The varroa mite Varroa destructor is a highly damaging ectoparasite of Apis mellifera. Originating in Asia - where it is a parasite of the eastern bee, Apis cerana - varroa made a host jump to Apis mellifera sometime in the last century and has since spread around the world. It was first discovered in the UK during 1992. It feeds on the blood of honey bee pupae and adults. In doing so, it activates and transmits viral diseases that reduce the life expectancy of the bees and causes colonies to decline.

Control measures in the UK have relied on pyrethroid pesticides, but resistance is now widespread. New, sustainable forms of management are required urgently. Varroa is the most important threat to bee keeping in this country.

Biological control technologies have potential to reduce reliance on synthetic pesticides as part of Integrated Pest Management (IPM). Biological control exploits the natural enemies of pests. However, because no natural enemies have been found causing population declines of varroa in honey bee colonies, little work has been done before on biological control.

Defra funded research at Warwick HRI and Rothamsted Research has investigated the natural enemies of mites to see if any have potential against varroa. We found that varroa is very susceptible to entomopathogenic fungi. These fungi are widespread in the environment where they help to naturally regulate populations of insects and mites. They do not harm fish, mammals, amphibians or other wildlife, are non-toxic to humans and produce no residues that may contaminate the environment. These features make them attractive as biological control agents. In agriculture, they are being exploited increasingly as microbial control agents of insect pests of glasshouse and field crops.

Varroa is highly susceptible to entomopathogenic fungi in lab experiments done at different spatial scales, and strains of fungi have been identified that kill varroa but have no effect on bees. Application techniques that allow varroa mites to be treated quickly and effectively in bee hives are possible. Work is now required to evaluate efficacy at the field scale, to develop formulation and application systems for fungal spores, and measure the fate and behaviour of fungal inoculum. Biological control could be an important part of IPM for bee health and needs to be taken further.

Varroa mite

Varroa mites on honey bee pupa