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Modeling Nuc Buildup

First published in: American Bee Journal, June 2018

Randy Oliver
ScientificBeekeeping.com


CONTENTS

The Players and the Numbers

A Simple Model

Wrap Up

Acknowledgements

References


My sons and I sell a lot of nucs each spring. But after a couple of months, not all have built up to the same extent. Is that because of the queen, the bees, or how we originally put the nuc together?

There are a number of ways to make a nuc (a nucleus hive)—with a laying queen, a queen cell, or by doing a “walkaway split” (in which the bees are forced to rear emergency queen cells). Plus, a nuc can be made with anywhere from two to five or more frames of bees, and with zero brood to as many frames of brood that the cluster can cover.

One might make nucs simply to arrest the swarm impulse, but in general, most beekeepers produce them in order to create fresh colonies to make up for winter losses, to make increase, or for sale. Therefore, the rate of buildup of the nuc is of critical importance—in general, the question is whether it can it build up in time to take advantage of the local honey flow?

The Players and the Numbers

A nuc is put together from three components—some number of frames covered with workers, some number of those also containing brood, and some sort of queen. For the purposes of predicting the growth rate of any nuc in this article, I’m going to make two assumptions—that the weather and nectar/pollen flow are favorable, and that the beekeeper adds additional frames of drawn comb as needed to prevent any restriction on the queen’s egg laying ability. I’ll also arbitrarily target a colony size of 20 frames of bees as the goal before the honey flow.

The Workers

If you want to count ‘em, there are typically between 2,000 and 2,500 bees on a fully-covered deep frame [1]. Frames containing brood in the center of the cluster are typically less densely covered, so for the purposes of this article, I’ll assume 2,000 bees per frame for all my calculations.

The workers in a nuc will typically consist of bees of mixed ages; however, if the nuc remains in the same yard from which the bees were taken, the field bees will fly home, thus leaving a nuc of mostly younger bees. For the purposes of my calculations, I’ll assume that the nuc is moved to a new yard in order to minimize such flyback drift [2].

Now consider a main limiting factor for nuc buildup—the number of thermoregulated brood cells within the cluster available to the queen for egg laying. A deep Langstroth frame with standard foundation contains slightly over 6,900 worker cells—if filled 90% with brood, that equals 6,200 cells of brood. A vigorous young queen laying at a rate of slightly over 2000 eggs per day might fill a frame with brood in 3 days—thus making that frame unavailable for more egg laying until that brood begins to emerge about 20 days later [3].

Practical application: the adult workers initially installed in a broodless nuc (such as a package or a hived swarm) are in a race against time in order to maintain a heated broodnest for at least 20 days until their replacements begin to emerge. Harbo [4] found that that you get the most brood per bee over the first 20 days from a package of a little over a pound of bees (which would initially cover a bit over 2 frames).

frames of brood at 19 days

I reworked Harbo’s data into the graph above to show how much brood was produced by package colonies of various sizes over the first 19 days after queen release. The black line indicates the “efficiency” of various package sizes at producing brood—note that peak efficiency occurred at 4,500 bees (which I converted to 2.3 frames covered with workers), but dropped as the package sizes increased to around 18 frames of workers (36,000 bees).

Practical application: although a larger cluster allows for more broodrearing, smaller clusters are more efficient at the job—you get more bang for your buck with smaller splits.

Harbo’s data clearly show that a nuc’s ability to rear brood is constrained by the number of frames covered and heated by the cluster. But as the original cohort of adult bees ages and dies, every day there are fewer and fewer workers to maintain that broodnest—until replacement workers begin to emerge 20-21 days later. And that’s assuming the queen of the new colony can begin egg laying immediately. And that introduces the subject of the queen…

The Queen

A nuc is typically made with a new queen. However, my sons and I also get excellent results from nucs made with queens in their second season.

Practical application: second-season-queen nucs are more likely to swarm, and the queens typically show signs of wearing out by the end of summer.

A nuc made with a laying queen (the bees’ own queen or an introduced caged queen) has the advantage of eggs being laid immediately (although as noted by Harbo [5], the nurses may consume eggs laid beyond the colony’s capacity to care for the larvae).

On the other hand, nucs made with either a queen cell, or a walkaway, will not benefit from new eggs until the new queen has mated and begun egg laying, as per the table below:

TIMING OF FIRST EGGS AND EMERGING BROOD BY QUEEN TYPE
Type of queen given to the split Minimum days until first eggs, assuming good mating weather Minimum days until first worker brood from the new queen begins to emerge
No queen cells (walkaway split) Typ. 22-26 42-46
Freshly-sealed queen cell 17-21 37-41
Ripe queen cell 10-14 30-34
Introduced laying queen 0-3 20-23
ScientificBeekeeping.com

Even once a queen has begun egg laying, we find that some are more productive than others. We ruthlessly cull our poorer-performing queens, keeping only those that show their worth.

