Varroa Mite
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''Varroa destructor'', the Varroa mite, is an external parasitic
mite Mites are small arachnids (eight-legged arthropods) of two large orders, the Acariformes and the Parasitiformes, which were historically grouped together in the subclass Acari. However, most recent genetic analyses do not recover the two as eac ...
that attacks and feeds on
honey bee A honey bee (also spelled honeybee) is a eusocial flying insect within the genus ''Apis'' of the bee clade, all native to mainland Afro-Eurasia. After bees spread naturally throughout Africa and Eurasia, humans became responsible for the ...
s and is one of the most damaging honey bee pests in the world. A significant mite infestation leads to the death of a honey bee colony, usually in the late autumn through early spring. Without management for Varroa mite, honey bee colonies typically collapse within 2 to 3 years in temperate climates. These mites can infest ''
Apis mellifera The western honey bee or European honey bee (''Apis mellifera'') is the most common of the 7–12 species of honey bees worldwide. The genus name ''Apis'' is Latin for 'bee', and ''mellifera'' is the Latin for 'honey-bearing' or 'honey-carrying', ...
'', the western honey bee, and ''
Apis cerana ''Apis cerana'', the eastern honey bee, Asiatic honey bee or Asian honey bee, is a species of honey bee native to South, Southeast and East Asia. This species is the sister species of '' Apis koschevnikovi'' and both are in the same subgenus as ...
'', the Asian honey bee. Due to very similar physical characteristics, this species was thought to be the closely related '' Varroa jacobsoni'' prior to 2000, but they were found to be two separate species after DNA analysis. Parasitism of bees by mites in the genus '' Varroa'' is called varroosis. The Varroa mite can reproduce only in a honey bee colony. It attaches to the body of the bee and weakens the bee. The species is a vector for at least five debilitating bee viruses, including
RNA virus An RNA virus is a virus characterized by a ribonucleic acid (RNA) based genome. The genome can be single-stranded RNA (ssRNA) or double-stranded (Double-stranded RNA, dsRNA). Notable human diseases caused by RNA viruses include influenza, SARS, ...
es such as the deformed wing virus (DWV). The Varroa mite is the parasite with possibly the most pronounced economic impact on the
beekeeping Beekeeping (or apiculture, from ) is the maintenance of bee colonies, commonly in artificial beehives. Honey bees in the genus '' Apis'' are the most commonly kept species but other honey producing bees such as '' Melipona'' stingless bees are ...
industry and is one of multiple stress factors contributing to the higher levels of bee losses around the world. Varroa mite has also been implicated as one of the multiple causes of
colony collapse disorder Colony collapse disorder (CCD) is an abnormal phenomenon that occurs when the majority of worker bees in a honey bee bee colony, colony disappear, leaving behind a queen bee, queen, plenty of food, and a few nurse bees to care for the remaining ...
. Management of this pest focuses on reducing mite numbers through monitoring to avoid significant hive losses or death. 3% of bees infested in a hive is considered an economic threshold where damage is high enough to warrant additional management. Miticides are available, though some are difficult to time correctly while avoiding harm to the hive, and resistance has occurred for others. Screened bottom boards on hives can be used for both monitoring and mite removal, and drone comb that mites prefer can be used as a trap to remove mites from the hive. Honey bee lines in breeding programs also show partial resistance to Varroa mite through increased hygienic behavior that is being incorporated as an additional management strategy.


