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 or 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 to 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 Tropilaelaps?
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, ….