Varroa mite infestation affects thermoregulation in honey bee colonies
Impact of varroa infestation on thermoregulation in honey bee colonies: new findings indicate that varroa mites reduce bees’ ability to effectively regulate their body temperature, making them more vulnerable to cold temperatures.
Honey bee colonies must develop strategies to survive various stress factors that can affect their health status and stability. Although several factors threatening colony well-being have been studied in the past, possible interactions or synergies between different factors have rarely been taken into account, as such studies are very complex. Researchers from the University of Udine in Italy examined more closely the relationship between the abiotic stress factor “cold exposure” and the biotic stress factor “varroa infestation” – both identified as major causes of winter colony losses, particularly in the Northern Hemisphere. The authors of the study focused on both the colony level and the level of individual worker bees.
To this end, they used a combination of field and laboratory trials, working with honey bees from two different apiaries in north-eastern Italy. One apiary was treated regularly against varroa infestation in order to control and reduce infestation levels as much as possible. The other apiary was not treated from the summer months through winter, with the exception of a single treatment in October to prevent colony collapse at that site. Data such as bee population size, level of varroa infestation, bee mortality, and the number of bees with deformed wings were recorded between August and December 2018. Temperatures within the brood nest of each colony were measured using temperature probes between August and November of the same year. In addition, ambient temperatures at the apiaries were also recorded.1
Furthermore, the researchers reared worker bee larvae at different temperatures in the laboratory (34.5 °C versus 32 °C – 94.1 °F versus 89.6 °F) or artificially exposed them to varroa mites during development in order to study the effects of both factors on survival. They also tested the effect of parasite exposure during the larval stage on the ability of adult bees to thermoregulate their body temperature by subsequently placing adult bees in a polystyrene box at 25 °C and using an infrared thermographic camera to assess the bees’ average body temperature. Worker bees with deformed wings were excluded from this experiment, and body temperature was compared with a focus on the thoracic region – the warmest part of the body.
The results of the field trial showed that mite infestation in untreated colonies – unsurprisingly – was significantly higher in November compared to treated colonies. This also resulted in a significantly higher viral load in parasitised bees and reduced survival. The average temperature in the brood nest was significantly lower in untreated colonies. In November, the difference in mean brood nest temperature between treated and untreated colonies was 4 degrees.1
The laboratory results provided further insights into the effects of larval varroa infestation and/or rearing temperature on the lifespan and thermoregulation of individual bees. Both larval varroa infestation and lower rearing temperatures resulted in reduced survival of affected bees, with the effect of larval mite infestation being stronger. Mite-infested bees exposed to a lower rearing temperature (32 °C – 89.6 °F) showed reduced survival compared with all other groups: control bees and bees exposed either to mite infestation or to low temperature alone.
As a completely novel finding, the authors report that bees infested by varroa during the pupal stage were less able to thermoregulate effectively in response to exposure to suboptimal temperatures. The weight of the thoracic muscles did not differ between the two groups, leading the researchers to believe that infested bees may not have sufficient access to the nutrients required for thermoregulation. Confirming this hypothesis, they found that sugar consumption by parasitised bees was significantly reduced compared to non-infected bees, referring to this phenomenon as “varroa-induced anorexia”.
A subsequent genetic analysis demonstrated that expression of insulin receptor substrate 1 (IRS-1) was upregulated in infested bees, confirming that varroa infestation directly affects bee metabolism.
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Sources:
Frizzera, Davide, et al. “Varroa destructor exacerbates the negative effect of cold contributing to honey bee mortality.” Available at SSRN 4208675.