IPM 3 Fighting Varroa 3: Strategy – Understanding Varroa Population Dynamics
Fighting Varroa: Continued
© 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.