Robbing Screens Part 9: Designing a Robbing Screen that Works in Conjunction with the Colony’s Guard Bees

Robbing Screens Part 9:

Designing a Robbing Screen that Works in

Conjunction with the Colony’s Guard Bees

Randy Oliver

ScientificBeekeeping.com

First Published in ABJ March 2025

The defenders of a colony against robbing are its resident guard force.  Do the current designs of robbing screens help or hinder those guard bees? 

Robbing Starts with an Attractive Scent

“Robber bees” don’t go looking for trouble — they are merely foragers attracted to a scent that they associate with a sugary reward, especially the scents of nectar.  A hive ripening nectar into honey strongly emits those scents, and each variety of honey emits volatile floral compounds that may attract the interest of foragers (Table 1).

The concept behind robbing screens is to misdirect foragers attracted to a scent away from the hive entrance, making it more difficult for them to find their way into the hive.  Our findings indicate that the designs that we tested indeed do so, but it was clearly evident that enough foragers can still figure out how to get in that robbing screens alone were not enough to prevent robbing — it was still up to the guard bees to keep any “potential thieves” that located the device’s opening from gaining entry to the hive itself.

History of the Robbing Screen

So let’s take a look at the original “robbing screen” invented by Dr. Harry Laidlaw (Figure 1).

Fig. 1 Two photos of Dr. Laidlaw’s original robbing screens (the first from [[2]] and the second from [[3]]).  Note how he left a part of the lower entrance open (rather than having a substitute entryway at the top).  And in the design on the left, he had a vertical metal barrier to separate bees attracted to the screen away from the entrance.

Having the entryway at the bottom allows the less-dense exhaust air to rise through the screen, thus diverting investigating potential robbers (guided by scent) away from the bottom entrance (refer to my smoke tests earlier in this series [[4]]).  So I built a copy of Laidlaw’s screen to test (Figure 2).

Fig. 2 For testing, I built a copy of Laidlaw’s original design, with a metal barrier between the screen and the small entrance to the side.

Of course we now needed to test it in the field against the other designs (Figure 3).

Fig. 3 I built some more top-viewing windows, so that we could compare all the designs side-by-side, moving the boxes to different locations for each run, and randomizing their placement.

The Laidlaw screen performed well, but some investigators still found their way in (Figure 4).

Fig. 4 All four types of robbing screens greatly reduced the rate of investigating bee entry, with the Mesh performing best, and the Laidlaw design on par with the rest.  Raw data in Table 1.

Table 2.  Raw data for the experiment.  The screen types are described at [[5]].

Although the robbing screens clearly made it difficult for potential thieves to locate their entryways, some invariably still got in!  And that’s why it’s necessary to have a security checkpoint of guard bees to prevent those that do locate the entrance from entering.

Practical application:  A robbing screen can make it more difficult for a potential thief to find its way to the hive entrance, but it’s up to the colony’s activated guard force at the nest cavity entrance to prevent robbing from progressing. 

Establishing a Security Checkpoint at the Nest Entryway

A point brought to my attention by Milan Jovanovic (one of the editors of Serbian Beekeeper Magazine) [[6]], was that the wide entrances and landing boards of Langstroth hives make it more difficult for the guard bees to intercept and defend the nest entrance against incoming thieves.  He’s not the only one — to minimize robbing or wasp predation, many beekeepers simply restrict the entrance opening to make it easier for the guard bees to defend it.  Ernie Daley, in an article in this Journal[[7]], noted that using an inner cover as a bottom board created a small, central lower entrance that benefitted his colonies. So I was surprised that Laidlaw, although providing a reduced entrance, placed it to the side of the hive, which would be away from the cluster of a weak colony.

Practical application:  It’s likely best for a robbing screen to not only have a restricted entryway, but for that entrance to be as close to the colony’s cluster as possible.

A second design consideration for a robbing screen is suggested by a key conclusion by Butler and Free [[8]]: “that guard bees recognize robber bees by their characteristic flight and behaviour rather than by their body odours”. 

We filmed foragers trained to anise scent exhibiting such back-and-forth flight as they investigated empty hive bodies exhausting that scent from their entrances [[9]].

