We're the bad guys - again!

[gavin]

To be fair, he only offers the possibility (as you rightly point out) along with other possible mechanisms -hence the reason I highlighted my word 'One'!

True, however he gave reaons why the other two possibilities discussed were less likely and seemed to favour this one.

[prakel]

Is anyone still with me? Should we shift this to Bee Health?

G.

Might be a good call. Amazing how two threads on different sites (but with plenty of cross over) starting from the same point can develop so differently.

[gavin]

They just wouldn't appreciate it over there.

[prakel]

True, however he gave reaons why the other two possibilities discussed were less likely and seemed to favour this one.

Yes he did! It's an interesting piece never the less, but I believe the real issue is that even if the Arnot Forest mites have either

a, developed avirulence or
b, been self selected as a result of more virulent strains dying with their hosts

the fact remains that the model is never going to work in multi-colony apiaries because you've instantly lost the necessary isolation to limit horizontal transmission!

[Jon]

Just to complicate things again,

Gav's definition of vertical transmission usually refers to mother daughter transmission of a disease like hepatitis (in humans.)

With bees it usually refers to swarming.

Abstract – The degree to which a disease evolves to be virulent depends, in part, on whether the
pathogen is transmitted horizontally or vertically. Eusocial insect colonies present a special case
since the fitness of the pathogen depends not only on the ability to infect and spread between individuals
within a colony, but also on the ability to spread to new individuals in other colonies. In honey bees,
intercolony transmission of pathogens occurs horizontally (by drifting or robbing) and vertically
(through swarming). Vertical transmission is likely the most important route of pathogen infection of
new colonies. Theory predicts that this should generally select for benign host-parasite relationships.
Indeed, most honey bee diseases exhibit low virulence. The only major exception is American
foulbrood (AFB). In light of current ideas in evolutionary epidemiology, we discuss the implications
of horizontal and vertical pathogen transmission for virulence of AFB and other honey bee
diseases.

Implications of horizontal and vertical pathogen
transmission for honey bee epidemiology
Ingemar FRIESa*, Scott CAMAZINE

http://www.helsinki.fi/science/ants/...26camazine.pdf

This paper

Invasion of Varroa destructor mites into mite-free honey bee colonies under the controlled conditions of a military training area

looks at how how mites move horizontally between colonies in an isolated setting.

http://www.ibra.org.uk/articles/Varroa-invasion

[chris]

Jon, if I have correctly understood it, the Fries ;Camazine paper says that host/parasite stability will most likely be achieved by vertical transmission via swarming. Greater virulence is the result of horizontal transmission which is itself encouraged by certain beekeeping practices.

My way of reading it, is that swarming is the key. I understand the various arguments against swarming, but it would appear that its suppression goes hand in hand with the necessity to treat for varroa .

[gavin]

Jon, if I have correctly understood it, the Fries ;Camazine paper says that host/parasite stability will most likely be achieved by vertical transmission via swarming. Greater virulence is the result of horizontal transmission which is itself encouraged by certain beekeeping practices.

Does anyone here agree with the authors that Varroa transmission is '+++' for vertical and just '+' for horizontal, whereas AFB is the other way round? Doesn't make sense to me, and just seems like a posteriori fitting to the data to support a hypothesis. Varroa spreads several miles (?) a year in the absence of beekeepers taking them further, depending on a reasonable spread of colonies.

I'd even question the classification of AFB as 'lethal' whereas Varroa is benign to lethal. There were plenty of observations of AFB-resistant stocks back in the time when it wasn't beekeeper or inspector-induced lethality.

[Jon]

It is very hard to get your head around this - in my case anyway.
vertical transmission should select for less virulent pathogens as a virulent pathogen will reduce colony strength to the extent that it would be less likely to swarm.
Horizontal transmission should select for more virulent pathogens as it is not critical to keep the original host alive as the pathogen can take up residence in a colony elsewhere.
Both vertical and horizontal transmission are going to be taking place at the same time in a given area and factors such as increased colony density or beekeeper practices such as making splits or nucs which are effectively horizontal transmission should shift the balance in favour of more virulent pathogens.
Increased colony density will also increase horizontal transmission through robbing or drifting.

With varroa, 80% of the mites are under cappings, 20% phoretic.
If a colony splits 50/50 to swarm, the swarm will have just 10% of the varroa load which in itself must be a significant boost.
The remainder of the colony with 90% of the mites has one or more queen cells and will be completely broodless by the time a new queen emerges, mates and starts to lay. It will be a further 8 days until the mites can get into the first cells being capped. Mites are much more vulnerable in the phoretic phase as they can fall off bees or be groomed off. Without brood, mite numbers cannot increase. (unless brought back to the colony through horizontal transmission via robbing or drifting)

I think there is some evidence that swarmy bees are more able to live with varroa but swarmy bees are no use to neither man nor beast.

AFB is a special case as it has a long lived spore which can infect a new colony decades after the original host colony has been killed so maybe it can afford to be more virulent.

[Mellifera Crofter]

As far as David Heaf's comments go, the mother to daughter transmission (vertical) takes place with natural swarming just as much as with controlled artificial swarming. He's plain wrong. Horizontal transmission describes mites leaping from colony to colony, almost always on a bee. So Varroa has both, but the spread around from colony to colony is mostly horizontal.

As to whether avirulence (less prolific types) can arise in Varroa (rather than an equilibrium be reached when the host gets better at suppressing the parasite), I just don't see how there will be selection driving that to happen. The issue I have with that idea is that any less virulent mites arising within a colony (eg by mutation) will always be outcompeted by her more virulent sisters. The competition within a colony will always cause avirulent types to be suppressed, so the advantage to the mite of 'allowing' the colony to survive never gets the chance.

... G.

It is very hard to get your head around this - in my case anyway.
vertical transmission should select for less virulent pathogens as a virulent pathogen will reduce colony strength to the extent that it would be less likely to swarm.
Horizontal transmission should select for more virulent pathogens as it is not critical to keep the original host alive as the pathogen can take up residence in a colony elsewhere.
...
With varroa, 80% of the mites are under cappings, 20% phoretic.
If a colony splits 50/50 to swarm, the swarm will have just 10% of the varroa load which in itself must be a significant boost.
The remainder of the colony with 90% of the mites has one or more queen cells and will be completely broodless by the time a new queen emerges, mates and starts to lay. It will be a further 8 days until the mites can get into the first cells being capped. Mites are much more vulnerable in the phoretic phase as they can fall off bees or be groomed off. Without brood, mite numbers cannot increase. (unless brought back to the colony through horizontal transmission via robbing or drifting)

...

Do I understand the two of you correctly? Gavin is talking about the idea of varroa nature changing as a result of colony efforts (and doesn't think it's possible), and Jon is talking about the effect of swarming on varroa presence - not varroa nature. You may have to correct me again, Gavin.

Kitta

[gavin]

Pretty much, Kitta. Jon started talking about (in his first three sentences) selection possibly changing Varroa then continued in the way you describe. He seems to accept that selection will shift Varroa genetics whereas I think that the dynamics within the hive will predominate and the theoretical shift due to whole colony behaviour simply not happen.

All good clean fun but I don't really know who is right.