It’s not about mites any more
“If one more speaker starts yapping about Varroa mites, I’m gettin’ up and walkin’ out!” The grumbler sitting next to me in the audience didn’t know me and didn’t know I was the next speaker. And yap I did. My topic? Deformed wing virus.
Honestly, I can’t blame the guy for feeling that way. I too am tired of mites. But in spite of endless boring discussions, I think we are still missing the point when it comes to managing Varroa. Mites are nothing more than bad guys with hypodermic needles. The true demon, deformed wing virus (DWV), rarely gets a mention.
I should clarify by saying DWV rarely gets a mention outside of academic circles. Among scientists and researchers, DWV is demanding more attention and taking its rightful position as villain-in-chief. What I’ve learned has caused me to re-think the way I handle mites. By looking at Varroa-mediated collapse not as a mite problem but as a virus problem, I’ve been able to better manage my bees.
The dream team
Many researchers agree that the ultimate destroyer of honey bee colonies is DWV, not Varroa mites. Apparently, when the virus is not present, honey bee colonies can withstand fairly heavy mite loads over long periods of time. Of course, having mites is not an ideal situation because mites feed on bees, weakening them in the process. Still, quick and total loss of a colony is not the usual result from Varroa mites alone.
Likewise, a colony of honey bees without Varroa mites can harbor DWV without succumbing to the disease. It appears that healthy honey bees have a natural resistance to the virus when it spreads via normal channels. What is normal? Normal channels include vertical transmission from queen to egg, and horizontal transmission via trophallaxis from worker to worker or from worker to larva1.
However, when both Varroa mites and DWV are present at the same time, the virus is transmitted through the bite of the mite into the tissue of the bee. Compare this to other diseases. You can catch the flu by breathing air or eating food that is tainted with the germs, or you may be able to fight it off. However, if you are actually injected with the pathogen, you have a much greater chance of becoming sick.
Casual contact or injected dose
In a similar way, honey bees can get DWV from casual contact within the hive, or they can be injected with it, courtesy of the Varroa mite. You will sometimes see the term “vectored infection” when the writer is referring to pathogens transmitted by the mites. In biology, a vector is an organism that spreads a disease without contracting the disease itself. Just as an anopheles mosquito is a vector for malaria and a deer tick is a vector for lyme disease, a Varroa mite is a vector for deformed wing virus.
Another term you sometimes see is “covert infection.” Basically, covert means hidden. So, depending on the author, it may refer to an infection that travels from bee to bee without a vector (the infectious pathway is hidden) or it may refer to a type of infection that does not produce obvious symptoms (signs of the disease are hidden).
Timing can make a difference
As I understand it, when the virus is transmitted naturally within the hive—from bee to bee—it is less likely to produce deformed wings and shortened abdomens than when transmitted by a mite.
At least part of the reason may be timing. If a bee contracts the disease as an adult, she will not get deformed wings and a shortened abdomen because those parts are already fully formed. Other aspects of the disease will still result, including a weakened immune system and a shortened lifespan.
In practical terms, bees can be infected with DWV without showing obvious signs of the disease, even if the disease was transmitted by mites. I’ve heard beekeepers say they saw no sign of deformed wings in their colony, therefore it wasn’t DWV that killed them. We need to remember that when and how the bee contracted the disease affects the visible symptoms, so a bee may die of the disease without having obvious physical deformities.
Is the DWV getting worse?
Many beekeepers are finding that to keep their colonies alive, mites have to be treated more often than they used to be. Some who used to treat once a year are now treating two and three times a year. This increase appears to be related not to stronger mites, but to increased virulence of the DWV. In other words, the virus seems to have increased in potency such that a colony cannot withstand as many mites as it used to. Since a colony can succumb to DWV with a just a small number of mites, it becomes necessary to keep the number of mites per hive at a very low level, much lower than in the past.
Why this is happening is unclear, but it may simply be a matter of numbers. As the virus spreads, more individuals exist and a new opportunity for mutation occurs with each replication. So, basically, with more individuals you have more chance for change. Some of these mutations could have increased the virulence of the disease to its host, the honey bee.
Another cause could be migratory colonies. If you have a chance mutation in, let’s say Florida, and another in California, instead of those being local problems, they are soon continental problems as we—human beings—assist the DWV in spreading the mutations to more and more bees.
Varroa and DWV working together
Of course, there are most likely other explanations as well, but the fact remains that the disease appears to be getting more deadly. Right now, the only way we have of slowing the disease is controlling the mites that carry it, so we are in something of a bind since controlling the mites hasn’t gotten any easier. Worse, some research has shown that the Varroa mites actually do better in the presence of high viral loads, because the disease keeps the bees weaker and less likely to defend against the mites2.
In my own apiary, I stopped thinking about managing mites and began to think about managing virus. This has helped me, especially with the timing of control measures. It also reminds me that killing the mites doesn’t kill the virus.
The most obvious case is the classic fall management conundrum. Most of us don’t want to treat colonies in August. Depending on where we live, it may be too hot, honey supers may be in place, winter is far off, the colony is huge and healthy, you’re hoping to pick up the fall flow, or you’re going on vacation. All of these reasons, and more, interfere with treating mites at the right time.
Instead of thinking about mites, think of DWV. Remember that here in North America, your long-lived winter bees will begin emerging in September and October. If they have deformed wing virus, they will not be able to care for the winter colony and, even if some survive the winter, they will pass the virus on to the early spring brood. So reducing the amount of deformed-wing pathogen must be completed by the end of August. Killing mites late in the year after the bees have already contracted deformed-wing virus, doesn’t help. By that time, the bees will succumb to the virus whether mites are present or not.
High winter losses
The scuttlebutt I’m hearing from beekeepers across the country is that pockets of high colony loss, up to 80 or 90%, are occurring in some areas. These losses are affecting commercial beekeepers, hobbyists, conventional, and natural beekeepers across the board. In other areas, sometimes relatively close by, losses are apparently at normal levels. What is going on? My own guess is that the heavily hit areas are hosting more virulent strains of DWV. I have no proof of this, but it will be interesting to see what happens next. Will these hard-hit areas recede and disappear, or will they expand?
As with many aspects of beekeeping, it is difficult to discern cause and effect. So when other factors come into play—exogenous variables like bad weather and poor forage—it is easy to assign colony loss to them. But throughout history, honey bees have shown amazing resilience when it comes to harsh environmental conditions like cold, snow, rain, heat, drought, wind, lack of forage, predation, and even viruses. But injected viruses? I just don’t know.
But even the classic signs of collapse by Varroa make more sense when you think of them as signs of collapse by virus. When a large colony collapses quickly in the fall or winter with plenty of food, a small and spotty nest, and practically no dead bees, it sounds more like disease than parasite.
Let me emphasize that much of this post is pure speculation on my part based on journal articles I’ve read and loss reports I’ve studied. You may come to different conclusions. Still, I think looking at the entire mite problem as actually a virus problem may help some beekeepers modify their management strategy. It’s time we evaluate the disease, not just the vector. After all, it was never about the mites.
Honey Bee Suite