The undervalued robbing screen
In my apiary, I consider robbing screens a necessity. Since I began using them about ten years ago, I haven’t had a single case of robbing. Prior to that, it was an annual fight to keep things under control.
This year in particular, I’ve had many beekeepers write in a panic asking how to stop the robbers. But, in truth, it is a zillion times easier to prevent robbing than it is to stop it.
Robbing usually begins in a nectar dearth when foragers from one colony find a hive that is weak or poorly defended. They detect the scent of stored honey inside and try to steal it. Fighting ensues, often with many deaths on both sides. If the defending colony becomes overrun, the marauders are free to take what they want.
Ripped and ragged cells
Robbing bees rip open honey cells instead of opening them nicely, honey flows out of the damaged cells, and the scent of honey is further disseminated. The scent will attract not only more bees, but other predators as well. Both the scent of honey and the odor of dead bees will attract hornets and yellowjackets. Once robbing begins, it feels like an avalanche. Everything gets worse in an overwhelming hurry.
Beekeepers can inadvertently initiate robbing by spilling a drop or two of honey or sugar syrup in the vicinity of the hives. Anytime you open hives in a dearth you are asking for trouble if you don’t take preventive measures.
Following the scent
Both wasps and honey bee robbers follow the scent of the hive. Since they don’t live there, they don’t know where the “door” is, so they sniff around. You can often see robbers examining the area just under the roof, the seam between two boxes, loose box joints, the area under the screened bottom, or any other place where the scent of the hive can leak out. They will keep trying and trying until they find their way in.
Robbing screens work by diverting legitimate hive traffic to an opening away from the real opening. The new openings are usually five or six inches above the main opening, and they are positioned above an impervious surface that prevents the hive odor from escaping.
However, a screen lower down near the “real” opening, allows the scent to escape. The robbing bees detect the scent through the screen and spend their time trying to find their way in. They don’t go near the diverted opening because it is covered by the impervious surface. Once the robbers move too far from the screen, the scent weakens, so they turn around and go back down.
Robbing screens traditionally have two diverted openings that open and close. Normally, you start with one open and one closed. If some robbers manage to find the opening, you can switch them and the learning process has to start all over again.
The first time I used robbing screens, I was skeptical, but they really do work. However, they work best if robbing never starts in the first place.
Use robbing screens early and often
I install my screens just before honey harvest because harvesting can sometimes begin a robbing spree. Then I leave the screens on until the fall when cold weather keeps the bees inside.
However, with nucs, splits, and captured swarms, I use a robbing screen from day one. These colonies are often small enough to get attacked by robbers during early spring or when there is a temporary lull in the nectar flow. I used to worry about those small colonies, but now I just add the screens pro-actively.
Types of robbing screens
A few different styles of robbing screen are on the market, but my favorite is by BeeSmart Designs. They are easy to put on and take off and they fit either 8-frame or 10-frame equipment. They come with push pins for attachment. If you buy the BeeSmart Designs ultimate bottom board, there are attachment points for the screen as well. Best, with the BeeSmart bottom board, the robbing screen can be replaced with a special mouse guard in the fall.
One of the problems with the wooden-framed robbing screens is that if you use a slatted rack, the rack and the lower brood box must be lined up precisely, because wood doesn’t bend around the edges and you can end up with a space at the sides large enough for a yellowjacket. You still need to have things basically lined up with the BeeSmart screen, but I find the plastic is a little more forgiving and easier to use.
Other advantages of a robbing screen
One of the overlooked advantages of a robbing screen is that it keeps out drifting bees as well. This can have a significant impact on varroa control. We know that drifting bees are one of the primary means of varroa dissemination. Drones especially are not very particular about where they bed down for the night, and they can easily travel from one hive to another. And because they’re drones, they’re usually allowed in.
If your colony is already overrun with mites, this might not make much difference. Perhaps you lose a few mites when drones leave, and you gain a few mites when drones arrive. However, if your mites are under control, drifting bees can make a huge difference.
In fact, if you have no mites to start with, one foundress mite can multiply into a sizeable infestation in one season. But if three foundress mites move in, they can produce three times as many mites in the same length of time. Multiple and repeated introductions can overwhelm even mite-resistant bees.
To me, it seems that robbing screens should be part of an IPM program to keep varroa under control. It is an additional technique we can use that is inexpensive, easy to implement, and chemical free. When paired with other IPM techniques, robbing screens can be a useful mite control tool.
Honey Bee Suite