beekeeping equipment

Tantalizing and weird things found on a bottom board

Even if you don’t use screened bottom boards for mites, they can give you many insights into what’s happening in your hive.

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.

This article first appeared in American Bee Journal, Volume 158 No 3, March 2018, pp. 285-287.

We usually associate 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. Until the beekeeper removed them, the wax moth larvae stayed on the board, happily munching through the plentiful comb debris.

This method of wax moth control eventually fell out of favor and beekeepers returned to solid bottom boards until varroa mites appeared. 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.

Differing opinions

Cluster Location: This varroa drawer hadn’t been cleaned in several months. By studying the pattern of debris, you can see how the colony decreased in size over the winter, as well as its current location.
Cluster Location: This varroa drawer hadn’t been cleaned in several months. By studying the pattern of debris, you can see how the colony decreased in size over the winter, as well as its current location. © Rusty Burlew.

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 the 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.


Harmless accumulations

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.

Bee Abdomens: This pile of bee abdomens accumulated in the back left corner of a solid bottom board. The small remaining colony was in the right front corner. I suspect this the work of a small mammal, such as a shrew, mouse, or vole.
Bee Abdomens: This pile of bee abdomens accumulated in the back left corner of a solid bottom board. The small remaining colony was in the right front corner. I suspect this the work of a small mammal, such as a shrew, mouse, or vole. © Rusty Burlew.

Deeper investigations

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.

Varroa Mites: Varroa mites are just one type of creature you may find on your bottom board. Although most species are harmless, some signal trouble inside the hive.
Varroa Mites: Varroa mites are just one type of creature you may find on your bottom board. Although most species are harmless, some signal trouble inside the hive. © Rusty Burlew.

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 to 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 are 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 airflow, 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.

Pollen Pellets on Bottom Board: I gave this colony some old frames of pollen, hoping they would clean them up. It took about a week, but they discarded every last cell of old pollen, hard as marbles.
Pollen Pellets on Bottom Board: I gave this colony some old frames of pollen, hoping they would clean them up. It took about a week, but they discarded every last cell of old pollen, hard as marbles. © Rusty Burlew.

Maintenance of varroa drawers

You must keep a varroa drawer clean, otherwise, you can’t see anything. In warm seasons, you can leave the drawer out most of the time, returning it when needed for diagnostics.

After examining the drawer, you should collect the accumulation in a bag or bucket. Otherwise, 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.

Greenery: The first clues I had about this mouse nest were fine pieces of green moss that filtered down onto the varroa tray.
Greenery: The first clues I had about this mouse nest were fine pieces of green moss that filtered down onto the varroa tray. © Rusty Burlew.

The false-bottom alternative

A piece of corrugated plastic cut to fit over the bottom board makes a good wintertime alternative to a screened bottom board. Some people slide the plastic sheets through the front opening as cool weather sets in. This system 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 stays 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 the position of the cluster.

Dead Mouse on Screen: The mice don’t always win. Sometime they are picked apart and discarded, like this one. Other times they are sealed in propolis.
Dead Mouse on Screen: The mice don’t always win. Sometimes they are picked apart and discarded, like this one. Other times they are sealed in propolis. © Rusty Burlew.

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.

HopGuard Cardboard: The fluffy mass on this varroa drawer is what’s left of cardboard strips of HopGuard.
HopGuard Cardboard: The fluffy mass on this varroa drawer is what’s left of cardboard strips of HopGuard. © Rusty Burlew.

Rusty Burlew
Honey Bee Suite


  1. 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.
  2. Hood WM. 2017. The Small Hive Beetle, Aethina tumida Murray. Hebden Bridge UK. Northern Bee Books.
  3. Storch H. 1951. At the Hive Entrance. Reprinted October 2014. CreateSpace Independent Publishing Platform.