Practical suggestion: start each season with at least twice as many colonies as you intend to take through the next winter. There is no reason to baby inferior queens—cull ‘em.

The Installed Brood

Returning now to the concept of the diminishing size of the founding cluster of bees in the nuc (due to aging and attrition), this problem can be addressed by including frames of brood. If some of that brood is already sealed, replacement workers may begin to emerge immediately. A single frame 65% covered with brood, will produce roughly 4,500 workers—enough to fully cover more than two full frames.

Practical application: the initial growth of a nuc is from the included brood—the earliest that any brood from the new queen can add to the population is 20 days later.

It’s easy to overestimate the actual amount of brood on a frame. The yellow ellipse represents 65% of a deep Langstroth frame covered with brood (~4,500 cells of brood total if the other side is equally covered).

In cool weather, you’re limited as to how much brood you can place in a nuc without the risk of it getting chilled, but in general, the more brood incorporated into a nuc, the more rapidly it can grow.

A Simple Model

I was curious a couple of years ago as to what the optimum configuration of a nuc would be for the most rapid overall growth. So I created a simple model in Excel to predict the buildup, using Lloyd Harris’ measured survivorship curve, in which the average worker survives for 35 days [6]. I also assumed that workers that have not yet engaged in broodrearing will have greater survivorship until after they’ve begun feeding brood [7] (this is a factor in walkaway splits that go broodless until they’ve reared a new queen). And then I tweaked the simple model until it matched personal observations and/or hard data from others.

I then went to the field to validate the model by comparing its simulations to measurements from actual nucs. We set up a group of nucs with every combination of bee coverage from three frames to five, and amounts of brood from zero frames to 4. I measured the brood with a 1” grid, and the amount of bees by counting the number of frames fully covered with bees once the nucs were settled.

As you may imagine, this took me and my assistant many hours. We then tracked buildup, again tediously measuring the amount of brood with a grid each week. To our dismay, we were blindsided by an unexpected spell of freezing weather, which shut down buildup, forcing us to end the experiment. Undeterred, we set up a repeat several weeks later. This time, we got hit instead by an extreme heat wave, which again made all our work moot. I’ve yet to make a third attempt.

That said, I did polish up the model for this article, and will share the results of a few simulations. For each of the simulations below, the assumptions are that it takes 2,000 bees to cover a frame, that a “frame of brood” is 65% covered with brood of all ages, and that the founding cohort of bees is a mixture of age groups, each of which exhibit a typical survivorship curve (that curve is extended for bees that emerge without brood present). I also assume optimal conditions (temperature, nectar, and pollen), as well as the lengthening days of spring, which appear to stimulate broodrearing [8]. The model also allows for adjustment for queen quality and conditions, but I didn’t change those for the simulations below. All simulations assume that the beekeeper adds drawn combs as necessary until the colony completely fills two Langstroth deeps (20 frames of bees).

DISCLAIMER: This is a very crude model. Please don’t take any of these simulations as hard values—they are only for comparison of the theoretical results resulting from changing the amounts of bees or brood (the upper two yellow cells), and/or the type of queen (L for laying, C for ripe cell, W for emergency queen).

PACKAGE BEES

The first simulations are for packages, first a 2-lb, then a 3-lb.

A 2- lb package would be expected to build up to 20 frames of strength in around 10 weeks. I’ve adjusted the model to fairly closely match the data from a very large study on package buildup by Nolan [9]. Note how the colony grows in fits and spurts as the queen runs out of open cells suitable for egg laying. Also note the recruitment of workers from the new queen (dark orange)—Nolan found that it generally took 35-47 days for their population to reach that of the original starting cohort (brown), and then grow explosively over the next 15-20 days; after that, growth was less predictable.

 

A 3-lb package would be expected to get a head start, since the cluster can cover more brood area. However, the prediction of reaching 20 frames of bees by eight weeks after installation occurred in only a few of Nolan’s test hives.

Practical application: There is an initial benefit of a larger package, but once the queen reaches her laying capacity, the advantage of a stronger start is lost. Nolan found that package colonies of any size typically reached their maximum population at around 10 weeks (70 days) after installation. Indeed, smaller packages (< 2 lbs) grew proportionally much greater than did larger (up to 10 lbs) package colonies over the first 8 weeks. The question to the beekeeper is how much time there is between package installation and the honey flow—if more than 10 weeks, save your money and get a 2-lb package.