Description and taxonomy

The adult female mite is reddish-brown in color, while the male is white. Varroa mites are flat, having a button shape. They are 1–1.8 mm long and 1.5–2 mm wide, and have eight legs. Varroa mites lack eyes. These mites have curved bodies that allow them to fit between the abdominal segments of adult bees. Host bee species can help differentiate mite species in the genus '' Varroa''; both ''V. destructor'' and '' Varroa jacobsoni'' parasitize ''
Apis cerana ''Apis cerana'', the eastern honey bee, Asiatic honey bee or Asian honey bee, is a species of honey bee native to South, Southeast and East Asia. This species is the sister species of '' Apis koschevnikovi'' and both are in the same subgenus as ...
'', the Asian honey bee, but the closely related mite species originally described as ''V. jacobsoni'' by
Anthonie Cornelis Oudemans Anthonie (Antoon) Cornelis Oudemans Jzn (November 12, 1858 – January 14, 1943) was a Dutch zoologist. Although he was a specialist in acarology, the study of the ticks and mites, he was often best known for his books on sea monsters and the do ...
in 1904 does not attack ''
Apis mellifera The western honey bee or European honey bee (''Apis mellifera'') is the most common of the 7–12 species of honey bees worldwide. The genus name ''Apis'' is Latin for 'bee', and ''mellifera'' is the Latin for 'honey-bearing' or 'honey-carrying', ...
'', the western honey bee, unlike ''V. destructor''. Until 2000, ''V. destructor'' was thought to be ''V. jacobsoni'' and resulted in some mislabeling in the scientific literature. The two species cannot be easily distinguished with physical traits and have 99.7% similar genomes, so DNA analysis is required instead. Because the more virulent and damaging species ''V. destructor'' could not be distinguished at the time, most pre-2000 research on western honey bees that refers to ''V. jacobsoni'' was actually research on ''V. destructor''. Other Varroa species '' V. underwoodi'' and '' V. rindereri'' can also parasitize
honey bee A honey bee (also spelled honeybee) is a eusocial flying insect within the genus ''Apis'' of the bee clade, all native to mainland Afro-Eurasia. After bees spread naturally throughout Africa and Eurasia, humans became responsible for the ...
species and can be distinguished from ''V. destructor'' and ''V. jacobsoni'' with slight differences in body size and
setae In biology, setae (; seta ; ) are any of a number of different bristle- or hair-like structures on living organisms. Animal setae Protostomes Depending partly on their form and function, protostome setae may be called macrotrichia, chaetae ...
characteristics, though each of the four species within the Varroa genus have similar physical characteristics. If a Varroa species is found on a western honey bee, it will typically be ''V. destructor'' except where ''V. underwoodi'' is present, such as in Papua New Guinea. The name "Varroa mite" is typically used as the
common name In biology, a common name of a taxon or organism (also known as a vernacular name, English name, colloquial name, country name, popular name, or farmer's name) is a name that is based on the normal language of everyday life; and is often con ...
for ''V. destructor'' after the species was considered separate from ''V. jacobsoni''. Varroa mite has two distinct genetic strains from when it switched hosts from the Asian honey bee to the western honey bee: Korean and Japanese. The Korean strain that occurred in 1952 is now found worldwide in high frequencies, while the Japanese strain that started around 1957 occurs in similar areas at much lower frequencies. Varroa mite has low genetic diversity, which is typical for an invasive species undergoing a range or host expansion.


Range

Varroa mites originally only occurred in Asia on the Asian honey bee, but this species has been introduced to many other countries on several continents, resulting in disastrous infestations of European honey bees. Introduction data prior to 2000 is unclear due to confusion with ''V. jacobsoni''. By 2020, ''V. destructor'' was confirmed to be present throughout North America (excluding Greenland), South America, most of Europe and Asia, and portions of Africa. The species was not present in Australia as well as Oman, Congo, Democratic Republic of Congo, and Malawi. It was suspected to not be present in Sudan and Somalia. Mites were found in 2022 in New South Wales in Australia.


