them what is called ‘Varroa Sensitive Hygiene’ (VSH). This is a phenotypic behavior where bees detect parasite-infested brood cells, open them, and remove the contents, stopping the mite replication process.
Varroa mite has been among the top causes of colony failure for more than a decade in the US.
National Survey of Managed Honey Bee Annual Colony Losss in the USA, Journal of Apicultural Research 2007-2019.
Philosophy of Varroa Infestation
There are two types of beekeeper, those that know their bees have Varroa and those who don’t – in which case they likely won’t have bees for much longer.
Even if you don’t see Varroa mites on your bees, there is a very high chance that your bees still have them. There are a number of mites that exist in bee hives, however, none are as problematic as Varroa. The easiest way to verify if your colonies have Varroa is to do a 24-hour sticky board on the bottom of each hive. Be sure to place a piece of 1/8th inch hardware cloth on top of the sticky board to prevent bees from sticking to it. A number of treatment options are available for managing Varroa mites in your bee colonies. It is uniformly agreed upon by beekeepers that an Integrated Pest Management (IPM) plan be used to mange mite loads.
Honey bee colonies risk re-infestation of Varroa mites all season, as healthy hives rob resources from weak hives. This process leads to heavy mite loads in the late summer and early autumn months.
In the late autumn months, prior to winter, as a last-ditch effort, colonies with heavy mite loads will abscond to alleviate themselves from Varroa mites. If your bees abscond, you are left with empty equipment.
It is normal for bees to have mites living with them in the hive. In fact, a sharp-eyed beekeeper can sometimes find a mite predator known as a Pseudoscorpion living among the bees inside the hive. This small arachnid will hitchhike on foraging bees and ride back to the nest. Once there, these little scorpion-like spiders will forage on mites, beetle larvae, ants and small flies. Beneficial insects such as this will not be present in an overly medicated bee hive. Many beekeepers are taught to routinely medicate with a variety of chemicals to keep Varroa mite loads down inside the colony. Some beekeepers take the opposite approach and do nothing, letting Darwinism take over.
BM Dooney Farms Varroa IPM Strategy
Simply doing nothing about Varroa mites will not yield positive results, if your expectations are to keep bees.
As Ecologists, we are dedicated to supporting the environment, which includes the area inside our Apiary and the one surrounding it. We are all part of the larger ecosystem. There are many options on the market for managing Varroa mites, and in this excerpt we are only going to talk about the path we have chosen. We manage this mite using a combination of approaches that overlap.
MECHANICAL BIOLOGICAL CHEMICAL GENETICS
Mechanical
We control this mite by applying screened bottom boards when the air temp is ideal – above 70F day and night, which translates into ON in the spring, OFF in the fall in the Southeastern US. These boards allow for the mites that fall off the bees to drop to the ground, rather than remaining inside the hive and hitching a ride back up into the uncapped cells.
Biological
We control this mite with the assistance of the mite predator, Pseudoscorpion. Studies have shown this little arachnid has a voracious appetite for Varroa mites, and about 25 predators can hold the mite population in check for a single hive. These are naturally occurring members of the Order Pseudoscorpion, there are more than 3,300 species known in more than 430 genera. They range worldwide – temperate to cold regions , above treeline, inside limestone caves, tropics to subtropics – they are EVERYWHERE on earth! And lucky for us, because they are very beneficial to humans – not just beekeepers benefit from their mite foraging! They eat many other insects we consider pests like carpet beetles, clothing moths, booklice, etc. You can encourage these mite predators by maintaining an Apiary that does not habitually expose their bees to chemical treatments.
Chemical
We control this mite with oxalic acid vapors. This is an organic chemical found naturally in the leaves of many plants in the Oxalis Family, hence the name. These plants are ones we all love like rhubarb, spinach, and amaranth, among others. There are 2 important things to remember if you wish to use this method:
- SOLID BOTTOM BOARDS MUST BE ON
- HONEY SUPERS MUST BE OFF
A word of caution: Oxalic acid vapor method is TOXIC to humans.
Personal protective equipment (PPE) is REQUIRED if you want to use this method, and this includes a full respirator that has a cartridge for organic vapor compounds..
