How the Auburn University Bee Center is Thinking About Tropilaelaps vs. Varroa Mites
Dan Aurell
Basics
It is easy to marvel at the abilities of the bees we work with. We admire their ability to build comb to tight specifications, their ability to remember and communicate the location of flowers and to exquisitely coordinate their activities among tens of thousands of individuals.
But have you ever thought of your bees as naïve? A definition of “naïve” is “showing a lack of experience, judgement or wisdom” (OED). In biology, calling an organism “naïve” has a slightly different meaning: it describes an organism that is not adapted to a parasite or disease. The honey bees we keep in North America are not adapted to fight back against parasitism by Varroa and Tropilaelaps mites (Figure 1 & 2). Our western honey bees (Apis mellifera) were safely out of contact with the parasitic mites in the genus Varroa, and in the genus Tropilaelaps, both living on Asian honey bee species. That changed when beekeepers brought western honey bees to East Asia in the past centuries. Varroa and Tropilaelaps mites (specifically Varroa destructor, Tropilaelaps mercedesae and Tropilaelaps clareae) managed to switch hosts to the western honey bee from their Asian honey bee hosts. Asian honey bee species have ingenious methods to kill parasitic mites and disrupt their reproduction (such as effective self-grooming by adult bees, and leaving mites trapped in capped brood cells). Our western honey bees lack these behavioral adaptations and the mites can build to damaging levels in colonies. Since the bees we keep are naïve to Varroa and Tropilaelaps, an important job of applied bee research labs such as the Auburn University Bee Center is to understand honey bee parasitic mites and develop ways to reduce the risk they pose to honey bee health and to beekeepers.
To help structure our thinking about the risks of Varroa vs. Tropilaelaps, let’s consider that risks can be split up into two components, in something called the “risk equation”:
Risk=Likelihood×Severity
It can be useful to think about two contributors to risk: the likelihood of a bad outcome occurring, and the severity of the outcome. It can also be helpful to consider these components of risk separately. Considering the potential arrival of parasitic mites to North America, different actions are required to address the likelihood of arrival, and the severity of impacts.
Likelihood of arrival
For Varroa, it is too late to manage the likelihood of arrival. It has already arrived and spread across North America (except a few places like the island of Newfoundland). For Varroa, all we can do is manage the second component of risk – the severity of impacts.
Tropilaelaps mites have not gotten to North America. Therefore, we have an opportunity to manage both the likelihood of its arrival and prepare to manage the severity of its impacts. Detection of Tropilaelaps mites in new regions over the past decades (like South Korea) and in this decade (like Russia’s Black Sea region and the Republic of Georgia; Brandorf et al., 2024, Janashia et al., 2024), shows that these mites have the potential to spread worldwide as has happened with Varroa. But, how does the likelihood of arrival in North America compare to that of Varroa?
Our thinking about the ability of Tropilaelaps and Varroa to arrive in new regions is strongly influenced by a difference in their biology. Whilei mites regularly feed on adult bees (even surviving on them for several months straight in places with long Winters), Tropilaelaps has never been documented to feed on adult bees. While Varroa can survive long term on adult bees, we think Tropilaelaps mites are very reliant on brood and can survive only a few days without this resource.
The movement of bees between countries is generally adult bees – whether this is mated queens with attendants, package bees or swarms that might stow away on ships. If package bees or attendants in queen cages that are being shipped between countries have Varroa on them, the mites are likely to survive the journey. In contrast, Tropilaelaps mites should have much reduced abilities to survive such a journey when adult bees are shipped on purpose or by accident. This should give us some confidence that Tropilaelaps mites would have a harder time arriving in North America. However, as with everything in science, we must be ready to update our thinking if we see new evidence. There are old and new threads that still need to be followed to further understand the ability of Tropilaelaps mites to survive off brood. First, they were once found on rats associated with bee nests; second, some researchers have suspicions that they might be able to feed on adult bees at the soft membranes near the wing, or on decaying adult bees (De Guzman et al., 2017, Gill et al., 2024).