Life cycle

Female mites enter brood cells to lay eggs on the comb wall after the cell is capped. Eggs are approximately 0.2 to 0.3 mm in diameter and cannot be seen without magnification. These eggs hatch into male and female protonymphs that are both transparent white. Immature mites can only feed on capped brood, so the life cycle cannot be completed during broodless periods. Protonymphs molt into deuteronymphs that more closely resemble the curved body of adults before they molt into adults. Development time from egg to adult is 67 days. Males will not leave brood cells and only mate with females present in the brood cell. Adult females can be found feeding both on brood and adult bees. After reaching the adult stage, females will leave the brood cell and enter a
phoretic Phoresis or phoresy is a temporary Commensalism, commensalistic relationship when an organism (a phoront or phoretic) attaches itself to a host organism solely for travel. It has been seen in tick, ticks and mite, mites since the 18th century, ...
stage where mites attach to adult bees in order to disperse. Mites will feed on adult bees at this time and can be transmitted from bee to bee during this stage. Nurse bees are preferred hosts in order to be moved to new brood cells. Because the nurse bee spends more time around the drone brood (i.e., male bees) rather than the worker brood, many more drones are infected with the mites. These phoretic females can also be transmitted to other hives through bee contact or hive equipment transfer. The phoretic stage can last for 4.511 days during brood production periods or up to five to six months when no brood is present in winter months. Female mites have a life expectancy of 27 days when brood is present. After the phoretic stage, female mites leave the adult bee and enter brood cells with bee larvae. Drone cells are preferred over workers. These females are called foundress mites, and they bury themselves in brood food provided by worker bees before the cell is capped. Brood cell capping begins egg cell activation for a foundress mite while she emerges to feed on the larva. She will lay a single unfertilized egg after feeding to produce a male mite. After laying this egg, fertilized eggs to produce females are laid approximately once a day. Both the mother and nymphs will feed on the developing pupa. Unless multiple foundress mites are present in a cell, mating occurs between siblings when they reach the adult stage. Once females mate, they are unable to receive additional sperm. Varroa mite's
genetic bottleneck A population bottleneck or genetic bottleneck is a sharp reduction in the size of a population due to environmental events such as famines, earthquakes, floods, fires, disease, and droughts; or human activities such as genocide, speciocide, wid ...
is also likely due to its habit of sibling mating. File:Varroa destructor protonymph (5048063601).jpg, ''V. destructor'' protonymph File:Varroa destructor deutonymph.jpg, Deutonymph of ''V. destructor'' File:Varroa destructor (Adult Male) (5048079279).jpg, ''V. destructor'' adult male File:Varroa on larvae.jpg, Varroa mite on bee larva File:Vorroa Mite on pupa.JPG, Varroa mites on pupa File:Female Varroa destructor on the head of a bee nymph (5048103407).jpg, Adult female in frontal view File:Varroa destructor ventral.jpg, Adult female in ventral view File:Varroa destructor dorsal.jpg, Adult female in dorsal view File:Varroa destructor microscope.jpg, Adult female mounted for microscopy


Host interactions

Adult mites feed on both adult bees and bee larvae by sucking on the fat body, an insect organ that stores glycogen and triglycerides with tissue abundant under epidermis and the surrounding internal body cavity. As the fat body is crucial for many bodily functions such as hormone and energy regulation, immunity, and pesticide detoxification, the mite's consumption of the fat body weakens both the adult bee and the larva. Feeding on fat body cells significantly decreases the weight of both the immature and adult bee. Infested adult worker bees have a shorter lifespan than ordinary worker bees, and they furthermore tend to be absent from the colony far more than ordinary bees, which could be due to their reduced ability to navigate or regulate their energy for flight. Infested bees are more likely to wander into other hives and further increase spread. Bees will occasionally drift into other nearby hives, but this rate is higher for Varroa infested bees. Adult mites live and feed under the abdominal plates of adult bees primarily on the underside of the abdominal region on the left side of the bee. Adult mites are more often identified as present in the hive when on top of the adult bee on the thorax, but mites in this location are likely not feeding, but rather attempting to transfer to another bee. Varroa mites have been found on flowers visited by worker bees, which may be a means by which phoretic mites spread short distances when other bees, including from other hives, visit. They have also been found on larvae of some wasp species, such as ''
Vespula vulgaris ''Vespula vulgaris'', known as the common wasp, is a species found in regions that include the United Kingdom, Ireland, Germany, India, China, New Zealand and Australia. It is sometimes known in English as the European wasp, but the same name i ...
'', and flower-feeding insects such as the
bumblebee A bumblebee (or bumble bee, bumble-bee, or humble-bee) is any of over 250 species in the genus ''Bombus'', part of Apidae, one of the bee families. This genus is the only Extant taxon, extant group in the tribe Bombini, though a few extinct r ...
, '' Bombus pensylvanicus'', the scarab beetle, '' Phanaeus vindex'', and the flower-fly, '' Palpada vinetorum''. There have not been any indications Varroa mites are able to complete their life cycle on these insects, but instead they become distributed to other areas while a mite is still alive on these insects.


