Varroa mites

Varroa mites (eg Varroa destructor and Varroa jacobsoni) are external parasites of honey bees. They can be seen with the naked eye. There are different species and a number of haplotypes, and they occur naturally in Asian honey bees. The Varroa species causing problems for Apis mellifera beekeeping in many countries is Varroa destructor (which was, until 2000, misidentified as Varroa jacobsoni). Some haplotypes of this species cause great damage to colonies of Apis mellifera by parasitising both adult and larval stages of the lifecycle and spreading harmful pathogens through and between colonies. This complex interrelationship between the Varroa mites and the damaging effects they have on a colony is sometimes termed varroosis. As far as we know, Apis mellifera has evolved no defences against this parasite which can cause devastating colony losses.

The whole life cycle of the Varroa mite is carried out within the honey bee colony, much of it protected by the brood cell cappings where they are hidden from the bees. The adult female Varroa mite moves into a cell immediately before sealing. There she lays a series of eggs. The first egg is always a male with the rest being female. The mites develop through nymphal stages before maturing to become adult. The mites feed on the honey bee larvae, sucking the haemolymph ('blood') by puncturing the body wall with their sharp mouthparts. This damage shortens the emerging bees' life and encourages the spread of pathogens through the colony. Brother and sister mites mate in the cells before the females emerge. The males die after mating and many of the females also die because they do not reach maturity before the honey bee adult emerges from the cell. Varroa mites select drone brood 10 times more often than worker brood. This is because the longer development period of the drone allows more young mites to reach maturity.

Colonies infested with Varroa mites can die unless mite numbers are controlled. Under the weight of a severe mite infestation the normal processes of foraging, brood rearing and colony defence diminish and the colony's entire social organisation begins to deteriorate - a process known as colony collapse. The signs of Varroa are not always obvious until colonies collapse and this can be very sudden. Although the host colony dies, the mites are able to move to new colonies during this time by climbing onto bees coming to rob honey from the dying colony thus spreading the mite infestation to many more colonies in the area. Evidence of how destructive Varroa can be, can be shown by the losses of hundreds of thousand of colonies worldwide since the parasite moved across the species barrier. The mite cannot be eradicated but can be controlled.

Small numbers of mites (less than 1000) cause no significant harm. Higher numbers than this increase the risk of mite damage. There is no clear harm threshold and there can be big differences in the effects of the mites between colonies. This is due, at least in part, to the presence of viruses and other pathogens as well as environmental factors. The mites are the vectors for a range of viruses that are naturally present in honey bee populations and often relatively benign and symptomless until they are combined with a Varroa mite infestation. Varroa also magnifies the effect of other pathogens. It is always helpful to keep vigorous colonies and, where possible, to select strains of bees that show Varroa tolerance.

Good husbandry and recognising signs of infestation are helpful starting points for successful Varroa control. Knowledge of the level of infestation allows beekeepers to plan their control methods. There are several methods of estimating mite numbers in a colony. However, regular collection and examination of floor debris or uncapping drone brood are probably the easiest ways of detecting Varroa infestation in managed colonies. Control methods can be divided into two groups - biotechnical (management) or varroacides (chemical) methods. In practise the best results are obtained by combining a range of methods that takes the level of infestation into account. This is termed Integrated Pest Management (or IPM).

In Africa, Varroa has been found in colonies in every country. However, there is some evidence that African and Africanised honey bees have some inherent defence against the Varroa mite although the basis of this is not yet clear. Five ideas have been proposed to explain this tolerance: (i) increased grooming behaviour - i.e. better removal of mites from adult bees (ii) removal of infested brood (iii) the shorter post-capping period (iv) limited brood attractiveness to the mite and (v) mite infertility.

Beekeepers and researchers need to discover a great deal more about this mite which has changed the face of beekeeping in many countries.

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