Bee Buzz Box September 2020 A History of Two-Queen Hives Part II – The first managed two-queen colonies

Clayton and Milton Farrar both professors of entomology https://eriebeekeeping.com/content/clayton-leon-farrar

Alan Wade

One of the earliest records of the existence of sustained presence of two queens in a hive comes from G.M. Doolittle’s 1889 book Scientific Queen-Rearingi, providing an account of queen supersedure:

After I had this experience with the colony that had ‘two Queens in a hive’ (which was a surprise to so many fifteen years ago, when it was thought that no colony ever tolerated but one laying queen at a time), I began to watch for a like circumstance to occur, which happened about a year from that time. In the latter case, as soon as I found the cells, they were sealed over, and not knowing just when they would hatch [sic emerge], I at once cut them out and gave them to nuclei. In a few days I looked in the hive again, when I found more cells started, which were again cut off and given to nuclei, just before it was time for them to hatch. In this way I kept the bees from their desired object for some two months, or until I saw that the old Queen was not going to live much longer, when I left one of the cells, which they had under headway, to mature. By this plan I got about sixty as fine Queens as I ever reared, and laid the foundation for my present plan of securing Queens…

Recognition of the potential of two-queen hives

Doolittle soon discovered that separating brood from the queen by a queen excluder resulted in colonies raising queen cells supersedure-style. Others, for example the notable Dr C.C. Miller, made observations on queens raised under induced supersedure impulse conditionsii. The simple procedure of isolating brood using queen excluders normally results in the establishment of a second or even or even a third colony queen provided entrances are provided to allow emerging queens to mate. This clear-visioned observation extended to the recognition that the process did not induce swarming as queens are raised under the ‘queen pheromone deficiency’ supersedure impulse – not the swarming impulse – as outlined in Part I.

One might wryly observe that Doolittle discovered that one had to ‘do little’ to establish a two-queen colony, that is to divide brood in a strong colony using an excluder and to allow one newly raised queen to mate. What was less clear was the way such colonies needed to be managed to make them truly productive and prevent their reversion to a single queen colony, that is where one queen supersedes the other. The skill and large effort required to ensure that two-queen colonies reach their full potential has proved and will continue to be a major barrier to adoption of two-queen system of hive management except in expert hands.

Healthy honey bees with optimal nutrition and a young queen have enormous honey gathering potential and are the backbone of commercial beekeeping. The presence of a second laying queen or, as already noted elsewhere, the parallel operation of two single queens in a strong hive split well in advance of a honey flow has the potential to further enhance colony production.

Farrar famously demonstrated the population response and nectar gathering potential of hives under different systems of management. He showed that the amount of honey gathered by bees could be directly linked to colony bee numbersiii. Factors that may boost those numbers include using young stock selected for queen laying potential and hygienic behaviouriv, optimising colony nutrition, hive insulation and siting of hives in sunny protected positions to minimise colony stress. Farrar pioneered practical means of operating two-queens knowing that, with the services of an extra queen, colonies with a formidable honey gathering capability would be produced.

In his notable 1968 paper on maximising colony strength, Farrar outlined the factors controlling colony buildup:

The time factors expressed in numbers of days required for the three classes of colonies to reach a maximum strength and enhance their maximum production efficiency are shown in Figure 1. These colony growth curves are based upon daily egg-laying rates, time of brood development, and the length of life for adult bees when healthy colonies are headed by good queens and abundantly supplied with honey, pollen, and hive space in a favorable position. The production efficiency per unit number of bees in the two-queen colony is equal to or slightly greater than that of full-strength single-queen colonies. When a measurable nectar flow develops about the time the second queen is introduced, the storing efficiency of the colony will be lowered, since more bees will be engaged in raising brood from the two queens. On the other hand, its production efficiency will be higher than that of single-queen colonies when united back to single-queen status. For 20 days such colonies have essentially double populations with brood from eggs laid by only one queen.

