Varroa destructor

Taxonomy

Taxonomic lineage

cellular organisms - Eukaryota - Fungi/Metazoa group - Metazoa - Eumetazoa - Bilateria - Coelomata - Protostomia - Panarthropoda - Arthropoda - Chelicerata - Arachnida - Acari - Parasitiformes - Mesostigmata - Monogynaspida - Dermanyssina - Dermanyssoidea - Varroidae - Varroa - Varroa destructor

Genus Varroa

The genus Varroa (the only genus in family Varroidae) is represented by four species:

V. jacobsoni Oudemans was first described as a natural ectoparasitic mite of the Eastern honey bee A. cerana. It's widely distributed on this bee throughout Asia and in Indonesia on A. nigrocincta.

V. underwoodi was first described from A. cerana in Nepal.

V. rindereri was described from A.koschevnikivi from Borneo.

V. destructor was described both from A. cerana and A. mellifera and was assumed to be V. jacobsoni until 2000. Therefore, all Varroa articles published before this year refer to V. jacobsoni.

Brief facts

Importance of honey bee

Thirty five percent of all crops that humans use for food depend on bees for pollination. Ninety percent of these crops are pollinated by managed honey bees. Therefore, although humankind will not die if all honey bees perish, the human diet will be greatly impoverished and resulting economic impact is difficult to underestimate. Moreover, ecological consequences of honey bees' disappearance are going to be enormous for pollination is a prerequisite of fertilization and seed development of many wild species of plants, which are at the base of food chains in the vast majority of terrestrial habitats.

Apis mellifera, honey bee taxonomy, facts, life cycle, anatomy at GeoChemBio

Importance of V. destructor

The Varroa mite is the most serious pest of honey bee known to date. The ectoparasitic mite V. destructor harms both brood and adult bees causing a disease called varroosis or parasitic mite syndrome and including a form of brood damage termed snotty brood. From the beekeeping point of view there is certain threshold for economic damage and for irreversible colony damage. At low infestation rates clinical symptoms are not visible, and infestation often remains undetected. Moderate infestation rates reduce the growth of the honey bee population and, therefore, the honey yield. Gradual increase in parasite population during the fall leads to greater losses of adult honey bees who impaired by the mite may die prematurely or never return to the hive because of learning disabilities. The final breakdown of a honey bee colony is associated with the typical parasitic mite syndrome such as scattered brood, crawling and crippled bees, supersedure of queens, etc. Several reports confirm that under temperate conditions untreated colonies may collapse due to varroosis 3-4 years after the initial infestation. It is clear that mite population growth is lower in subtropical and tropical climates.

New host-parasite system is unbalanced

The varroa mite (V. jacobsoni) was first described by Oudemans (1904) from Java on A. cerana. V. destructor is a new species derived from V. jacobsoni shifting from its natural host, A. cerana to the Western honey bee, A. mellifera. Details of the speciation are not clear. Most likely the shift occurred in the first part of 21st century after both bee species came to a sympatric co-existence in Eastern Russia or the Far East. Because V. destructor is a new parasite, a balanced host-parasite relationship is lacking in A. mellifera colonies and mite is winning. However, natural selection of honey bees results in survival of bee strains that apply greater pressures on the parasite and manage to control its reproduction sufficiently well to avoid the colony collapse. The occurrence of natural tolerance to varroosis is well known in Africanized Honey Bees (AHBs) in South American tropical countries such as Brazil. Recently, long-term survival of unmanaged European Honey Bees (EHBs) was reported from France and the United States.

Distribution

In 1951, the varroa was found in Singapore, in 1962-63 in Hong Kong and the Philippines and spread rapidly from there. In 1979, the first mite was found in Maryland (United States). Today, V. varroa has spread almost worldwide within a short time period and it may now be difficult to find a Varroa-free colony anywhere, other than in Australia. There are two main "haplotypes" or "quasi clonal" populations of V. destructor: Korean and Japanese. The former is distributed worldwide and is very virulent, while the latter has only been reported in Japan, Thailand and North- and South America and is considered to be less harmful. A hypothesis has been suggested that the two distinct forms of the mites developed as a result of V. jacobsoni shifting to the new host, A. mellifera, on two separate occasions.

