Application of Metarhizium anisopliae as a potential biological control of Varroa destructor in Italy

The ectoparasitic mite Varroa destructor is a serious threat to European honey bees. Traditionally, Varroa mites has been controlled with synthetic pesticides, but residues in honey bee products and recently arisen resistances to chemicals call for new control methods. One alternative approach is the use of biological control methods, such as acaropathogenic fungi. In this field study, we evaluated a special isolate of M. anisopliae var. anisopliae BIPESCO 5 as a possible biological control for V. destructor. The colonies were divided in two treatments of eight replicates. During the first month of the experiment, every colony of the ‘Metarhizium’ group was dusted weekly with 10 g of conidia in the concentration of 1 1010 conidia per gram. After 14 days, all the colonies were split, moving the frames with broods to new nuclei and leaving the remaining brood-less colonies with the queens. The brood-less colonies were exposed to a follow-up oxalic acid treatment to assess the residual mite level, while the nuclei were transferred in a new apiary and were observed for a further 24 days until all broods hatched and were then treated with oxalic acid. The results showed significant lower mite levels in the treatment group than in the control (<60%). In addition, M. anisopliae had no impact on colony strength and development. An increased number of dead bees found in the under baskets of the treated colonies did not affect any of the colony strength parameters. To conclude, these results lead us to consider BIPESCO 5 a promising biological control agent against Varroa.