“We kept hearing anecdotally that these Californian honeybees were surviving with way fewer treatments. I wanted to test them rigorously and understand the driving force behind what the beekeepers were seeing,” said Genesis Chong-Echavez, a UCR graduate student and lead author of the study, in an article from the university.

WHY IT MATTERS: Varroa mites can devastate Canadian beekeepers’ hives, and go-to control methods have become less effective, leading producers to look for new methods to protect honeybees.

Varroa mites are an invasive parasite that has plagued North American beekeepers since the late 1980s. The mites weaken the bees by feeding on their fat stores, and also can carry viruses. Varroa mites are a consistent contender among the top four causes of winter bee loss in Canada, as noted by the Canadian Association of Professional Apiculturists.

Chong-Echavez’s team found bee colonies led by locally-raised Californian hybrid honeybee queens had about 68 per cent fewer mites, on average, than hives with commercial queens.

While these populations were not entirely varroa mite-free, they were more than five times less likely to hit the threshold at which chemical treatment is necessary.

Local bee larvae attract fewer mites

The resistant bees came from a genetically-mixed population established in southern California — often from “feral” colonies living in trees, the UC Riverside article said. They were found to have mixed ancestry steming from African, eastern European, Middle Eastern and western European genetics.

Varroa mites must enter bee brood cells to reproduce. In lab experiments with developing honeybee larvae, researchers found mites were less attracted to the locally-adapted bees than commercial bees.

“What surprised me most was the differences showed up even at the larval stage,” Chong-Echavez said. “This suggests the resistance mechanism may go deeper than some kind of behaviour and may be genetically built into the bees themselves.”

The research team next intends to investigate the signals that may make the locally-adapted larvae less attractive to mites.

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The study followed nine honey bee colonies during a “particularly hot” Arizona summer to assess the limits of the bees’ ability to control the hive’s temperature according to a release from the University of Chicago Press Journals.

Honey bees maintain their hive temperature between 32°C and 36°C, according to Oregon State University’s extension service. Outside that temperature window, bee larvae and pupae won’t develop and may die.

When the temperature inside the hive gets too hot, the bees line up at the entrance and fan their wings. Other bees will also bring water to the hive to assist with cooling.

Temperature swings

In the study, researchers found that while the bees were able to keep the average temperature around their brood within the acceptable range, the temperature inside the hive still swung widely throughout the day. Developing bees in the centre of the brood experienced about 1.7 hours below the optimal range and 1.6 above the range per day.

Young bees toward the edge of the brood saw nearly eight hours per day outside the safe temperature window.

Larger colonies were better able to regulate temperature than small ones.

Colonies exposed to higher peak air temperatures and greater internal temperature swings saw their populations decline, the researchers found. They concluded that temperatures exceeding 40°C can impair the hive’s ability to regulate temperatures.

This could also shorten adult bees’ lifespan.

The authors of the study noted that extreme heat events are expected to be come more common due to a warming climate.

To help bees regulate temperature, the researchers suggested placing hives in shaded areas, improving hive design and insulation and providing supplemental water. Access to high quality forage may also become increasingly important, they added.

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From Asia to Europe

Tropi’s natural host is the giant honey-bee (Apis dorsata), common across South and Southeast Asia. At some point, the mite jumped to the western honey-bee (Apis mellifera), the species kept by beekeepers around the world. Because this host is widespread, the parasite has steadily moved westwards.

It has now been detected in Ukraine, Georgia and southern Russia, and is suspected to be in Iran and Turkey. From there, it is expected to enter eastern Europe, then spread across the continent. Australia and North America are also at risk.

Why tropi spreads so fast

Like varroa, tropi is a tiny mite that breeds inside capped brood cells, the life stages of the honey-bee when the late larvae and pupae develop inside honeycomb cells that are sealed by a layer of wax. The mite feeds on bee pupae and transmits lethal viruses, such as deformed wing virus – the deadliest of the bee viruses. But there are crucial differences.

Varroa can survive on adult bees for long periods, but tropi cannot. Outside brood cells, it lives only a few days, scurrying across the comb in search of a new larva.

Because tropi spends more time in capped cells, it reproduces quickly. A capped cell that contains a female varroa will result in one or two mated varroa offspring emerging with the adult bee. Tropi offspring develop faster inside a capped cell than varroa offspring, so a tropi “mother” may result in more offspring emerging than a varroa infested cell, more quickly overwhelming the colony.

As a result, colonies infested with tropi can collapse far faster than those plagued by varroa.

Current control methods

In parts of Asia where the parasite is already established, small-scale and commercial beekeepers often manage it by caging the queen for about five weeks.

With no eggs being laid, no brood develops, leaving the mites without a food source. This method is practical where beekeepers manage dozens of hives, but not in places like Europe where commercial operations often involve thousands.

Another option is treating the beehive with formic acid, which penetrates brood cell caps and kills the mite without necessarily harming the developing bee, provided concentrations are kept low. This treatment may offer beekeepers a practical tool.

Why varroa treatments won’t work

Many wonder whether the chemicals used against varroa could also fight tropi. The answer is, mostly no.

Varroa spends much of its life outside of a capped cell clinging to adult bees, where it comes into contact with mite-killing chemicals known as miticides spread through the colony on bee bodies. By contrast, tropi rarely attaches to adults, instead darting across comb surfaces.

Because of this, it is far less exposed to chemical residues. Treatments designed for varroa are often ineffective against the faster-breeding tropi.

Managing both mites together will be particularly difficult. Combining treatments risks harming colonies or contaminating honey. For instance, formic acid for tropi and insecticides such as amitraz for varroa might interact at even low levels, killing the bees as well as the parasites.

There is also the danger of resistance. Over-use of varroa treatments has already produced resistant strains, reducing the effectiveness of several once-reliable chemicals. Introducing more compounds to fight tropi, without careful integrated pest management, could accelerate this process and leave beekeepers with few effective tools.

The wider impact

The spread of tropi will not only devastate beekeepers but also agriculture more broadly. Honey-bees are critical pollinators of many crops. Heavier hive losses will raise costs for both honey production and pollination services, affecting food prices and availability.

Research is underway in countries such as Thailand and China to develop better management strategies. But unless effective and practical treatments are found soon, the spread of this new mite around the world could be catastrophic.

The story of varroa shows how quickly a single parasite can transform global beekeeping. Tropi has the potential to be even worse: it spreads faster, kills colonies more quickly, and is harder to control with existing methods.

— Jean-Pierre Scheerlinck is an honorary professor fellow at Melbourne Veterinary School at the University of Melbourne. The author would like to acknowledge the contribution of Robert Owen, a beekeeper who completed a PhD on the varroa mite at the University of Melbourne in 2022, to this article.

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