We are still free of Tropilaelaps mites, but for how long?

Inside: Here’s the scoop on Tropilaelaps, the Asian parasitic mite. This tiny creature is a close relative of varroa, feeds on developing honey bees, and spreads deadly viruses. But for now, it’s still restricted to tropical climates outside of North and South America.

Global trade can bring unwanted gifts

Global trade has spoiled us. We can buy anything from anywhere, and before we can say Acarapis woodi, the orders plop onto our front porches. And if we don’t like what arrives, we can send it back. International shopping is so effortless, we seldom think about it.

Unfortunately, the part about sending it back doesn’t work well for invasive species and the pests and pathogens they carry. Many items we didn’t order arrive beneath our radar, settling in our homelands attached to something else, hidden from view, or not recognized as a problem. Others sneak in despite our best efforts at monitoring and inspecting. Think varroa mites, for example. A single mistake can have lasting consequences.

One creature patiently waiting in the emigration queue is Tropilaelaps (tro-pih-lay-laps), also called the Asian parasitic mite. As I listen to scuttlebutt, I sense many beekeepers understand this creature is eager to pounce, but few know the specifics. They sometimes ask, “How worried should we be?” Well, that’s a good question, and one many bee scientists are asking too.

An assortment of unpleasantness

The first piece of bad news? Tropilaelaps is a close relative of the varroa mite. Too close for comfort, you might say. Like varroa, the Tropilaelaps mite is an ectoparasite, attaching onto or burrowing into the outer surface of its host. Once affixed to a tender larva or pupa, the mite feeds on the developing bee.

The adult mites lay eggs inside a honey bee brood cell. When the immature nymphs eclose from their eggs, they feed immediately, weakening the honey bee brood and infecting them with viruses such as the infamous deformed wing virus. They also carry black queen cell virus and perhaps others. Does that sound familiar?

A super-fast reproduction rate

The similarities between varroa and Tropilaelaps are disquieting, but their differences are not comforting either. Since the Tropilaelaps mite has a shorter life cycle, it can produce offspring at a faster rate than varroa. That means it can produce more mites per bee in a shorter time. In addition, a higher proportion of Tropilaelaps females (about 70%) produce at least one offspring, whereas the rate in varroa is about 50%. Further adding to their amazing fecundity, mating is optional. At least for some individuals, if they can’t find a mate, they just reproduce the parthenogenic way.¹

In addition, Tropilaelaps are so small, most haven’t the strength to latch onto adult bees. Instead, they skip the lengthy “phoretic” period² that varroa mites prefer (5-7 days when brood is present, 5-6 months without).³ Instead of riding around while slurping the fat bodies of adult bees, they simply get on with more reproduction.

Because they don’t dally on adult bees, most Tropilaelaps move into a new brood cell within 24 hours of leaving the first one, quickly infecting up to 90% of all brood cells in a hive. Some beekeepers who have both kinds of mites in their colonies report Tropilaelaps may rapidly outnumber varroa 25 to 1.⁴

How a Tropilaelaps-infected colony dies

A Tropilaelaps-infected colony collapses in much the same way as a varroa-infected colony. A beekeeper may notice an overall population drop, an irregular brood pattern, perforated cell cappings, uncapped larvae dead in the cells, adult bees with shrunken abdomens, thoraces, or deformed wings, and listless or crawling bees. Many of these symptoms result from the viruses that both types of mites carry.

In addition, Tropilaelaps-infected colonies may have many chewed larvae visible in the cells. These die and rot, causing a revolting dead-animal odor we don’t associate with varroa. This chewed look starts when the mites feed. Unlike varroa mites that use the same puncture wound over and over, Tropilaelaps open a fresh wound at every feeding. Tragically, the brood succumbs to being munched alive.

The sheer number of mites means the worker bees cannot keep up with removing all the dead bodies. At first, they may heave dead larvae onto the landing board, but after a while, the workers give up and allow the brood rot in place.⁵

Jumping from one species to another

Recall that varroa mites were originally parasites of Apis cerana, the Asian honey bee. Because the Asian honey bee and varroa co-evolved over centuries, a balance of power developed between them. But when beekeepers introduced the European honey bee into Asian honey bee territory, the varroa mite crossed over.

Unlike Asian honey bees, European honey bees don’t have an extensive defense mechanism against varroa. As we now know, they lack the genetic wiring to handle either the parasite or the associated viral diseases as well as Apis cerana.

Like varroa, Tropilaelaps also crossed species when Apis mellifera expanded into a territory ruled by Apis dorsata and Apis laboriosa. Apis dorsata (the giant or rock honey bee) is a native of South and Southeast Asia. The word “giant” is appropriate because this bee is larger than most other honey bee species, measuring about 0.7-0.8 inches (17-20 mm) long. The other common host is Apis laboriosa (the Himalayan giant honey bee) which is an astonishing 1.2 inches (3 cm) long.