He further elaborated on the size of colonies needed to realise honey flows:

Good nectar flows are often not recognised because colony populations are too small to show gains. The producer of package bees can profitably manage colonies within the range of 10,000 to 20,000 bees, periodically shaking marked bees. This is because colonies with 10,000 bees raise proportionately more brood than larger colonies. The beekeeper who keeps colonies for honey production or plant pollination must direct his management towards developing all colonies to full strength for particular crops. Package bees used to establish new colonies require 11 to 13 weeks to reach full production efficiency. The division of strong colonies six to eight weeks in advance of the principal nectar flow to establish two-queen colonies is a means of obtaining relatively higher brood production early in the season and high production efficiency when nectar is available.

In a recent two-part seriesv we examined the origins of doubled and two-queen hives noting, in rather more detail, Alexander’s success in establishing ‘any number of queens in a hive without a queen excluder’.

In retrospect, and despite his technique of acclimatising additional queens prior to their introduction now being recognised as being entirely feasible, we can now state that his success was contingent upon good spring buildup conditions and very long honey flows.

While these productivity gains attendant to operating hives with two queens had become self evident, it was not until the 1930s that the great bee researcher Clayton Leon Farrar was able to directly correlate bee numbers, read colony strength, with honey yield. In a remarkable series of experiments he was able to not only quantify the impact of colony strength on honey production (Figure 1a) but also explain how colonies build and why extra strong colonies are far more productive per bee than average strength colonies while weak colonies struggle to store any surplus honey (Figure 1b). He then went on to plot the trajectories of colony buildup starting with broodless bees (a swarm or package bees), an overwintered single queen colony, and a colony split to form two colonies that were united as a two-queen colony (Figure 1c). How to build bees so that the population peaks coming into the main honey flow requires a very nuanced appreciation of the buildup processvi.

Figure 1 Farrar’s plots of colony performance:
(a) colony production in relation to colony strength and increased productivity per bee as population increases;
(b) increasing ratio of total bee numbers to amount of brood (expressed as decreasing brood to adult bees) as population expands freeing up bees to become foragers (linear relationship) and, for a young laying queen, daily egg laying rate increasing almost linearly until population exceeds about 30,000 bees (parabolic relationship); and
(c) population expansion over two-three months under different systems of queen management again assuming a highly fecund young queen.

The reliable two-queen hive

One of the best acknowledged early reports of successful operation of two-queen hives came with the publication of Ducat’s bookvii La Ruche Gratte: Ciel a Plusieurs Reines – The skyscraper hive with several queens. Eva Craneviii reports that the publication caused quite a stir when it was published in 1946 (an English translationix The Skyscraper Hive appeared in 1948). Butler, however refuted the publicity given to Garcia in statingx:

…a good many beekeepers will heartily disagree with M. Dugat‘s system of colony management. His system is indeed little more than an extension of the two-queen system of management which has been tried out very thoroughly in the United States and other countries since at least the beginning of this century, and has not been found to yield any notable increase in honey crop per queen over various single-queen systems.

By this time, however, Farrarxi had not only already demonstrated that two-queen colonies could be readily set up and greatly increase honey yield, but had also developed the main practical means for their successful operation. His experiment spanned an eleven year period (1934-1946), a monumental study. Farrar was joined by John Edward Eckert (1937)xii and later by Floyd Moeller (1961)xiii – also by Charles Harold Gilbert (1938), see below – in realising the superior honey performance of populous colonies, Farrar being undoubtably the great pioneer of two-queen hives.

Farrar’s experimental setup

Farrar’s system employed shallow ten-frame supers, and in a parallel setup square shallow twelve-frame supers throughout, to maximise flexibility for rotation of supers. He emphasised the importance of establishing hives on firm level bases and his 1958 paper provides detailed year-round operational details required for their setup and management.

To set up his two queen colonies (Figure 2), Farrar first split strong colonies 6-7 weeks in advance of the main honey flow. He did this by moving up about half the brood, mainly sealed, above an inner cover with a separate top entrance and introduced a new laying queen.

Farrar surmised his approach in stating:

Two-queen colony management represents an intensive system of honey production designed to obtain the maximum yield from each hive unit during any two-week honey flow, the honey product on per unit number of bees increases with the colony. In other words, one large colony will produce more honey than two or more smaller colonies having the same aggregate number of bees. Since small colonies may increase their populations, under a long honey flow the relative production between the two classes of colonies need not remain static. Colonies with large populations throughout a honey flow, whether long or short, make the greatest total gain.