Virus transmission

Open wounds left by the feeding become sites for disease and virus infections. The mites are vectors for at least five and possibly up to 18 debilitating bee viruses, including
RNA virus An RNA virus is a virus characterized by a ribonucleic acid (RNA) based genome. The genome can be single-stranded RNA (ssRNA) or double-stranded (Double-stranded RNA, dsRNA). Notable human diseases caused by RNA viruses include influenza, SARS, ...
es such as the deformed wing virus. Prior to the widespread introduction of Varroa mite, honey bee viruses were typically considered a minor issue. Virus particles are directly injected into the bee's body cavity and mites can also cause immunosuppression that increases infection in host bees. Varroa mites can transmit the following viruses: * Kashmir bee virus * Sacbrood virus * Acute bee paralysis virus * Israeli acute paralysis virus * Deformed wing virus Deformed wing virus is one of the most prominent and damaging honey bee viruses transmitted by Varroa mites. It causes crumpled deformed wings that resemble sticks and also causes shortened abdomens.


Colony collapse disorder

There is some evidence that harm from both Varroa mite and associated viruses they transmit may be a contributing factor that leads to
colony collapse disorder Colony collapse disorder (CCD) is an abnormal phenomenon that occurs when the majority of worker bees in a honey bee bee colony, colony disappear, leaving behind a queen bee, queen, plenty of food, and a few nurse bees to care for the remaining ...
(CCD). While the exact causes of CCD are not known, infection of colonies from multiple pathogens and interaction of those pathogens with environmental stresses is considered by entomologists to be one of the likely causes of CCD. Most scientists agree there is not a single cause of CCD.


Management

Mite populations undergo
exponential growth Exponential growth occurs when a quantity grows as an exponential function of time. The quantity grows at a rate directly proportional to its present size. For example, when it is 3 times as big as it is now, it will be growing 3 times as fast ...
when bee broods are available, and exponential decline when no brood is available. In 12 weeks, the number of mites in a
western honey bee The western honey bee or European honey bee (''Apis mellifera'') is the most common of the 7–12 species of honey bees worldwide. The genus name ''Apis'' is Latin for 'bee', and ''mellifera'' is the Latin for 'honey-bearing' or 'honey-carrying', ...
hive can multiply by roughly 12. Mites often invade colonies in the summer, leading to high mite populations in autumn. High mite populations in the autumn can cause a crisis when drone rearing ceases and the mites switch to worker larvae, causing a quick population crash and often hive death. Various management methods are used for Varroa mite
integrated pest management Integrated pest management (IPM), also known as integrated pest control (IPC) integrates both chemical and non-chemical practices for economic control of pests. The UN's Food and Agriculture Organization defines IPM as "the careful consideratio ...
to monitor and manage damage to hives.


Monitoring

Beekeepers use several methods for monitoring levels of Varroa mites in a colony. They involve either estimating the total number of mites in a hive by using a sticky board under a screen bottom board to capture mites falling from the hive or estimating the number of mites per bee with powdered sugar or an ethanol wash. Monitoring for mites with a sticky board can be used to estimate the total number of mites in a colony over 72 hours using the equation: :p(c)=\frac where ''b'' is the number of mites found on the sticky board and ''c'' is the number of estimated mites in the colony. However, the bee population in a colony also needs to be known to determine what population of mites is tolerable with this method. Mite counts from a known quantity of bees (i.e., 300 bees) collected from brood comb are instead often used to determine mite severity. Mites are dislodged from a sample of bees using non-lethal or lethal means. The bees are shaken in a container of either powdered sugar, alcohol, or soapy water to dislodge and count mites. Powdered sugar is generally considered non-lethal to honey bees, but lethal methods such as alcohol can be more effective at dislodging mites. 3% of the colony being infested is considered an economic threshold damaging enough to warrant further management such as miticides, though beekeepers may use other management tactics in the 02% infestation range to keep mite populations low.


