Backfilling: the sign of the swarm

Here is a photo that perfectly illustrates the concept of backfilling. You can see that the frame once contained brood in the center with an arc of nectar above and to the sides. But now, most of the brood has emerged and the empty brood cells, which are slightly darker, are being filled with nectar instead of more brood.

When this happens in spring, it means the colony is getting ready to swarm. In fact, backfilling is the most useful indicator of swarming because it happens early enough that a knowledgeable beekeeper may be able to prevent the swarm.

Bees backfill the brood nest to shrink it. Without so many places to lay eggs, the queen slows her egg production and the brood nest contracts. This is important because after the swarm leaves, the brood nest will be small enough that the remaining bees will be able to care for it. Backfilling provides a way of scaling down the entire brood-rearing operation so a smaller workforce can still get the job done.

One of the most effective ways to counter backfilling is to open the brood nest. Opening the brood nest is accomplished by adding empty frames between frames of brood. So while the bees are busy contracting the nest, you go in and expand it. When a breach in the nest occurs, the bees go in and draw comb and the queen fills it with eggs—all of which delays swarming and may prevent it altogether.

To avoid chilling the brood, opening the brood nest should only be done when there are enough bees to cover the expanded nest. However, backfilling normally occurs when a colony is preparing to swarm, which means there are usually plenty of bees and moderate nighttime temperatures.

Thanks, Nan, for sending a great illustration.


This colony is backfilling the brood nest in preparations for swarming. Photo courtesy Shady Grove Farm.

This colony is backfilling the brood nest in preparation for swarming. Photo courtesy Shady Grove Farm.


  • This is so helpful. Even though we’re very new to beekeeping, in the 5 or so bee books I’ve read, never have I read about “backfilling”. Really, really helpful. Thanks again!

  • Rusty,

    This is a perfect example of how information may not register if you don’t have a use for it at the time. I read about backfilling during winter, when I was nowhere near considering splitting or swarming. So Tuesday, my reaction was just, “Oh look they’re backfilling. Just what they should be doing as brood emerges,” with never a thought of the implications for swarming.

    Lesson driven home! Now I can summarize my entire question as, “Next year, should I just wait to see backfilling before I try to make splits or start to worry about swarming?” From the tangled history of cold spells, failed splits etc, it sounds as if I could have saved a whole lot of trouble!


    • Nan,

      Hard question. I don’t think I would wait to see backfilling. Usually, I try to be proactive about swarming, but if I misread a hive, the backfilling lets me know. This year I split two proactively, and now one of those is actively backfilling, so if it ever stops raining, I will go in there again.

  • Next year I’m going to try adding foundation to open the broodnest, instead of empty frames.

    I added a total of ten empty frames in attempts to open the broodnest. In one hive that looked like it was getting ready to swarm, I pulled out a nuc and inserted five empty frames. The hive didn’t swarm (success!) BUT a month later it had a population of nearly 50% drones! Turns out they drew all five frames as 100% drone comb, and when I pulled them out they were actively rearing a second generation. I ended up feeding the drone larvae to our chickens and replacing the frames with worker-sized foundation, but the colony lost a fair amount of resources producing drones and also built up a fair mite load.

    The two other hives that I added empty frames to also drew them as 100% drone comb. Maybe it is unique to the genetics of my bees, but in the future I think I will stick to foundation during swarm-prep season and only give empty frames to new swarms (which seem to draw them quite nicely).


    • Wow, Mark, that’s a lot of drones. Apparently, that’s one of the reasons foundation became popular in the first place—to control drones. But still, it’s hard to believe they made frame after frame of solid drones. Lucky you!

      • Hmm, my reaction would be rather the opposite: Clearly I haven’t been giving the bees the opportunity to rear an appropriate amount of drones, or they wouldn’t be going crazy like this.

        I observed the same thing when I switched a conventional nuc to foundationless. The first frames were all drones. But if you don’t interfere, they quickly get back to making workers, once the hive has the right ratio of drones to workers.

  • Rusty,

    Thanks, that is reassuring. I hope at some point to have developed the ability you describe, to “read a hive.” But I will never again overlook the significance of backfilling.

    Mark, I don’t know if it’s related, but in the later brood frames this particular hive produced, it seemed there was a higher proportion of drone cells. And the backfilled frames of honey are covered with fat ravenous drones.

    I wonder, when adding a third brood box, if it would be worth just using old drawn comb, if you had enough clean.

    Thanks for the insights! Rusty, my sympathy for the rain, as we now have YET ANOTHER cold spell (hi 30’s, lo 40’s nights)!


  • Rusty,

    Will the hive change directions from swarm prep to continuing to build up if you catch the backfilling in time? Last week my new hive was packed wall to wall in the 10 frame medium I started them in. I added another box by moving the center 3 frames of brood into the top box and adding empty frames to the 1,2 and 10 position in bottom box.