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  • I found bees clustered underneath the screened bottom board on two hives just two weeks ago. My theory is the returning queen took a slight wrong turn and then was joined by many of her colony. Yes, they built comb. I rebuilt the hive with a new bottom board and carefully knocked/swept the bees into the top of the hive. Fingers crossed (I never saw a queen).

  • Rusty! Thank you for your site! I look forward to reading each new post and am constantly scanning the index for answers to questions. I am a new-bee and just started with 3 packages here in Northern Illinois.

    Question: the varroa boards don’t always fit perfectly and on one of my hives a few bees were always seen going in and out of that darkened area and crawling around on the board. I ended up duct taping it closed because all I envisioned were mites getting an easy taxi ride back up into the hive.

    Do you think this could be the case and should I try to keep the tray sealed up the best I can?

    PS: This bee stuff is great fun!!!

    • Jill,

      Take the tray out for the summer. You don’t need it now anyway. Soon after it is gone, the bees will stop going under there.

      • Make them fit better to keep the bees out. I do that with my trays. They have oil in them. A bee goes in, it won’t come out and neither do any mites, larvae, or beetles. It may be a small loss of bees, but the hive stays healthy.

  • Rusty,

    I have a screened bottom board and a styrofoam pull-out board. My issue seems to be carpenter ants. I pulled out the drawer today and it looked like a full fledged colony had taken up residence, they were even starting to make burrows in the styrofoam. In your opinion, IS IT an issue, and what would your recommend if it is?

    • Gerry,

      The ants are eating stuff that dropped out of the hive. Just remove the styrofoam for the summer. You don’t need now anyway.

    • Gerry,

      The ants are eating stuff that dropped out of the hive. Just remove the styrofoam for the summer. You don’t need now anyway.

      • Rusty,
        Right you are! The board was spotless as a matter of fact. I removed it like you suggested. I think the tunnelling if left unchecked would have compromised the integrity of the styrofoam, and for the summer I like your idea of increased ventilation, as it is in a well protected spot with little wind exposure.

      • Rusty

        A follow up I didn’t expect. I noticed a carpenter ant, then another enter the entrance of the styrofamed cover. So I flipped it over and voila! A mass of ants scurrying about on the cover! Is this common? Does this behaviour negatively impact the colony? Suggestions?

  • Very nice article. Some in my area have gotten rid of their screened bottom boards for various reasons. #1 seems to be due to less brood towards the bottom of the frames. Cold weather in the winter comes in at #2. That being said, I think they should try slatted bottoms. For the latter, I have overwintered a colony that had only a handful of bees and I had inadvertently left the screened bottom open. Not on purpose mind you. They made it and I got honey off that hive.

    I live in southeast Idaho where the winters are pretty cold and snowy.

    • Ken,

      Good example of cold-weather “management,” even though it was accidental. I still believe way too much emphasis is put on trying to keep bees warm, when other issues are more important.

  • Hi Rusty,

    good topic, this is something I have wrestled with on and off for a while. I went with screened bottom boards in one apiary a couple years ago and lost all 10 hives in the winter, when the leaves dropped it was very windy and snow drifting in winter. Could be non related, the was sigh of Verroa, so could have been Visous related.
    I like the summer ventilation aspect, all most always when I get a lot of bearding in the summer on hot days, changing from a solid to a screen bottom board fixes the issue.
    I am now experimenting with a new bottom that has a drawer.
    Screened bottom, 5 inch high sides, Drawer (4 inch) that seals well for the winter.
    Benefits hope for:
    Pull out Drawer for inspection , and dumping of hive droppings.
    Air space under the brood nest for air exchange.
    No mold in the debris in the spring, laying on the solid bottom board. Some what requiring the hive be tore down and scrape the junk off. This was the undesirable I most wanted to get away from. time , effort
    In the Hot season partial open drawer for more air flow, I have also began using supers with a 5/8 hole in them for heat escape.

