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Varroa - a 'mitey' pest of bees

Globalization has opened up new markets for honey and bee products but it has also resulted in the spread of a parasitic pest which threatens beekeepers' colonies, wild bee populations and consequently the effective pollination of many crops worldwide. International trade in live bees, unrestricted movement of hives and lack of beekeeper awareness are largely responsible for the increasing occurrence of Varroa jacobsoni, the Asian predatory mite. In the last thirty years, the varroa has spread beyond the Asian continent, where it is indigenous, to Europe, North and South America and North and South Africa. And, in April this year, despite strict legislation to prevent the live importation of bees, Bee colonies in Asia have adapted to varroavarroa was also reported in New Zealand. Australia remains the only continent free of varroa and has extensive monitoring and quarantine procedures in place to prevent the entry and spread of exotic Asian bees.

The Varroa mite is an indigenous parasite of Apis cerana (oriental honeybees). Wherever colonies of these bees are kept in the region, there is a possibility of infestation although, through millions of years of being parasitized by the mite, some degree of resistance appears to have developed and most colonies are seemingly unaffected. Colonies heavily infested with Varroa produce little or no honey but beekeepers are more likely to lose entire colonies as the bees naturally abandon hives or nest sites to rid themselves of mite-infested broods.

It is the European honeybee (Apis mellifera) that has proved particularly susceptible to the parasitic mites and associated secondary diseases, and varroa now poses a significant threat to beekeepers worldwide. In addition, feral bee populations in many regions have been virtually eliminated and a wide variety of crops, dependent on pollination from bees, are threatened by these losses.

Chemical control

Effective chemical miticides (e.g. fluvalinate - a synthetic pyrethroid) are available but, with the risk of honey contamination and increasing insecticide resistance, the integration of alternative control methods are strongly advocated. The unnecessary application of chemicals can be avoided, for instance, by only treating hives with a pest density above a particular threshold.

Formic acid, an organic acid, is the only treatment for killing the mites within the brood cells although correct conditions are required for full efficacy. However, a US patent is pending for a gel, which is easier and safer to handle than the liquid and emits formic acid vapours over two to three weeks, reducing the labour associated with its use. Essential oils, such as eucalyptus, nutmeg, peppermint and spearmint have been tested to kill mites by direct contact and also to impair reproduction. Grenadian research has recently involved the testing of locally produced oils including cinnamon, mace and nutmeg and have considered the commercialization of an 'apimiticide' made from coconut oil, beeswax and nutmeg oil all produced in Grenada. A natural mineral block, impregnated with thymol and other oils, has been licensed for use in the EU.

Tobacco smoke is a cheap but only partially effective method of control that can be repeated at regular intervals. However, an American entomologist has tested a variety of plants and has found that smoke from burning dried grapefruit leaves kill the majority of mites whilst leaving the bees unaffected.

Biotechnical control

A variety of biotechnical control methods can be also used, particularly by small-scale beekeepers who want to minimise use of chemical treatments. Most methods work by trapping mites in brood combs which are then removed from the colony, or by causing mites to drop off adult bees by mechanical means. Heavily infested colonies may require additional chemical control and these techniques do depend on a high degree of beekeeping skill and are labour-intensive to apply.

Mites dislodged through grooming or control methods have the potential to re-attach to bees so an insert, known as the Beltsville screen, has recently been developed to allow mites to fall onto the hive bottom. The technique has been shown to reduce mite populations by about 15% and is an effective and complementary control method that can be used in IPM practices.

Biological control

No naturally occurring pests or pathogens specific to V. jacobsoni have so far been identified but mites are known to be susceptible to a range of fungi and bacteria. A project in the UK by IACR Rothamsted is currently underway to screen strains of fungi and the bacterium Bacillus thuringiensis of known activity against mites. Useful strains will then be tested for their effects on beneficial insects, including honeybees and ladybirds. Although identifying suitable biocontrol agents may take some time, the potential benefits are considerable, particularly as a biocontrol agent found to be effective under UK conditions could be equally effective worldwide.

Genetic resistance

Mite resistance in bees would be the most sustainable and cost-effective solution to the mite problem but progress in this area of research has been constrained, as the incidence of varroa in A. mellifera is rare. However, recent progress has been made by scientists in the US with hardy honeybees from the mite-infested Primorski region of Russia's Far East, which appear to offer natural genetic resistance that could be bred into U.S. honeybees. Commercial bee colony suppliers in three states are currently evaluating the bees for valued traits including temperament, honey production, and pollination skills.

Varroa is not a problem that is about to go away and beekeepers worldwide will have to continue to monitor hives on a regular basis. Whatever the methods of control chosen, it is vital that they do not endanger the bees or the beekeeper, nor give rise to residues in the honey and/or wax. Unwanted residues would contaminate not only bee products and but also beekeepers' reputation for producing products that are pure and natural.

For further information:
Varroa WWW Hub

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