Varroa destructor, parasite of the honeybee Apis mellifera

Introduction

The varroa mite, Varroa destructor (Anderson and Trueman) is a voracious parasite of the European honeybee, Apis mellifera. It feeds on the bee haemolymph causing grave physical damage and is a catalyst for viral infections within the colony. Untreated, colonies are destroyed by this pest in just a few years and many, many thousands of hives have already been lost throughout the world to attack from this mite. Varroa is without doubt the most serious threat to honeybee health worldwide.

Origins

The original host of the varroa mite is known to have been the Asian honeybee, Apis cerana but infestations do not cause death or collapse of the colony. A.cerana can tolerate varroa mite infestation as the reproductive rate of the mite is not too high (the mites only reproduce in drone brood and do not infest worker cells) and the adult bees also remove mites by grooming and cleaning behaviours.

In the European honeybee, Apis mellifera, however, the varroa mite can infest both drone and worker brood and A. mellifera exhibits little grooming and hygienic behaviour to rid themselves and the hive of mites. Since the parasite jumped hosts to A. mellifera probably sometime in the early 1900’s it has spread rapidly to almost all areas of the world where beekeeping is practiced today.

For years it was thought that the mite, then referred to as Varroa jacobsoni (Oudemans), originated in Java, parasitising Apis cerana and had then later spread to Apis mellifera. However, recent research by Anderson and Trueman in Australia shows that there exist a number of related varroa mite races, originating in Korea and Japan; these are the destructive mite species that have spread around the globe. Varroa jacobsoni itself does parasitise A. cerana but is apparently limited to Java and surrounding islands.

Disease

How the varroa mite infiltrates the hive

Varroa is transported within and between bee colonies on adult bees. Shortly before the brood cells are to be capped the mites detach themselves from the adult bees and go into the cells where they secrete themselves in the brood food provided by nurse bees. Once the cells are capped the young larvae ingests the brood food, liberating the mite or mites. These parasites then pierce the cuticle of the bee larvae and feed off the haemolymph. Only after the first blood meal can the female mite lay her eggs, which quickly hatch and infect the cell with mites (see Varroa lifecycle below).

A single brood cell can contain as many as ten mites of different generations. These sucking parasites weaken the bee brood, impairing normal development. When varroa infestation is severe, worker bees and drones emerge with shortened abdomens, misshapen wings or other deformities. Young bees such as these have only a brief life expectancy and are generally immediately rejected by the colony.

Diagnosis

The varroa mite is sometimes difficult to detect on the adult bee because of its colour and small size. The mite is red-brown, oval in shape and around 1.5 mm by 1 —1.2 mm.

Varroa mites conceal themselves usually between the sclerites or other inaccessible areas on the adult bee body. The bees have great difficulty in removing them by grooming. Varroa infestation is therefore easily overlooked by the beekeeper. Infested hives may seem strong and high honey yield should not be taken as an indication of freedom from this pest. Often heavily infested bee colonies will bring in a good honey crop and yet within weeks the colony is dead.

The stress and physical damage caused by varroa infestation can be devastating but the association of viral and bacterial infection alongside varroa infestation is the real cause of colony demise in many cases. The stress of the varroa accentuates these other diseases.

Poor foraging weather, lack of food, water and space can all be contributory stress factors in honeybee disease. While there are currently no treatments known for viral disorders in honeybees it is possible to limit the effects of these diseases by controlling the contributory stress factors.

Untreated for varroa infestation, a honeybee colony is fated to collapse in the third or fourth year after initial varroa infestation.

In the first instance it is important to examine hive floor debris for mites.
In heavy infestations, mites can be seen on adult bees, on wax combs and in cells.
As varroa is more attracted to drone brood than workers uncapping and examining samples of drone brood may be used as a diagnostic tool for varroa infectation.
A sudden crash in adult bee numbers may be an indication of varroa.
Bees with twisted or shrivelled wings, small abdomens or other deformities may be the result of varroa plus viral infections.
Poor general colony health, irregular brood pattern may be attributed to varroa plus attack by other disease organisms (viruses, bacteria, fungi) sometimes referred to as Parasitic Mite Syndrome or PMS.
Diagnostic treatment of the honeybee colony may be performed with approved acaricides and methods.

Transfer Of Varroa Mites

The close social contact between bees within the hive facilitates the transfer of varroa mites from one host to another.

