Varroa Management Planning: Tools and Strategies (Bee Science)

Varroa Management Planning: Tools and Strategies

Dewey Caron:

Hi, I am Dr. Dewey Caron. I come to you from Portland, Oregon. I present another audio postcard in my new series of once-monthly Beekeeping Today mini-series podcasts, Bee Science with Dr. Dewey Caron. This is the fourth installment in this series. Each episode seeks to blend research, field experience, and seasonal context, focusing on the why behind honey bee biology and behavior. As always, I welcome your suggestions for any timely topics.

The mini-series this month is MIT, part two. My overall message is simple. If you do not get anything else from this episode, remember: have a plan. Part of the plan, and a critical tool to use, is the Honey Bee Health Coalition Tools for Varroa Management. The ninth edition will soon be out. Details on our mite tools are in this free document. Much has changed in the last year, so I recommend you get the latest, our ninth edition, as you download it.

The last Bee Science episode on mites discussed how our annual Varroa control management plan should incorporate IPM, Integrated Pest Management, or IPPM, Integrated Pest and Pollinator Management. That approach is four-part. Pest control begins with, one, knowing the enemy, which I covered in detail. The second tenet is understanding the number of pests, or the amount of reproducing pest involved at any specific point in time by sampling. The third aspect is establishing the risk that might be presently in play for the pest at the time of sampling. The number we are looking for is a threshold. Hopefully, the sampling does not exceed an EIL, or an economic injury level. Our fourth consideration, then, is digging into our tool bag and selecting the most appropriate tool if we have determined the risk is such that our pest level may be exceeding that economic injury level. So that is what we will cover today.

So what is an EIL? EIL basically is the number, or the level of reproduction, of a pest that is at a level of, or exceeding, where the pest is causing harm to the bees. That might then negatively affect our beekeeping objective, whether it is just enjoyment of bees, return from honey or their products, or affecting the pollination numbers of our colonies, for example. It can be an elusive number because we have different objectives. In different periods or seasons, we might even vary. Based on our assessments, our pest management control plan during the season will vary.

During the overwintering period, we suspend sampling, the second tenet, as it can be harmful to the bees and the number misleading. However, we can and should seek to reduce the survival of phoretic mites, those in the traveling or dispersal phase of the mite life cycle, on the adult bodies of those overwintering bees in their cluster. Preventative treatment is accomplished with oxalic acid during the overwintering period. Our rationale is to reduce the surviving population of female mites. Once the bees start rearing brood, and some colonies never completely cease brood rearing during winter, but it will be reduced, and then begin their spring growth spurt, surviving female mites will also begin rearing their young. The mite numbers will grow. Make no mistake. But the lower the early spring inoculum, that is, how many surviving females there are, the lesser the chance, and the longer it will take, for mites to reach a high population. So it is essentially a prophylactic control, but it is smart preventative management.

This concept is similar. It mirrors doing a brood break, or when we start a package or a nuc colony in the spring. The lower the starting number of mites in that expanding, growing bee colony that we have established, or from our brood break, means a smaller overall mite population size at the mites’ fall peak. As indicated, our material of choice during the winter is oxalic acid, used as oxalic acid vaporization, which we call OAV, or as oxalic acid dribble, or OAD. There is no residual effect, so the oxalic acid must contact the mites themselves.

During winter, this is a challenge. The female mites are deeply embedded beneath the ventral abdominal sclerites of host bees, with only a sliver of their abdomen exposed. We can use a single treatment of either OAV or OAD when brood is not likely to be present, or we might elect to bracket the most likely broodless period with, for example, one treatment in December, another in January, and maybe a third in February. Two or three applications improve our chances of exposing mites to the acid, including those fully embedded between the sclerites.

Although oxalic acid has been used for three decades, in Europe at least, the mode of action of oxalic acid against Varroa mites is not yet clearly understood. It seems that direct contact with the low pH of oxalic acid solutions has a deleterious effect on the mites. We do not need to use a high concentration of oxalic acid, as oxalic acid is seventy times more harmful to mites compared to the harm it might cause to their bee host, to our honey bees.