Association with other pathogens

V. destructor is a vector for various honey bee viruses. So far, about 18 different viruses have been isolated from honey bees and for Kashmir Bee Virus (KBV), Sacbrood Virus, Acute Bee Paralysis Virus (ABPV), Israel Acute Paralysis Virus (IAPV) and Deformed Wing Virus (DWV), it has been proven that they can be vectored by V. destructor. In the absence of the mite, these viruses cause covert infections and have been considered a minor problem to honey bee health. Feeding activities of the mite not only increase titers of these viruses in the bee organism by spreading the viral particles, they also activate viral replication, increase virulence of the viruses and, as a result, lead to overt diseases with which weakened immune system of the bees is unable to cope.

External morphology

Varroa mites exhibit distinct sexual dimorphism and many morphological adaptations to their host. The body of both sexes is divided into two distinct parts - idiosoma and gnathosoma. The female mites have flattened, ellipsoidal idiosoma with greater width than length (measuring 1-1.77 mm long and 1.5-1.99 wide). The male body is pear-shaped (0.7-0.88 mm wide) and shows only weak sclerotisation, which is mainly present in the legs and the dorsal shield. Males are smaller than females in all developmental stages. Their legs are longer in relation to the body size than the legs of females. Adult females are brown to dark brown and adult males are light yellow with lightly tanned legs. The mite gnathosoma consists of mouthparts with two sensory pedipalps and two chelicerae. Females use teeth on third movable digits to open the wound on the host. In males, the movable digit is transformed into a spermatodactyl, a canula-like structure that allows the transfer of sperm into the female genital tract.

Life cycle (developmental stages)

Varroa mite is obligate parasite throughout its life cycle, it does not have a free-living stage and is always found in close association with its host.

There are 3 distinct phases in V. destructor female life cycle: ontogenesis, phoretic phase and reproductive phase. The female can have 2 or 3 successful reproductive phases under natural conditions and up to 7 in the laboratory. The male mite has only one reproductive episode when he copulates with and fertilizes his sisters in the brood cell. After this, he dies.

Phoretic phase

Female mite hidden under bee' sternites sucks host's hemolymph while being transported to brood cells in the beehive. This phase on average lasts about 5.9 days. Nurse bees are preferred fare of the mites. This phase is more important for dissemination of the mites than for reproduction. During their time on the adult bees, mites not only are vulnerable to various environmental factors but also can be dislodged or killed by bees during their grooming and allo-grooming. Original host, A. cerana, exhibits especially vigorous grooming activities.

Host finding

When in the vicinity of brood cells in the beehive, female mite uses various chemical as well as non-chemical (size and shape of the cell) cues to find a cell suitable for egg laying, which is a cell with 5th instar larva of the honey bee about to be capped. In the original host, A. cerana, Varroa mites infest only drone cells. In the new host, A. mellifera, the mites are able to parasitize not only drone but also worker pupae. However, the drone cells are preferred.

The mites are so sensitive to smell of real bee pupae that not a single experiment succeeded in luring the Varroa females from the adult bees to a dummy containing a certain blend of attractants. Therefore, development of Varroa traps is still in the future.

Reproductive phase &

ontogenesis

The reproductive phase of the mother mite and ontogenesis (development of a living organism from egg to sexually mature stage) of her offspring takes place exclusively in the sealed brood cell of the bee.

Cell invasion

15-20 h hours prior cell capping in case of worker cells and 40-50 h in case of drone cell. Upon detecting a potential host, the female disembarks from its carrier and enters the cell, passes between the larva and the cell wall to the bottom of the cell and becomes stuck upside down within larval food. Respiration takes place by the peritreme (part of the integument of an insect which surrounds the spiracles) in snorkel-like manner. Multiple mites can invade one cell.

Oogenesis &

vitellogenesis

First 70 hours after cell capping. Approx. 5 h after cell capping the larvae has consumed the rest of the larval food and the mite starts to suck hemolymph from the larva. Within a few hours oogenesis starts, followed by vitellogenesis.

Egg laying &

offspring

development

The female mite lays up to 5 eggs in worker brood and up to 6 eggs in drone brood. Eggs are laid one by one every 30 hours. During this period, the mite creates a wound in the pupa's cuticle, normally on the 2nd or 5th (reports vary) abdominal segment. She cares to keep it open and accessible for her son and daughters, which is critical for survival of all developmental stages. The mite and her offspring feed on hemolymph 2-3 times a day for 3-9 minutes. Between feeding they rest on fecal accumulation site, which is located on the cell wall in proximity of the feeding site.

