The mite Varroa destructor is an economically devastating ectoparasite of the Western Honeybee (Apis mellifera). It was originally known only from Apis cerana (which is found in southern and eastern Asia), but expanded its host range to include A. mellifera during the first half of the 20th century, spreading rapidly around the world, and is currently considered the single greatest threat to apiculture. Varroa mites have been considered a problem for beekeeping since around the late 1960s; by the 1970s, they had reached Western Europe and South America and by the 1980s they had reached the United States. On A. cerana, both V. jacobsoni and V. destructor apparently only parasitize drone (i.e., male) brood, whereas, for unknown reasons, the two mtDNA lineages of V. destructor that are capable of reproducing on A. mellifera utilize both drone and worker brood. (Rosenkranz et al. 2010 and references therein) Today, it can be safely assumed that all honey bee colonies within the mite’s range harbor varroa mites. As a consequence of mite infestation, dramatic colony losses have repeatedly occurred in affected countries (vanEngelsdorp and Meixner 2010 and references therein).
From hatching out of the egg until the adult molt, developmental time is about 5.8 and 6.6 days for female and male mites, respectively. The mother mite creates a hole in the cuticle of the pupa for the nymphs to feed through. This behavior is part of ‘‘parental care” and necessary because the soft chelicerae of the nymphal stages cannot perforate the pupal cuticle and the male’s chelicerae are modified for sperm transfer. (Rosenkranz et al. 2010 and references therein).
Varroa destructor lacks a free-living stage, being totally dependent on its honeybee host. There are two distinct phases in the life cycle of females: A phoretic phase on adult bees (during which the mite is transported by its host) and a reproductive phase within the sealed drone and worker brood cells. Males and nymphs are found only within the sealed brood cells in which bees are developing. The mites suck substantial amounts of hemolymph ("blood") from both adult bees and from the developing bees within the sealed brood cells. Shortly after leaving the brood cell on a young bee, the mites preferentially infest nurse bees for transport to the brood cells. Freshly hatched infested bees are less attractive than older ones and the middle-aged nurse bees are the most infested group of adult bees in breeding colonies. Drone brood are infested at a much higher rate than worker brood. Efforts to identify cues used by varroa females that cause them to switch from bees to brood, which might be used to develop an effective varroa trapping system, have so far been largely unsuccessful (Rosenkranz et al. 2010 and references therein).
Varroa destructor resembles V. jacobsoni, with which it was confused until the end of the 20th century. Relative to V. jacobsoni, V. destructor is significantly larger and differs substantially with respect to mtDNA COI sequence, as well as at other genetic loci investigated. Varroa jacobsoni is rarely found on A. mellifera. Only a couple of lineages of V. destructor appear to have shifted hosts from A. cerana to A. mellifera. Varroa destructor now occurs nearly everywhere A. mellifera is found, but as of 2010 it had not yet been detected in Australia.
Rosenkranz et al. (2010) review the morphology and reproductive systems of Varroa mites. These mites show a distinct sexual dimorphism in body form and males are smaller in all developmental stages and have proportionally longer legs than females.
Only two of several known mitochondrial haplotypes of Varroa destructor have been found to be capable of reproducing on Apis mellifera (the others being limited to V. destructor's original host, A. cerana). Solignac et al. (2005) analyzed microsatellite markers and mtDNA of V. destructor from 45 populations in 17 countries. They found that the two V. destructor halotypes on A. mellifera also have characteristic and diagnistic alleles at numerous microsatellite loci. They also found genetic evidence suggesting that there has been at least one host transfer from A. mellifera back to A. cerana.
Once inside a 5th instar honeybee larva brood cell and several hours after it has been capped, the female Varroa mite begins to suck hemolymph ("blood") from the larva. Within a few hours, internal egg development is initiated and about 70 hours after the cell is capped, the mite lays her first egg. This first egg is normally unfertilized (females store sperm internally and are able to control whether or not an egg is fertilized). Like honeybees themselves, Varroa mites have a haplo-diploid sex determination system in which unfertilized (and hence haploid, i.e., with a single set of chromosomes) eggs develop into males and fertilized (and hence diploid, i.e., with two sets of chromosomes) eggs develop into females. The first egg is typically unfertilized and develops into a haploid male, while subsequent eggs are fertilized (and therefore female) and laid in 30 hour intervals. Up to five eggs in worker brood and up to six eggs in drone brood are considered typical. (Rosenkranz et al. 2010 and references therein)
Varroa mites become sexually mature immediately after the last molt. Males reach maturity before the females and wait for the first adult female, which molts to adulthood some 20 hours later. Before copulation starts, the male cleans his chelicerae (fang-like mouthparts characteristic of mites, spiders and relatives). He touches the female with his first pair of legs and climbs onto her back. He then slips around to her underside, a repositioning that is often facilitated by the female raising her body. In this "belly-to-belly" position, the male locates the female's gonopores (which are distinct from the genital opening through which the eggs are deposited). He then takes the spermatophore out of his genital opening and transfers it into the gonopore of the female using his chelicerae. Multiple mating is common until the next female is mature and available. (Rosenkranz et al. 2010 and references therein)
Varroa destructor has a variety of negative impacts on honeybees (and, therefore, on human apiculture). The loss of hemolymph during development within the brood cell significantly decreases the weight of the hatching bee, which has a variety of downstream effects such as shortened lifespan of workers. This mite is also a vector for a variety of honeybee viruses, such as Deformed Wing Virus (DWV) and Israeli acute paralysis virus (IAPV) (vanEngelsdorp and Meixner 2010 and references therein). There is strong suspicion that V. destructor plays a significant role in Colony Collapse Disorder (Schäfer et al. 2010), having a synergistic effect in combination with other causes such as other pathogens, environmental factors, and stressful colony management practices. Some feral, unmanaged A. mellifera populations appear to have evolved a degree of resistance to varroa mites, after initial sharp declines, through natural selection and there is some hope that studying these examples could provide valuable insights that could be applied to managed colonies. (Rosenkranz et al. 2010 and references therein) On the other hand, a number of authors have noted that the level of mite infestation that causes significant colony damage appears to have decreased over time in at least some areas (vanEngelsdorp and Meixner 2010 and references therein). Clearly, the host-parasite relationship is complex and may vary through space and time as it evolves.