While indicating that the upper colony could raise its own queen or that a good queen cell – or a mailed caged queen – could be used, he strongly advocated using bees with a new queen in a well-established a nucleus colony. This technique establishes and builds the upper unit quickly. At the same time he under-supered the new top unit with two empty supers of drawn comb to facilitate expansion of the founding lower unit. Once established, the two units were united by replacing the dividing inner cover with a queen excluder, with additional supers also being added to the upper second-queen unit as needed. Farrar noted that the system – in some later developments – should be able to employ about half the number of shallow supers using full depth supers.

From here on in brood chambers were reversed regularly – until the colony got too large – and honey supers were under-supered as the season progressed to maximise brood production and to facilitate ready removal of finished supers of honey. Honey was harvested regularly, since supers fill very quickly in two-queen system operations. Special care is required to always ensure that queens are kept quite separate during hive manipulations. The option of using an additional queen excluder above the second (upper) queen, though while not needed, will assist in keeping track of queens in extremely large and populous hives. Towards the end of the main honey flow excluders were removed and honey supers were progressively removed to facilitate reversion of colonies to strong single-queen colonies for overwintering.

Figure 2 Farrar 1946 system showing key steps in operation of two-queen colonies depicting twelve-frame shallow super operation: ic = inner cover; x = excluder; e = entrance
(a) strong single-queen colony employing good previous season queen built early in the season with brood supers is regularly reversed to arrest swarming and to encourage the queen to lay at near capacity;
(b) colony split, new queen is established in upper unit above inner cover and lower unit supered to facilitate expansion;
(c) two-queen unit is further supered and dynamically operated during the honey flow;
(d) brood and super chambers are rearranged towards end of the honey flow; and
(e) colony is consolidated to a strong overwintering unit with all excluders removed.

The sophistication of his operation is given brief insight to in one of Eva Cranes earlier visits to Americaxiv:

We visited the apiary which Dr Farrar uses for his ‘methods of management’ experiments, which have recently been described in detail in Bee World (see Farrar, (1953)). I had read and heard much about these colonies; even so, I was impressed by what I saw. The colonies, with 80-90 000 bees, were in shallow 12-frame Dadant bodies, 8-9 per hive. The two-queen units had already been united when we saw them. When the hives are to be inspected, a truck drives between the rows, and each hive in turn is lifted and turned over sideways on to a platform, and examined while on its side.

By the late 1930s Charles Harold Gilbertxv, and indeed also Farrar, had explored many top-bottom two-queen tier hive arrangements, for example placing weak overwintered colonies over a screened inner cover, introducing a second queen to upper units as a requeening ploy and using two-queen setups to control swarming. Gilbert’s modification of the Farrar system (Figure 3) employed standard ten frame full depth Langstroth hive bodies and substituted the inner cover with a double screen. Use of a double screen in lieu of a mat allowed for more effective warming of the upper nucleus colony. Initial division of the colonies is essential to avoid loss of one of the queensxvi. Ripe queen cells introduced to young brood over a double screen, well separated from the main brood nest, resulted in reliable establishment of a second queenxvii.

In Gilbert’s scheme the double screen was replaced with an excluder after several weeks once both queens were laying well. He also employed of a riser rim over the excluder with an opening and an attached landing board obviating the need to drill auger holes to provide the upper brood nest free flight access.

In referring to Eckert’s and Farrar’s experience of increasing bee populations using two queens, Gilbert noted that:

Success is based upon greatly increased bee population resulting from the combined efforts of two queens working in the same hive. The method may appear new and revolutionary to some beekeepers but, according to Eckert its use dates back many years in the history of beekeeping.

He also noted preemptively:

that two-queen colonies require regular manipulation, and that serious difficulties arise when they were neglected. Supering requirements increase greatly during a heavy honey flow, and it was almost impossible to give each unit enough supers.

Gilbert’s use of double brood boxes reflects the common American practice of running a double brood box in single-queen hives. While a single full depth nine or ten frame brood chamber (or an eight frame full depth super combined with a shallow or ideal super) normally suffices, there remain many adherents to double brood chambers.