    So far it seems to be working, only have 4 hives in the test and only since this spring.
    One thing I have noticed is that pollen pellets at times fall thru the screen, so some small pollen loss looks apparent.
    Future change, the drawer “should” come out the back So the playing around with the drawer is easier, seems obvious now. 🙂
    Concern, seems ants can get in under the screen, in the drawer area, and may have a place to accumulate out of reach of the bees.

    • Hi Brian,

      I have screened bottoms that slide out as well, both the drawer and the screen. They came from Kelley Bee Supply at least 10 years ago. I don’t know if they still make them or not.

  • Thank you, Rusty. I use Freeman beetle traps, and have been wondering if I should plan to return to solid BB for the winter, and now I have ideas for keeping the screened boards and not having to switch back and forth every year. And…I’m going to read up on slatted racks. I love how one well-written article leads me to another on your site!

  • Thanks again
    I’ve heard advice to make the varroa tray sticky with oil or vaseline so the varroa and debris can’t crawl away or be blown away in the wind. Do you think it’s worth the effort?

  • Rusty, 2 questions:

    Here in western WA, do you leave the bottom of the hive unaltered in winter, for the ventilation the air leaks provide?
    Have you tried feeding those pollen pellets back to your bees?


    • Hi Kitty!

      I leave my screened bottom boards open all winter, with no varroa tray inserted, unless the temperatures drop down into the 20s for an extended period. A day or two in the 20s is okay, but it if goes on longer than that, I just slide in the varroa trays.

      If you are referring to the pollen I collected in my pollen trap, no. I still have those pollen pellets in my freezer. The best laid plans…

  • This is my first season as a beekeeper. I live in Colo at 7700 feet. I have a screened bottom and with temps now dropping into the high 40s at night, I slid in my styrofoam board below by screened bottom. This morning, I pulled it out to examine it and there was water on it (no rain in the interim). Is this normal? Inner cover is not damp. Hive is very full and still foraging. I’m nervous about winter and moisture in the hive. When should I use styrofoam bottom board? When should moisture quilt go on? Love your willingness to share your knowledge and ideas.

    • Carin,

      I always see moisture at the top before the bottom, so I don’t really know what’s going on there. I had to put quilts on my largest hives already because the inner lids were soaking, even though we are still in the high 40s at night. Try using both the styrofoam board and the quilt and see if the problem goes away. Try it for a few days and let me know.

  • Hi Rusty

    Thanks for your note back. I tried the quilt and left my styrofoam board in and still found water on it in the morning. My husband and I think that the water is collecting on the styrofoam board because of the temp difference between outside and the large bundle of bees right directly above the screened bottom board. I am doing two things after reading many of your posts: adding a slatted rack above my screened bottom board which will move the brood nest and big bee bundle up higher in the hive; and adding a wood backing (1/2”) beneath the styrofoam board (3/16’) for added insulation. I should have mentioned that my hive boxes are on a hive stand that holds them about 18-20 inches above the ground. Do you have any other suggestions or concerns with my path forward as winter approaches? I appreciate your thoughtful approach on everything. Carin

  • This article was very helpful, thank you! I am a new beekeeper and this is my first fall season but we have had a few extremely cold days here in Ohio already. We had a warm day this weekend and I went to quickly check the bees. I didn’t plan on opening the hive, just listen and look. I saw lots of what looked like the golden bee feces on the entrance but didn’t hear or see any bees. It was windy so I didn’t open anything but I did pull the varroa drawer out. There were a few dead yellowjackets there, lots of debris, and what looked like some type of web. I’m thinking that this isn’t a good sign but would like to wait for a decent day to open everything up and make sure. Thoughts? Suggestions? Thanks in advance! 🙂

    • Charlotte,

      The stuff or the bottom board is normal and nothing to worry about. Webs, debris, yellojackets, or an assortment of miscellaneous bugs don’t mean anything on the bottom board, a place that will attract all kinds of wildlife.