First egg,

haploid male

In about 70 hours (~3 days) after cell capping. The female lays her first egg that is normally unfertilized and develops into haploid male.

Second egg,

diploid female

In about 100 hours (~4 days) after cell capping. The female lays her second, this time, fertilized egg that develops into female.

Mixed

developmental

stages

In a few hours after oviposition, eggs hatch into protonymphs that develop into deutonymphs, which in their turn, molt into adults. Each nymphal stage is divided into mobile feeding and immobile pharate phase. The latter is called proto- or deutochrysalis, respectively. All mobile stages are eight-legged and actively feed and grow. Each chrysalis stage is terminated with the rejection of a skin (exuvium). The exuvium of female deutochrysalis can be used to calculate the number of new adult daughters.

During this time, the mother mite continues to lay other eggs.

In about 6.6 days after first egg oviposition, male deutochrysalis molts into male. First female molts to instant adulthood in about 10-20 hours after the male (~5.8 days after oviposition).

All stages of development of the parasite can be observed at the same time within a normal Varroa family not until the bee pupa has dark eyes.

Mating &

insemination

Male waits for the female to emerge at the fecal accumulation site. Male initiates mating as soon as the first female arrives. Multiple mating is common until the next freshly molted daughter female arrives at the fecal accumulation site and distracts the male from the older female by her pheromones. Up to 35 sperm spermatozoa are deposited into female's gonopore by means of chelicerae. Within 2 days after insemination, the roundish prospematozoa migrate into the female spermatheca.

Exit

In 12 days after capping, young bee leaves the cell. Male mite dies, whereas mother mite together her inseminated daughters leave the cell to start phoretic phase of their life cycle. The average reproductive rate is highly variable and depends on mite fertility and fecundity. Under natural condition, average reproduction rate is 1.3-1.45 in single infested worker brood and, due to the longer capping period, 2.2-2.6 in drone brood. Juvenile mortality is among most important factors that moderate reproductive success of Varroa. In A. cerana, mites sometimes become "entombed" in the drone cell when weakened by parasitism young drone fails to open the cell cap and dies. Up to 25% of the reproducing mite population could be killed by entombing.

Methods of control

Biological methods

Biological methods rely on peculiarities of mite biology by controlling its population at points of its life cycle where the mite is most vulnerable. One of the best strategies is to breed bees that would be naturally resistant to the mite parasitism. Even small changes in duration of bee's post-capping stage or in bee's hygienic habits can prove crucial for mite eradication.

Several eco-friendly approaches were proposed. Unfortunately most of them are very labor and time consuming and are not suitable for commercial honey producers.

A biotechnological method, the "trapping comb technique" (Maul et al., 1983, 1988) has been suggested. The queen is restricted to a single brood comb which attracts majority of the reproductive mites in the colony. After 3 brood cycles more than 90% of the mite population can be trapped and subsequently removed from the colony by this technique.

Partial control in lightly infested apiaries can be obtained with smoke from a burning plant material that cause mite knockdown (for example, tobacco). Smoke dislodges mites and can be used periodically to remove those that subsequently emerge from brood cells. A sticky board is used in conjunction with smoke to trap dislodged mites.

Chemical control

Over the last 15 years, the most noted synthetic or so-called hard acaricides against V. destructor are the organophosphate coumaphos (Checkmite®, Asuntol®, Perizin®), the pyrethroids tau-fluvalinate (Apisan®, Klartan®, Mavrik®) and Flumethrin (Bayvarol®), as well as the formamidine amitraz. Disadvantages of using hard acaricides include: possibility of harming bees, persistence and accumulation in the environment and in bee products, and development of resistant mites.

Organic acids and essential oils, such as formic acid, oxalic acid, lactic acid and thymol, represent soft acaricides for the control of Varroa. Advantages are efficacy, low risk of accumulation in bee products, low risk of eliciting mite resistance. Among disadvantages are difficulties in treatment implementation and harmfulness to humans (formic acid).

Treatment concepts

periodic treatment;

no chemical treatment during the nectar flow;

"soft" acaricides should be preferred;

in temperate climates, treatment need to be performed before development of overwintering bees to ensure healthy spring bees and good start for the colony;

perform periodic diagnosis and mite population survey methods to keep an eye on mite population dynamics and catch re-infestations in time;

avoid repetition of one treatment method, use different control methods to prevent development of resistant mites.