At the Auburn Bee Center, we currently think that the chance of Tropilaelaps mites coming on ships or bee shipments is low for any individual shipment, even from an infested area. However, we think a precautionary approach is warranted, and that live bee shipments from known or suspected infested areas should be avoided. On top of that, it is important to avoid any illicit or accidental imports of bees into North America. As noted in June’s Bee Culture article, we have trained bee inspectors in the Apiary Inspectors of America on detection techniques to promote a speedy response if it ever arrives.
From a beekeeper perspective, what can you do to reduce the chance of Tropilaelaps arriving? Please don’t be tempted to illicitly import queens. Don’t put queens from another country in your pocket to bring them home! Let’s work together to avoid further scenarios like Varroa mites arriving in the U.S. You can also visit this Tropilaelaps online resources page (https://www.honeybeepests.org/tropi) to get acquainted with the visual appearance of the mite so that we have more vigilant beekeepers in case it were to arrive.
Severity of impacts
Having considered the likelihood of Tropilaelaps mites getting here compared to Varroa, we can move on to the expected severity of impacts if they get here.
As you know, Varroa has wreaked havoc on bee health. While effective Varroa control is possible, tools like resistant bees still need to be further broadened in use, nonchemical controls are labor intensive, and the array of chemical treatments that can practically be used at any moment of the year is limited by weather conditions, colony conditions, honey production periods and resistance. Overall, Varroa continues to cause severe problems.
With Tropilaelaps, we can make some predictions on severity based on what we know about its biology. Aligning with observations that Tropilaelaps does not feed on adult honey bees, places with cold Winters seem to have reduced issues with Tropilaelaps. Both Thailand and South Korea have both Varroa and Tropilaelaps mites. In South Korea, Varroa is the mite that regularly gets to damaging levels in colonies. In Thailand, Tropilaelaps is the predominant mite. We think this is because Winters in cold parts of Korea effectively interrupt the Tropilaelaps life cycle.
Varroa can be severe in any kind of climate, though their populations grow more in places with a longer brood rearing season. For Tropilaelaps mites, their biology and observations from countries with different climates indicate that the impact depends a lot on climate. If you are a stationary beekeeper in a cold Winter area and your colonies get a proper brood break in Winter we think you would be relatively protected from Tropilaelaps. If you keep bees in a mild Winter area and your colonies rear brood all Winter, we think your colonies would be very vulnerable if Tropilaelaps arrived. Results from the Winter Capped Brood Monitoring project (https://aub.ie/winterbrood) can help show the completeness of brood breaks in different areas of the eastern U.S. Keep in mind that many commercial beekeepers winter their colonies in southern or coastal U.S. and would thus be quite vulnerable. Indoor shed wintering (also quite common among commercial beekeepers) could be extremely useful to break the brood cycle, but only if the brood cycle is completely broken by shed conditions, which apparently is not always the case.
In areas where brood is reared year-round, the faster reproductive rate of Tropilaelaps mites compared to Varroa would make it a major concern. Like with Varroa, we see colonies collapse from high Tropilaelaps infestations. Over two-month experiment periods in South Korea and Thailand, we observed the number of Tropilaelaps-infested cells in untreated colonies increasing 11-fold (Korea) or 21-fold (Thailand) while Varroa only increased 2.7-fold (Korea) compared to their starting numbers (Figure 3, Aurell et al., unpublished). This rapid increase in populations means that, to successfully control Tropilaelaps mites, we need to have interventions that are more effective than those we have for Varroa, or we need to intervene more frequently.
In our arsenal against Tropilaelaps, we can currently promote two things with confidence: brood breaks and formic acid. Brood breaks could involve starting colonies from packages, modifying splitting practices or caging queens. Formic acid works well, we believe, because it turns into a vapor and can thus be delivered to Tropilaelaps mites in capped cells. Since Tropilaelaps mites spend such a short period of time outside the capped brood cells, we think this may be part of why contact miticides have often not worked well. An additional thing to consider: could the strides that are being made in breeding Varroa-resistant bees help us against Tropilaelaps if it were to arrive? We suspect Varroa resistance traits might help, but we don’t yet know!