Free’s later observations [[10]] are informative as to the optimal design for a robbing screen (I boldfaced his most germane finding):

[Our] observations strongly suggest that the identity of robbers is normally established by their behaviour, and then confirmed by their ‘foreign’ odour. This same mechanism acts as a safeguard to members of the guards’ own colony which approach the hive in a hesitant manner, although under what are probably exceptional conditions some of them may be killed by the guards. The practice among beekeepers of reducing the size of the entrance to a hive during the robbing season therefore not only leaves a smaller area for the guards of the colony to defend, but also causes congestion at the entrance and accentuates the innate hesitant flight of robber bees, which are not as well orientated to the entrance as the bees of the colony and so facilitates their recognition.

Practical application: Butler and Free’s observations suggest that for a robbing screen to be effective, it should promote some congestion, as well as allow the guard bees to visually monitor for the swaying investigatory flight pattern of investigating potential thieves.

As I pointed out in my article last month, we’ve observed that guard bees tend not to take up positions at the remote entryways of the robbing guards that we’ve tested.  I suspect that the reason is that the guards take up station where light enters the nest cavity — not at a remote opening.

Activation of the Guards

Colonies tend to adjust their number of guard bees at the entrance dependent upon nectar availability, since robbing pressure becomes more intense during dearths.  When there’s a strong nectar flow on, guards (if there are any) accept any and all incomers, but during a dearth they reject most non-nestmates.  However, in an apiary of closely spaced hives, there is often considerable drifting of confused bees between the colonies.  This influx of bees with foreign odors may accustom the guard bees to the odors of non-threatening “intruders.”

As found by Dr. Free, guard bees tend to allow drifted bees from other hives to freely enter, and they are not concerned about the occasional “thief” who sneaks in to nab a bit of stored nectar.  Dr. Free ingeniously experimented with training foragers to feeders within empty hives, then adding combs of bees from a different hive, or the parent hive, to the empty hive, and observing the changes in behaviors of the foragers and the guard bees (which were sometimes sisters of the foragers):

In the above experiments it has been found that bees which foraged for sugar syrup or honey at a hive other than their own did not assume the typical swaying flight of robber bees at the entrance to it, even when it contained an enclosed colony of bees having the same or different odour as themselves. The foragers adopted this behaviour when there was a congestion of bees at the hive entrance.

Free’s observation of the deterrent effect of congestion at the hive entrance is something to keep in mind (Figure 5).

Fig. 5 There are reasons that colonies, even in warm regions, prefer nest cavities with small openings — the bee activity there is intimidating to investigating foragers (who aren’t looking for trouble), and easier for the guards to monitor and defend.  Photograph by JarrahTree at wikimedia.org. 

Working as a security guard can be boring, until something activates them into taking action.  As pointed out by Free, guard bees don’t do much until they are “activated.”  Certain cues alert them that their colony is being threatened, and trigger them to “wake up and take action.“  The main cues (other than a predator or beekeeper scraping or jolting the hive) are:

• An excessive amount of the odor of foreign bees entering the hive (sense of smell).
• Swaying or jerky movements in front of the entrance (sense of sight).

Practical application:  It is this last cue that may be most important to activate the guards against potential “robbers,” and where the designs of the screens that we tested fail.  It was very simple for us to test for the presence of guards by simply looking down into the substitute entryways, or by seeing if there was any response to a bee lure waved by us above it — very seldom did we observe any response from an activated guard!

By this point in time, I had some ideas about improving the design of robbing screens, based upon the physics of air flow and the biology of bee behavior (rather than mere assumptions).  This involved addressing four issues:

1. Investigatory potential honey thieves cue to the attractive floral scents exiting a hive. Those scent plumes rise by convention, and thus guide investigating thieves right to an upper entrance.  Mesh screens work better than perforated plates.
2. The device should be field tested on scent-baited empty hives to confirm its effectiveness at frustrating and deterring potential “honey thieves” from finding their way in.
3. Ideally, the gateway should be placed so as to minimize confusion of the colony’s own returning foragers or workers returning from their first flight.
4. The gateway should be designed and located to work in conjunction with the colony’s guard bees. Perhaps there should be a single small entrance (easier to defend, and encouraging a little thief-deterring congestion), placed centrally (nearest to the broodnest), and blocking light from above (to help that day’s guard bees to recognize it as the entrance to the nest cavity and take up their defensive positions there).

