Climate Change and Honey Bees: Pest and Disease Management

Thursday 8 December 2022

Both practical beekeepers and academic researchers are focusing efforts on understanding the impact that climate change will have on honey bees.  Last week, we concentrated on one aspect of honey bee biology as it relates to overwintering success and climate change.  Additionally, concerns over antimicrobial resistance, incidences of brood disease, honey production and emergence of novel pathogens have all been mentioned in the same breath as climate change.  As weather patterns shift, some unforeseen problems, or perhaps even benefits, for honey bees may yet emerge!  One ongoing battle for beekeepers, and the greatest risk to honey bees, must be considered in the context of climate change.  That is the Varroa mite!  This week, we will examine, through the lens of our changing climate, this deadly pest.

Climate Change and Honey Bees: Pest and Disease Management

Most of the pests and diseases which beekeepers must manage are variable across the season.  Chalkbrood is often thought of as a spring problem.  Vairimorpha disease (formerly Nosema) will fluctuate across the season with typically higher levels in the spring reducing to lowest levels in the autumn.  Strong honey flow and hive growth in the early summer may precede an EFB outbreak.  In consideration of what is understood of honey bee pests and diseases, the suggestion that changes in weather patterns impact overall honey bee health, is not inconceivable.

A small but growing body of research warns of an increased threat from the honey bee antagonist, Varroa destructor, due to climate change.  The caution expressed by these researchers is reenforced by beekeeper’s experience.  The observed trends in Atlantic Canada are for earlier springs and longer falls.  This extended season provides a lengthened period of brood production which could benefit the colony in terms of overall population size.  The concern is, due to the Varroa mite life cycle being linked to brood production, this provides an increased opportunity for this pest to grow in numbers during a longer beekeeping season.

Varroa destructor Antagonist of honey bees (Photo: ATTTA 2022®)

The reproductive phase of the Varroa mite takes place in the brood cells of honey bees. A foundress mite will enter the cell containing a worker, or preferentially a drone, pupa.  She will hide under the pupa to avoid inspecting nurse bees.  Now safely ensconced within the capped cell, the female Varroa mite will produce between 1 and five daughters depending on conditions and type of brood cell (drones cells allow higher production). 

A typical female Varroa mite will have, at a maximum, 7 reproductive cycles in her life.  The number of cycles is dependent on the length of time in the capped cell which will be 12 or 15 days for worker or drone cells respectively.  This, in part, demonstrates the reproductive potential of a single mite.  A high fecundity female producing 5 daughters per cycle, for a season containing seven reproductive periods, would equal near 80 000 daughters.  If a longer season means one more reproductive cycle this number would be an unimaginable 390 000 mites.  This example is theoretical only to make a point and luckily there are factors, including beekeeper management, which limit this type of unchecked growth. 

The behaviour of Varroa mites changes across the beekeeping season.  Sexually mature mites, mothers and daughters, will focus on reproduction and be found mainly in the brood during the early season as the colony is expanding.  As brood production slows, increasingly, mites will be found on the honey bees.  This increased concentration of mites on the honey bees will be reflected in beekeepers seeing more mites in washes.  Also, in part, why the economic threshold for treatment increases later in the season.

So it is known that warmer spring and fall temperatures increase the level of V. destructor infestation in the bee colonies[1].  Longer beekeeping seasons will require added vigilance in controlling Varroa mites.  Continual monitoring and perhaps looking at mid season treatment options are becoming more important.  These challenges are not a hypothetical, future problem but upon us now.  One proposed contributing factors to the high 2021/2022 overwintering losses is the preceding longer beekeeping season leading to increased Varroa mite populations. 

Awareness is the first step in addressing climate change!  Longer growing seasons, supportive of Varroa mite reproduction, needs additional management focus.  As discussed last week, the asynchrony between flower phenology and honey bee foraging behaviour extending further into autumn, also presents challenges.  Long term changes in weather patterns may have a greater effect on honey bees as their behaviours are strongly influence by temperature.  Research and information will be required to support the beekeeping industry through these new challenges!  

[1] Hillayová, M.K., Korený, Ľ. and Škvarenina, J., 2022. The local environmental factors impact the infestation of bee colonies by mite Varroa destructor. Ecological Indicators, 141, p.109104.

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