The varroa mite, Varroa jacobsoni
, is an external parasite
of bees and is found in Europe
, South America
, and, since the 1980s also in North America
. It is one of the most serious pests affecting bees
and is thought to contribute significantly to the 50% drop in the number of honey bees in recent years in the UK and Europe. The financial cost to agriculture is enormous - the honey bee cross-pollinates
many crops and its usefulness is thought to be worth upwards of £200 million pounds per year in the UK and many billions of dollars in the USA.
The female varroa mite is red/brown in colour and about the size of a pin head (roughly 1.2 x 1.5mm). It feeds preferentially off adult drone bees and developing larvae (brood), and will only infect the queen and worker bees in cases of heavy infestation. It sucks the haemolymph causing loss of vigour and deformity, especially of the wings, rendering the bees unable to fly. Whole colonies can be quickly wiped out and it is important to spot varroa early to try to contain the spread of this dangerous parasite.
The male mite is about half the size of the female (around 0.7mm) and is pale grey or yellow. Adult males cannot feed because their mouth parts (chelicerae) are specially modified to transfer sperm to the female, and they die soon after mating. They never emerge from the brood cells of the hive.
Outward signs of infection include:
- Varroa mite faeces on the insides of the brood cells where bees have emerged
- Dead bee larvae and pupae in stages of decay in brood combs
- Varroa mites, maybe as many as 10, can be seen on the underside of bees and brood from uncapped cells
- Deformed adult bees, characteristically with misshapen wings, crawling around the hive
A fertilized female varroa mite is carried to the hive by a host bee until she finds a brood cell which contains a young bee larva at a stage just before the cell is capped.
- She remains here, feeding off the larva for 2 days before laying 4 - 6 eggs.
- As the eggs develop, they and the original adult feed and develop on the maturing bee.
- There are 4 developmental stages - egg, two eight-legged nymphal stages (protonymph and deutonymph) and the adult.
- It takes 6 - 7 days for the females to develop from egg to maturity and 5 - 6 for the males.
- The adults mate inside the brood cell, after which the male mite dies and the female attaches herself to an adult bee.
- Female mites born in the summer live 2 - 3 months, those born in the autumn (fall), 5 - 8 months. They can live for 5 days without a bee to parasitise.
Chemical control of the mites includes a product called Apistan which contains the pyrethroid insecticide, tau-fluvalinate. The manufacturers say it is harmless to the bees and leaves no residue in the honey, and can control the problem within 6 - 8 weeks by killing the mites as they emerge from the brood cells.
Essential oils, such as wintergreen, tea tree oil and spearmint oil, have been used to control mite populations. The oils work directly on contact with the mite or indirectly on the development of the varroa larvae when the bees are fed with the essential oil diluted in syrup.
For information on how to use essential oils see: http://www.wvu.edu/~agexten/ipm/insects/pollinat/varroa/varroa2.htm
http://www.beefarmers.co.uk/vofbeesreport.htm for a report on The Economic Value of Bees in the UK