As a vapor, oxalic acid coats the body of the honey bee, which is the safest method for the bee to get exposure. Oxalic acid as an oral treatment (known as oxalic acid dribble) is another option for managing this mite. However this method can be toxic to the bee because it can disrupt the healthy microflora in the honey bee gut, which has been linked to increased malnutrition and disease susceptibility. By using a short-term vapor exposure, the bees are unharmed by this process and will groom themselves after the vapors crystallize on their bodies, but they do not ingest the crystals during this process. The mites, however, are not so lucky – the oxalic acid vapors burn their appendages, so the mites fall off the bees and hit the bottom boards, to die a short time later, within 24h, from lack of feeding.
Chemically eradicating Varroa from your Apiary by continuous & habitual application of chemical treatments may be possible, but it is neither ecologically nor economically sustainable.
Genetics
By allowing our bees to be exposed to Varroa mites, our bees have the opportunity to change their behaviors as a response to best manage this mite – in other words, adaptation. Our bees display Varroa Sensitive Hygiene (VSH) and Excessive Grooming Behaviors (EGB) as documented by USDA Managed Pollinator Coordinated Agricultural Projects (Hunt et al. 2007; Oxley et al. 2010; Arechavaleta-Velasco et al. 2012; Hamiduzzaman et al. 2017). These bees use extensive grooming activities to hunt down parasites, pathogens, and diseased bees (even inside capped cells) to expel them from the hive. We select nucleus colonies from the most healthy and productive hives that display strong resistance to Varroa mites. New Queens are naturally mated by Drones of the most Varroa resistant colonies.
When to treat with OAV?
Ideally, when the bees kick their drones out, that is your cue the window of time has arrived. As you know, the bees do not allow the drones to overwinter, and will stop allowing them into the hive. This is also when brooding is lowest, which is another target for successful application of this vapor treatment. That time of year typically falls sometime in October for North America. Targeted treatment is done annually to support the environment, rather than chronic exposure to treatments that risk the development of tolerance among the bees, although tolerance to this type of treatment is highly unlikely because it does not rely upon toxicity thresholds for mode of action. Oxalic acid vapors have a physical impact to the mite that does not involve chemical tolerance. Our approach is further supported by Darwinian Evolution – you want your bees to have exposure to this mite so they can evolve resistance, like has been observed with hygienic & infected cell cleaning behaviors in honey bees.
As a reminder, your hives will continually bring in Varroa mites as stronger hives rob weaker hives throughout the season – that includes the hives you ARE managing inside your Apiary, and those you are NOT that surround you.
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Curious To Learn More? Read These!
- Conlon, B. H. et al. The role of epistatic interactions underpinning resistance to parasitic Varroa mites in haploid honey bee (Apis mellifera) drones. Journal of Evolutionary Biology 31, 801-809 (2018).
- Conlon, B. H. et al. A gene for resistance to the Varroa mite (Acari) in honey bee (Apis mellifera) pupae Molecular Ecology 28, 2958-2966 (2018).
- Devi, S. et al. Different methods for the management of Varroa mite (Varroa destructor) in honey bee colony. Journal of Entomology and Zoology Studies ,https://www.entomoljournal.com/archives/2019/vol7issue4/PartC/7-3-331-917.pdf
- Kulhanek, K. et al. A national survey of managed honey bee 2015–2016 annual colony losses in the USA. Journal of Apicultural Research 56, 328-340 (2017).
- Seeley, T. The Lives of Bees. Princeton, NJ, USA. (2019)
- Seitz, N. et al. A national survey of managed honey bee 2014–2015 annual colony losses in the USA. Journal of Apicultural Research 54, 292-304 (2015).
- Spotler, A., Gupta, P., MAyer, M., Reinsch, N. & Bienefeld, K. Genome-Wide Association Study of a Varroa-Specific Defense Behavior in Honey bees (Apis mellifera). Journal of Heredity 107, 220-227 (2016).
- Steinhauer, N. A. et al. A national survey of managed honey bee 2012–2013 annual colony losses in the USA: results from the Bee Informed Partnership. Journal of Apicultural Research 53, 1-18 (2014).
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