Dunham’s modified system for honey productionxviii was a further adaptation of the Farrar two-queen system. Instead of maintaining two queens and supering each of the brood nests separately in tower hives – a labour intensive element throughout extended honey flows – hives were reorganised and reduced to a single-queen condition but only where the flow was of limited duration, as for example occurs in most years where box eucalypt flows are well defined and last less than 6-8 weeks. The nuanced arguments put forward by Dunham, for example the need for young and productive queens and contingency for managing colonies of different strengths and dynamic management of backup colonies as part of any scheme to ‘condition hives’ suggests that there are natural limits to the number of such hives that can be effectively managed. Close reading of the condition of individual hives is essential and a very flexible approach to their operation is needed especially during a honey flow.

Figure 3 Gilbert’s system operation of two-queen colonies for ten-frame shallow super operation: OQ = old queen; NQ = new queen; HS = honey store; ds = double screen; x = excluder; e = entrance:
(a) strong single-queen colony is established using good previous season queen (OQ) and is built early in the season;
(b) the colony is split, a new queen is introduced to the upper unit above a double screen and lower unit is supered above excluder to facilitate expansion;
(c) the two-queen unit is further supered and dynamically operated during honey flows; and
(d) the brood and super chambers are rearranged towards end of honey flow.

Dunham made further modifications to the Farrar system both in terms of the gear employed and in terms of strategic timing of two-queen hive operations. Eckert and Shawxix in reviewing Dunham’sxx earliest 1943 publication noted that he:

…set up his two-queen unit in much the same way as did Farrar, but instead of a regular inner cover he used a double-screened divider with a quarter-inch of space between the two screens. The full screen allows more heat from below and aids in the development of the weaker colony above…

and that:

…the goal sought in Dunham’s system of management of two-queen colonies is to reach the desired strength before the main honey flow while the equipment is still light in weight. There must be a continuous light nectar flow to provide adequate stores and uninterrupted brood rearing. By the end of the spring breeding period, the ideal two-queen colony will have 10 to 14 frames of brood in the lower unit, 8 to 10 frames in the upper unit, and the hive crowded with bees.

By 1947 Dunham had modified the two-queen system, realising that maintaining a second queen once the population had peaked complicated operations during the honey flow. He had simply united the brood nests.

Eckert and Shawxxi pointedly concluded:

Some beekeepers operate two-queen colonies in specially built hives, some of which have the queen confined to brood compartments on the sides with storage chambers in the center. Supers are added to the center stack as they are needed during the season. Some difficulty is generally encountered in keeping honey-free brood chambers and enough room for good queens to maintain a high rate of brood rearing. Such hives are too heavy to move where migratory beekeeping is practiced but could be used to advantage in permanent apiaries. Most beekeepers, however, prefer to use standard hive bodies because they are interchangeable and more easily managed.

Then at the end of the season Eckert and Shaw found a singular new use for strong colonies to overwinter new queens:

In the more temperate regions, weaker colonies with good queens can be wintered above a stronger colony if separated from it by a double wire-screened division board or a thin liner cover with the bee escape covered with a double wire screen. In fact, in some areas many commercial beekeepers divide their stronger colonies six to eight weeks before the close of the brood-rearing period and place the divide over the parent colony, separating it with double screen or a thin division board. The upper colonies are always given a separate entrance. The divides are given young queens or, if time permits, ripe queen cells. The divisions should consist of five or six combs of brood and bees and enough honey to winter on, unless both colonies can store sufficient honey for this purpose.

In a contemporary variant of the Farrer system, Victor Croker and David Leemhuis at Australian Honeybeexxii introduce a ripe queen cell raised from select stock to several frames of young or ‘wet’ brood lifted above an excluder in each hive. This allows each nucleus colony on top of each stack to quickly establish new queens by provisioning each fledgling broodnest with stores from below and – as we have noted– to benefit from the rising warmth of brood with the old queen below. Australian Honey Bee do not run the resultant two-queen colonies as production hives, instead employing the newly raised queens to directly requeen each hive. In their operation, and once the new queens are well established, the brood chambers with the old queens are simply offset to new stands to allow field bees in the offset hive to drift back to their parent stands. At the same time each top unit with a new queen is moved to the bottom of the stack. This provisions every hive in the apiary with a new queen.

The offset hives – with the old queens and depleted of field bees – are moved to another apiary where they are are readily searched and made ready to accept a new batch of cells.

