Does a screened bottom board have any value? My vote is a resounding “Yes!” even though they are basically useless as a way to control varroa mites. A screened bottom board, especially when used with a sliding varroa drawer beneath it, can be a valuable diagnostic tool for the beekeeper. And for the beginner, it can provide a fascinating look at life inside the hive.
We usually associate the development of screened bottom boards with varroa mites, but according to Wyatt Mangum,1 the screened bottom board with a removable drawer was first used in 1853 for the control of wax moths. Wax moth larvae fell to the floor of the hive, through the screen, and landed on the drawer. Happy with plentiful wax debris to examine, the larvae stayed there until they were removed by the beekeeper.
This method of wax moth control eventually fell out of favor and beekeepers returned to solid bottom boards until varroa mites came along. Suddenly, screened bottom boards with removable drawers became must-have bee equipment.
The current thinking is that although some mites lose their grip and fall through the screen, the number of mites lost this way isn’t enough to make a difference. Mite counts based on the number of individuals that fall through the hive during a measured interval also have been questioned. The number of mites on a board is highly variable based on the time of year and the size of the colony, making it impossible to devise reliable rules for treatment based on mite drop alone.
As a result of disappointing mite control, many beekeepers are abandoning their screened bottoms and returning to solid bottom boards. But those who stick with them recognize their utility as a diagnostic tool. And because they are so valuable, beekeepers have found ways to alter screened bottom boards to minimize their shortcomings.
Some beekeepers find screened bottom boards useful for the control of small hive beetles in much the same way they were once used for wax moths.2 The larvae fall through the screen and land on the board. For beetles, the board can be replaced with an oil tray to drown the larvae.
Paradoxically, another benefit of screened bottom boards, ventilation, is also seen as one of their shortcomings. Some beekeepers, especially in colder climates, believe the screens allow too much heat to escape through the bottom of the hive, even with a varroa drawer in place. Other cold climate beekeepers—those who enjoy the advantages of the varroa drawers—fit a piece of foam board or other rigid insulation between the screen and the drawer during the winter months.
Unfortunately, sometimes beekeepers find clusters of bees hanging from the underside of a screened bottom board. This is most apt to happen in warm weather and appears to be a form of bearding. Usually, the bees go back inside when daily temperatures begin to drop. Although I have never seen it, some beekeepers report newly constructed honeycombs in this inconvenient location.
How the drawer is useful
A sliding varroa drawer can reveal secrets from within the hive. It is similar to an observation window or to a sampling port, like those on a wine cask. It provides a non-invasive way to learn what is going on inside your hive. Together with other observations, such as entrance activity or heat measurements, you can piece together a picture of colony activity.
Of course, the stuff that lands on screened bottom boards is the same stuff that lands on solid bottom boards. But the difference is your ability to see it easily and to evaluate the pattern of accumulation. For example, in mid-winter you can scrape the accumulation from a solid bottom board through the hive entrance, but the pile you get is not very instructive. But pull out a varroa drawer and you can easily see the size and location of your cluster.
In warmer months, the screen separates the bees from their debris. Honey bees are tireless housekeepers, and what they don’t manage to carry out, drops to the bottom. During summer, your bees are wizards at keeping the bottom board clean, and so they often remove evidence before you see it. But with a screened bottom, the bees are prevented from reaching the pieces that fall through. They have no choice but to leave them there for you.
Regardless of the type of bottom board you have, many different things will collect there. Most items are benign. Like trash from your house, it is the detritus of creatures living in a confined space. The accumulation is also seasonal. For example, you may find bits of wax nearly any time of year, but newly secreted wax scales are more common in the spring, while wax cappings are plentiful in winter.
You may also see drips of honey, bee feces, sugar crystals, legs and wings of dead insects, bits of propolis, small pools of condensation, or pollen pellets. The pollen pellets may be fresh or they may be rock hard, depending on what the bees are doing.
In his book, At the Hive Entrance, Heinrich Storch3 refers to the lines of debris between frames as “bands of decay,” and reminds us that the color of the bands varies with the age of the comb that is breaking off. Older, darker combs leave a darker pile and that, by itself, is nothing to worry about. He also reminds us that random dead bees are also normal, as is a modest accumulation of legs and wings.
