Tropilaelaps mite
A worse threat than varroa?

Since the varroa mite met the European honey bee in the 20th century, bee populations around the world have been decimated. Now the honey bees may be facing a new threat, one which shares many characteristics as the varroa mite - Tropilaelaps (or tropi mite).

There of 4 species in the Tropilaelaps genus, 2 of which (Tropilaelaps clareae and Tropilaelaps mercedesae) are considered serious threats to the European honey bee. Tropilaelaps hail from tropical and sub-tropical Asia, where their primary host is the Giant Asian honey bee. Just like varroa, Tropilaelaps is an ectoparasitic mite that feeds on the heamolymph of developing bees. However, unlike its well-established cousin, the Tropilaelaps mite does not additionally feed on adult bees, as its mouthpiece is unable to penetrate the exoskeleton membrane wall. Due to this, the tropi mite spends most of its life in brood cells. The damage caused to developing bees is like that attributed to varroa. This includes fatality (up to 50% of brood), stunted growth, lower body weight and severely reduced lifespan. Tropilaelaps also vector viruses such as DWV.

Lifecycle

It takes approximately 7 days for the tropi mite egg to mature to adulthood. The shorter reproduction cycle presents a significant problem for beekeepers, as it results in even faster rates of infestation than varroa. In its native geography, Tropilaelaps is considered a worse threat than varroa and it is believed that if both mites infest the same colony, the tropi mite will outcompete varroa. As the young, parasitized bee emerges from the uncapped cell, 3-4 hatched tropi mites, as well as the foundress, also make their way out. The majority of the newly hatched tropi mites mate and re-enter new brood cells within 1-2 days after emerging. Without feeding, they die within 3 days. Tropilaelaps require brood to be present to stay alive, which is a limiting factor for their spread.

Detection

As the mite spends most of its life in the brood cell, detection is difficult. Successful varroa detection methods, such as the sugar shake or alcohol wash are inneffective. Instead, beekeepers must rely upon brood uncapping, bump testing, sticky board inspection and colony examination. Tell tale signs of the Tropilaelaps infestation are irregular brood patterns and perforated brood cappings caused by sanitary behaviour of the bees. It is common for heavily infested colonies to abscond from the hive.

Defence

In geographies with high infestation rates, beekeepers are forced to treat with miticides every two weeks. The miticide cycle (against varroa) commonly used by European and U.S beekeepers (2 to 3 treatments per year), is far too infrequent to stop infestation. Furthermore, some commonly used miticides, such as Amitraz have demonstrated no effect on Tropilaelaps populations. As such, stronger border controls are viewed as the best weapon against the spread of the pest. Scientists are conducting research to evaluate other forms of defence such as hyperthermy, which may prove to be more practical and effective solutions.

Spread and distribution

The spread of Tropilaelaps can occur due to beekeeper management practices, such as moving brood combs between colonies or transporting colonies, or from bee behaviour. The biology of the mite does support phoretic behaviour, however, as it does not feed on adult bees, spread via robbing, absconding, swarming and drifting are not as common as with varroa. This may be one reason as to why the tropi mite has not spread at the same pace as varroa.

Despite their geographic distribution increasing significantly over the last 40 years, and several cases in locations such as Kenya, Tropilaelaps are still found only in Asia. However, many scientists fear that due to longer brood seasons, technical developments in global trade, and the mites rapid reproductive cycle, it is just a matter of time until the tropi mite makes its way to the rest of the world.

Conclusion

While the potential impact of Tropilaelaps remains to be seen, the threat is unquestionable. Strict border controls may be the best solution for the short term, but how sustainable are they for the long run? It is encouraging that scientists are looking for other sustainable solutions, so that that a legacy like the varroa mite, may be avoided.

Resources

Chemical and cultural control of Tropilaelaps mercedesae mites in honeybee (Apis mellifera) colonies in Northern Thailand

Jeffery S. Pettis,  Robyn Rose,  Veeranan Chaimanee, Xiao-Yue Hong, 2017
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5681254/

Tropilaelaps mites
Bee aware org
https://beeaware.org.au/archive-pest/tropilaelaps-2/

Ecology, Life History, and Management of Tropilaelaps Mites
Lilia I. de Guzman, Geoffrey R. Williams, Kitiphong Khongphinitbunjong, Panuwan Chantawannakul
Journal of Economic Entomology, Volume 110, Issue 2, April 2017, Pages 319–332,
https://academic.oup.com/jee/article/110/2/319/3063341

Tropilaelaps mite: an emerging threat to European honey bee
Panuwan Chantawannakul, Samuel Ramsey, Dennis van Engelsdorp, Kitiphong Khongphinitbunjong, Patcharin Phokasem
https://www.sciencedirect.com/science/article/abs/pii/S2214574517300810

Vatorex, Grant Morgan
25 January, 2022
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