Cook et al. (2007) estimated that preventing Varroa destructor from establishing in Australia over the next 30 years would avoid costs of between 16 million and 40 million dollars (U.S.) per year.
The genus Varroa includes at least four species of obligate ectoparasitic mites. Varroa jacobsoni was described from Java in 1904 as a parasite of Apis cerana and has a wide distribution on this bee throughout Asia and on A. nigrocincta in Indonesia. Varroa underwoodi was first described from A. cerana in Nepal in 1987. Varroa rindereri was described from Apis koschevnikovi in Borneo in 1996. Varroa destructor was described from both A. cerana (its original host) and A. mellifera (a new host) in 2000; prior to its recognition, V. destructor was mistakenly lumped together with V. jacobsoni and most literature referring to V. jacobsoni prior to 2000 probably refers to the species now known as V. destructor. (Anderson and Trueman 2000; Rosenkranz et al. 2010 and references therein) Oldroyd (1999) discusses aspects of the evolution of the varroa mite-honeybee association and notes that A. mellifera is the only Apis species believed to have escaped natural parasitism.
Varroa destructor, the Varroa mite is an external parasitic mite that attacks and feeds on the honey bees Apis mellifera and Apis cerana. The disease caused by the mites 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 by sucking fat bodies.[2] The species is a vector for at least five debilitating bee viruses,[2] including RNA viruses such as the deformed wing virus (DWV). A significant mite infestation leads to the death of a honey bee colony, usually in the late autumn through early spring. The Varroa mite is the parasite with possibly the most pronounced economic impact on the beekeeping industry. Varroa is considered to be one of multiple stress factors[3] contributing to the higher levels of bee losses around the world.
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V. destructor protonymph
Deutonymph of V. destructor
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V. destructor adult male
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Adult female in lateral view
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.[4] Varroa mites lack eyes.[5] One mite weighs approximately 0.453 mg.
Mites reproduce on a 10-day cycle. The female mite enters a honey bee brood cell. As soon as the cell is capped, the Varroa mite lays eggs on the larva. The young mites, typically several females and one male, hatch in about the same time as the young bee develops and leave the cell with the host. When the young bee emerges from the cell after pupation, the Varroa mites also leave and spread to other bees and larvae. The mite preferentially infest drone cells, allowing the mite to reproduce one more time with the extra three days it takes a drone to emerge compared to a worker bee. This can cause genetic defects such as useless wings or viruses and fungi in the bee.
Adult mites suck on the fat body of both adult bees and bee larvae for sustenance. As the fat body is crucial for many bodily functions such as hormone and energy regulation, immunity, and pesticide detoxification, the bee is left in a severely weakened state. Adult mites live and feed under the abdominal plates of adult bees primarily on the underside of the metasoma 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 mesosoma region, but research suggests that mites in this location are not feeding, but rather attempting to transfer to another bee.[2]
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,[2] including RNA viruses such as the deformed wing virus. With the exception of some resistance in the Russian strains and bees that have Varroa-sensitive hygiene (about 10% of colonies naturally have it), European Apis mellifera bees are almost completely defenseless against these parasites. (Russian honey bees are one-third to one-half less susceptible to mite reproduction).[6]
The model for the population dynamics is exponential growth 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 hive can multiply by (roughly) 12. Mites often invade colonies in the summer, leading to high mite populations in autumn.[7] 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.[8]
Once infected with a V. destructor mite, the honey bee may be damaged two ways. Firstly, the mite's consumption of the fat body weakens both the adult bee and the larva; in particular, it significantly decreases the weight of both the hatching and adult bee. Additionally, infected 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. Secondly, the mites are vectors of various viruses, in particular the deformed wing virus.[9]
After the initial developmental stages, when the young bee matures, it leaves the brood cell and takes the mite with it. V. destructor then leaves the young bee for an older one, preferably for a nurse bee, because nurse bees spend more time near the brood, giving the mite more ample opportunity to reproduce. In fact, because the nurse bee spends more time around the drone brood rather than the worker brood, many more drones are infected with the mites.[9]
Varroa mites have been found on tricial larvae of some wasp species, such as Vespula vulgaris, and flower-feeding insects such as the bumblebee, Bombus pensylvanicus, the scarab beetle, Phanaeus vindex, and the flower-fly, Palpada vinetorum.[10] It parasitizes the young larvae and feeds on the internal organs of the hosts. Although the Varroa mite cannot reproduce on these insects, its presence on them may be a means by which it spreads short distances (phoresy).