Although there are four species of Tropilaelaps found within these Asian regions, as far as we know, only two parasitize Apis mellifera, T. mercedesae, and T. clareae. Of these two, T. mercedesae has a larger distribution and probably a greater risk of spreading.

A sizeable problem

According to Dr. Samuel Ramsey of the University of Colorado at Boulder, body size is a critical issue for European honey bees. All else being equal, a giant bee can withstand more parasitism than a peewee bee. In theory, at least, five mites munching on a large larva will do less damage than five mites chewing on a small one.

When varroa moved from A. cerana to A. mellifera, size wasn’t an issue because both species have roughly the same dimensions. But where the difference is great, as in A. dorsata vs. A. mellifera, the host’s size dictates the amount of damage a single mite can deliver. A large larva may survive an amount of parasitism that would kill a smaller one.

How to recognize a Tropilaelaps mite

Although both varroa and Tropilaelaps are reddish-brown mites, they have radically different shapes. Varroa looks more crablike. The head is on a long side, much like a yummy blue crab. Conversely, Tropilaelaps has a beetle-style body, with the head at a narrow end.

If you compare the ventral side of a varroa with the ventral side of a crab, you can see the eerie similarities. They are both arthropods, but not otherwise closely related. Mites, spiders, and lice are Arachnids (terrestrial creatures), whereas crabs, lobster, and crayfish are crustaceans (aquatic species). Despite their many differences, I can’t eat a crab without envisioning those enormous spiders that drape from the trees in my yard. (Then there was the grade school science teacher who said a crab was just a spider gone swimming.)

You could confuse Tropilaelaps with the fly, Braula coeca, although the fly is quite a bit larger. Or you might confuse Tropilaelaps with the harmless pollen mite, Mellitiphis alvearius, except for its rounder shape.

Aside from a different physique, Tropilaelaps are tiny compared to varroa, perhaps a third as large. From end to end, Tropilaelaps extend about 1 mm, and from side to side they reach half that far, about 0.5 mm. Compare this to typical varroa mites that measure roughly 1.1 mm long and nearly 2 mm wide. (Remember, long and short sides do not correspond because of their body shapes.)

In both varroa and Tropilaelaps, the males are smaller than the females. Male and female Tropilaelaps have similar body shapes, but male varroa do not resemble their females. Instead, the males are almost round and measure about 0.7 mm by 0.7 mm.

You can see Tropilaelaps after a powdered sugar shake. Just remember they are tiny and move fast, so you need to be quick, too.

Rates of colony failure

Most of us have noticed that an untreated colony infected with varroa mites collapses in one to two years. The differences in survival times are likely related to brood breaks, swarming, or perhaps various levels of genetic resistance. But the colony gradually weakens until it finally folds, often in the fall or winter.

We’ve learned to recognize various characteristics of a varroa-weakened colony. We might notice a decreasing bee population, holes in brood cappings, chewed cappings, and mite feces in brood cells. We may see an abnormal brood pattern, deformed wings, shrunken abdomens, or dead mites on the bottom board. In spite of the damage, the queen may persist, and you may see capped brood and honey stores.

With untreated varroa, a backyard beekeeper may even get a honey crop before his colony gives up the ghost. But a honey bee colony with Tropilaelaps has a much shorter shelf life, more on the order of several months. Left untreated, a colony with Tropilaelaps would be unlikely to produce honey or even make it to the fall, perhaps collapsing during its first summer.

How do beekeepers treat these mites?

Successful management of any organism depends on finding its weak spot. The most glaring weakness of Tropilaelaps is its dependence on a constant supply of bee brood. Without brood, the mites starve because, unlike varroa mites, they don’t feed on adult bees.

Some beekeepers speculate that this single weakness has slowed Tropilaelaps distribution around the globe, and it may be the reason we don’t yet have them in the Americas. Since the mites cannot live longer than about 36 hours without bee brood, they can’t survive on shipped queens or in bee packages. Unless brood is present, Tropilaelaps simply can’t go along for the ride.

For now, jurisdictions with Tropilaelaps don’t allow shipping or receiving of bees with brood, and shipments of queens and packages are subject to two-day holding periods to assure none of the mites can survive.

For the same reason, regular brood breaks are the best control for Tropilaelaps. By sequestering the queen long enough to assure periodic two-day broodless periods, beekeepers can successfully manage mite populations. This is similar to managing varroa with brood breaks except the intervals are shorter yet more frequent.

Other control measures that work include the same miticides we now use for varroa. Those shown to work include acaricides containing formic acid, thymol, hop beta acids, fluvalinate, and amitraz. In addition, beekeepers in Asia have good results with sublimated sulfur.⁶

How fast is Tropilaelaps spreading?

