Res. Popul. Ecol. (1968) X, 31--39

W O R K E R B R O O D S U R V I V A L I N HONEYBEES1, 2

Hiromi FUKUDA and Sh6ichi F. SAKAGAMI a

Zoological Institute, Hokkaido University, Sapporo, Japan

INTRODUCTION

There exist a large number of papers dealing with quantitative aspects of the

colony life of honeybees, giving raw data on or estimates for the numbers and biomasses

of individuals, both young and adults. However, these studies have so far been carried

out relatively independently from recent advances in population ecology. Obviously

the honeybee possesses a mode of life so aberrant that various theories and procedures

in population ecology cannot be applied to it without appropriate modifications. But

it should be admitted that the sound progress in apicultural research must be based

upon the comprehensive ecological principles. On the other hand, population ecology

should be enriched by studies on the honeybee, one of the best studied insects. Keeping

this idea in mind, we made studies on the seasonal changes of longevity of adult

workers, which is basic to estimate seasonal trends of population fluctuation (FUKUDA

and SEKIGUCHI, 1966; SEKIGUCHI and SAKAGAMI, 1966). However, the preparation

of a complete life table requires survival data for both adults and young. There are

few previous contributions concerning the brood survival in honeybee, and the results

given in such studies, especially those on unsealed brood (=eggs and feeding larvae),

are not always appropriate for our purpose. The present paper deals with some

observations made in order to determine the survival in successive immature stages

of honeybee workers.

MATERIALS AND METHODS

All observations were made using the colonies of the unpure Italian race, Apis mellifera ligustica, kept in LANGSTROTtt'S hives on the campus of Hokkaido University,

Sapporo, in 1967.

A comb with ample empty cells was introduced in the center of a colony headed

by a prolific queen. Twenty four hours after introduction, the comb was taken from

the colony. An area of the comb covering 100 cells, most of which had received an

egg, was marked by driving nails at four corners (Fig. 2). Thereafter, the comb was

1 Contribution No. 801 from the Zoological Institute, Faculty of Science, Hokkaido University, Sapporo, Japan.

2 Population and bioeconomic studies on the honeybee colonies. I. We thank Mr. H. TAKAHASm, Laboratory of Agricultural Physics, Hokkaido University, for his kind help as to the meteorological condition in Sapporo, and Prof. C.D. MICHBS~R, University of Kansas, who read through the manuscript.

32

introduced in another colony and the survival of immature stages was periodically

recorded until the disappearance of all individuals within the area, by emergence or

by death. This procedure was repeated in three different periods, using eight combs,

abbreviated N1, P~, N2, N~, N3, P2, P~, and N~. In these combs, the observation area

chosen was at the center of the brood rearing area, except for N~ in which the

observation area was at the middle of the comb but occupying the lower part of the

brood rearing area. The arrangement of combs in the colonies used for observation

is given below, from left to right side of the hive, using the following abbreviation:

s=comb with ample stored honey, b=comb with stored honey and brood, u = c o m b

not utilized or nearly so. The comb arrangement in the colony, in which N~ and P1

were introduced, is schematically illustrated in Fig. 1.

Fig. 1. Comb arrangement in the colony in which combs NI and P~ were introduced. The observation areas are represented by black quadrangles.

NI: Introduced into the center of brood rearing combs in a colony with 2 kg of

worker population (mean weight of a single worker, including the weight of honey

crop, is about 100mg), and ten combs arranged, bbbN1 bbbsP~. Attachment of workers

on combs was uniformly dense, except for the outer sides of the two outermost combs.

Observed June 10-July 2.

P~: Introduced into the same colony used for N1, and observed during the same

period. The comb was inserted so that the observation area faced the side wall of the

hive, separated from the central brood rearing combs by one comb of stored honey.

N2: Introduced into the center of brood rearing combs of a colony with 1.5 kg

workers and nine combs arranged ssbbN2bbbs. Workers on the combs were uniformly

dense, except for the outer storage combs where they were sparser. Observed August

14-September 2.

N,, : Introduced into a colony similar to that which received N2. All conditions

were same as for N2, except for the position of the observation area as mentioned above.

N3: Introduced into the center of brood rearing combs of a colony with 1.8 kg

33

workers and nine combs arranged ssbbN3bssP~. Workers were dense on b, but sparser

on s than in the colonies mentioned. Observed September 10-October 2.

P2: Introduced into the colony same to N3 and observed during the same period.

The comb was first introduced in the center of brood area, then one day after,

transferred in an outermost side as in P1.

P3: Introduced into a weak colony with only 0 .3kg workers and nine combs

arranged P3uuusbsgu. Workers were virtually confined to the central combs, sbs, being very sparse on the other combs. Observed during the same period as Ns and P2.

N~: Introduced into the center of the brood rearing combs of a colony with 1.5 kg

workers and nine combs arranged ussN~bssu. Workers were dense on the central

combs sN~b, about half as dense on stored honey combs, very sparse on the two

outermost ones. Observed during the same period as N~P3. This comb differed

from the others in having many cells with two eggs in the observation area (Fig. 2).

The three observation periods represent different "bee seasons" under local climate

and floral conditions: Period I (June 10-July 2)--Period of population increase with

heavy honey flow mainly from white clover; Period II (August 14-September 2)--Period

after population peak, with decreased honey flow; Period III (September 10-October 2)

--Period of autumn contraction, with very sparse honey flow.

