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How does it work?

Among pathologies of varroa, only fungi have been found to be lethal. Varroa is a soft-bodied animal that strives in humid habitats, two conditions favourable to fungi development. Among several strains studied, two fungi have revealed to be fatal to varroa, harmless to bees and adapted to the warm and relatively low-humidity environment of a beehive: Hirsutella thompsonii and Metarhizium anisopliae.[1]

Fungi can be disseminated in the beehive either by feeding the bees with syrup containing the spores, spraying or fixing strips in the hive. The last method has revealed to be most efficient.[1]

Bees are a good carrier of fungi and it is known that workers happen to enter foreign beehives: This could contribute to further dissemination of the fungi, thus to a global remediation against varroa.[1]

Efficiency

Varroa mortality reached 90% within 4-5 days in laboratory tests. The treatment is more efficient when the mite is not sealed in the brood cell, but resides on adult bees, thus in early spring and late autumn. The fungi are resilient to winter temperatures. Bee colonies showed to be stronger than control colonies after a fungi treatment.[1]

Trials with commercial agricultural pest control products based on fungi including Metarhizium anisopliae sold in the Netherlands[2] did not reach Kanga's success, but is has to be noted that the method used by Gerritsen & Cornelissen differed in several points: extreme humidity in some of the trial hives, spraying instead of dusting/diffusing strips, different strain of Metarhizium anisopliae. It may be that different strains of the same fungus species are able to develop with different levels of success in varroa or hives, thus have to be selected very carefully for efficient varroa control.

Risks

The bees did not show any health alteration after the treatments. They seem to be immune to the fungi at all development stages.[1][2] Nevertheless, further trials need to confirm the absence of sublethal effects and that the spreading of fungi in the beehive is not detrimental to the quality of bee products for human consumption: If any problem is discovered after dissemination, it will be very difficult to remove the fungi from the environment.

Practicability

Strips offer the same ease of treatment than current chemotherapeutic treatments.

Nevertheless, bees have been observed to remove chemotherapeutic strips or seal diffusers with wax and propolis[3] if they feel disturbed by the alien object or substance. Experience has to show how bees react to fungi diffuser strips.

Further advice

The method needs to be refined to optimise doses and application conditions.

The strips should be only delivered on prescription by vets in order to prevent inadequate or abusive use of the therapy.

Since dead mites are a favourable ground for development of fungi[1], it is not judicious to use bottom screens as a complementary method.

References

  1. 1.0 1.1 1.2 1.3 1.4 1.5 Kanga L., Jones W., James R., 2005: Enlisting fungi to protect the honey bee. Florida A&M University & United States Department of Agriculture. In: Biologist, Volume 52 Number 2, May 2005. http://www.iob.org/userfiles/File/biologist_archive/Biol_52_2_Kanga.pdf
  2. 2.0 2.1 Gerritsen L., Cornelissen B., 2006: Biological control of Varroa destructor by fungi. Applied Plant Research, Bee Unit, Wageningen,. In: PROC. NETH. ENTOMOL. SOC. MEET. - VOLUME 17 - 2006. http://www.nev.nl/sete/sete-2006/125-132-GerritCor-2006.pdf
  3. van Dugteren P. 2009: personal correspondence per email with Piet van Dugteren, biologist and beekeeper, on 2010-01-14

See also:

Defra, 2005: Fungal control of Varroa jacobsoni (destructor) - HH0819SHB. In collaboration with the University of Warwik. http://randd.defra.gov.uk/Default.aspx?Menu=Menu&Module=More&Location=None&Completed=0&ProjectID=10939

Meikle W. G. et al., 2009: Effects of multiple applications of a Beauveria based biopesticide on Varroa destructor (Acari: Varroidae) densities in honey bee (Hymenoptera: Apidae) colonies. International Bee Research Association, 2009. http://www.physorg.com/news171872868.html

Hansen J., 2009: Fungus against mites parasitic to honey bees. Faculty of Agricultural Science, Århus University, 2009. http://www.agrsci.org/index.php/ny_navigation/nyheder/fungus_against_mites_parasitic_to_honey_bees




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