Dennis VanEngelsdorp – Everything Varroa (Part 3)

Everything Varroa

by Dennis van Engelsdrop

Part 3

Population Dynamics

Let’s talk about population dynamics in terms of what happens with mites in your colonies over the year; at the population and colony level. We’ll also look at the field data from five years of intensive survey through various efforts, including the national honey board and the bee informed partnership efforts.

We know this female mite, smells the right larva, and comes up when it’s capped. What she does next is remarkable: she bites a hole in the developing larva, which is actually a feeding well. Immediately across from that well, she poops on the cell. That way, all of her offspring, who don’t have the mouthparts as hard as they need to to cut through that larval bee, they crawl up and smell for that fecal matter and know that the food is exactly opposite. It’s a remarkable communication structure. All those varroa are feeding over that same feeding well. Of course, that feeding well is also an opening so you can get bacterial infections there but it’s also a way the bees can introduce viruses into the colonies. One of the best signs when you look at a cell that a worker bee has just emerged from, you’ll see an immature mite because it’s still white, but you’ll often see a lot of fecal matter. In fact, if you have a dead-out in the spring or middle of the fall, those are probably your strongest colonies. Those are usually all varroa mite. The best way to know is if you look on the bottom and there are white dots, which are called fecal pellets. That’s a good way of knowing you have high mite levels; you have a lot of varroa poop, you have a lot of varroa mites.

But of course in that feeding, we transmit a lot of bee viruses. One of the things my lab has been doing is the National Honey Bee Disease Survey. Every state that wants to participate gets funding to take 24 samples semi-randomly selected and they send us live bees in bee boxes. We do alcohol washes for varroa mites and nosema. We’ll find the viruses and pesticides. Then we’ll look at the association of the viruses and varroa mites. As your mite population goes up, the prevalence of the number of bees in that colony that have the viruses goes up. It’s a close relationship. The more mites, the more viruses you’ll have. That’s not always true for all the viruses. For example, the Lake Sinai virus because it isn’t transmitted by varroa mites. However, for DWV (Deformed Wing Virus), it’s a very strong association. Right now, we have a hard time finding bee colonies in the country that don’t have detectable levels of varroa mite. So 90% of colonies in the country have some level of DWV. But, it’s not a very virulent virus; it doesn’t kill colonies until it has very high titer counts, which is different than the acute paralysis viruses, like APC (Acute Paralysis Virus). We don’t often find them but when we do we find them quickly. Not all viruses are the same. These viruses are driving a lot of these problems.

This next slide is the data from all the colonies we’ve sampled. Blue is for stationary and red is for migratory beekeepers with the National Honey Bee Disease Survey over the last five years. There’s a huge variation and pattern in them; if you look at the green section, that five mites per hundred, a critical number. We now think that when you hit over three mites per hundred, your bee population is in trouble. However, when you get over five mites per hundred, your bees are not only in trouble but even if you intervene, you should expect to see losses. Those thresholds are very different than the thresholds I would’ve suggested four years ago and much different than the thresholds twenty years ago. So we think that the change in thresholds are ten to twenty mites ten years ago. What’s changed are the viruses. The viruses have been in the bees before varroa. Some would say that the viruses have a benefit for the bees. Japanese researches have shown that the bees that are more likely to be the guard bees are more likely to get viruses, so in fact viruses might benefit the colonies to protect it as long as it doesn’t get too widely dispersed. As long as you have a situation where viruses aren’t getting spread from sister to sister but only from mother to daughter, because that’s the only way it’ll get spread without varroa mite, you expect those viruses to be benign. If it killed the host, it would kill itself. However, when you start to see transmission horizontally (sister to sister), which varroa mites allow them to do then you expect to be much more virulent much quicker. For example, if I make a lot of copies of myself somehow then I’m more likely to get sucked up by a varroa mite so when my colony dies I move over with the bee and invade my neighbor. There’s a lot of pressure for those viruses to mutate and become virulent. They’re the ones who are going to survive in a world through horizontal transmission. It’s not that the mites have changed but the viruses that the mites are transmitting have changed this story to low levels mites that are causing measurable damage to the cause.

If you mathematically model a population of mites, you can expect colonies to die every 1 in 3 years depending on where you are. If you’re in the tropics and you have brood production all year round, you expect your colonies to die from the mites within one year. If you’re in central Europe, you expect them to die every two years and in the temperate regions, every 3 to four years. That’s why Northern states in Europe don’t have high mortality rates than Southern ones, like Denmark has low winter mortality rates because the varroa aren’t growing as aggressively. The population is doubling every month.

It’s important to understand how we measure mites. We don’t do a sugar roll but we collect the bees in the same way and do an alcohol wash in the lab, a much more efficient way of doing it if you’re doing big samples.

The most important message from this video is that the need for a half cup measuring cup because that totals to about 320 bees. When you’re taking samples to send in kits, you need the 300 bee count otherwise the sample is not as accurate. It’s really important to recognize, no matter your method, it’s number of mites per adult bee. We have to look at the population dynamics of adult bees and the brood in the colony relative to the mites. If we have an adult bee population of 30,000 or 10,000 that have the exact same number of mites in them, the mites per bees is going to be very different because you have more bees diluting. The bees that are most critical for winter survivorship start getting produced in August. This is when eggs get laid, larvae start to mature, and take you through the winter. If you’d like to have the healthiest bees possible, you need to try and make sure that the production window is as disease-free as possible. If that bee is supposed to live 6 months over the winter and you lose 10% of your life, that explains why we lose so many bees in March.