Overall, we think that if Tropilaelaps mites got to the U.S., the beekeeping industry would suffer severe impacts. Some cold Winter areas are relatively protected, but most of the nation’s colonies are kept in areas where continuous brood rearing seasons would leave the bees much more vulnerable. American beekeepers could surely find ways to manage Tropilaelaps, but it would involve serious upheaval and the reorganization of colony management.
To reduce the impact of the mites in case they get here, we have been researching important questions around detection and management. On detection, we have shown that DNA-based testing can effectively detect Tropilaelaps DNA in easy-to-collect colony samples (Aurell et al., unpublished). Recent findings on management include that caging queens for 24 days was highly effective at controlling Tropilaelaps in both Thailand and South Korea (Tokach et al., 2024; Aurell et al., unpublished). This can surely be extended to other types of brood breaks (from Winter, from walk-away splits, etc.). Relatedly, Winter Capped Brood Monitoring has shown that Winter brood breaks are absent or intermittent in wide areas of the southeastern U.S., so additional measures like beekeeper-induced brood breaks and chemical treatments would probably be needed. Going forward, we plan to work on formic acid dosage and on new treatments. Our work on Tropilaelaps is not coming at the expense of work on Varroa, which is already here and causing serious problems, so we are pursuing Varroa work on multiple fronts as well.
We need to keep communicating about both mites in a way that recognizes uncertainties. There has been substantial research on Varroa since the 1980s; even so we are still learning new things like that it feeds on the hemolymph of honey bee brood but on the fat body of adult honey bees (Han et al., 2024). The amount of research on Tropilaelaps is still quite limited. A search of Web of Science reveals 4,030 articles on Varroa mites, and only 157 on Tropilaelaps mites. This means there are more unknowns, and starting sentences with “we think we know” is often better than “we know.”
Several ingredients have been necessary for our group’s overseas research on Tropilaelaps mites to be successful. First, collaborations have been essential. We have had the privilege to work with Dr. Bajaree Chuttong at Chiang Mai University in Thailand, and with Dr. Chuleui Jung at Gyeongkuk National University in South Korea. Second, we have a team of researchers willing to spend multiple months away from home who have proven eager to accept the challenge. Third, funding from groups including USDA ARS, USDA APHIS, Project Apis m. and the North Dakota Department of Agriculture have been essential to support our research in far-flung places.
While our bees may be naïve to Varroa and Tropilaelaps mites, the Auburn University Bee Center is continuing to work to answer important questions about these consequential parasites. We are targeting our activities to address the impact of Varroa (already here), to reduce the likelihood that Tropilaelaps arrives (as it is not here yet), and to reduce its impact in case it does arrive.
References
Brandorf, A., Ivoilova, M.M., Yañez, O., Neumann, P., and Soroker, V. 2024. First report of established mite populations, Tropilaelaps mercedesae, in Europe. Journal of Apicultural Research 64: 842-844.
De Guzman, L.I., Williams, G.R., Khongphinitbunjong, K., and Chantawannakul, P. 2017. Ecology, life history, and management of Tropilaelaps mites. Journal of Economic Entomology 110: 319–32.
Gill, M.C., Chuttong, B., Davies, P., Earl, A., Tonge, G., and Etheridge, D. 2024. An in vitro investigation into the survival of Tropilaelaps mercedesae on a range of matrices. (Non-peer-reviewed preprint).
Han, B., Wu, J., Wei, Q., Liu, F., Cui, L., Rueppell, O., and Xu, S. 2024. Life-history stage determines the diet of ectoparasitic mites on their honey bee hosts. Nature Communications 15: 725.
Janashia, I., Uzunov, A., Chen, C., Costa, C., and Cilia, G. 2024. First report on Tropilaelaps mercedesae presence in Georgia: The mite is heading westward! Journal of Apicultural Science 68: 1-6.
Tokach, R., Chuttong, B., Aurell, D., Panyaraksa, L., Williams, G.R., 2024.
