VSH: Varroa Sensitive Hygiene

We do not yet fully know how hygienic workers recognize the presence of a reproducing mite in a capped cell. Probably it is a smell from the wounded / diseased pupa, and not an odour from the mite itself. Mites that do not start reproduction will not be discovered and removed.  Hygienic bees indeed showed to be more sensitive for odours than non-hygienic bees. In breeding programs, aimed at producing colonies with improved hygienic behaviour often a pin-test or a frozen brood test is incorporated. In these tests the researcher wounds or kills a number of pupae inside capped cells by pinching with a needle or by freezing an area of brood with liquid nitrogen. Next the rate at which the colonies remove the damaged or killed brood is taken as a measure for their level of hygiene, and this is used for selection. Colonies that showed improved hygienic behaviour were however not consistently more resistant to varroa mites. Which may implicate that VSH at least partly works in a different way than the hygienic response to killed or injured pupae.

Varroa never comes alone: viruses play their part  

The deformed wing virus complex (a virus complex of genetically very closely related viruses which can partly merge into each other), consisting of DWV, VDV-1 (varroa destructor virus-1) and KV (Kakugo virus) plays a crucial role in the final collapse of colonies having a severe infestation of varroa mites. In 2012 Martin et al. showed that the DWV virus complex quickly changed composition upon the arrival of Varroa destructor on a few islands of Hawaii: the genetic diversity of the virus complex decreased rapidly, and only a very virulent remained. The same will have happened likewise in all places where the varroa mite had appeared before. This may also be one of the explanations why nowadays a varroa infestation does more harm than the same infestation would have done in the beginning years of the varroa epidemic 30 years ago.

VSH bees know the difference

Shöning et al (2012) investigated to which level VSH workers discovered and removed mites with ‘normal’ DWV in comparison with mites carrying the much more virulent DWV/VDV-1 recombinant. VSH bees were found to preferentially   open and remove the mites with the latter virus form. Mites that were reproducing (and which therefore did collect haemolymph from the pupa) but not carried the latter virus form were mostly neglected by the VSH bees.This appears to be a good strategy at colony level, since removing any pupa hosting a mite in its cell would probably be a too big sacrifice. Maybe this may also explain part of the difference between VSH and ‘pin-test-hygienic’ bees, the latter ones not discriminating virulent and non-virulent cell contents.

Transfer of the virus from mite to pupa selects for the virulent form

Ryabov et al. (2014) showed in an elegant and very complete study that the transfer of DWV from mite to bee (pupa), which happens when the pupa is pierced or bitten by the mite, resulted in a strong amplification of only the virulent form of the virus inside the bee, also when the mite carried a mixture of many different forms of the virus. They were also able to show that the same happened when the virus mixture was introduced into the pupa by injection, again only the virulent form started to reproduce inside the pupa. They concluded that the high virus titres (number of virus particle, in billions) of the virulent recombinant of DWV did not arise in the mite but in the pupa, although the viruses also replicate (but slower) in the mite. However, it was because of the action (biting the pupa and transmitting the virus mixture) of the mites. Especially these pupae become symptomatic (deformed wings), but also: especially these pupae are detected and removed by VSH bees.

The combat: virulence and resistance

The fact that through the involvement of the mites the virus complex becomes increasingly virulent also causes that the damage done by the mites increases. However, VSH is put in action  in this battle not against the varroa mites per se, but targeted against mites containing the virulent virus recombinant. Mites that do not transmit the virulent virus remain undiscovered or at least left unaffected. Moreover, VSH appears in this way to help varroa mites: by removing the virulent virus form the damage done by the remaining mites becomes less, and more mites can reproduce in a colony before a collapse follows. Maybe similar to the situation in the early days of varroa in Europe.

DWV/VDV-1 takes up the gauntlet

If VSH indeed combats the virulent form of the DWV/VDV-1 complex (by especially removing these) then VSH is principally a resistance behaviour against the virulent virus. In a recent very elegant study by Mondet et al (2015) carried out in France and New Zealand, the transcription of genes in the antennae of bees showing VSH compared to those never showing hygienic behaviours was studied (the groups were selected by observation of behaviours in an observation bee hive). It was shown that many genes playing roles in sensing odours were increasingly active in VSH bees, as well as metabolic and motor activity genes. The pattern of activity of the transcriptomes was more alike that found in nurse bees than in forager bees (the age of VSH activity, 15-18 days age is between that of nurses and foragers, (Wilson-Rich et al, 2009). But in addition a very remarkable outcome was that antennae of bees not showing hygienic behaviours contained high loads of virus particles of DWV/VDV-1 virus, much higher than the antennae of bees showing hygienic behaviour. The hygienic and non-hygienic bees originated from the same colony, so it seems that the virus in the antennae inhibits the expression of those genes that regulate VSH. The virus takes the VSH threat seriously, and takes up the gauntlet!  

In short, VSH is a trait of bees in the battle against varroa mites, but because varroa seldom comes alone but almost always in company with (virulent) viruses the VSH trait is a weapon against these viruses.  This research also shows that host parasite relationships are complicated and that diverse players in such webs can affect each other and may shift balances. Maybe this also holds for some other bee traits which play a role in varroa resistance and tolerance, like grooming and the reduction of the reproduction success of mites in resistant bee brood cells.