For the Love of Bees and Beekeeping archives
Varroa Mites: Is the Cure Worse than the Disease?
by Keith Delaplane
It’s a moment for reflection when one learns that something one has always believed has been blown to bits by a piece of new information. This has been happening to me a lot lately, especially as new research keeps coming out shedding light on the mysterious web of causes surrounding bee decline. An example came out not long ago at the hands of Dennis vanEngelsdorp and his co-authors in a paper describing factors contributing to bee mortality in the U.S.1. It was shown that increasing levels of the miticide coumaphos in brood were associated with colonies less likely to express signs of Colony Collapse Disorder (CCD) and die. Coumaphos is registered for Varroa control in the U.S. under the product name Check-Mite+, but it and other synthetic miticides have been implicated by some as problems in their own right – vilified as just so many more man-made chemicals that interfere with the bee’s biology, encourage chemical resistant mites, and probably do the bees more harm than good. I’ve never been extreme in accepting this point of view, but I was certainly sympathetic to it and I had to look over vanEngelsdorp’s results twice to make sure I was reading them right.
Now the most obvious explanation for results like this is that commercial beekeepers are no slouches when it comes to feeding colonies, managing queens, and controlling mites. With livelihoods at risk and financial stakes high, these professionals are going to make sure that all the easy explanations for mite outbreaks are eliminated. In light of intense mite control, the stress of a move to California becomes comparatively tolerable. And if it is true that coumaphos does bad things to bees, it is also true that Varroa mites do worse, and I came away from this paper thinking it was less an endorsement for chemicals as an indictment against Varroa.
So, if things reduce to some kind of mathematical formula with Risk from Mites on one side of the equation and Risk from Miticides on the other, then it’s important that we do everything in our power to minimize the risk from miticides. And the way to begin that is to get accurate information.
There has been interest in low-grade or so-called sublethal effects of bee hive chemicals for a while now. Some of this comes from the fact that high-grade or acute toxicity is easy to diagnose – piles of dead bees. But what about low-grade toxicity that exacts a slow, mysterious, and cryptic cost on colony health? Until bee decline has been fully understood, it is important that we uncover as many of these “sleeper” toxicities as possible. There is, for instance, evidence that fluvalinate (Apistan) increases mortality of young drones, reduces their body weight, and decreases their sperm count2 and that coumaphos (Check-Mite+) reduces queen larva acceptance rate, body weight of virgin queens3, and longevity of adult queens.4 We know that in-hive acaricides affect gene expression5 and immune response6, rendering the bees susceptible to diseases and other pesticides.
We also know that fluvalinate and coumaphos can interact with each other to elevate the bee toxicity of fluvalinate to damaging levels.7 This bit of news was particularly startling to this bee expert who has spent decades advising beekeepers to “rotate your miticides to reduce chemical resistance to mites” – only to find out that the practice could lead to damage to the very bees we are trying to protect. What’s an advisor to do?
My group weighed in on the question of sublethal effects on bees with a study comparing three legal bee hive chemicals – the miticides fluvalinate (Apistan) and coumaphos (Check-Mite+) and the popular wood preservative copper naphthenate8 used to protect woodenware against decay and termites. This paper is publicly available at the open access journal PLoS One.
Now, a personal note here about copper naphthenate. Back in the early 1990s I wrote and hosted a PBS television show and book on beginning beekeeping called A Year in the Life of an Apiary. Of all the footage filmed and ink printed on the details of building hives, installing bees, managing growth, controlling disease, harvesting honey, and marketing the crop – the one sequence that has garnered more comments than all the rest combined is a brief section where I belabor the point of treating woodenware with copper naphthenate. I can’t count how many times I’ve been asked “What was that green stuff you were painting hives with?” so it is with pardonable bias that I included this compound in our study as I consider the original toxicology work done on it rather sketchy and I had lingering curiosities whether the material was in fact safe for bees.
In order to eliminate beginning miticide residues, we set up experimental bee colonies with factory-new equipment and took the extra precaution of starting each comb with a one-inch strip of wax-less plastic foundation (beeswax foundation may harbor residues of bee hive chemicals). Each colony was ....
On Screen Hive Floors
by Keith Delaplane
If my columns the last couple months have had a theme it might be called: Great Ideas that Didn’t Work. So it’s high time I interject some optimism on the matter. Now, it may not be a lot of optimism, but when it comes to low-chemical Varroa mite control, the name of the game is numerous simultaneous pest control components, any one of which may be insufficient to keep mites at bay, but when used together might do the trick.
So for this month I’m happy to bring up the subject of screen hive floors. Only a decade ago most hive floors were simply solid, but nowadays it is common for suppliers to offer screened hive floors, and their use has widely increased. It is generally thought that screen hive floors allow improved ventilation, increase brood production, and give a measure of Varroa mite control.
Screened hive floors have been used in some parts of the world for years1, but it is safe to say they have never been mainstream until lately: the only mention they garner in Eva Crane’s authoritative 1990 Bees and Beekeeping2 is a 1981 reference to “netting” floor boards used in Norway. Aside from their use in ventilation, they have also been tested for their effects on overwintering3 and moisture content in honey4.