Clearly any scheme that facilitates hive renewal and that minimises the amount of labour required is well worth pursuing. Wedmore’s 1945 classic book, A Manual of Beekeeping for English-speaking Bee-keepersxxiii reviews nearly 1600 hive manipulation practices. He makes a specific recommendation about employing young queens, about using the similar Demaree Plan to establish new queens and to strengthen swarms and weak colonies with good queens in tiered top-bottom two-queen systems. He draws conclusions similar to those made by Gilbert and by Eckert and Shaw.

Having placed two-queen hive on a sound practical footing, the next part in this series will chronicle post 1950 endeavours of the likes of New Zealand researcher G.M. Walton and the renowned American apiarist Robert Banker to demonstrate the commercial viability of large-scale two-queen hive apiary operations.

Readings

iDoolittle, G.M. (1889). Scientific queen-rearing as practically applied being a method by which the best of queen-bees are reared in perfect accord with nature’s ways, 184pp., Chicago, Ills. Thomas G. Newman & Son, 923 & 925 West Madison Street. https://ia800200.us.archive.org/12/items/cu31924003263724/cu31924003263724.pdf

iiMiller, C.C. (1911). Fifty years among the bees. A.I. Root Company, Medina, Ohio, ‘Rearing queens in hive with laying queen’, pp.310-312. http://biobees.com/library/general_beekeeping/beekeeping_books_articles/50_years_bees.pdf
Collison, C. (2005). Do you know? Swarming and nectar. Bee Culture 133(4):63-65.
Collison, C.H. (8 May and 30 June 2014). Swarm management. Publication 1817, 4pp. Extension Service of Mississippi State University, cooperating with US Department of Agriculture. https://extension.tennessee.edu/Rutherford/Documents/p1817%20swarm%20management%20by%20Dr.%20Clarence%20Collison.pdf
Collison, C.H. (2018). Swarming behavior. A closer look. Bee Culture. https://www.beeculture.com/a-closer-look-20/

iiiFarrar, C.L. (1931). A measure of some factors affecting the development of the honeybee colony, PhD Thesis, Massachusetts State College, 139pp. https://scholarworks.umass.edu/dissertations_1/889
Farrar, C.L. (1932). The influence of the colony’s strength on brood-rearing. Annual Report of the Beekeepers’ Association Ontario, 1930 and 1931. Ontario Department of Agriculture: pp.126-130.
Farrar, C.L. (1937). The influence of colony populations on honey production. Journal of Apicultural Research 54(12):945-954. https://naldc.nal.usda.gov/download/IND43969007/PDF
Farrar, C.L. (1944). Productive management of honeybee colonies in the Northern States. Circular No. 702, 2 July 1944, Washington, D.C. United States Department of Agriculture 28pp. https://archive.org/details/productivemanage702farr and http://www.nnjbees.org/wp-content/uploads/2016/10/Productive-management-of-honeybee-colonies-Farrar.pdf
https://babel.hathitrust.org/cgi/pt?id=uiug.30112019274536
Farrar, C.L. (1946a). Productive management of honey-bee colonies, Entomology Research Division No. E-693, revision to USDA Agricultural Research Service Department of Agriculture Bulletin No. 702, 1946 in cooperation with the Wisconsin Agricultural Experiment Station. Published online by USDA at Beesource (2019). https://beesource.com/resources/usda/productive-management-of-honey-bee-colonies/ and https://beesource.com/resources/usda/productive-management-of-honey-bee-colonies/productive-management-of-honey-bee-colonies-support-material/
Farrar, C.L. (1968). Productive management of honey bee colonies. American Bee Journal 108:95-97, 141-143. reprinted as Farrar, C.L. (1968). Productive management of honey bee colonies. Apiacta 4:22-28. http://www.fiitea.org/cgi-bin/index.cgi?sid=&zone=cms&action=search&categ_id=55&search_ordine=descriere Tegart, D. (1984). Two-queen hive management using package bees in the Peace River area, Alberta, Canada. Bee World 65(2):80-84. http://www.nmhoney.com/nmhoney/Sub_Files/Two%20Queen%20Hive%20Management%20BW652pp80-84.pdf
Donaldson-Matasci, M.C., DeGrandi-Hoffman, G. and Dornhaus, A. (2013). Bigger is better: honeybee colonies as distributed information-gathering systems. Animal Behaviour 85(3):585-592. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4511854/