A 2017 study in Argentina, with Diana Sammataro of USDA in the Tucson area, compared the susceptibility of a Varroa population exposed to sixty-four consecutive treatments with oxalic acid, what they called their focal mite population, versus the susceptibility of a Varroa population never exposed to it, their naive mite population. They did this in order to evaluate whether mites might be developing resistance to oxalic acid. As a result, none was found.

We have come to understand aggressive control during the spring is the best management practice for healthy bees. That should be in your plan. Both our bee population and mite population grow rapidly in spring. While the ten- to fifteen-thousand adult bee population may quadruple, or grow five times in size to reach a peak of perhaps fifty or sixty thousand bees, the population of surviving female mites, which might only number single or even low double digits, will grow more greatly and reach a thousand to five thousand or more mites at peak. Peak mite population occurs after the host bee population peaks.

In the spring colony, foundress mites select drone cells about to be capped, so, as a result, she can produce three mature daughters versus a single mature daughter when the worker cell is used to reproduce. Middle-aged worker bees are in short supply in a spring colony. Nurse bees are in high demand, as are foragers to bring in pollen, which is the driver of that spring growth. So sniffing bees and recapping bees, those middle-aged bees important to hygienic behavior, are in short supply. And in early spring, adult bees with high amounts of fat body enable the phoretic mites to spend less time on their adult bee host. Meaning our tools, which largely target those phoretic mites, those on bee bodies, not the reproducing mites in cells, are going to be less effective.

The IPM approach toward reducing an eventual peak mite population means flattening the mite population growth curve. We have few tools, and it is very hard to reduce a high population of mites in the fall, later in the season. It should be in your plan. Start low and keep mite numbers low for healthier bees.

We need to do a reality check here. It is a fact that it is not the mite number surviving the winter, or their growth in the spring colony, that puts our colony at risk. It is the mites’ ability to spread the viruses of DWV, deformed wing virus, or the paralysis virus groups, that is so critical. But still, waiting until the number of mites is large in the fall is too late. The virus has at that point perhaps grown to very high numbers. We therefore believe, although there are no effective means to control the viral load of honey bee colonies, regular and effective control of mites can help our bees survive.

Integrated Pest Management, or Integrated Pest and Pollinator Management, strategies are required to effectively control mites. A study in Italy and Slovenia evaluated the effect of two brood interruption techniques, that is, queen caging and drone trapping, followed by an oxalic acid treatment. They measured mite fall, colony strength, and viral load of both deformed wing virus and the acute bee paralysis viruses. Results showed that although both the queen-caging and trapping-comb techniques followed by the oxalic acid treatment did effectively control Varroa mites, they did not reduce viral loads. Repeat: they did not reduce the viral loads. The authors indicated further studies should be carried out to evaluate the long-term effects on viral loads.

So, what are our tools to flatten the mite growth curve in the spring? We have, and perhaps our best choice is a non-chemical method, and that is drone brood removal. In drone brood removal, we allow the colonies to raise drones in a place where we can find them again. We put in a frame that has the foundation of drone-sized cells, or an empty frame, and the bees then will construct some worker cells and some drone cells. Or we can put a medium-sized frame into a regular standard-size box, and that space below the bottom bar will be filled in by the bees with comb. In the spring, that will usually be drone comb.

So what we do is let them raise the drones, but then we intercede and remove the drone brood before adult drones emerge from their cells, so while the drones are still capped. By knowing where our bees are rearing their drones—they will raise drones anywhere, on one or several combs usually—but by putting in a special frame or a special modification, we know that is where the drones will be. So we are managing them rather than doing the hunting and pecking to find where the bees are raising their drones.

This technique, that is drone trapping, should begin as soon as capped drone cells are present, as you are seeing them in the colony. And if started and not continued—in other words, you try to start this process but you do not follow through by removing the drones at the capped stage—you are actually creating a factory to raise more mites. So, for the faint of heart, do not start this, because you will just raise more drones, which means mites will have a better chance of creating more mites through their reproduction.

Two other management techniques can be used: creating a brood break or queen caging. These let us obtain a period without capped brood in our bees. They are not usually performed in very early spring. An important advantage is that they also work in conjunction with a proactive swarm-management and control effort. And this is going to be the topic of our next Bee Science next month.

