Monitoring Honey Bee Colony Losses

Thursday 5 November 2020

For the past few weeks, we have talked about options for the control of Varroa destructor.  The importance of managing this pest has been emphasized in recent publications which investigate the causes of honey bee colony losses.  One of the main messages of these reviews is that Varroa mites are the most serious challenge facing beekeepers around the globe.  Let’s have a closer look at these reports and consider the implications of this new knowledge for our region.

Monitoring Honey Bee Colony Losses

Of significance to the beekeeping community, the journal Diversity has devoted a special issue to “Monitoring Honey Bee Colony Losses.”  This is a comprehensive review of over 300 recent, international, scientific papers around the topic of colony loss and presented as 11 individual articles.  In reviewing this substantial body of relevant literature, these researchers have identified five common themes which cover the causes of colony loss globally.  These include:

1. Additional stress from diseases and pests as a result of the movement of bees.  As an example, Varroa mite is identified as the number one threat to honey bees which has spread across the globe, with a very few exceptions, due to human movement of honey bees.  The writers have termed this phenomenon as “human-driven spread of pathogenic and pest organisms.”

2. Landscape or habitat changes.  The review points out that changes in land use can have both a negative and positive effect on honey bees.  We can see this in our region as many beekeepers benefit from unused agricultural land to place apiaries.  Alternatively, large monocrops replacing areas of mixed-use agriculture are demonstrably damaging to honey bees.

3. Intensification of agricultural practices.  This category of colony loss includes the damage caused to honey bees from the use of agrochemicals in the form of pesticides, herbicides and fertilizers.  The significant breadth of the substances to which honey bees are exposed is emphasised.  The point is made by one piece of research which found 150 different pesticides in the sample of honey bees tested.  In this study the average was two pesticides per bee with a maximum of seven.

4. Climate change is given brief mention as the extreme weather events around the globe become more frequent.  We saw a possible effect of our changing climate this beekeeping season with areas of Atlantic Canada suffering a 50-year drought.

5. Invasive species of plant are highlighted as a cause of honey bee colony loss.  This along with invasive species of animal are of concern with the most current, and well publicized, threat to beekeepers coming with the spread of the Giant Asian Hornet.

Also mentioned are the importance of beekeeping practices as factors which could both cause and prevent colony losses.

Two other important points are made in this analysis.  One is the cumulative effects of individual sublethal stressors.  The suggestion made is that individually many of the stressors associated with honey bee colony loss would not, in themselves, cause the loss of a colony but it is the combined effects of many stressors which kill the bees.  This has been described by many honey bee advocates using the clichéd expression, “death by a thousand cuts.”  The second point is that research should focus on relevant concentrations of environmental chemicals damaging bees.  Traditional toxicological studies often focus on high concentrations of a substance to identify its lethal dose but honey bees are being exposed to low environmental concentrations of a number of substances.  The authors of this work refer to this as a “pesticide cocktail” and they emphasize the need for addition research examining the effects of real-life exposures of chemicals.

Factors Associated with Honey Bee Colony Losses

Another group has just published an additional review entitled, “Factors Associated with Honey Bee Colony Losses: A Mini‐Review. These writers examined 150 relevant, scientific journal articles to reach their conclusions. This work recognizes the importance of the honey bee globally to agriculture and food production and the worrying trends in colony losses.  These workers agree that the phenomenon of colony loss is not attributed to a single identifiable cause although, they make some strong suggestions that the Varroa mite is the primary threat to honey bee colonies.  Other pest and diseases are also mentioned, including Nosema, as problematic.

This work reviews the causes, or drivers, of honey bee colony losses under two broad headings:

1. Role of Pests and Diseases as Drivers Leading to Honey Bee Colony Losses

2. Anthropogenic Direct Drivers Associated with Honey Bee Colony Decline

Under these headings, these researchers cover much of the same ground as the previously discussed work.  They have included additional information on these drivers with some good details on the combined effects of all these stressors on honey bees.  They have stated, as with the previous work, that bees are facing many stressors which combined are contributing to colony losses.  


Combined these two separate reports have reviewed a huge body of work and determined that the Varroa mite is the most serious pest of honey bees.  Additionally, the consensus from both of these large reviews, is that there is more that one stressor leading to honey bee colony loss.  Moderate levels of Varroa mite infestation can lead to colony loss due to the combined damage of the mite and the other related contributors to honey bee ill-health.  Varroa mite infestation in honey bees has been linked to behavioral changes, perturbed navigation and homing ability, reduction in fertility, increase in viral load and disease.  Considering these disease factors on top of the damaging loss of fat bodies, it is no wonder of the devastation caused by even moderate Varroa infestation.

Many of the causal factors linked to colony losses may be beyond the control of beekeepers.  What is within our control is the Varroa mite!  This current research strengthens the case for vigilant monitoring and treatment of this overwhelmingly agreed single contributor to colony losses.

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