Chemical measures

Varroa mites can be treated with commercially available
acaricide Acaricides are pesticides that kill members of the arachnid subclass '' Acari'', which includes ticks and mites. Acaricides are used both in medicine and agriculture, although the desired selective toxicity differs between the two fields. Termi ...
s that must be timed carefully to minimize the contamination of
honey Honey is a sweet and viscous substance made by several species of bees, the best-known of which are honey bees. Honey is made and stored to nourish bee colonies. Bees produce honey by gathering and then refining the sugary secretions of pl ...
that might be consumed by humans. The four most common synthetic pesticides used for mite treatments with formulations specific for honey bee colony use are
amitraz Amitraz (development code BTS27419) is a non-systemic acaricide and insecticideCorta, E., Bakkali, A., Berrueta, L. A., Gallo, B., & Vicente, F. (1999). Kinetics and mechanism of amitraz hydrolysis in aqueous media by HPLC and GC-MS. Talanta, 48(1 ...
, coumaphos, and two
pyrethroids A pyrethroid is an organic compound similar to the natural pyrethrins, which are produced by the flowers of pyrethrums (''Chrysanthemum cinerariaefolium'' and '' C. coccineum''). Pyrethroids are used as commercial and household insecticides. In ...
, flumethrin and tau-fluvalinate, while naturally occurring compounds include
formic acid Formic acid (), systematically named methanoic acid, is the simplest carboxylic acid. It has the chemical formula HCOOH and structure . This acid is an important intermediate in chemical synthesis and occurs naturally, most notably in some an ...
,
oxalic acid Oxalic acid is an organic acid with the systematic name ethanedioic acid and chemical formula , also written as or or . It is the simplest dicarboxylic acid. It is a white crystalline solid that forms a colorless solution in water. Its name i ...
, essential oils such as
thymol Thymol (also known as 2-isopropyl-5-methylphenol, IPMP), , is a toxic monoterpenoid phenol derivative of ''p''-Cymene, isomeric with carvacrol. It occurs naturally in the oil of thyme, and it is extracted from ''Thymus vulgaris'' (common thy ...
and beta acids from
hops Hops are the flowers (also called seed cones or strobiles) of the hop plant ''Humulus lupulus'', a member of the Cannabaceae family of flowering plants. They are used primarily as a bittering, flavouring, and stability agent in beer, to whic ...
resin (e.g. lupulone). Many of these products whether synthetic or naturally produced can negatively affect honey bee brood or queens. These products often are applied through impregnated plastic strips or as powders spread between brood frames. Synthetic compounds often have high efficacy against Varroa mites, but resistance has occurred for all of these products in different areas of the world.
Pyrethroids A pyrethroid is an organic compound similar to the natural pyrethrins, which are produced by the flowers of pyrethrums (''Chrysanthemum cinerariaefolium'' and '' C. coccineum''). Pyrethroids are used as commercial and household insecticides. In ...
are used because a concentration that will kill mites has relatively low toxicity to honey bees. Compounds derived from plants have also been assessed for mite management. Thymol is one essential oil with efficacy against mites, but can be harmful to bees at high temperatures. Other essential oils such as garlic, oregano, and neem oil have had some efficacy in field trials, though most essential oils that have been tested have little to no effect. Essential oil use is widespread in hives with many of those uses being off-label or in violation of pesticide regulations in various countries. Hop beta acids are lupulones obtained from hop plants and have been used in products marketed for mite control. Resistance to pyrethroids has occurred in the Czech Republic and the UK due to a single amino acid substitution on Varroa mite's genome. Underlying mechanisms for resistance in other pesticides, such as coumaphos, are still unknown.


