IPM 3 Fighting Varroa 3: Strategy – Understanding Varroa Population Dynamics

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What can we do to keep the mite population under control?

The first thing to keep in mind is that any mite control methods we use today are merely stopgap measures until the bees are genetically able to take over the battle for themselves (kind of like our “strategy” in Iraq). My feeling is that we need to rethink our paradigm, and switch from the strategy of blasting the mites with a chemical bomb after they’ve built up to dangerous levels, to a strategy of controlling mite buildup from the get go.

Those of you who keep asking for some new miracle control for the mite are ignoring the reality that we have at least two kinds of bees who are already demonstrating exactly how to do it! Apis cerana and the Africanized bees keep varroa in check unaided by us egotistical humans who think we need to solve everything ourselves! My guess is, that if we had focused our resources toward the development of resistant bees, rather than on chemical control, we wouldn’t be needing this discussion on mite control today.

The bees have already provided us a model of what works. Maybe we should attempt to emulate it. In essence, these bees use a brass knuckles approach every day to make life miserable for the mite. Our modeling of mite population dynamics confirms for us just why their strategies are successful. To wit:

Tactics used by mite-tolerant bees

1. Resistant bees are hell on phoretic mites, always grooming them off their bodies, harassing them, and increasing daily mite mortality. This action helps rid the colony of overwintering mites, and slows mite buildup in spring by keeping individual mites from breeding multiple times. We should seek out and breed from bees that exhibit strong grooming behavior (I think this should be a top priority). We can assist the bees with natural* chemical treatments during spring buildup, by using screened bottoms, and with an oxalic acid dribble in the fall.
2. Resistant bees rip out worker pupae if mites try to reproduce on them. This keeps baseline mite buildup in worker brood to a minimum. We can select for strong varroa sensitive hygiene.
3. Mite-infested drones of Apis cerana essentially sacrifice themselves by dying under the tough cell cap. Whole colonies abscond and leave mites behind. We should seek out and breed from bees whose infested drones die within the cocoon. We can help by minimizing the amount of drone comb, and by practicing drone brood removal during spring and summer.
4. Resistant bees never let mites build up to high enough levels to cause virus epidemics. We can monitor mite levels, and use natural chemical treatments if necessary in August to bring down mite levels.
5. Resistant bees produce fewer collapsing colonies to rob, and due to heavy grooming, are less likely to carry mites home. Ideally, we would isolate our apiaries. Realistically, we can minimize swarming by mite-susceptible colonies that will eventually collapse, we can prevent our own colonies from collapsing and spreading their mites, and we can monitor our colonies spring and fall to watch for sudden jumps in mite levels.
6. Resistant bees abscond frequently, and leave behind disease spores in the old combs. We can breed from bees that exhibit immunity to viruses. We can rotate out old combs that may be contaminated with chemicals, or harbor spores. We can also help suppress tracheal mite, nosema, and other diseases with good husbandry or medication.

* “Natural” chemicals such as oxalic or formic acid, thymol or essential oils, powdered sugar; as opposed to synthetics such as fluvalinate or coumaphos that leave harmful residues in the wax combs. I’m not touting “organic” beekeeping, just common sense methods based on good scientific research.

There you have it—a summary of the strategy and tactics that are successfully used by mite tolerant bees. These bees rarely allow varroa infestation to exceed the 2% level at any time of the year. We don’t have to reinvent the wheel! Mite tolerant bees are out there right now—let’s identify them and breed from them. In the interim, let’s follow their example, and help the rest of our colonies to survive and thrive!

A brass knuckles fighter doesn’t just bide his time, waiting for his one good shot with the “silver bullet”—instead, he sizes up his opponent and just keeps pounding away, never letting the opponent get the upper hand. The way we “pound” varroa is by an incremental approach. For example, if you plant a bed of lettuce in spring you know that you’re going to have to weed it to keep the weeds from shading the crop and stealing all the nutrients. The incremental approach would be to do a little hand weeding every week, and keep knocking the weeds back. The silver bullet approach would be to wait until the weeds grew tall, and then use your 50 horsepower garden tractor to mow ‘em down just before they smothered the lettuce. You’d save time, and still get a crop, but the lettuce would be small, stressed, and diseased. Varroa affects your bees the same way that weeds hurt your lettuce.

So let’s compare two strategies: brass knuckles vs. silver bullet. If you were to use the “brass knuckles” strategy—recruiting mite-fightin’ bees that can kill an extra 1% of the mite population each day (an additional 7% kill a week, or 35% a month), you’d retard the mite growth rate to less than 1½% a month. At that rate, 200 overwintered mites wouldn’t even reach a population of 2500 by the end of a full 6-month season of broodrearing! If you go a step further, and help the bees with biotechnical methods, or spring treatments, you’ll keep the upper hand on the mites. An incremental approach of hitting the mite with little jabs all through spring will win the fight. Mites only get out of hand because we ignore ‘em until they are in our face!

Compare this to the silver bullet strategy we’ve been using, letting them build up to 10 or 20 times their initial population over the course of a season, then gamble that your one good shot with the silver bullet in the fall will kill the 90-95% necessary for the colony to survive! Both approaches knock the mites back to the starting point, but the silver bullet approach results in a stressed colony being hammered by the mites for much of the season, which greatly decreases your honey crop, allows viruses to rampage, and greatly increases your stress level.