      Droppings near the entrance can happen after a cold snap. Yellow droppings are normal, dark droppings can signal dysentery or some other condition. I usually check the color before I get concerned.

  • Hi Rusty,

    I like your blog and I refer to it as soon as I have a question about anything on beekeeping, thanks for your work! 🙂

    I just wanted you to know that here, in Quebec, Canada, we have winters way colder and longer than yours (freezing nights from October to May) so the big problem here, is not temperature, it’s humidity, cause it ends up building ice into the hive. To avoid that, we need a lot of ventilation so pretty much all the beekeepers here let their screened bottom board open without any sort of isolation to maximize the flow of air and for the colony to get rid of that moisture. Since snow covers much of the hive, winds are not a problem and since the hot air rises, the bees are cozy and there’s no cold drift that reaches the cluster. Another good advantage for this device 😉

    Also, I’m currently monitoring my small colony without opening the hive cause it’s still too cold out here and I’m wondering if there any differences between wax bits that fall on the board that comes from a honey cell and a brood cell? In other words, do you think there’s a way to know if the colony is currently rearing brood by looking at the bottom board?

    Thanks again for what you’re doing!

  • Hello,

    We have just got our hive of bees and have been checking the observation board quite frequently. This afternoon we checked the observation board and found a large buildup of brown debris on one side of the board. We were wondering if you could give us some insight on what this might be.


    Ps. If you could email us soon, I would love to send some photos of the board via email.

  • Rusty,

    I have my hives all with screen bottom boards, each side edge sitting on concrete blocks. This is leaving openness for airflow to keep them cooler during the summer heat. I have put salt on the ground just below to kill any mites, larvae, whatever falls through the hardware cloth. By having the screen bottom uncovered below, am I making them vulnerable to something that could harm them? If so, what?

    Thank you,

    • Jana,

      The whole point of screened bottom boards is to have them open, both for ventilation and debris removal. They also provide protection from predators.

      • Hi Rusty, remaining on this topic, I live in coastal NC and all hives are on screened bottom boards with a slide-in corrugated plastic IPM board. Last summer I pulled all IPM boards for increased ventilation. Currently, I run a hive monitor on one hive that includes an indie sensor that measures temperature and humidity. On a 90-degree day with 85% humidity, the inside temp was 94.5 and humidity 65-70%. The question is by pulling IPM board for increased ventilation is it harder for bees to keep humidity low? Does the plastic board serve as an aid, or should I pull it, because the board doesn’t have much effect? Thank you!

        • Brad,

          Removing the IPM board should make it easier to control humidity. Much of the inside humidity comes from bee respiration.

  • 1st year beek here. My screened bottom board became really stuck. I finally moved everything and got it out, washed with hot water, and it separated. Is there anything I can use to keep it from getting stuck? BTW, I think I’m hopelessly queenless and honey bound. Western NYer. Any advice?

  • Lived in Homedale ID many years ago and had bee boards on our roof. We just wonder if that practice continues today? Miss those times! Thank you.

  • Rusty, thank you so much for your wonderful blog. I spend hours reading your well written and highly educational posts. I live in Northern Wisconsin (gardening zone 4a) and I have been a beekeeper for four years. I have screened bottom boards on my hives and I like them for ventilation purposes in the summer and for observation of the clusters in the winter. This winter I have noticed something that I had not seen before on the white bottom insert of one of my hives: dark colored pools of liquid (condensation?). I use moisture quilts on my hives but there still is a little condensation that drips down on the bottom. However, I have never seen it in this color, or any color for that matter. It is almost the middle of February and there is a thick layer of dead bees on the screened bottom… I wonder if the decaying bees could have something to do with the color of the moisture dripping down. Thank you in advance, Laura

  • Hello. Love your blog.

    My bottom board has small red flecks among the other debris. It’s not there all the time – comes and goes. Never what I’d call alot.

    Any ideas?

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