ivGerdts, J., Dewar, R.L., Finstrom, M.S., Edwards, T. and Angove, M. (2018). Hygienic behaviour selection via freeze-killed honey bee brood not associated with chalkbrood resistance in eastern Australia. PLoS ONE 13(11):e0203969https://doi.org/10.1371/journal.pone.0203969
Gerdts, J. (2014). Hygienic behaviour in 50 AI breeder colonies of the Australian queen bee breeding program, Queensland Australia, 12pp. https://honeybee.org.au/wp-content/uploads/2014/12/AQBBP-HygienicTestingReport_BeeSci2014FINAL.pdf

vWade (December 2019d). Doubling hives Part I – Doubled versus two-queen hives. Canberra Region Beekeepers Newsletter https://actbeekeepers.asn.au/bee-buzz-box-december-2019-doubling-hives-part-i-doubled-vs-two-queen-hives/?preview_id=6911&preview_nonce=066f240efa&preview=true&_thumbnail_id=7059
Wade, A. (February 2020a). Bee Buzz Box February 2020: Doubling Hives Part II – The Wells System. – Part II The Wells System. Canberra Region Beekeepers Newsletter. https://actbeekeepers.asn.au/bee-buzz-box-february-2020-doubling-hives-part-ii-the-wells-system/?preview_id=8466&preview_nonce=f4183004ba&preview=true&_thumbnail_id=7087

viWade, A. (2019b). Building bees for the honey flow. Presentation to Canberra Regional Beekeepers, 18 September 2019. Powerpoint https://drive.google.com/file/d/1ZNLo8xAV5b6kRD4klCD5lIUhBTLPxHaq/view

viiDugat, M. (1946). La ruche gratte ciel á plusieurs reines: une nouvelle methode d’apiculture intensive. 3rd edition published by Marlieux, Abbaye de N.-D. des Dombes, 1947. https://catalog.hathitrust.org/Record/009058161
Dugat, M (1948a). Skyscraper hive [Ruche gratte-ciel]. Au Bon Meil [With good honey] (2020) extract from French Hunter No. 600, p.132. https://www.aubonmiel.com/ruche-gratte-ciel/

viiiCrane, E. (1980). Multiple queen hives and hyper hives in Perspectives in world agriculture: Apiculture, Chapter 10. pp.261-294. Farnham Royal, UK: Commonwealth Agricultural Bureaux. https://www.evacranetrust.org/uploads/document/b00fb1cb4f88f874217e28dcf04930fb5639e1bd.pdf

ixDugat, M. (1948b). The skyscraper hive. Authorized translation of La Ruche Gratteciel à Plusiers Reines by Norman C. Reeves. Faber and Faber, 24 Russell Square, London. https://www.abebooks.com/servlet/SearchResults?an=dugat+father&bi=h&ds=5&n=100121503&sortby=1&tn=skyscraper+hive&cm_sp=mbc-_-ats-_-filter

xButler, C.G. (17 July 1948). The Skyscraper Hive. Nature 162(4107):87. https://doi.org/10.1038/162087c0