The only way we can tell if the bees (with our help) are successfully keeping the mite in check is by monitoring mite levels at appropriate times. In the next installment of this series, I’ll compare the various methods for monitoring mite levels, and make suggestions as to timing. By the way, I’m going to help you put all these pieces together in (probably) the May issue, so you can make a personalized yearly plan for varroa control. I also should have a website up by next month, so you can see updates to these articles.


I want to be clear that I could not be writing this series of articles without the gracious help of many other beekeepers and researchers. Indeed, this series is a learning experience for me day by day, as I find new resources of information, printed or verbal. I’d especially like to thank Eric Mussen, Marion Ellis, Rob Currie, Marla Spivak, Gloria DeGrandi-Hoffman, Sue Cobey, Tom and Suki Glenn, Dewey Caron, Bob Harrison, Dave Cushman, Bill Truesdell, and Jerry Hayes. Newbie George Fergusson has been invaluable in helping me distill the information. I greatly appreciate the support and indulgence of editor Joe Graham, who lets me make last-minute revisions right up ‘til press time. For those of you that I forgot to mention, my apologies.
Resources for Information


© Randy Oliver 2006

Joe Beekeeper typically has a gnawing feeling in his gut that the ways he’s been dealing with the varroa mite are starting to fail. I heard a complaint at a recent convention: “How long can we continue in a business where 30% of our assets die each year?” Many commercial beekeepers are using a combination of the latest ag chemical on a stick, and prayer that it will work once again. They’ve got no fallback position, and it’s scary. They are correct in their assessment that the “silver bullet” model of mite control is going the way of the Polaroid camera—it worked great for a while, but no longer does the job we demand.

In my two previous articles in this series, I’ve introduced a view of the future: integrated pest management (IPM), and the fact that we can breed bees that have the innate ability to fight varroa. I feel it’s only fair to tell you that varroa IPM is a work in progress. I can’t offer each and every one of you a turnkey, failsafe method of keeping varroa under control. There are far too many variables, such as length of season in your area, climate, humidity, strain of bees, colony stress, your management practices, the size of your operation, and your time and money constraints. This is a learning process for me, for bee scientists, and for beekeepers in general. What I can give you is information—the theory and science behind the methods, and strategies and techniques that have been proven to work by others. It’s going to be up to you to put the pieces together for yourself (although I’ll help you to do that later in this series).

I may sometimes sound as though I’m proselytizing. So let me be clear. I have only two agendas: (1) to help beekeepers be successful and profitable through the application of current scientific research, and (2) to keep honey’s wholesome name untarnished by not contaminating it with chemicals. We all saw what happened to this year’s fresh spinach crop when the news media announced that one little farm’s harvest was contaminated—the rest of the unfortunate spinach farmers couldn’t give their crop away! The same thing will happen with honey unless we clean up our chemical act.

Initially, I planned to write this article on the topic of monitoring mite levels (with sticky boards, etc.). In the process of attempting to draw a graph (Figure 2) to illustrate how the mite population fluctuates over the course of a year, I searched for good data so that I could plot the curve accurately. That search led me to research on mite population dynamics. I came to realize that the key to developing an effectual strategy for mite control is to have a firm grasp of mite population dynamics. Only then can you intelligently weigh the likely efficacy of various mite control strategies and methods. This is hardly an academic issue. In my own beekeeping business I have limited time and money to invest in mite control, so I want to get the most bang for my buck. By understanding mite dynamics, I can do so.

What are the population dynamics of varroa in a honeybee colony?

Let’s start by seeing just why it is typical for varroa to become a problem in the fall. Please refer to Figure 1.

Both the mite and bee population are at their lowest just before the first brood emerges in spring. The bee population climbs at a quicker rate than the mite population until midsummer, when the bees start to ramp down. The mites get off to a slower start, and then hit their stride during drone rearing season in spring and summer. Note how the mite to bee infestation ratio climbs dramatically in early September. When that occurs, the bees really feel the impact of varroa—brood is stressed or dies, viruses run rampant, and the generation of bees that will form the winter cluster is weakened and vulnerable. For a review of the insults that varroa parasitism visits upon a honeybee colony, see the excellent New Zealand guide cited at the end of this article.

A key point to remember is that the relative infestation (percent, or mites per 100 bees) is more important than total mite population—a large colony can handle more mites than a small one. At much above a 2% infestation in spring, honey production drops off severely. At much above 5% in fall, colony winter survival suffers (although the fall “economic injury threshold” numbers by various authors range from 1% to 11%) (Currie & Gatien 2006). We will return to percent infestation, and economic injury levels in my next article.

Unchecked, varroa can really multiply! A 12-fold increase is typical in a short season consisting of 128 days of brood rearing (Martin 1998). However, its population can increase 100- to 300-fold if broodrearing is continuous! (Martin and Kemp 1997).

There are also major confounding factors. Some years, mite populations are low across the board (possibly due to hot, dry weather) and no treatment is required (Harris, et al 2003; and personal observations). In any apiary, there is usually huge colony-to-colony variation in mite levels, especially if one is using a variety of queen lines. If there is a reservoir of collapsing colonies nearby, mite invasion can make your best mite-fighting efforts moot. Finally, tracheal mites, nosema, viruses, and chemically contaminated combs can cause even relatively low mite levels to be fatal to the colony.

It is easy to find all this information overwhelming! Unfortunately, as our “Silver Bullet” chemicals fail to control mites with a yearly “no-brainer” treatment, beekeepers will be forced to exercise their brains in order to stay in business! This article is by far the most difficult one in the series for me to attempt to condense the state of scientific knowledge into practical recommendations for Joe Beekeeper. So let me start with models of varroa population dynamics.