Since I love to invent and build things in the shop, I modified Laidlaw’s design slightly (Figure 6).

Fig. 6  To try out the New design, Rose and I fabricated a batch for testing.  We placed a restricted “guard station” entryway in the center, blocking any light coming through the screen above, and incorporated Laidlaw’s metal barriers.

Testing the “New” Design

Issue # 1.  Moving the entrance away from the attractive scent

We had noticed again and again that returning foragers and investigating “potential thieves” tended to orient to the upper portion of the robbing screens, and used the scent emanating from the upper substitute entrance to find their way in.  So we ran smoke tests on the New design (Figure 7).

Fig. 7 There were gentle shifting breezes when we ran the smoke tests.  The smoke was slightly more buoyant than the ambient air (similar as how colony exhaust air will also be less dense), so it slowly rose up into the screened area (where the scent of nectar would attract investigating foragers).

Unlike as with the guards with upper openings that we previously tested, we rarely observed smoke exiting the low entryway, even as the smoke shifted from side to side with the breezes.  But would it work the same with bees seeking the scent of honey?  Take a look at Figure 8.

Fig. 8  Despite there being an open entryway at the bottom, bees orienting to the scent hovered above and in front of the screen.  We were surprised, however, how much more quickly the returning foragers figured out how to “get in.”

Issue # 2. Testing efficiency at diverting and frustrating investigating foragers

We again ran comparative tests to count how many potential thieves entered empty scented-syrup baited hives, using matched pairs — one unscreened Control, and the other with the New design (Table 3).

Table 3.  Twelve times as many scent-trained foragers seeking the odor of anise entered the control hives compared to those entering hives fitted with the New design of robbing screen, suggesting that this design appears to be reasonably effective at preventing “robbers” from locating the entrance.

Issue # 3.   Minimizing confusion at the entrance

When a robbing screen is first placed on a hive, the returning foragers have a hard time trying to figure out how to get in (Figure 9).

Fig. 9  With the robbing screens that we tested, there was often a pileup of confused and frustrated returning foragers (sometimes lasting for several days).  A substantial proportion of those bees may “just give up” and drift to nearby hives.

We didn’t see as much traffic congestion with the New design (Figure 10).

Fig. 10 Unlike as with the first three designs we tested, which confused their foragers for many days, foragers adapted very quickly to this design (flight often looked close to normal within an hour).  Note how bees seeking the hive scent go to the top of the screen, whereas returning foragers fly right into the entrance. Regarding the bees inside the screen, with time the traffic pattern stabilizes, and we see far fewer.

Since the foragers adapted so quickly to using the low-central entryway position, we hoped that there would be less drifting of first-time flyers, but we were apparently wrong (Figure 11).

Fig. 11  When we tagged 100 guard bees on the top bars, to our great surprise (I keep repeating that phrase), we recovered 20 tags over the next two days on the magnetic traps (shown below, after removal of the discs for counting) that we placed on two adjacent hives (10 discs in the first few hours after tagging the bees!).

Oh well, I guess no design is perfect, and the weather turned cold, so we unfortunately couldn’t repeat the test.

Issue #4.  Setting up a “security checkpoint” of guard bees scanning for potential thieves

Here’s where the New design really shone.  In our initial tests with heavy robbing pressure in November, with weak hives containing only four combs of bees, we observed that guard bees quickly set up security checkpoints at the entryways of the New design (Figure 12).

Fig. 12 With the small entryway near the center of the hive, guard bees made it clear that they weren’t going to let any robbers in.

Encouraged by these results, but with the weather getting too cold for further testing, I phoned my beekeeper friend Rob Stone in sunny Southern California, and asked how strong the robbing pressure was from the feral Africanized bees surrounding his apiaries.  He said “intense.” So I shipped him down a few robbing screens of each design and asked him to install them on his hives.  A few days later I flew down to observe and film them (Figure 13).