xiFarrar, C.L. (1936). Two-queen vs. single-queen colony management. Gleanings in Bee Culture 64(10):593-596. Cited by Gilbert (1940), Eckert and Shaw (1960) and Farrar (1958). loc. cit. Details of the original paper are also cited by Rahmlow, H.J. (ed.) (1937). Wisconsin Horticulture 27(7):145-176. http://digicoll.library.wisc.edu/cgi-bin/WI/WI-idx?type=turn&entity=WI.WIHortv27.p0197&id=WI.WIHortv27&isize=text
Farrar, C.L. (May 1946b). Two-queen colony management. United States Department of Agriculture. Agricultural Research Administration, Bureau of Entomology and Plant Quarantine, 14pp. http://ufdc.ufl.edu/AA00026061/00001/14j
and https://archive.org/stream/twoquyma00unit/twoquyma00unit_djvu.txt
and https://www.scribd.com/document/313855449/Two-queen-colony-managemet
and http://www.immenfreunde.de/docs/2queen.pdf
Farrar, C.L. (1953). Two-queen colony management. American Bee Journal 93(3):108-110, 117 reprinted as Farrar, C.L. (1953). Two-queen colony management. Bee World 34(10):189-194. https://doi.org/10.1080/0005772X.1953.11094821
Cale, G.H. (1952). Two-queen? American Bee Journal 92(6):236-237. Cited by Farrar (1953). loc. cit.
Cale, G.H. (1922). The relation of queens to seasonal management. American Bee Journal 62(2):61-63.
Schaefer, C.W. and Farrar, C.L. (1941). The use of pollen traps and pollen supplements in developing honey bee colonies. US Bureau of Entomology and Plant Quarantine No. E-531. Cited by Haydak and Tanquary (1943). p.7. loc. cit.
Schaefer, H.A. (1943). Two-queen system. American Bee Journal 83(6):234-235. Cited by Butz, V.M. and Dietz, A. (1994). The mechanism of queen elimination in two-queen honey bee (Apis mellifera L.) colonies. [Nouvelles Observations Sur Les Abeilles]. Journal of Apicultural Research 33(2):87-94. https://doi.org/10.1080/00218839.1994.11100855
Schaefer, C.W. and Farrar, C.L. (1946). The use of pollen traps and pollen supplements in developing honeybee colonies. US Bureau of Entomology and Plant Quarantine, E531, rev. 13pp. 33:866-870. Cited by Moeller (1980a). loc. cit.
Farrar, C.L. and Schaefer, C.W. (1958). Two-queen colony management for production of honey. United States Department of Agricultural Research Service August 1958, 33-48, 9pp. Cited by Farrar (1953) loc.cit.
Chaudry, M. and Johansen, C.A. (1971). Management practices affecting efficiency of the honey bee Apis mellifera (Hymenoptera: Apidae). Scientific Paper No. 3674, Washington Agricultural Experiment Station, Washington State University, USDA ARS-33-48, 11pp. Melanderia 6:32pp.
Haydak, M.H. and Tanquary, M. (June 1943). Pollen and pollen substitutes in the nutrition of the honeybee. Technical Bulletin 160, 23 pp. Division of Entomology and Economic Zoology, University of Minnesota, Agricultural Experiment Station https://conservancy.umn.edu/bitstream/handle/11299/204094/mn1000_agexpstn_tb_160.pdf?sequence=1
Farrar, C.L. (1958). Two-queen-colony management for honey production. August 1958 ARS-33-48 USDA Agricultural Research Service. https://archive.org/stream/two queencolonyma48farr/twoqueencolonyma48farr_djvu.txt
Kornely, R. (1969). Care of the two-queen-colony in spring. Gleanings in Bee Culture 97(1):16-20.

xiiEckert, J.E. (1937). The duo or two-queen hive. Gleanings in Bee Culture 65:137.

xiiiMoeller, F.E. (1961). The relationship between colony populations and honey production as affected by honey bee stock lines. US Department of Agriculture, Agricultural Research Service, Production Research Report 55, 24pp. https://babel.hathitrust.org/cgi/pt?id=uc1.aa0001977776;view=1up;seq=2

xivCrane, E. (1954). An American bee journey. Bee World 35(7):125-137. https://doi.org/10.1080/0005772X.1969.11097261 and https://www.evacranetrust.org/uploads/document/ab43420d3fa91f1e2ee69e8e63dc32907c536a85.pdf

