Introduction Varroosis is an ectoparasitosis of adult bees and

Introduction

            Varroosis
is an ectoparasitosis of adult bees and bee larvae. The pathogen responsible is
Varroa destructor, this parasite has
a sexual dimorphism. The female of the parasite is larger than the male and
their morphology is perfectly adapted to their host (Rosenkranz et. al 2010). Their life cycle is missing a free-living stage and can be divided into two main phases. First one is the phoretic
stage, located on the adult bees, this stage is used by the female of the
parasite in order to infestate other
larva cells or other colonies of bees, at
this stage the adult bee is used as a carrier and as food source (Rosenkranz et. al 2010, Piou et. al. 2016). 
After a certain amount of time (that may vary between one and ten days)
the female mite switches to the reproduction phase leaving the adult bee and
entering a cell with brood larva (Piou et. al. 2016). According to Rosenkranz et. al 2010 and Kuenen and Calderone, 2000 the mite
prefers the brood larva in detriment of worker larva because the brood larva
has a longer period of incubation and that gives the mite more time to reproduce (Rosenkranz et. al 2010; Kuenen and
Calderone, 2000; Martin and Cook, 1996).

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            The reproductive
cycle of the parasite is perfectly adapted to
its host, the female mite chose’s and infestate a brood cell, a few hours
before the cell is closed by the nurse bees. A few hours after the cell is
sealed the Varroa female feeds of the brood larva hemolymph. This step is
necessary to activate its ovary system, in less than three days after the cell
is closed appears the first egg (which is always a haploid male mite), after
that she lays up to 5 eggs at regular intervals of approximately 30h each (the
mite prefers drones because they require 24 days to hatch, which means more
time to reproduce). The mother and her offspring continue to feed with the
hemolymph of the bee larva and the male mite will mate with his sisters, except
the case in which two adult female mites infest the same cell (Frey et. al.
2013; Andino et. al., 2016).

            The
original host of the parasite is Apis
cerana and until the mite infestation started to produce economical damage
to the beekeepers at the end of the 20th century, the parasite had little
attention. At first, the mite responsible for the damage of the colonies was
considered V. jacopsoni, and only
after further studies was revealed the existence of more families of Varroa and
has been concluded that the mite responsible for the damage is V. destructor (Anderson and Trueman,
2000).

Types of genetic
resistance

            When V. destructor started to produce
economic damage (both in the production of the colony and the colony numbers)
the beekeepers called the chemical option. They started using various chemical
substances in order to fight this parasite. But this has brought new problems.
In time the mites started to gain resistance to the treatments leaving the
beekeepers with a limited number of options. The chemical substances can also
harm the bees not just the parasite and their residue can be found in the bee
products. Also buying each year substances to treat the bees is pretty
expensive. So the researchers have reached the conclusion the obtaining bees
resistant to Varroa destructor is the
only long term viable solution (Tsuruda et. al., 2012).

            Until
now the researchers have identified and studied several resistance mechanisms
that allow bees to counteract the effects of infestation.

            1.The
entombing of drone brood- Until now this behavior has been observed only on the
original host of the parasite A. cerana,
this behavior seams to manifest more when a brood cell is infested with 2 or 3
mites. In this case the drone brood seems to remain without sufficient energy
to be able to hatch and die in the cell, as a result the parasites also remain
blocked with the dead brood. Also A. cerana
workers seem to delay the dead brood removal (Rath, 1999; Boecking and Spivak,
1999).

            2.
Effective grooming and hygienic behavior

            3. No
reproduction in worker brood at all