Varroa mites originally only occurred in Asia, on the Asian honeybee, Apis cerana, but this species has been introduced to many other countries on several continents, resulting in disastrous infestations of European honeybees.[11]
In early 2010, an isolated subspecies of honey bee was discovered in the oasis of Kufra (southeastern Libya), this newly discovered bee is the only known subspecies that is Varroa free.[21] The Hawaiian islands of Maui and Kauai are free of the mite.[22]
Until recently, V. destructor was thought to be a closely related mite species called Varroa jacobsoni. Both species parasitize the Asian honey bee, A. cerana. However, the species originally described as V. jacobsoni by Anthonie Cornelis Oudemans in 1904 is not the same species that also attacks A. mellifera. The jump to A. mellifera probably first took place in the Philippines in the early 1960s, where imported A. mellifera came into close contact with infected A. cerana. Until 2000, scientists had not identified V. destructor as a separate species. This late identification in 2000 by Anderson and Trueman corrected some previous confusion and mislabeling in the scientific literature.[1]
The infestation and subsequent parasitic disease caused by Varroa mites is called varroosis. Sometimes, the incorrect names varroatosis or varroasis are used. A parasitic disease name must be formed from the taxonomic name of the parasite and the suffix -osis[23] as provided in the Standardised Nomenclature by the World Association for the Advancement of Veterinary Parasitology.[24] For example, the World Organisation for Animal Health (OIE) uses the name varroosis in the OIE Terrestrial Manual.[25]
Treatments have met with limited success. First, the bees were medicated with fluvalinate, a synthetic pyrethroid, which had about 95% mite falls. However, the last 5% became resistant to it, and later, almost immune. Fluvalinate was followed by coumaphos, which mites also have developed resistance to.[26]
Several methods exist for monitoring levels of Varroa mites in a colony.[27] For a powdered sugar roll,[28] the sampler collects about 300 bees using a 1/2-cup measuring cup and places them in a jar with a wire mesh screen lid (1/8") along with 2 tablespoons of powdered sugar. They then gently swirl the bees for about a minute before turning the jar upside down and shaking for two minutes over a tray to capture the mites as they fall. Those mites are then counted, and the count is divided by three to find the number of mites per 100 bees. The sugar roll is typically done with the intent to prevent killing the sampled bees, but whether the vigorous shaking causes damage is not known.
For an alcohol wash, which is the most effective method, the sampler collects about 300 bees using the same cup. The bees are submerged in alcohol with a concentration of 70% or higher. A lid is placed over the jar to seal it, and the mixture is shaken vigorously for two minutes before it is poured over a 1/8" wire mesh screen into a tray. The mites are then counted, and the resulting number is also divided by three. This method kills all sampled bees. The sticky board method does not kill any bees. For this method, a sticky board with a thick coating of petroleum jelly is placed under the brood chamber under a screened bottom board (or similar 1/8" wire mesh screen). The board is retrieved after three days, and the beekeeper takes a count of the mites on the board. This number is divided by three to find the average 24-hour mite drop. This method does not kill any bees, but takes longer for results.
Varroa mites can be treated with commercially available acaricides. Acaricides must be applied carefully to minimize the contamination of honey that might be consumed by humans. Proper use of miticides also slows the development of resistance by the mites.
However, the most effective long-term way of protecting bees against V. destructor is by breeding bees that are resistant to these mites.[9]
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.
Researchers have been able to use RNA interference to knock out genes in the Varroa mite. Efforts also have been made to breed for changes in the honey bees.[37] Two strains have been developed in the United States that can detect damaged pupae under cappings and remove them before the infestation spreads further.[38][39] The “IN”/Indiana strain is under development at Purdue University to develop lines that groom off and bite phoretic Varroa to kill the mites.[40][41]
Varroa destructor, the Varroa mite is an external parasitic mite that attacks and feeds on the honey bees Apis mellifera and Apis cerana. The disease caused by the mites 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 by sucking fat bodies. The species is a vector for at least five debilitating bee viruses, including RNA viruses such as the deformed wing virus (DWV). A significant mite infestation leads to the death of a honey bee colony, usually in the late autumn through early spring. The Varroa mite is the parasite with possibly the most pronounced economic impact on the beekeeping industry. Varroa is considered to be one of multiple stress factors contributing to the higher levels of bee losses around the world.
