About Varroa Mite
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 the multiple stress factors contributing to the higher levels of bee losses around the world.
Reproduction, feeding, infection, and hive mortality.
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 at 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 infests 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 in the mesosoma region, but research suggests that mites in this location are not feeding, but rather attempting to transfer to another bee.
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 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).
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. 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.
Low-temperature scanning electron micrograph of V. destructor on a honey bee host
Once infected with a V. destructor mite, the honey bee may be damaged in 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.
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.
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. 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).