The arrival of Varroa mites in North America suggested new purposes for screen hive floors. Tibor Szabo noted their usefulness as a means to protect bees from sticky sheets placed on hive floors to catch and count dropping mites5. Noting that up to 50% of the mites dropping onto bottom boards are still alive, Pettis and Shimanuki6 reasoned that if a barrier could be placed between these mites and the colony that they would not be able to reinfest the bees. What they came up with was a screen insert that fit between a standard (solid) bottom board and the hive body (Fig. 1). The device was not properly a screen floor, per se, rather a screen suspended above the floor. It did not open to the ground and debris (and presumably mites) could accumulate beneath it. The authors monitored mite populations every month with standard sticky sheets and found a downward trend in mite numbers in colonies with screens in June and July, but screens did not prevent an exponential jump in mite numbers in fall. Curiously, they found that brood production was significantly higher in colonies with screen floors. Pettis and Shimanuki concluded that screen floors could not stand alone as a mite control measure, but could be a helpful in an integrated control approach.
My lab got in on the action with a project my then-student Jamie Ellis published in 20017. We joined the floor modification of Pettis and Shimanuki with two miticides to examine their combined effects on Varroa. Like the previous authors, we found a slight increase in brood production with screen floors and, similarly, a slight but non-significant drop in mite levels, in our case 15%. We had evidence that Apistan was not working in our test apiaries, but in those colonies treated with Apistan plus a bottom screen mite control was restored to an average of 44%. We concluded that the modified screen floor exerted a small but beneficial downward pressure on mite populations and helped compensate for chemical resistance in mites.
Up to this point the published research on screen floors in the U.S. had centered around the modified design of Pettis and Shimanuki. It was Harbo and Harris8 who foregrounded the idea of the simpler design known today in which the bottom is fully screened and open to the ground (Fig. 2). These authors compared mite population growth and brood production in colonies with solid bottom boards or an open screen and found a trend for fewer mites in colonies with screen floors. Interestingly, the open screen group also had a lower fraction of their mite population in brood cells – a proxy indicator of the fecundity of the mite population, given that mites can only reproduce in brood cells.
Another paper came along describing the usefulness of screen floors integrated with mite-resistant queens9. There was a consistent downward effect of screens on mite populations, albeit rarely statistically significant, but this time there was also evidence that screens interacted with mite-resistant queens in a way to elevate mite control above that found with only one component – an effect sometimes called a synergy or a positive interaction. But not all was rosy with screens. In one experiment this same paper detected a significant reduction in stored pollen and honey in colonies with screen hive floors.
This discussion by no means exhausts the literature on screen hive floors, but it is clear that consistent trends are detectable. Screens rarely provide a dramatic level of mite control, but they do predictably put a downward pressure on mites and tend to increase brood production. I put together a summary below of some of the published findings about screen hive floors (Table 1).
Now for me a curiosity remains – ...
On Small-Cell Foundation
by Keith Delaplane
The worldwide spread of the Varroa mite has morphed beekeeping from one of the most chemical-averse agricultural industries to one of the most chemical-dependent. It is widely thought that the only practical control for mites is the use of toxic miticides inside the hive. It is equally agreed that this is a sorry state of affairs because it’s a fine line between killing one arthropod and sparing another when both live in the same space.
As a result, there has been an outpouring of creativity among scientists and beekeepers with the aim of controlling mites without pesticides. One of the most visible of these projects has been the use of small-cell foundation.
The idea behind small-cell foundation is the fact that mites can only reproduce in bee brood cells, and a few studies have shown that, if given a choice, Varroa mites prefer comparatively large brood cells. This preference seems to hold whether mites are given a choice of brood cells made by European bees (larger cell size) versus African bees (smaller cell size)1 or if mites are experimentally given a choice of three cell sizes: 4.8 mm, 5.2 mm, or 5.3 mm2. It is reasonable to assume that colony mite population growth would be correspondingly reduced in colonies with brood cells smaller than the mites’ natural range of choices.
These observations ultimately led to a commercially-available product, a small-cell foundation that measures 4.9 mm per cell compared to conventional foundations ranging around 5.2 mm to 5.4 mm. This product is available in bee supply catalogs and many beekeepers use it as part of an overall Varroa control strategy.
The trouble is, the practice has not held up to experimental challenge. I was part of a team that tested small-cell foundation as a means for reducing colony mite populations3, and I want to give an overview of our experience here.
We set up three independent studies with colonies with one of two brood cell types: small-cell (4.9 mm cell width) or conventional-cell (5.3 mm). In one study, ending colony bee population was significantly higher in small-cell colonies than conventional-cell. However, the main interest for small-cell foundation is its effects on mites, and on this count small-cell didn’t do so well. In fact, small-cell colonies were significantly higher for mite population in brood, percentage of mite population in brood, and mites per 100 adult bees. We were forced to conclude that small-cell foundation does not slow Varroa population growth. This conclusion is reinforced by the fact that the experiment was replicated independently three times with start dates ranging between spring and fall and test periods ranging from 12-40 weeks.