Fig. 13  Rob watching the entrance to one of the New robbing screens.

We’ve now closely watched the guard activity at dozens of entrances, including swinging a dead-bee lure in front of them (you can watch some video at [[11]]).  Unless the guards have already been activated by robbing pressure or hive jolting, there is little or no response.  But when a colony’s been exposed to robbing pressure, at least one guard will immediately rush out and follow the lure’s movements.  Its motion appears to immediately activate additional guards, who quickly rush to its side, looking in all directions, and help to attack the lure (often flying out to intercept it; Figure 14).

Fig. 14  You can barely make out the fishing leader and hook holding the dead bee lure, which we swung in front of the entrances to simulate a potential nectar thief.  Note the guards that immediately attacked it, as well as the activated guards who rushed out the entrance at the ready.

But that’s not all!  We noticed another unusual phenomenon (Figure 15).

Fig. 15  Not only did the guard bees protect passage through the entryway, but some activated guards would patrol the outsides of the screens!  Uncoordinated individual guards would chase down and drive away any foreign bees that landed on the screen!  There are at least three of them doing so in this photo.  Watch a video at [[12]].

Conclusions

OK, I’m wrapping up our investigations into robbing screens.  We found that it was of benefit for a robbing screen to have:

• A small, centrally-located entryway,
• Placed at the level of the bottom board, and
• Shaded from guard-distracting light.
• With the misdirection screen placed well above the entryway, so that the exhaust air rises well-away from the entryway. The screen should have a large “opening ratio.”

I publish our research and findings for the benefit of beekeepers everywhere — any beekeeper or manufacturer building robbing screens is welcome to use them.

Back to the Drawing Board and Shop

The above said, my assistant Rose wondered whether we could make the New design even more useful to the beekeeper.  So back to the drawing board and workshop (Figure 16)!

Fig. 16  We’re currently working on prototypes for a tricked-out model to cover all conditions.   Rose is thinking of producing them as a side gig, so she’s going to build a batch for others to try out.

The “Oliver Screen” will be designed for long life, hot-wax dipped, and have four adjustable seasonal settings:

• A screened anti-robbing and wasp setting.
• An unscreened minimal-ventilation restricted-entrance setting for winter (or year-round).
• An unscreened wide-open entrance setting for hot-weather or honey ripening.
• A screened closed-entrance “moving” setting for ventilated transport.

If you’re interested in trying one, send an email to endrobbing@gmail.com to get put on the list (Rose will respect your privacy, and contact you after she’s happy with the design, builds a hundred, and figures out a price).

Citations and Notes

[1] Manyi-Loh, C.E, et al (2011) Volatile compounds in honey: A review on their involvement in aroma, botanical origin determination and potential biomedical activities. International Journal of Molecular Sciences 12(12): 9514-9532.

[2] Laidlaw, H & J Eckert (1962) Queen Rearing.  Univ. Calif. Press (p. 98)

[3] Laidlaw, H (1992) Production of queen and package bees, p. 1041 in The Hive and the Honey Bee, Joe Graham, editor. Dadant & Sons.

[4] Robbing Screens Part 5:  They don’t just affect robbers.  ABJ November 2024

[5] Robbing Screens Part 7:  Can they alone prevent robbing?  ABJ January 2025

[6] Jovanović, Milan (2005) Troyan Tragedy of Bee Colony.  Serbian Beekeeper August 2005: 32-37.

[7] Daley, E (2022) Inner Cover Bottom Floor.  ABJ 162(4): 457-459.

[8] Butler, C & J.B. Free (1952) The behaviour of worker honeybees at the hive entrance. Behaviour 262-292.

[9] View at https://www.youtube.com/shorts/eLsXk2AZChg?feature=share

[10] Free JB (1954) The behaviour of robber honeybees. Behaviour 7(2/3): 233-40.

[11] https://youtu.be/vOetLg7IlpU

[12] https://youtube.com/shorts/AtQ0o8k_vnA?feature=share