xvGilbert, C.H. (1938). The two-queen hive. Gleanings in Bee Culture 66:406-419. Medina, Ohio.
Gilbert, C.H. (July 1940). The two-queen hive and commercial honey production. University of Wyoming Agricultural Research Station Bulletin 239:3-15, Wyoming. https://mountainscholar.org/bitstream/handle/20.500.11919/335/AESB_1940_Bulletin_239.pdf?sequence=1&isAllowed=y and https://repository.uwyo.edu/cgi/viewcontent.cgi?referer=https://www.google.com.au/&httpsredir=1&article=1187&context=ag_exp_sta_bulletins
Lanyi, S. (1939). Doubled honey-earning by a new two-queen system. Bee World 20(11):123-125. https://doi.org/10.1080/0005772X.1939.11093925 Harvey, M. (2009). Kerkhof hives: Advantages of a two-queen set up. Bee Culture 37(4):65-66. https://discovery.csiro.au/primo-explore/fulldisplay?docid=TN_proquest214713243&context=PC&vid=CSIRO&lang=en_US&search_scope=All&adaptor=primo_central_multiple_fe&tab=default&query=any,contains,Harvey,%20M.%20(2009).%20%20Kerkhof%20hives:%20Advantages%20of%20a%20two-queen%20set%20up.&offset=0
Goolsbey, A. (2001). 2-Queen system. Bee Culture 129(5):24-29. https://discovery.csiro.au/primo-explore/fulldisplay?docid=TN_proquest214712867&context=PC&vid=CSIRO&lang=en_US&search_scope=All&adaptor=primo_central_multiple_fe&tab=default&query=any,contains,Goolsbey,%20A.%20(2001).%20%202-Queen%20system.&offset=0 Labesque, S. (2004). 2 Queener. Bee Culture 132(4):31-34. https://discovery.csiro.au/primo-explore/fulldisplay?docid=TN_proquest214704962&context=PC&vid=CSIRO&lang=en_US&search_scope=All&adaptor=primo_central_multiple_fe&tab=default&query=any,contains,Labesque,%20S.%20(2004).%20%202%20Queener&offset=0

xviSzabo, T.I. (1982). Requeening honeybee colonies with queen cells. Journal of Apicultural Research 21(40):208-211.

xviiVictor Croker and David Leehumis (pers. comm.) employ this technique for introducing cells in the first stage of a sophisticated program to requeen their apiaries.

xviiiDunham, W.E. (March 1947). Modified two-queen system for honey production. Bulletin of the Agricultural Extension Service, the Ohio State University 281:1-16. https://scholar.google.com.au/scholar?hl=en&as_sdt=0%2C5&q=Dunham%2C+W.E.+%281948%29.++Modified+two-queen+system+for+honey+production.&btnG=
Dunham, W.E. (1948). Modified two-queen system for honey production. Gleanings in Bee Culture 76:277-281.
Dunham, W.E. (1951). The Ohio modified two-queen system. Gleanings in Bee Culture 79(4):212-214.
Dunham, W.E. (1953). The modified two-queen system for honey production. American Bee Journal 93(3):111-112.

xixEckert, J.E. and Shaw, F.R. (1960). Beekeeping: Successor to Beekeeping by Everett F. Phillips, MacMillan Publishing Co. Inc., New York, 546 pp.
Eckert, J.E. and Shaw, F.R. (1969). Beekeeping, The Macmillan Company, New York. https://archive.org/stream/in.ernet.dli.2015.140016/2015.140016.Beekeeping-Successor-To-Beekeeping_djvu.txt

xxDunham, W.E. (1943a). The modified two-queen system. American Bee Journal 83(5):192-194. Cited by Eckert and Shaw (1960). pp.195-196 and Eckert and Shaw (1969). p.197. loc.cit.
Dunham, W.E. (1944). Bees: Maintenance of colonies, control of colony population for honey production and pollination. Ohio State University Extension Bulletin 254:2-32. Cited by Eckert and Shaw (1960). loc. cit.
Dunham, W.E. (1943b). Versatility of the modified two-queen system. Gleanings in Bee Culture. 71:264-268. https://books.google.com.au/books?id=WSNbAAAAIAAJ&focus=searchwithinvolume&q=Dunham%2C+W.E.+%281943%29.++Versatility+of+the+modified+two-queen+system.

xxiEckert, J.E. and Shaw, F.R. (1969). p.217. loc. cit.

xxiiTheir technique will be described in a short upcoming series on recent adaptations of the two-queen system to raising queen bees, a scheme that integrates running production colonies with requeening and that avoids having to find old queens in populous colonies.

xxiiiWedmore, E.B. (1945). A Manual of beekeeping for English-speaking beekeepers. 2nd Revised edition, republished 1979 by Bee Books New & Old, Steventon, Hampshire as A manual of beekeeping, 2nd Edn revised, pp.294-295, 312. Butler and Tanner Ltd, Frome and London.

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