When it Comes to Protecting Honey Bees, Timing is Everything

Impact

Managed honey bees pollinate 80 per cent of insect-pollinated crops and provide Ontario farmers with $395 million in pollination services. Data show that commercial beekeepers (operating 50 or more colonies) lost half their colonies over the winter of 2023/24. Alliance research has determined that adjusting the timing of pest/disease treatments could help the province’s 4,000+ beekeepers strengthen their hives and increase survival of their honey bee colonies.

With support from the Ontario Agri-Food Innovation Alliance, Dr. Ernesto Guzman and his team at the University of Guelph’s Honey Bee Research Centre have been analyzing a massive dataset drawn from commercial and research colonies between 2015 and 2020. Their goal is to better understand pest and pathogen (disease-causing microorganisms) prevalence at different points of the season, and how it affects bee death.

As an output of their data analysis, the Ontario Agricultural College researcher and his team are also developing and distributing new pest monitoring and management strategies for the beekeeping industry. 

“The bee health situation right now is difficult,” Guzman says. “But beekeepers might not always know that these kinds of resources and knowledge are available. So, in addition to pests, I think we also have a communication issue.”

What bees are up against

One of the main pests in Guzman’s sights is the varroa mite—a.k.a., Varroa destructor, widely considered the world’s single most important honey bee parasite. 

Not only do the mites feed on adults and developing bees, explains Guzman, but they can spread viruses, such as deformed wing virus (DWV), which leaves bees with shrunken, poorly formed wings. 

Another varroa-transmitted pathogen is Israeli acute paralysis virus, which results in bee paralysis, trembling and death. 

On the pathogen front, Guzman is also looking at Nosema ceranae, a fungus that infects a bee’s midgut (part of the digestive tract) and is spread via spores from bee fecal matter within the hive. While Nosema doesn’t cause death outright, it can cause colonies to grow slowly—or not at all. 

“If you have a colony that doesn’t grow, or grows very slowly in the springtime, then it’s not going to be productive at all,” says Guzman. “And that, of course, usually means the colony also won’t be strong enough to survive the winter months.”

Data reveals varroa pest prevalence

Guzman’s first major finding was that the presence of pests increased from early spring to late fall; particularly, for varroa mite. 

“Varroa mite is found in only about 10 to 20 per cent of colonies in the spring months. But then we see that number increase to 80 per cent of colonies in the fall,” he explains. 

Why the enormous jump in mite populations?

Guzman says part of the reason could be that the most-used method of mite detection isn’t sensitive enough. In other words, beekeepers and apiary inspectors can’t detect the pest in colonies that are likely infested with varroa mites but at very low levels. 

Along with growing varroa mite infestations, the colony data also showed that 50 per cent of all bee samples had deformed wing virus (DWV), and that the levels of viral infection are 13 times higher in the fall than in early spring. 

“The varroa mite and DWV prevalence is critical, because the fall is when beekeepers are preparing their colonies to survive the winter,” Guzman explains. “And if you have high levels of varroa mites and DWV, the bees definitely aren’t living the five or six months they need to during the winter.” 

A closer look at Nosema ceranae

The current Ontario treatment threshold for Nosema ceranae is one million spores per bee. But according to Guzman, their data shows colonies have the equivalent of two million spores per bee in the spring each year—double the treatment threshold. 

And while the team saw no correlation between infection rates and winter mortality, they did find that the pathogen plays a huge role in slowing down colony population growth. 

“Not all colonies start at the same strength in the spring, but doubling your population over the course of the season just isn’t enough. You may start with 10,000 bees in a colony, but you’d want that to grow by at least four or five times,” says Guzman. “Whether you’re using colonies for honey production or pollination services, if they don’t grow, or grow very slowly, then they won’t be productive.” 

And when you add varroa mite infestations, along with varroa-transmitted viruses, beekeepers have—in Guzman’s words—a “perfect storm” on their hands. 

Treating pests and pathogens on time 

Guzman and his team are now planning knowledge translation activities around their findings. 

They intend to host a series of in-person talks at beekeeping industry conferences and prepare a series of articles and videos to be featured on the Honey Bee Research Centre’s website. 

Guzman says these resources will discuss in detail how to diagnose varroa mite infestations and Nosema infections and will cover integrated control strategies. 

The main takeaway: timing is everything when it comes to monitoring and managing pests and pathogens. 

“For a colony to have a chance of surviving the winter, they should be mite-free, or close to mite-free, between the end of August and early October. That’s the critical window,” explains Guzman. “Beekeepers should be treating their colonies no later than August and consider treating again in the spring just to slow down the mite growth curve.

“But emphasizing the need to be monitoring mite and Nosema presence at different times of the year, beekeepers can be better prepared to act on time and ensure they have colonies that not only survive the winter but are set up to thrive in the spring.” 

This research is funded by the Ontario Agri-food Research Initiative (OAFRI) and the Ontario Agri-Food Innovation Alliance, a collaboration among the Government of Ontario, its agency Agricultural Research and Innovation Ontario (ARIO) and the University of Guelph.