Elucidating the regulation of reproduction in Varroa mites: uncovering potential control strategies.

Insect pollination of crops is absolutely critical to food production with an estimated annual global value of $361 billion. Honey bees (Apis mellifera) are undergoing a serious bee health crisis threatening global food security. Huge annual losses of bee colonies in the US (45%, 34%, 40% in 2012/13, 2013/14 and 2015/16) are now being matched by similar losses in Europe. The status of bee health is recognised as critical with unsustainable losses of bees. Though the cause of the bee health crisis is multifactorial, it is generally accepted that the external parasite varroa mite (Varroa destructor) and its transmission of viral pathogens is one of, if not the, major cause of honey bee colony loss. Because of the impact on global food production, varroosis is arguably the most serious disease of livestock in any species.

Despite varroa's unquestionable significance, our understanding of its physiology could be considered rudimentary relative to its importance. Progress in varroa research is substantially hampered by the lack an artificial feeding and rearing system which would supply researchers with adequate numbers of consistent varroa year round and provide an ideal experimental system for studying varroa physiology, reproduction, disease transmission and the development and testing of new control strategies. We have recently developed an artificial feeding system for varroa that far exceeds any currently available. Subsequently, we succeeded in inducing varroa in the artificial feeding system to lay eggs by exposing them to the odour cues from either bee larvae or hormone-treated pupae, but not from standard pupae. These eggs hatched and developed through to adult varroa. We now need to capitalize on these initial findings and better understand the fundamentals of varroa reproduction to allow our artificial feeding system to fully evolve into an artificial rearing system.

To this end, the overall aim of the project is to understand the control of reproduction in varroa and assess if this information could be utilised as a control measure. By analysing the volatile compounds emanating from cohorts of larvae, pupae and juvenile hormone-treated pupae and applying a differential chemometrics approach, we will chemically characterize components(s) of the "Oviposition Kairomone" (OK). The effect of the larval volatiles or OK on induction of oviposition, the gender of the egg and cessation of egg laying in varroa will be studied by a combination of bioassay and chemical analysis. The role of juvenile hormone in the production of these bioactive volatiles or OK by bee larvae will be elucidated. We will investigate if the varroa detects this odour cue from the bee larvae with its legs and if this signal causes egg laying in varroa through induced sex hormone production (ecdysteroids) via a neuroendocrine factor. The optimum concentration of larval volatiles or OK required to induce varroa egg laying in the artificial rearing system will be determined by chemical analysis and bioassay and then a suitable method for delivery of that concentration range developed.

There are strains of bees in France, Sweden and Germany exhibiting a trait termed "Suppressed Mite Reproduction" by which varroa have reduced reproduction. By performing the oviposition bioassay and chemical analysis, we will test the hypothesis that altered levels of larval volatiles or OK are the basis for the reduced varroa reproduction on these bee strains. Finally, we will assess if larval volatiles or OK can induce varroa to lay eggs even when in an inappropriate scenario such as while on adult bees.