Tropilaelaps prefer tropical and semi-tropical environments, such as exist in their native Southeast Asia. Because these areas are warm year-round, the hives produce brood year-round. So far, the mites have not spread into cooler areas that don’t support continuous bee brood production.

Although the distribution of Tropilaelaps has increased in the last fifty years, the expansion has not been dramatic. For now, temperatures cool enough to halt bee brood production prevent them from spreading. However, the thought that keeps beekeepers awake at night is a mite that could survive longer than 36 hours without bee brood. How much farther could they spread if they could go 48 hours without brood? Or 60?

So far, Tropilaelaps have migrated to Kenya, Papua New Guinea, Myanmar, Pakistan, the Philippines, South Korea, Vietnam, and Afghanistan.

It is not clear how Tropilaelaps spreads in nature. Because only about 3-4% of adult mites attach themselves to adult bees, only a small percentage move between colonies using that method. Regardless of the low rate, however, robbing or drifting bees could easily move those individuals to other colonies. On a local scale, beekeepers disperse mites when they make splits or move frames between hives.

A potential addition to beekeeper angst

Oddly, I find beekeepers have a lackadaisical attitude toward this new mite. Many argue that we already have widely available mite controls, we’ve developed hygienic bees, and we practice management techniques such as brood breaks — all things that can help control both varroa and Tropilaelaps.

Others believe that because Tropilaelaps cannot survive over 36 hours without honey bee brood, it would be impossible to distribute them around the globe. But that’s naïve, like saying another pandemic couldn’t happen. All it would take is one small mutation to negate that problem, and one mutation isn’t hard to imagine in a constantly reproducing population of billions.

Another group of beekeepers argues that we already have enough to worry about, so why bother? Well, that’s the truth. But not considering the possibility won’t make Tropilaelaps go away. Thinking is free, and this is a good topic to ponder.

The recent statistics on colony losses are sobering. Overall, we’re not doing great with the problems we already have, so I can’t imagine we could add another mite and not notice. Especially one that breeds faster, moves like lightning, and destroys a greater percentage of colonies than varroa. Surely, the arrival of Tropilaelaps would not improve our colony loss rate even if we prepare — let alone ignore — the possibility.

Don’t panic, but stay informed

Although I agree we shouldn’t panic, I think we should take the threat seriously enough to educate ourselves about it. There’s no reason not to learn the signs of an infestation, the mite’s life cycle, how it impacts honey bees, and the control methods that are now working. 

The fact remains that the Tropilaelaps mite plays nasty with honey bees, so anyone who cares about Apis mellifera should learn and prepare — at least mentally — for its arrival. If we sweep this mite under the carpet, if we “wait and see” because it’s not here today, we may pay dearly tomorrow.

In an apiary ruled by Tropilaelaps, any beekeeper who is “a little late” with mite meds or brood breaks won’t be able to squeak by — they breed too fast for that. But if I’m wrong and Tropilaelaps turns out to be “just another mite,” then I will celebrate. Sometimes it’s best to be wrong.

Notes and references

1. de Guzman LI, Phokasem P, Khongphinitbunjong K, Frake AM, & Chantawannakul P. (2018). Successful reproduction of unmated Tropilaelaps mercedesae and its implication on mite population growth in Apis mellifera colonies. Journal of Invertebrate Pathology, 153, 35-37. https://doi.org/10.1016/j.jip.2018.02.010
2. Here, I use the term “phoretic” loosely. If the mites are doing more than just riding, i.e. they are also eating, phoretic may be the wrong term. However, the word is commonly used to describe varroa mites that are riding on adult bees regardless of feeding status, as I’ve used it here.
3. https://bee-health.extension.org/varroa-mite-reproductive-biology/
4. https://www.mdbeekeepers.org/product/dr-ramseys-mite-research/
5. Ashley N, Mortensen AN, Burleson S, Chelliah G, Johnson K, Schmehl DR, and Ellis JD. 2022. Tropilaelaps Mite Tropilaelaps Spp. Delfinado & Baker (Arachnida: Mesostigmata: Laelapidae). University of Florida, IFAS, Entomology and Nematology Department, Featured Creatures, EENY 568.
6. Pettis, J. S., Rose, R., & Chaimanee, V. (2016). Chemical and cultural control of Tropilaelaps mercedesae mites in honeybee (Apis mellifera) colonies in Northern Thailand. PLoS ONE, 12(11). https://doi.org/10.1371/journal.pone.0188063

About Me

I backed my love of bee science with a bachelor’s degree in Agronomic Crops and a master’s in Environmental Studies. I write extensively about bees, including a current column in American Bee Journal and past columns in Two Million Blossoms and Bee Craft. I’ve endured multiple courses in melittology and made extensive identifications of North American bees for iNaturalist and other organizations. My master beekeeper certificate issued from U Montana. I’m also an English nerd. More here.