Mechanical control

Varroa mites can also be controlled through nonchemical means. Most of these controls are intended to reduce the mite population to a manageable level, not to eliminate the mites completely. Screened bottom boards are used both for monitoring and can modestly reduce mite populations by 1114%. Mites which fall from the comb or bees can land outside the hive instead landing on a solid bottom board that would allow them to easily return to the nest. Varroa infest drone cells at a higher rate than worker brood cells, so drone cells can be used as a trap for mite removal. Beekeepers can also introduce a frame with drone foundation cells that encourage bees to construct more drone cells. When the drone cells are capped, the frame can be removed to freeze out mites. This labor-intensive process can reduce mite levels by about 5093%, but if trap cells are not removed early enough before mites emerge, mite populations can spike. This method is only viable in spring and early summer when drones are produced. Heat is also sometimes used as a control method. The mites cannot survive temperatures near , but brief exposure to these temperatures do not harm honey bees. Devices are marketed intended to heat brood to these temperatures, though the efficacy of many of these products has not been reviewed. Powdered sugar used for estimating mite counts in hives has also been considered for mite management as it or other inert dusts were believed to initiate grooming responses. Long-term studies do not show any efficacy for reducing mite populations.


Genetic methods


Honey bee genetics

The Asian honey bee, is more hygienic with respect to Varroa mite than western honey bees, which is in part why mite infestations are more pronounced in western honey bee colonies. Efforts also have been made to breed hygienic honey bees
heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, the offspring cell (biology), cells or orga ...
behavior traits, such as those with resistance to Varroa mites. Honey bee lines with resistance include Minnesota Hygienic Bees, Russian Honey Bees, and Varroa sensitive hygiene. Hygienic behaviors include workers removing pupae heavily infested with mites, which kills both the developing bee and immature mites, and grooming or removal from the brood cell, which increases adult mite mortality. Mites removed from host pupae are at an incorrect life stage to re-infest another pupa. An extended
phoretic Phoresis or phoresy is a temporary Commensalism, commensalistic relationship when an organism (a phoront or phoretic) attaches itself to a host organism solely for travel. It has been seen in tick, ticks and mite, mites since the 18th century, ...
period in adult female mites has also been noticed. Hygienic behavior is effective against diseases such as
American foulbrood American foulbrood (AFB, ''Histolysis infectiosa perniciosa larvae apium'', ''Pestis americana larvae apium''), caused by the Endospore, spore-forming bacterium ''Paenibacillus larvae'', is a highly infectious honey bee brood disease. It is the ...
or chalkbrood, but the efficacy of this behavior against mites is not well-quantified; colonies with this behavior alone do not necessarily result in Varroa mite resistant colonies that can survive without
miticide Acaricides are pesticides that kill members of the arachnid subclass ''Acari'', which includes ticks and mites. Acaricides are used both in medicine and agriculture, although the desired selective toxicity differs between the two fields. Termino ...
treatments. The efficacy of this behavior can vary between bee lines in comparison studies with Minnesota hygienic bees removing 66% of infested pupae, while Varroa sensitive hygiene bees removed 85% of infested pupae. There are minimal trade-off costs to hives that have this hygienic behavior, so it is being actively pursued in bee breeding programs.


Mite genetics

Researchers have been able to use
RNA interference RNA interference (RNAi) is a biological process in which RNA molecules are involved in sequence-specific suppression of gene expression by double-stranded RNA, through translational or transcriptional repression. Historically, RNAi was known by ...
by feeding honey bees mixtures of
double-stranded RNA Double-stranded RNA (dsRNA) is RNA with two complementary strands found in cells. It is similar to DNA but with the replacement of thymine by uracil and the adding of one oxygen atom. Despite the structural similarities, much less is known about ...
that target expression of several Varroa mite genes, such as
cytoskeleton The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is compos ...
arrangement, transfer of energy, and transcription. This can reduce infestation to 50% without harm to honey bees and is being pursued as an additional control method for Varroa mite.


See also

*''
Acarapis woodi ''Acarapis woodi'' is an internal parasite affecting honey bees, the symptoms of infestation were originally observed on the Isle of Wight in 1904, but was not described until 1921. ''Acarapis woodi'' mites live and reproduce in the tracheae o ...
'' - Tracheal mite *'' Nosema ceranae'' *'' Nosema apis'' * Mite biting bees * Thymovar *https://www.varroaresistant.uk/


References

{{Authority control Mesostigmata Arachnids of Asia Beekeeping Western honey bee pests Parasites of bees Parasitic acari Agricultural pest mites Animals described in 2000