    I inspected them last night and really confused. They drew out new comb on frames 3 and 7. There are eggs in every cell on these frames. But the center frames are backfilled with a starter queen cup?

    Should I inspect them sooner than a week to look for more swarm cells? Do I need to look in the bottom box for swarm cells? I stopped when I saw the queen.


    • Margali,

      You can often stop the backfilling by manipulating the frames, which is why backfill is such a useful indicator of swarming.

      You pyramided your hive which often works well. It’s hard to tell exactly what is going on, but I wouldn’t worry too much about the queen cup. Some bees build these just to have them ready in case they need them. If they are drawing new comb and the queen is actively laying in it, I doubt a swarm is imminent.

      Swarm cells are usually found at the bottom or sides of the brood box if you have just one, or between the two brood boxes if you have two.

  • Rusty,

    Is it likely that I saw backfilling on one of my two package hives installed 4/26 or one caught swarm hived on 5/4? I saw backfilling this week when inspecting my hives. Would it happen so soon? I did not know what it was and did not note it in my records and can’t remember which of the three. But we remember seeing liquid in newly hatched empty cells. We are new at this and are at a loss but fascinated just the same. TonyG Port Townsend.

    • Tony,

      I suppose it is possible, though unlikely. The reason is that you’ve only had time for one full brood cycle (21 days) in that period. So the bees would have had to rear the one cycle and then immediately backfill, which seems odd. On the other hand, bees do unexpected things. I would look for other signs of swarming before I got too worried.

  • I found this exact pattern in one of my hives today and then a few frames down discovered a capped swarm cell. I didn’t have any extra gear with me, so I couldn’t do anything about it. I’m tempted to set up a swarm trap and just let bees swarm. I’ll keep an eye on them every day and maybe I’ll get lucky and find the bees hanging off a branch and then I can just re-hive them. That’s my wish.

  • There is an active colony in my backyard. The last several days I am finding bees come to die on my patio. Is this because the hive is overcrowded? I’m concerned about the probability of them in the grass as I wouldn’t want anyone to step on a stinger.

    • Beth,

      In a healthy colony of honey bees, about a 1000 die each day, mostly from age (workers only live 4 to 6 weeks) so I wouldn’t be too concerned about seeing dead bees on your patio. It is entirely normal.

  • Rusty, I recently attended an event in Norther California where Randy Oliver was a featured guest and I learned something very interesting that I thought I would share. Randy was asked a question about swarms and he said that the reason honeybees swarm is not necessarily due to overcrowding in the hive but due to a lack of open brood pheromones. He went on to say that when you have a very prolific laying queen she can completely fill the open comb and once all of the cells are closed the pheromone levels are very low. He said that sometimes the workers will actually kill the queen if this happens. He said to prevent this you can swap some of those frames of sealed brood with empty frames of built out comb from the upper brood box giving the queen more space to lay and this would counter their inclination to swarm. I don’t know if I got all of the details exactly correct but this is how I understood what he was saying. I am taking an intermediate beekeeping class from him on Sunday and I will have a chance to ask him more about it.

    • Paul,

      I believe this is akin to “opening” or “spreading” the brood nest. See “How to open the brood nest.” In any case, open brood pheromone has a powerful influence on the colony in many ways. I will be interested to hear what else Randy has to say.

  • Rusty,

    I received a 5 frame nuc 4 days ago. Lots of brood lots of bees. I placed them in an 8 frame deep. I checked them today to see if i should add a super and there is massive amounts of backfilling already occurring. I can’t believe how fast it occurred.

    I am new to beekeeping and I don’t have any drawn comb frames. The 3 empty frames are being drawn with comb but slowly and they too have nectar. I tried splitting the brood with these frames of foundation but I am unsure of what else I can do? I hardly saw any brood and most of them were drone cells. Could I try making a split?

    • Brad,

      I think what you are seeing is not technically backfilling, but perhaps just a response to the upset of being plopped down in a new hive in a new place. Give them some time to sort themselves out before trying anything as complicated as a split. And do not spread the brood nest with empty frames! This allows too much cold air into the nest, making it hard for the nurses to keep the brood warm. Spreading the brood nest can be useful in a mature colony, but not in one recovering from so much manipulation.

      If you saw little brood and most of it was drones, splitting is not appropriate. As a new beekeeper, you should sit back a little and let your bees sort out some of their problems. Too much beekeeper interfere can cause many problems, and until you become more skilled at “reading” the hive, you should let them lead you.

      You can learn tons by just watching. Don’t overestimate your ability at this point. Your skill will increase dramatically over time if you don’t panic.

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