NUCS WITH A RIPE QUEEN CELL

The reason that I originally created a model was to figure out how best to make up our own nucs, using ripe queen cells. We have less than two months for them to build up before our main honeyflow. Note that in all the following simulations there is a yellowish cohort of bees, representing those that emerge from the originally-installed frames of brood–pay attention to this cohort, as it turns out to be the key to nuc growth.

 

A 5-frame nuc with the outer frames completely covered with bees, given a single 65% frame of brood and a ripe queen cell, takes around 65 days to reach 20 frames. This figure is realistic if the weather cooperates, but what I’m interested in is using it for comparison to the next two simulations.

 

Adding a second frame of brood drops the buildup time to only 55 days. Compare the yellowish cohort of workers that emerge from the original brood frames to those in the graphs above and below. This simulation typifies the ratio of brood to bees that we usually use, since we typically split our hives when they contain 20 frames of bees, with 10 containing some amount of brood.

Practical application: note that by three weeks after creating the nuc above, that it will be seriously crowding a 5-frame nuc box (due to the emergence of the original brood). At this time the new queen will have nearly refilled the broodnest, and if the nuc is not soon moved to a larger hive, it will begin preparation for swarming.

That said, how important is the original cohort of workers, since many will die of old age before the new queen even begins laying eggs?

 

 

In the simulation above, I reduced the starting worker strength to only 4 frames. Note that the nuc grows nearly as fast as the one started with 5 frames of bees—it appears that the 5th frame of bees is wasted on a nuc with 2 frames of brood.

Let’s increase the amount of initial brood:

 

Adding a third frame of brood allows the colony to reach 20-frame strength in only 48 days That difference has a large effect upon how much honey the colony will produce in my area, since our flow is typically over within 90 days of when we make our nucs. Adding an additional 5th frame of bees (not shown) gave no benefit, since there is no laying queen to take advantage of those bee-covered cells.

Practical application: note that the nuc above would crowd a 5-frame nuc box within a week. A nuc given three good frames of brood should be placed in a full brood chamber rather than a 5-frame nuc box. Such strong nucs can also quickly starve during unfavorable weather or if there is no nectar flow—be generous with combs of honey or sugar syrup.

As I suspected, the amount of brood originally placed in a nuc is the most important factor, since it allows for the emergence of a large cohort of new workers to carry on broodrearing once the original bees have dwindled from age and wear.

Take home lesson: if you want rapid buildup, don’t short your nucs on brood. The more, the better, so long as there are enough workers to prevent chilling, and adequate honey reserves

SMALL NUCS

At this point, allow me to be clear that there are times that you may wish to create small nucs—such as for the purpose of mating queens (perhaps in mini nucs). Or one can make small late-season nucs in order to hold young queens for later sale, or to overwinter as queenright nucs with a queen that have never been given enough room to reach her laying capacity.

Practical biology: a queen’s “age” is not a function of chronology, but rather determined by how many eggs she’s fertilized [10]. A queen in a nuc kept small is unable to lay very many eggs, and thus remains “young” until she is placed in a hive that allows her to reach her laying potential.

Such a late-season nuc intentionally created with a small cluster has the advantage of generally not growing fast enough to plug out and swarm before winter (but they may require feeding).

Practical application: remember about Nolan’s findings about small packages? My simulation for a 3-frame nuc with a single frame of brood (not shown) takes around 72 days to build to 20 frames. Such weak nucs made during swarm season can be used to create a nice reserve of spare queens to have on hand, without needing to worry about them swarming in short order. You can keep them weak by pulling out frames of brood—simply shaking off the bees if you can’t find the queen.

Conversely, we also make extra-strong nucs. Something that we’ve learned in recent years is to not kill the old queen when we make up nucs—instead, we now save any productive second-season queens, waiting to replace them until after the honey flow. Since these nucs are started with a queen in full lay, they take off right out of the gate, as in the simulation below, in which I entered a laying queen rather than a queen cell.

 

Now we’re down to only 42 days to approach maximum strength. Starting with 3 frames of brood reduces that figure to only 33 days (not shown). The difference is due to recruitment of new workers from eggs laid by the queen starting at least 10 days earlier than you can hope for in a nuc given a queen cell.

Practical application: this trick allows us to get an additional season out of productive second-year queens—allowing us to requeen the colony at our leisure after the honey flow. With the earlier honey flows that we are experiencing with climate change, these “super nucs” really help us.

WALKAWAY SPLITS

Many beekeepers simply split their colonies in two prior to swarming, and allow the queenless half to rear a new queen from scratch.

