Varroosis

Also known as:

Varroatosis Colony Collapse Syndrome

Description

Varroosis is the clinical syndrome caused by the infestation of colonies of honey bees (Apis mellifera) by the mite Varroa destructor, formerly Varroa jacobsoni. Varroosis is a complex syndrome that may result in so-called Colony Collapse Syndrome or complete absconcion from the hive. V. destructor was traditionally a parasite of the Asian honey bee (Apis cerana) which is much more resistant to infestations but it has spread to all parts of the World except for Australia.

Pathophysiology

The fertilised adult female mite enters a cell just before it is sealed and initially feeds on the brood food. When the egg is laid and the cell sealed, the mite bites through the exoskeleton of the larva (usually towards its caudal end) and begins to suck its haemolymph. She lays eggs and she and her offspring continue to feed on the developing bee. In heavy infestations, several female mites may enter a single cell. The next generation of mite offspring are produced in the cell and these individuals continue to inter-breed. A female mite first produces a male offspring then 5-6 female offspring, giving the mite a high biotic potential. When the developed bee breaks out of the cell, it carries adult female mites into the hive but immature stages and male mites are left to die. The female must therefore be fertilised before she leaves the males behind, one reason why the infestation is sometimes romantically called Amazon disease. The female mites show a predilection for the cells of drones, probably because these brood take longer to develop and therefore allow a further generation of mites to develop in the cell. However, unlike Varroa infestations of the Asian honey bee, it is able to develop in any type of cell. Developing larvae and pupae that suffer mite infestations are often abnormal, often being smaller, weaker and having incomplete wings when they emerge.

The female mites preferentially attach to worker bees of the nursery group, probably due to the differential expression of alkenes on the surfaces of these and the older pollen gatherers. The mites suck haemolymph from the adult bees and also transfer numerous pathogenic viruses between individual bees. The feeding process may also make these worker bees more susceptible to the diseases caused by these viruses after they are introduced. The female mites have a variable lifespan depending on the time of year. In winter when the bees are hibernating, they may survive of up to five months but in summer, they generally live for 3-4 weeks.

Epidemiology

Mites are transferred between colonies by worker bees in areas of high hive density. Since its first reported infestation in the UK in 1992, V. destructor has now spread across the whole country except for the Isles of Scilly, the Isle of Man and some Scottish islands. Mites can also be spread iatrogenically as they survive on fomites for up to 72 hours.