In addition to drone trapping, you can use chemical tools in the early spring. What you want to do, again, is flatten that mite growth curve. The single synthetic miticide we have that is still effective is amitraz. The tool Apivar, and a new and improved Apivar 2.0, is a flash treatment that takes six weeks. Apivar 2.0 is a better hanger arrangement. The second product with amitraz is Amiflex. You put a glob of this paste-like material on a wooden pallet on top of a frame. This is like the pest control operator that comes in to control cockroaches. They put these gobs of what looks like toothpaste in areas where the cockroach is frequent. Amiflex was developed for commercial appeal, but is very useful for hobbyists too. It also is a flash treatment.

Importantly, both amitraz products, Apivar 2.0 or Amiflex, do not need the cooling-off period of the original Apivar. That means supers can be added immediately following treatment. With original Apivar, we had to wait a couple of weeks. Now you do not. This makes amitraz a more useful miticide for spring control.

But what about amitraz resistance that we have been hearing about? The acaricide amitraz has been among the most preferred Varroa treatments for more than a decade because of its high effectiveness and convenience of application. Mite resistance to amitraz showed first in commercial apiaries due to their higher use frequency and use of different formulations, which is the reason why resistance developed there first, at least we find, in commercial operations. As a result, over-reliance and higher levels of repeated seasonal use has, we think, led to this Varroa resistance to amitraz.

Frank Rinkevich and associates at the USDA lab in Baton Rouge have documented the developing resistance in numerous bee talks that they have presented and in their publications. Recently, in a 2023 paper, they demonstrated the why. They found that resistance is due to a Y215H mutation—Y215H mutation—leading to a change in the beta-octopamine receptor in a Varroa mite. That means that the Varroa mites can tolerate the levels that we are using for this material. You can review this subject in their most recent publication in Scientific Reports by Rogan Tokash, Frank Rinkevich, and fellow researchers. This is in the end notes.

A new, interesting publication from the USDA Pollinator Health Research Lab at UC Davis reported that using a synergist, an inhibiting compound, specifically verapamil, in combination with amitraz increases amitraz toxicity. It was even effective in increasing the killing probability against amitraz-resistant mites. Verapamil works by inhibiting ATP-binding efflux transporters, which are part of the mites’ defensive mechanism. Verapamil acts as a gatekeeper that pumps toxins, for example amitraz, out of the mite cells. More exposure means greater pesticide kill power.

Why is this important? A pesticide is a poison, of course. Toxicity is due to both risk and exposure. So the inhibitor acts to increase mite exposure. We can use less pesticide to get greater kill power. Unfortunately, the bad news is the inhibitor, verapamil. Their view is that it is not specific to just the living Varroa mites. It can also negatively affect the ability of honey bees to tolerate their exposure to pesticides. Research is ongoing to develop specialized targeted synergists that specifically inhibit those processes in Varroa, in other words, that would just affect Varroa without harming the bee host.

So-called softer chemicals approved in organic treatment management, or for those who wish to practice a reduced-chemical treatment approach to mite control, are acids and essential oils based on thyme oil, the essential oil thyme oil. Why would pesticides be approved for organic honey production? In animal husbandry, such as with our care of our honey bees, management should avoid creating undue stress on the animal under your care. Mites, with their association with viruses, are a stressor for honey bees, and in fact kill colonies. In organic animal management, you should attempt to manage so your hosts do not stress or die. Thus, the softer chemicals of acids and essential oils are permitted in a management plan.

So of the so-called softer chemicals, we might use formic acid. Formic acid is available as strips. We can use either one strip or two strips at one time. It would be one strip followed by another strip. The most effective kill power is when the two pads are applied at one time. The essential oils, Apiguard and Apilife Var, are also so-called softer chemicals. These are based on the essential oil thyme oil. Apilife Var has additional materials as well as the thyme oil, but thyme oil is the major ingredient. These work as vapors. They are put onto the colony and then you have to exchange them because they degrade. The vapors escape and the material then is gone in about a week for Apilife Var and about ten to twelve days for the tin that we apply with Apiguard. So this means going to your colonies two or three times to complete a one-month treatment period.