The hive debris is often visited by a number of harmless creatures who are there for a free meal but nothing more. Such characters include earwigs, springtails, spiders, flies, assorted ants, and random beetles. It is important to remember that while some species of beetles, ants, and flies are predators of honey bees, most are not. Even a few scattered parasites, such as Braula coeca, the bee louse, are usually nothing to worry about.
Objects to investigate include varroa mites, small hive beetle larvae, wax moth larvae, chalkbrood mummies, the empty skeletons of honey bee pupae, mouse droppings, greenery such as moss, bee eggs, large numbers of bee heads or abdomens, honey bee pupae, or excessive honey.
While mites, small hive beetles, wax moths, and chalkbrood mummies are self-explanatory, some of the other debris is more cryptic. Honey bee eggs could mean the queen is laying more eggs than the workers can raise or may signal some other queen problem. Accumulations of greenery can mean mice are living inside. Empty larval skeletons or dripping honey can signal small hive beetles, while bee heads or abdomens may point to shrews or mice. Excessive bee parts could indicate yellowjackets. None of these clues point to a sure thing, but each warrants a second look by the beekeeper.
If you are in the unfortunate position of doing a postmortem on a colony, the bottom debris is a good place to start. You may find a dead queen, a layer of yellowjackets, piles of hollowed out abdomens, or any number of items that may give you a clue about why your colony died.
Variations on the varroa drawer
By providing additional ventilation to a hive, a screened bottom board also helps control excess moisture. A number of tweaks on the design have allowed beekeepers to have the advantage of some ventilation without having too much. What is too much depends on local conditions, so trial and error is necessary.
As mentioned above, some beekeepers fit winter insulation below the screen. Others replace the easily removed varroa drawer with a tighter-fitting winter board and seal any air leaks with tape and staples. Other beekeepers prefer to surround the bottom board with a floor-length skirt of tar paper that prevents drafts and collects solar heat. In the spring, they can quickly revert to the regular screened bottom board and all its advantages.
For those who want to control the flow of air during other seasons, the varroa board can be shortened or inserted only partway into the hive. The varroa board itself can also be drilled with holes. Holes allow increased air flow, but leave enough of the board intact to collect hive debris for evaluation. Alterations of this type are often used by hobbyists who have the time and inclination to experiment.
Maintenance of varroa drawers
To be useful, a varroa drawer must be kept clean. In warm seasons, you can leave the drawer out most of the time, returning it when needed for diagnostics. When you’re done looking, accumulation should be collected in a bag or bucket because hive debris dumped on the ground near hives can attract unwanted visitors. Everything from large animals to small parasites can latch onto the odor, so it’s best to remove it from the apiary. After you clear the board, a splash of water or a quick wipe with a wet rag makes them easier to read the next time.
The false-bottom alternative
One wintertime alternative to a screened bottom board is simply a piece of corrugated plastic cut to size and slid into the bottom entrance to cover the floor. It is not as revealing as a drawer under a screen because the bees can walk on it and clean it. It also reduces the size of the entrance, potentially making it crowded.
But for short-term winter diagnostics, a slide-in plastic board works well. The colony is clustered for the coldest parts of winter, so you can check the board by simply removing the entrance reducer or mouse guard, and sliding it out. Evidence of mice, shrews, and some other parasites will be clearly visible, as will be the position of the cluster.
Think before you ditch
Before you completely ditch your screened bottom boards and varroa trays, at least consider their advantages. Although they are not without problems, screens and drawers offer a view into the hive that is otherwise hard to achieve.
Honey Bee Suite
- Mangum WA. 2015. A Brief History of Beekeeping in the United States. In JM Graham (ED) The Hive and the Honey Bee (pp. 25-50). Hamilton IL. Dadant & Sons.
- Hood WM. 2017. The Small Hive Beetle, Aethina tumida Murray. Hebden Bridge UK. Northern Bee Books.
- Storch H. 1951. At the Hive Entrance. Reprinted October 2014. CreateSpace Independent Publishing Platform.