Varroa isn’t the only mite that has jumped hosts to Western honey bees
The video was nothing short of frightening. Tiny, brown, slender-bodied creatures were scurrying hither-dither on the rims of the comb, and they were scurrying fast. They were everywhere, and they had places to be. I listened, sitting in the audience at Apimondia, Montreal, as Dr. Samuel Ramsey introduced this strange new parasite while we all watched the clip. I mourned for the mite-stricken colony, which was near certain death; however, the infesting mites were not the usual suspects. They were not varroa, but something worse. They were Tropilaelaps. Tropilaelaps mites were first identified on rats near honey bee colonies in the mid-20th century.1 Now, we know that the mites come in at least four species: T. thaii, T. koenigerum, T. clareae, and T. mercedesae, with the latter being the biggest threat to managed bees.2,3 They are normally parasites of the giant honey bees (Apis breviligula, A. dorsata, and A. laboriosa), but at some point, they jumped hosts to A. mellifera. The mites are a nuisance to the giant honey bees, but infestations are generally not severe because the giant honey bees have evolved resistance mechanisms, such as colony migration, brood entombment, and open nesting, all of which our beloved A. mellifera lacks.he video was nothing short of frightening. Tiny, brown, slender-bodied creatures were scurrying hither-dither on the rims of the comb, and they were scurrying fast. They were everywhere, and they had places to be. I listened, sitting in the audience at Apimondia, Montreal, as Dr. Samuel Ramsey introduced this strange new parasite while we all watched the clip. I mourned for the mite-stricken colony, which was near certain death; however, the infesting mites were not the usual suspects. They were not varroa, but something worse. They were Tropilaelaps.
This probably sounds too familiar. An almost identical scenario occurred with varroa jumping hosts from the Asian honey bee, A. cerana, to A. mellifera. Just as the giant honey bees have evolved strategies to resist Tropilaelaps, the Asian honey bees have evolved strategies to resist varroa. Unfortunately, A. mellifera is not so hardy against either parasite. When varroa spread west, it devastated the industry. Today, Tropilaelaps has not yet spread to Europe, Australia nor the Americas, but it is probably only a matter of time.
In his talk, Ramsey warned that we know dangerously little about Tropilaelaps, and that we should be doing more to learn about it. A lot of what we do know is information we’ve borrowed from varroa, but sometimes even that can be utterly wrong. For example, for decades we thought that varroa fed only on hemolymph, but because of Ramsey’s research, we now know that varroa actually feeds on the bee’s fat body.4 What varroa eats might seem like small potatoes, but it’s a fundamental shift in our understanding of the parasite’s basic biology. And the value of basic biology shouldn’t be underrated — all applications are founded by basic science — a philosophy not lost on Ramsey.
Ramsey has been developing a new way to study the life cycle and behaviour of Tropilaelaps and varroa mites using carefully-crafted brood cells with built-in windows to peer inside. He lets the mites naturally invade, then records their activities with a high-resolution video camera to study their behaviors. The mites have no privacy, and the videos he showed us were astonishing. It felt like getting a glimpse at the hidden world of torture beneath the cap. The creeping mites made the freshly capped larvae writhe grotesquely. It’s all the larvae can do, as they have nowhere to go. They look totally helpless as the mites go about their business, tearing and poking feeding holes, laying eggs, copulating, and starting their families.
Sometimes, borrowing information from varroa is fine, but Tropilaelaps is also a unique monster. For example, with his videos, Ramsey and his colleagues can see that while varroa creates one or two communal feeding holes on a developing bee for the whole family to use, Tropilaelaps tears many holes all over the larva’s body. Other researchers have found that while brood parasitized by varroa are usually stunted but don’t die, considerable brood death can be caused by Tropilaelaps. If they do live, the emerged adults typically suffer from greater damage or deformities. Furthermore, while varroa can feed on adult bees and live for weeks as hitch-hikers, Tropilaelaps can’t feed on adults and they re-infest new brood cells within a few days. And these differences in infestation strategies will affect how we (and the bees) can manage the parasite.
Some of Tropilaelaps’ quirks are both a blessing and a curse. Since the mites spend very little time outside brood cells, only treatment methods that can target brood cells are expected to be effective. But the fact that Tropilaelaps can’t piggyback for very long (four to nine days) also means it’s tougher for them to survive the journey with a sea-going swarm (ships accidentally transporting swarms is one way that dispersal can fly under the radar) or illegally imported queens and packages, making it easier for countries to protect against invasion.
For the same reason, we didn’t expect Tropilaelaps to persist in geographic regions that have broodless winters, since this means the mites should be systematically wiped out once a year (unlike varroa, which can overwinter wedged between the bee’s abdominal tergites, or exoskeletal plates). This was good news for Canada and the Northern U.S., but we have already been proven wrong. If Tropilaelaps does move west, it will be a formidable foe.
In Asia, where Tropilaelaps and varroa coexist, Tropilaelaps is the more prevalent mite. It is the one on which beekeepers focus ….
