A beekeeper called Kasia told me recently that we are not talking enough about possible new threats to beekeeping in the form of exotic pests and diseases, at least not locally in our beekeeping associations. I agree with her and decided to raise awareness.
Varroa destructor remains our biggest everyday challenge, but what if one or more of the Tropilaelaps (tropi) species arrive in the UK? Would you recognise one if you saw it scuttling around in one of your hives, and what would the implications be? If you thought you saw one in Scotland your first response would be to inform Scottish Government and let the bee inspectors know since this is a notifiable pest. To that end at least you do need to be able to recognise them.
You can see the differences between varroa (left) and tropi (right) in the photo, and one of the most noticeable one, apart from smaller size and longer shape, is the position of the legs. Varroa has legs on one side of its body and moves like a crab, whereas tropi has legs on both sides of its body which probably accounts for its fast gymnastic movements making it harder to spot. They are both 8-legged arachnid mites and hail from Asia.
There are at least four species of tropi which are; T. Koenigerum, T. Thaii, T. Clareae, and T. Mercedesae; but only the latter two have so far have become parasites of the Western honey bee, Apis Mellifera. In their native Asia, tropi are parasites of the giant honey bees but these large bees evolved coping strategies to resist them such as leaving home and migrating to another area, nesting in the open, and keeping infested brood entombed1..
In the middle of last century, scientists first discovered tropi on rats and there has been at least one siting of them in a mouse nest1. This discovery has serious implications for tropi overwinter survival, but let’s look more closely at the biology and other differences between varroa and tropi first.
Tropi are parasites of immature honey bees only and do not feed on adult bees during the short period that they are hitching rides to the next cell. They spend only between 4-9 days in the phoretic/dispersal stage so any treatments would need to target mites in the cells. Whereas varroa tend to open a feed hole or two on larvae which are kept open for others to feed through, tropi tear open many feed holes causing much more larval damage. If larvae do survive, they suffer greater deformities than the stunted growth caused by varroa. They both spread viruses, including DWV, and cause colonies to collapse.
Rapid Population Increase.
However, although the life cycle of tropi is shorter than that of varroa, fertility is higher. Tropi reproduce in both worker and drone cells and have a short reproductive cycle of about a week then mother and both male and female mites leave the cell. Varroa males die within the cell and never leave. So, the population of tropi increases at a much faster rate than varroa.
Worryingly, research, reported by Dr Scott Mc Art2,demonstrates how tropi mites reduce the foraging abilities of adult honey bees. In experiments, non-infested bees flew for more than 3 times longer, and almost 4 times further, than those infested with tropi. It took the latter 50% longer to get home thereby increasing the negative impacts on foraging.
Tropi Survives Harsh Winters.
COLOSS studies show that tropi can be found in Iran, Russia, Uzbekistan, Afghanistan, China, South Korea, and Papua New Guinea (PNG). It could easily arrive and thrive in the West. After all, Australia is a major honey bee exporter and very close to PNG. Despite stringent security and testing of bee imports it could slip through undetected.
Back to the big question of tropi’s survival over long broodless winters. Some literature suggests that tropi has not adapted to broodless periods so should not be a problem in our northern climes, but how is it that they can survive long cold winters in Russia and Iran? Researchers in South Korea1 are looking at this enigma and considering mice and rats as enablers and the possibility that tropi can survive over winter on rodents. We all know how mice like to cosy in to bee nests over winter. And we ought not to forget the bubonic plague of the Middle Ages when rats played host to the plague-transmitting fleas and causing mayhem by traveling stowaway on sailing ships and spreading disease in Europe.
Scientists on the Case.
The good news is that the scientists are working hard on this problem right now and Dr Samuel Ramsey is leading a research project in Thailand this year. You can listen to the superb account of his discoveries so far in this video. https://www.youtube.com/watch?v=piiaCqjnRR8
One of my favourites and most interesting UK journals is the BBKA News, and it is currently focussing on current threats to beekeeping such as Asian hornet, and potential threats such as tropi. August’s issue had so many useful articles on Asian hornet that I filed the whole magazine in my Bee Health folder rather than the individual pages. July’s issue features an inspring article by Maggie Gill3, Regional Bee Inspector for Wales, describing recent work in Thailand by herself and four colleagues sponsored by Bee Diseases Insurance Ltd. I accessed an interesting recording of the visit on the BDI’s website www.beediseasesinsurance.co.uk . It really was an exciting fact finding mission and here is the link: https://youtu.be/7e8EHLlHNds
The team’s goal was finding out if the current field and laboratory monitoring procedures currently used in this country were effective enough to detect tropi, and if there were any scenarios that allowed it to survive long enough to reach the UK. They found that the “bump” method used to bang frames onto a white collecting tray onto which the tropi fell was hard on the brood. When they returned after 24 hours to the bumped frames, they noticed large amounts of dead brood on the floor that had been pulled out by the bees. They found it very hard to find tropi and concluded that the best methods of detecting it were uncapping brood using tweezers, and monitoring mite drop on sticky floor inserts. These were time consuming but successful. They found that tropi didn’t survive for long on clothing. But what about survival rates on rats? It will be interesting to find out if rodents do indeed play a role in mite survival over winter.
How to Prepare.
The best thing we can do now is learn as much as possible about the biology and life history of tropilaelaps, no matter which part of the world we live in, and be prepared for something much worse than varroa.
1 McAfee, A, Another Mite to Fight, American Bee Journal, December 2019, pp. 1343-1345.
2McArt, S, How Tropilaelaps Mites Reduce Foraging Efficiency of Adult Bees, American Bee Journal, September 2021, pp.981-984.
3Gill, M, Tropilaelaps: An Underestimated Threat? BBKA News, July 2023, pp.222-224.