Varroa destructor Anderson & Trueman, 2000
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).
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).
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 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.
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)
Evolution and Systematics
Systematics and Taxonomy
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.
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.
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.