by Dennis van Engelsdrop
If we look at varroa mites right now, we’ll see that it has a worldwide distribution. This map was made in 2010 and since then, Madagascar and most of Africa, have varroa mite but they don’t seem to cause as much trouble there as other places. Hawaii has varroa mite now but there are only a small places that don’t like the island of Newfoundland, islands in the Caribbean, but mostly, mites are everywhere we keep bees except for Australia.
Varroa destructor did not come from apis mellifera, the European honeybee. It originated with apis cerana, the Asian honeybee. In apis cerana, varroa caused very little damage. In fact, not all varroa mite in apis cerana are the same. There are these different haplotypes, or varieties, of varroa mite. One of those is the Japanese haplotype, the mite that was introduced into Paraguay and was the first North and South American variant. That variant is not very lethal; it’s gentle. Unfortunately, there’s the Korean haplotype, which predominates the world today. It’s more aggressive, reproductive, and successful type of varroa mite.
In 1904, varroa jacobsoni was first described as apis cerana by scientists. When a scientist names something they have a voucher specimen but it turns out there were several types of species of varroa and the varroa we had originally was not varroa jacobsoni, which is where the new name varroa destructor because scientists found out it was a different variation. Both varroa jacobsoni and cerana live well on apis cerana. Apis cerana is the Asian honeybee; a much smaller bee with smaller colonies. In terms of honey production, it produces maybe 2 kilograms of honey a year, so it’s not very productive. But, the bees live with mites very handily. One of the reasons they do is that the capping, the length of development time for these bees, only means that mites that go into droned brood will be reproductive. Otherwise, the workers develop so fast that even though the varroa mite will invade that cell, it won’t be able to have any children. Also, if that drone brood has more than one invading mite, somehow the bees sense that and they’ll cap that drone brood very heavily with a heavy capping and that prevents that drone from emerging (you’ll see these distinctive little holes). So basically, they’re exhuming that drone and the mites in the cell so then both drone and the varroa die. This apis cerana has developed this behavior which allow it to control the mite in its natural habitat. We have to be very careful about bringing stuff in; that people have done more to spread this mite than anything else. We started here in Indonesia in the 1900s then the Russian soldiers brought apis melliferae (the European honey bee) into Asia and the mite jumped species. The Russian soldiers brought some of these colonies back into Germany and we can actually base the spread of mites distributing across Europe because they’re right beside a military base outside of Berlin. The Russians brought them over to Germany and it spread throughout Europe. We’ve had introductions into Brazil and in the 80s, we saw introductions into the US and Canada, and Hawaii in 2007. We can see that it’s spread internationally because of human trade.
So what causes it? We can see the shipments of queens in packages. We also know that moving colonies is a great way of moving things around the world. We’re reliant, in this country and many others, on a moveable pollination force. Half the colonies in this country are in almonds. That means if there’s any new disease or any new parasite that gets into the country and into the almond orchards, they’re across the country in that next year. We have to be on the lookout for new diseases very vigilantly to stop it before it becomes a national issue.
We talk about trucking our bees over long periods of time, but to give it some perspective, us moving bees from Florida to California is the same thing as moving bees from South Korea to Bangladesh twice a year. Or going from Brussels to Moscow twice a year. This is a huge amount of distance of travel with our bees several times in the year. In our survey, we consistently show that people who are migratory beekeepers lose fewer bees than non-migratory beekeepers. So certainly, we think some of the spread, especially in Europe, can be traced along the beekeeper migratory rates. In Europe, there’s a big migration into the Black forest to collect honey dew. The migratory nature is what makes the industry necessary. But even without people, mites are able to spread pretty quickly. There was a great study done in Germany where they had an infested colony and they watched colonies placed in several time points and distances from that hive to see when they would become infested. So within one month, you saw colonies within 100 meters infested; within two and half months, five hundred meters away were infested; within three months, 6 or 7 kilometers away. This is a huge ability to move very quickly across the landscape because a lot of those mites that were moving are on drones, who are famously unfaithful — they’ll go to any colony they want.
How Varroa Mites Feed
Let’s go into the biology of the varroa mite and how it feeds. We have the mites who live on the adult bee, the phoretic stage. When the bees are feeding the larva, they come in to smell, and when the smell is right they’ll jump off, bury underneath that larvae, and live in that brood food until it gets capped. The mite will then crawl up and start to lay eggs. Eventually, all her daughter mites that are fully matured when their outer cover is solid and brown — otherwise it gets dehydrated and the exoskeleton is there to keep it wet — will come out.
Here we have the life stage of a worker bee: 3 days as an egg, a larval uncapped stage — a growth stage when the nurse bees are feeding them like crazy and they’ll get fat. It’s the only stage in which the bee grows — it gets capped and becomes the pupil stage — it spins a cocoon of that larva, it goes to the bathroom for the first time. It then undergoes this amazing transformation because all of its organs dissolve and becomes a bag of fat, which gets organized into legs, arms, hearts, organs, etc. It’s a remarkable time for the pupil bee. The adult emerges somewhere around day 21. The female mites on the nurse bee enters the cell a day or two before capping. After capping, about 3 days later, it lays its first egg. It then lays an egg every 30 hours thereafter. However, only those female mites that get sparitized will survive hatching out with the emerged worker bee. The first egg she lays is unfertilized, exactly like honey bees, where fertilized eggs become female and unfertilized eggs become male. So that male comes out and becomes a white color who will always remain that color. They will die after the adult bee emerges. The female eggs gets laid and they’ll mature. They’ll then mate with their brother as a result of that. Some of the mites don’t get mated, so they’re virgins. That means they could invade another cell, lay male eggs, and mate with their son. Generally, those are not successful. About 10% of mites haven’t been mated and will lay duds; they’ll just lay males. Are there ways that we can induce this sterility so the mites aren’t mating? We have phoretic mites that feed on larval bees and if the larva smells right they’ll stay in that cell, but if it doesn’t, they’ll leave with the worker and go on to another properly aged cell. Right after it gets capped, that fifth in-star larva releases a special scent and that scent allows the varroa’s ovaries to activate. It starts to produce its first age. If it doesn’t get that shot of smell in the first 24 hours, the ovaries don’t develop and she’ll never start laying eggs. That activation is a really important step.
You can see the male is much smaller than the female. He actually will court her and spends a lot of time figuring out where their sex organs meet. They have the transfer of sperm, she stores it, just like a queen bee does, to fertilize the eggs for the rest of his for life. The fact that it’s the length of time under the cappings is what limits the number of mature mites that can develop. The longer you’re under the cappings, the more your daughters are going to be fully sparitized. This explains why varroas prefer drone bees. If you can get under a drone cell that’s capped for 13-16 days, you can have, on average, 2.2-2.6 kids that are fully sparitized and able to survive with the drone that emerges. The workers are only capped between 12-12.5 days, which means they have half as many kids that can successfully emerge. That’s why drones are this great breeding ground and that’s why varroa mite really do prefer drones. Varroa mites do invade queen cells, which aren’t capped long enough to have any viable offspring.