In this graph, we see that 80% of mites are in the brood so if we take that percentage, it’s about 4 mites per hundred. That’s pretty close to the national average. Looking at the peak population, we have 30,000 bees, 400 mites, which totals 1.3 mites per hundred. If you’re doing a sugar roll in your yard, for every 2 colonies, 1 will have a mite level of 1 per average, which is very low. You don’t want to enter a new season with more than 1. Every other colony should not have detectable levels of mites. If you have higher than that, you’re starting at a big disadvantage and will become very hard to recover. In October, we see that the population has decreased drastically, about 12,000 bees, with the brood decreasing rapidly. That’s 33 mites per hundred times 80% that’s 6.6 mites per hundred, which is in keeping but you’ll have very little brood. This is a critical time for those colonies to survive. Untreated colonies will die between 1-3 years. Often, if [beekeepers] send us their debri from their winter, we’ll find it’s loaded with mites.

So, we have a level of 5 mites per 100, which has come from several different studies. A German study has shown that after 5 mites per 100 that even if you treat it, you’ll have serious losses. Another Argentinian study came up with the same threshold. Our tech transfer team also said that when beekeepers suffer 5 mites or more, they’ll start seeing heavy losses very quickly. For many independent sources globally, that 5 seems to be the stabilizing point where you start seeing critical problems. Anything that’s less than 3 is grey and less than 5 is pink. Coming in August, you’ll see that more than half of the colonies for the rest of the year until December, have levels that are higher than we think that even treatments will prevent losses from occurring in those situations. There’s little question that nationally, on average, according to this national honey bee disease survey, we have mite populations that are exceeding what we think causes damage.

One of the methods we’ve been using is the centennial apiary program or “self-monitoring.” You can do this as a group and send in samples from your apiary or you can do it individually. The yellow is personal results from a beekeeper in Maryland, the orange is everyone else’s average mite levels who was part of the program that year, and the grey is the aphis national average. Yellow has sent his samples in for June and he has only a quarter of the mite levels of everyone else. In July, he’s still low and by August he’s still at half of where everyone else is. But in October, his levels go through the roof. What’s happening is that mite problems are not your own but of your neighbor’s. There are a lot of beekeepers that out of goodwill have decided not to treat their colonies. We think that this transmission of mites across the landscape goes into different colonies. Out in the environment, we have mite mines acting as untreated colonies; they blow up and spread their goodwill to all the colonies within 3 kilometers. These explosions are happening all the time. Some of this data is from the aphis survey but a lot of it is from our tech transfer team, which does several different things: we do our big national survey and we put people in with commercial beekeepers to help them monitor their colonies over the course of the year. They’re actively taking samples. If you’re a part of one of these teams, you get these reports every year that talks about the number of samples taken within your operation. It also gives your average mite levels this year and last year. It also anonymously compares you and other people in the program within your region. Typically, when people join our tech transfer team, their mite levels exceed thresholds very frequently. But, if you tease this apart, you’ll see the averages are a bit betraying. You’ll see not just the average but the range of mites (each dot represents a colony). However, there are outliers that are waiting to explode and transfer to your neighbor. If you were going through 100 colonies, there is a reasonable error rate of 1-2% with a couple of these landmines blowing up in July and August, and their spreading to the neighbors, by September or October, you’ll have 20 or 30 of these mines going off and spreading. It’s important that all beekeepers have an active and vigilant monitoring program.


In the national honey bee disease survey, we regularly collect, test, and analyze bee bred for pesticides. How do you summarize all the products in there? So we calculated something called a hazard potent, which takes into account the toxicity of the product and how much of the product we found. We can then graph the average hazard quotient in colonies that had no detectable mites, less than 3, 5, 10, between 10-25, and 25. We often hear from beekeepers that something is happening in the environment, like the mites are eating that pesticide, having super-kids, and the population is growing like crazy. If you have a pesticide that locks out 30% of your worker force then you don’t have more mites in the colony, you have the same number of mites and fewer bees. If you have certain colonies within the landscape that have had a high exposure to pesticide, it means you have another potential explosion.

Here’s a project we just did this year that dealt with fungicides on blueberries in Maine and the results were not at all what we expected. We have the control, which is they went to get the blueberries from Florida at an organic farm and then the experimental was that they also went to a blueberry farm, but where the berries were sprayed with pesticides. It had slightly more mites but not that much more than the experimental. So, when we look at blueberries, it was the control not the experimental that grew much faster. Another possibility is when we have these outyards where we’re dropping colonies into the landscape and pulling colonies out to spread into the system, we’re creating mini-landmines that blow up in the landscape and spread into the neighboring colonies. It’s both your neighbors and internals practices that explain these big spikes that we see in certain colonies that spread the mites.

We asked commercial beekeepers how they monitor, keeping in mind most of them lost 33%: 17 of them visually inspected mites, some did a drop where they stuck a sticky board in for treatment. It was only the people who started doing sugar rolls and alcohol washes that had noticeably lower loss rates the following winter. There’s no real difference between the two methods in terms of accuracy but the people doing alcohol are spending more money doing and are listening more carefully. Something about doing mass sampling, sending them out, and getting back in a report makes that data more actionable than only doing sugar rolls.

How/when to do alcohol washes: I would monitor my population of mites two weeks after every pollination event. I would do it pre-supering to make sure that I don’t have detectable mite levels. If I have more than one mite for every other colony, than I have a problem. It will manifest itself in August. Mid-season, if you can, I would check for levels that are well in excess of we think would cause damage. Two weeks after your fall treatment, you want to come in and make sure your mite treatment works.

The time of year you need to sample is the time of year when you don’t have time to do it, so I suggest joining one of these groups. Within two weeks, you get results. If I could suggest anything, I would talk to other members to the tech transfer team, especially if you have over 200 colonies.

Drone brood pull is a great way to know if you have varroa mite, but you can’t tell the exact infestation rate and whether it’s important or not.