Transfer of the mite between colonies of bees can occur in several ways:

Attachment to the bee in flight
Varroa mites attach themselves to the abdomen or thorax of the adult bees by gripping. Spines on their legs also entwine with hairs on the body surface of the bee. Varroa mites can achieve wide geographic distribution by securing themselves underneath or between the sclerites of the bee and being carried in flight.

Carried by a robber bee
A robber bee that has been infested with varroa mites can transfer them to previously uninfested hives during the process of pillaging. Also a robber bee may become the unsuspecting host when stealing stores from an infected hive.

Drifting Bees
Varroa could also be transmitted during swarming or by drifting bees. Drones especially can carry mites from one hive to another, sometimes over large distances.

The spread of the varroa mite can also be accelerated by the following ways:

Transport of hives by migratory beekeeping
Bees being moved between colonies
Where social structure has already been weakened by varroa. These hives are more vulnerable to robber bees, which pick up and then disperse the mites to their own and other colonies.

The Varroa Lifecycle

Entering an uncapped cell
A fecund adult female varroa leaves an adult bee and enters the uncapped cell of a five to five ½ day old larva of a worker or a drone, between 30 and 60 hours prior to capping. In heavy infestations, more than one female mite enters the cell. Drone cells in particular are often infected by more than one female.

A fecund adult female varroa leaves an adult bee and enters the uncapped cell of a five to five ½ day old larva of a worker or a drone, between 30 and 60 hours prior to capping.

Inside brood food
Inside the cell, the mite submerges itself in the brood food, supplied to the developing larva by nurse bees. Oxygen is still available via channels called peritremes that protrude from the ventral surface of the mite through the semi-liquid brood food.
Feeding on the captive host
Shortly after the cell is capped the larva consumes the food, thereby releasing the varroa mite, which then proceeds to feed on its captive host.
Laying Eggs
Varroa eggs are laid on the walls of the cell. The first egg is laid around 60 hours after operculation (when the bee larva has completed cocoon spinning) and subsequent eggs are deposited at approximately 30 hour intervals.

Larva grows six then eight legs
The six-legged larval stage of the mite develops within the egg during the first 24 hours and then develops into an eight-legged protonymph before hatching.

Reaching the adult stage
The protonymph gorges on the haemolymph of the bee pupa for 1½ - 2½ days before moulting to a deutonymph. The parasite continues feeding for another three to four days and then moults to the adult stage. Total development time from egg to adult is approximately 6 ¾ days for males and six days for females.

Adult males are small, globular and pale yellow/gray in appearance. Their mouth parts are specialised for spermatocyte transfer only so males are unable to feed. Mature females are larger and are red-brown in colour. In temperate climates, mites hatched in summer may live two or three months, while those hatching in winter or during broodless periods can survive for five to eight months, during which time they are not reproductive.

Mating
Both the male and the first female mite reach adulthood 10 days after operculation. Mating takes place between the male and the mature female mites in the cell.
Emerging from the cell
After mating, the mature female mites attach themselves to the young adult bee as it emerges from its cell. The male and remaining immature female mites stay in the cell and perish. Between one and three fecund female mites leave the worker bee cell, while three or four may emerge from drone cells.

Control Of Varroa Mites

The destruction of many wild bee colonies has wide and serious implications for the future of seed crops, fruit orchards and any produce that relies on the bee for pollination. While there is no known way to prevent varroa mites from infesting a beehive, the mites can be controlled. Controlling the spread of varroa is an environmental issue of worldwide concern.

There are several key strategies for the effective control of varroa:

Monitoring the infestation level of the colony.
This will indicate whether the mite population is building up to levels that will harm the colony. It will also indicate if the current method of control is not proving effective.
Use a combination of methods.
The most effective control of varroa can be gained by using a combination of both biomechanical methods and chemical methods. These work in different ways and can be practised at different times of the year.
Use approved varroacides such as Apistan® or Apiguard®.
These are proven to work and to be safe for bee and the user. It is also important to follow manufacturers' instructions. Incorrect use may result in residues in the hive products and it may promote the development of mite resistance.
Use essential oil or organic acid treatments with great care.
If legal to do so, in rotation with registered acaricide products in a concerted Integrated Pest Management strategy.
Use biomechanical methods.
Drone trapping and restricting queen movement can be a useful diagnostic and seconday control measure.
Use a co-ordinated approach.
Developing a treatment programme with other beekeepers in the area will help reduce the likelihood of re-infestation.

Vita (Europe) Limited is developing a number of preparations for varroa control [as well as products for control of other honeybee diseases]. Research is continuing into two new acaricides, one of which is a near-natural product.

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