There is yet another, perhaps more appropriate acid to use, the less caustic oxalic acid. During winter, we suggested use of OAV or OAD, the vaporization or the dribble. Now we are talking about OAE, oxalic acid extended. There is a material that is available. It is called VarroxSan. It consists of strips that we tent over frames, roughly one strip for every two to two-and-a-half frames that include brood.

Why should we think of wanting to use oxalic acid for an extended period in our colony? Can we use OAV, vaporization, and dribble, OAD? Short answer is yes, but since there is no residual with oxalic, we have to use the vaporization and dribble repeatedly. Roughly, use it two times a week, because phoretic mites are on average on a host adult bee only seven days, and once a week might not be aggressive enough. You may need to use it five, six, seven times.

What has changed, however, in oxalic acid is the amount that we need to use. The original registration by the USDA lab in Beltsville was one gram per box of bees. Mustafa Bozkurt, a student from Turkey working on his master’s at Oregon State University, used different doses of oxalic acid. He used one, two, and four grams in one season, and in the second, three and four grams, applied once a week over three weeks in August at Oregon State University’s apiary in Corvallis, Oregon. And you could do the same for spring, for example. Each experimental group included eight honey bee colonies. Control colonies received no oxalic acid.

The Varroa levels in colonies treated with one and two grams of oxalic acid were not significantly different from each other, and not different from the control group in both years. In other words, they were not very effective. The four-gram oxalic acid effectively helped suppress Varroa mite growth, but it also appeared detrimental to honey bee larval growth. In the study, then, Mustafa concluded that three grams was the best. It would cause less harm and still provide the best control. So now recommendations are to use greater amounts and, with oxalic acid extended, or when we use the vaporization or dribble treatment, one or two times for four to eight total treatments. Thus, we are going to be using oxalic acid at greater frequency. There is a much higher level of oxalic acid than has historically been used in the past, particularly at that one-gram level.

Can we expect development of resistance to oxalic acid? Our belief is the selection pressure for resistance against these natural miticides, the acids, for example, and the essential oils, should be low due to their rapid degradation inside the colony and historically because we have been using very little, with their superior kill power and less harm to the host. That is completely opposite of what has happened with our synthetic acaricides, initially Apistan, then coumaphos, and now amitraz.

We are aware of populations in Cuba, Africanized honey bees, a population on a Swedish island, and bees in the south of France that need little to no miticides. Many beekeepers in the UK are not treating, alongside other individuals and beekeepers that are treating. These bees resistant to mites use different means of achieving success against Varroa mites. Some means that have been documented include bees that exhibit high hygienic behavior, bees with heightened grooming behavior, bees that stay at smaller colony size with higher swarming frequency, bees with reduced breeding time, or bee stock with the ability for suppression of mite reproduction.

A systematic review in 2024 in Veterinary Research by researchers at Leipzig University in Germany reviewed papers over the past thirty years, in other words, a review of the whole literature, using the calculation of seventy percent or greater efficacy, and concluded that Varroa mites show no resistance, no evidence of resistance developing, against oxalic acid. A study in Argentina compared the susceptibility of a Varroa population exposed to sixty-four consecutive treatments of oxalic acid, that focal population, to those that were naive, and did not find any. I referenced this a little bit earlier.

So should we advocate oxalic acid full speed ahead? There are rumors that in certain instances oxalic acid is losing its efficacy. Associated with Varroa are certain bacteria that are capable of actually degrading oxalic acid. So if they are present, what it means is there would be less available, less exposure to the bees. Oxalotrophy, that is what it is called. Oxalotrophy is a rare trait among bacteria, but common in bacteria that are associated with Varroa mites. Oxalotrophy in bacteria is the metabolic capacity to use oxalate, which is a salt or ester of oxalic acid, as its primary source for the carbon and energy necessary for its growth. These bacteria, known as oxalotrophs, break down highly insoluble calcium oxalate, thus resulting in it not being available, the oxalic acid not being available to kill mites.

So I repeat once more: what is your plan? How are you going to flatten the mites’ growth curve? It should be an integrated plan. Mechanical removal of drone brood and use of one or more of the organic chemicals or the synthetic amitraz, and through the spring buildup of our bees, a plan to keep the mite numbers low. The message with this Bee Science is: start early when mite populations are low, and be sure to keep them low.