Can fungal secreted metabolites be used to control the honey bee parasite Varroa destructor? : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University
(Lincoln University, 2026) Maan, Alex
Varroa destructor (Varroa) is an ectoparasitic mite of honey bees and a vector of deadly bee viruses. Varroa infestation can lead to colony losses, threatening pollination and global food security. Chemical miticides have previously been effective control methods, however their efficacy is declining due to the rapid development and spread of miticide‑resistant Varroa populations. New control methods are therefore needed. Biological control has been trialled using live entomopathogenic fungi (EPF) that exhibit virulence towards Varroa, but remains limited by the hostile environmental conditions of the hive towards live organisms. A logical follow‑up to this approach, and the aim of this research, was to assess the direct application of crude fungal extracts and specific fungal metabolites.
To achieve this, appropriate Varroa bioassay protocols were first established along with an effective Varroa collection method. Iteration of the bioassay protocols, such as maintaining a high relative humidity, and providing a food source led to increased baseline Varroa survival and reduced toxicity when exposed to the commercially available miticide Amitraz, suggesting a more ecologically valid outcome. Fungal extracts derived from EPF and other fungal species were then assessed against Varroa. Only one, a Metarhizium anisopliae extract, exhibited miticide potential. However, the bioactivity was attributed to destruxins, a well‑known virulence factor of Metarhizium species that can negatively affect bees.
Subsequently, a fungal metabolite exhibited rapid activity against Varroa upon contact exposure and showed an acceptable safety profile in honey bees. However, the price of this metabolite would currently limit its viability as a commercial Varroa treatment. Natural production of this metabolite is restricted by low titres caused by feedback inhibition. Therefore, to reduce the metabolite production costs, a screening of fungal isolates was conducted alongside an in situ extraction method to alleviate feedback inhibition.