The work of Martin and Kryger4 seemed to support small-cell foundation when they observed that mortality of male offspring mites was increased under conditions which constrict the space between the bee pupa and male. However, these same authors pointed out that, “reducing cell sizes as a mite control method will probably fail to be effective since the bees are likely to respond by rearing correspondingly smaller bees.” We found some evidence to support this. In one of our trials we compared average body weight of bees reared in the two cell types and found that average bee weight was smaller in small-cell colonies.
These results were pretty convincing to me, but other authors have chimed in too. Small-cell foundation was shown to be ineffective in reducing mite population growth in independent tests in Florida5 and New Zealand6. In fact, I am unaware of any publicly-accessible peer-reviewed papers that directly support it.
If our results were convincing to ...
On Comb Replacement
by Keith Delaplane
One of the brightest spots on the American beekeeping landscape is the Bee Informed Partnership http://beeinformed.org. BIP is a USDA-funded consortium of university and USDA bee scientists focused on surveying the beekeeping industry to identify management factors that affect colony health and survival. The nice thing about BIP surveys is that the data are summarized and the trends publicized on the BIP website. One need only peruse the BIP homepage to get an appreciation of the power of data and its ability to strip away guesswork and make sense of complicated phenomena. There one finds not only annual colony loss reports, but clear and succinct summaries of noteworthy trends in the surveys that give clues to good management.
It is one of these succinct summaries that caught my attention the other day. The online BIP article, “Brood comb management and treatment of dead outs: National management survey 2011-2012” informs us that beekeepers who replaced 50% or more of the brood combs in their colonies experienced 30.7% colony overwinter loss; beekeepers who replaced 10% of brood combs lost 21%, and beekeepers who replaced none of their combs lost 22%. The 30.7% loss rate suffered by the 50% replacers was statistically significant.
I admit these results surprised me, as I count myself among the many voices down the years who have advocated that beekeepers should regularly replace their brood combs. My opinions on this matter draw mainly from the research of my first graduate student, subsequent lab manager, and now authority in her own right, Jennifer Berry. One of the chapters of Jennifer’s master’s thesis dealt with the effects of old brood combs on colony strength.1 In each of three years she set up apiaries of 21-24 four-frame Langstroth nucleus colonies and assigned each colony one of two treatments – establishment on first-year, newly-drawn beeswax brood combs or establishment on old black combs of unknown age (Fig. 1). She then tracked colony brood production, brood survivorship, emergent adult weight, and adult populations. We measured brood production by overlaying a plexiglass grid marked in square centimeters over the brood and visually adding it up (Fig. 2). We measured brood survivorship by overlaying a sheet of transparent acetate onto a comb and marking on it the location of 10-40 cells of live, uncapped larvae (Fig. 3). Three days later we returned the sheet of acetate to its corresponding frame and made note of surviving brood to determine percentage survivorship.
Colonies housed on new comb produced more brood and heavier ...
The Rise and Fall of the Dust-ructor
by Keith Delaplane
It was an idea whose time had come. Varroa mites were raging as the front-runner of beekeeping problems; the synthetic chemicals used to control them were coming under scrutiny as problems in their own right, and non-chemical alternative remedies were looking smarter and better all the time. And what could be a safer alternative than powdered sugar?
The idea of treating bees (and mites) with finely ground dusts such as wheat flour or confectioner’s sugar had been around a while, the idea being that dust impedes temperature-sensing organs on the mite’s forelegs that it uses to locate bee hosts, or impairs the mite’s ability to keep its grip on the bee1, or induces a grooming response from bees that dislodges mites2. Moreover, once a mite is dislodged and falls onto a dusty hive floor, it may have trouble moving around and eventually die of starvation. A handful of early studies suggested a degree of efficacy in dislodging mites with dust, either for diagnostic purposes or outright control, but these references were for the most part hidden away in non-English literature or obscure conference proceedings. But by the early 2000s there were new studies giving the matter more exposure in mainstream journals. Mite dislodging rates between 77%3 to more than 90%4,2 were being reported, and American beekeepers and bee scientists were taking notice. But no one can pretend that it was a revolution taking place. For starters, there was no consensus on such details as mode of delivery, quantity of dust, timing and intervals of treatment, or even the basic question whether dusting worked.
A convincing field-scale study finally came out in 2009 from Florida – and the results were not promising5. Amanda Ellis and her co-workers dusted the top bars of brood combs every two weeks from April until the following February and found no difference in colony strength or mite populations between dusted colonies and non-dusted controls. When I read these results, I was ready to write off powdered sugar once and for all, but that was not to be.
My intrepid staffers Brett Nolan, Ohad Afik, and Jennifer Berry weren’t quite as pessimistic as I and reasoned that several questions remained unresolved. To begin, they argued that (1) the efficacy of dusting had not been adequately tested in the context of a brood-free period (bee colonies in Florida are rarely brood-free), and it was exactly a brood-free period when one could expect maximum control when the whole mite population was on adult bees and vulnerable to dislodgement. They also argued that (2) more than one delivery method should be tested – especially one that could work at a commercial scale, and finally, they thought that (3) more than one treatment interval should be tested. In short – they dreamed up a whole new experiment6, and who was I to resist such youthful initiative?