Practical caution: as documented by Winston [11], with Africanized bees, as many of two-thirds of walkaway splits may swarm upon the emergence of the emergency queens. This phenomenon appears to be less common with European stocks, but suggests that it may be of benefit to remove excess queen cells. Winston’s work also suggests that the workers later cull any emergency queen cells that were started with larvae over one day of age—thus the fear that the first queen to emerge in a walkaway split will be substandard may be unfounded.

So let’s take a look at walkaway splits. The queenright half would be expected to perform similarly to the “super nuc” above. On the other hand, buildup would be much slower for the queenless half. In the simulation below, I assume that it gets 4 frames of brood–since few 2nd-year queens can produce more than 8 frames 65% covered with brood (which would require a sustained egg laying rate of over 1,800 eggs per day).

Note the lag before you see new workers emerging (dark orange), due to the minimum 24-day before an emergency queen can be expected to start serious egg laying. Loading up the queenless half with brood doesn’t help much (not shown). The result would be better if there were already near-ripe queen cells in the queenless split.

Practical application: it takes slightly longer for the 10-frame queenless walkaway split to build up than it does for a 5-frame nuc with a ripe queen cell. I suggest that all beekeepers learn to rear their own queen cells, and breed from their best colonies—see “Queens for Pennies” at ScientificBeekeeping.com.

And don’t forget to take advantage of the induced queenless window in splits for varroa control—see http://scientificbeekeeping.com/the-varroa-problem-part-15/.

Wrap Up

The generic simulations above assumed perfect conditions, with the goal of building up in time to take advantage of the honey flow.   That said, conditions and best management practices vary greatly from region to region. My hives are full of brood and drones and are ready for splitting when I pull them from almonds in mid March—which allows me two months before my main honey flow, after which we experience a dearth for the rest of the season. In other areas of the country, the main flow may come earlier or later, or there may even be a pollen and nectar flow in the fall. Thus, one must take into consideration the local environment, the phenology of the local nectar and pollen flows, the intended goal(s) from making splits, whether one is willing to perform supplemental feeding, and how one manages varroa. My hope is that the above simulations help you to better understand the results of the options involved in making nucs.

Acknowledgements

Thanks to Pete Borst for research assistance, and to John Harbo, Stephen Martin, and Lloyd Harris for their data.

References

[1] Delaplane, KS, et al (2013) Standard methods for estimating strength parameters of Apis mellifera colonies. Journal of Apicultural Research 52(1): 1-12.

Plus, personal observations by shaking frames of bees, counting and weighing them.

[2] As with package bees, the manner in which the bees are collected will influence the age distribution of the starting cohort. If the workers are collected by shaking over an excluder, or if frames transferred to nucs are disturbed or left to sit for an appreciable time, the older workers will fly back to the parent hive, leaving mostly younger workers in the starting cohort for the nuc (or package). Starting with younger bees would be advantageous. We prefer to make our nucs in cool weather (we actually make many while it’s raining or snowing), since what we then put into the nuc, stays in the nuc.

[3] Although many books state that worker development takes 21 days, the hard data that I’ve seen suggests that worker emergence starts as early as 19.5 days, predominately occurs on Day 20, but may extend until nearly 21 days. Temperature may be a factor.

Harbo, JR (1992) Breeding honey bees (Hymenoptera: Apidae) for more rapid development of larvae and pupae. Journal of Economic Entomology 85(6): 2125–2130.

Martin, SJ (1997), graphic in Varroa–Fight the Mite. Munn, P and Jones, R (Editors), IBRA.

[4] Harbo, JR (1986) Effect of population size on brood production, worker survival and honey gain in colonies of honey bees. Journal of Apicultural Research 25: 22-29.

[5] Harbo, JR (1986) op cit.

[6] Harris, JL (2010) The effect of requeening in late July on honey bee colony development on the Northern Great Plains of North America after removal from an indoor winter storage facility . Journal of Apicultural Research and Bee World 49(2): 159-169.

[7] http://scientificbeekeeping.com/understanding-colony-buildup-and-decline-part-9a/

[8] Kefuss, JA (1978) Influence of photoperiod on the behaviour and brood-rearing activities of honeybees in a flight room. Journal of Apicultural Research, 17(3): 137-151

[9] Nolan, WJ (1932) The development of package-bee colonies. USDA Technical Bulletin No. 309.

[10] A queen’s “age” is not a function of days or years, but rather a function of how many eggs she’s had to fertilize. I highly recommend that every beekeeper read the excellent study by Dr. Boris Baer:

Baer, B, et al (2016) Sperm use economy of honeybee (Apis mellifera) queens. Ecology and Evolution 6(9): 2877–2885. https://doi.org/10.1002/ece3.2075 Open access.

[11] Winston, ML (1979) Events following queen removal in colonies of Africanized honeybees in South America. Insectes Sociaux 26: 373-381.

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