In one aspect our procedure is not immune from criticism. Honeybee worker larvae

are fed by adult workers for 5-6 days. Then the cells containing them are sealed by

workers. The new adults emerge from the cells mostly 20-21 days after oviposition

through orifices mainly made by themselves. Therefore we cannot directly observe

the survival of brood during the sealed brood period without removing cell caps,

though the appearance of caps often tells the inner condition. We considered larvae

or pupae within cells as alive when the cell caps remained normal. Also we could

not observe directly all new adults at the time of emergence. We assumed successful

emergence, when hitherto capped cells were opened 19-23 days after oviposition. A

few cells with abnormal caps were recorded and the contents examined 23 days after

oviposition. All of them contained dead pupae.

We admit that this procedure involves circular reasoning. It might be expected

that caps of a few cells, normal in appearance, were opened by adult workers on days

19 23 in order to remove dead pupae. However, the low mortality of sealed brood

under normal conditions is already well known in apicultural practice. Our primary

aim was to determine the survival of unsealed brood under undisturbed conditions, so

that we did not use a wire cloth protector to exclude interference by adult workers.

One of us (S. F.S.) has repeatedly observed the emergence of new adults in observation

hives and confirmed that nearly all cells opened during days 19-23 were brought to

successful emergence. To assume the successful emergence for all cells, caps of which

were detatched during days 19-23, should not invalidate too much our aim.

34

R E S U L T S

Table 1 shows the survival of each stage and Table 2 the variability of duration

of each stage, using the records of successfully emerging individuals alone. As indicated

above, brood cells are sealed usually 8-9 days after oviposition. Pupation occurs about

four days after sealing, so that we cannot separate larval and pupal stages without

removing cell caps. We adopted the categories of unsealed (=feeding larvae) and

sealed brood (=post-feeding larvae and pupae) used in apicultural practice. This

distinction may also be more advantageous for population studies than the use of real

post-embryonic stages, because it not only facilitates the study, but also represents

T ab l e 1.

Comb I obse rved ]

S t age & ] D a y s s ince a ~ . NI

...... ov ipos i t ion " 2 " -

E g g 95

Day 1 1

.~ 2 2

3 3

(4) b

(5) b 69 r

~ S u r v i v a l

~ U n s e a l d brood

�9 ~ "~ D a y 1 4

" ~ 2 5

r 4 7

~ 2 5 8 i~ ~ Su rv iva l

r Sealed brood 78 y. =. D a y 1 9 78

2 10 78

9 17

12 20 (15) ~ (23)

Z Su rv iva l

T o t a l w o r k e r s p roduced

S u r v i v a l i t y in succe s s ive i m m a t u r e s t age s .

U n d e r n o r m a l condi t ion U n d e r a b n o r m a l condi t ion

.h~ T o t a l and I N~ N~ 3 m e a n ratio] Na P1 P2 Pa

I

99 87 101 e 382

94 99 87 92 372

93 99 87 90 369

91 96 86 90 363

(95) (84) (360)

95. 8% 96.0% 96.6%

91 95 84

87 93 81

87 93 80

79 93 80

78 92 80

(85.7) e (96.8) (95.

82.1 92.9 92.

89. 1% 94. 2%

90 360

86 347

84 344

80 332

80 330

2) (88.9) (91.7)

0 79.2 (86.4)

92 80 80 330

91 80 80 329

90 80 80 328

77 90 80 78 325

77 90 80 78 325

67 a 85 98 94

64 85 95 49

61 73 93 27

60 64 58 6

(58) (46) 0

(57) (45)

85. 196 75. 396 45. 9?6 O. 096

57 64 45

43 30

55 42 18

43 41 0

(75.4) (64. 1) (0.0)

64.2 48.2 0.0

43 41 0

43 41

43 41

43 41

i 43 40

(39)

(98.7)e (97.8)(100.0) (97.5)(98.5)[(100.0) (95.1)

?7 90 80 78 325 43 39

Surv iva l 81.1 90.9 92.0 77.2 85. 1 64.2 45. 9

a S h o w n by t h e m o s t f r e q u e n t l y obse rved ca se (cf. Tab le 2). b C a s e s excep t iona l . c O b s e r v a t i o n s on s o m e rep laced e g g s a r e inc luded. a D a t a on t he cells w i t h two e g g s a r e exc luded . e Su rv iva l in e a c h s t age .

Table 2. Duration of each immature stage. (E=egg, U=unsealed brood, S=sealed brood)

35

Duration of stages

(in days)

Total E U S

19 3 4 12 3 5 11

Number of cases observed and ~6 ratio

Combs observed under normal condition

N1 Nz N~ Na

Combs observed under abnormal condition

Total and mean ratio in normal

cases

2 (2. 6) 3 (3.8) 5 (1.5) 2 (2. 6) 7 (7.8) 9 (2.8) 4 (5.2) 7 (7.8) 3 (3.8) 14 (4.3)

Na P1

1 (2.6) 1 (2.6)

20 3 4 3 5 3 6 4 4 4 5

13 12 62(80.5) 11 1 (1.3) 12 1 (1.3) 11 1 (1.3)

85 (84.4)

21 3 5 13 3 6 12 4 5 12

2 (2.2) 1 (1.3) 4 (5.1) 7 (2.2) 74(82.3) 68(84.9) 59(75.6) 263(80.9)

1 (0.3) 3 (3. 3) 2 (2.5) 5 (6.4) 11 (3.4)

1 (o. 3) 79(87,8) 71 (88. 7) 68(87.1) 283(87.1) !

2 (2.2) 2 (2.5) 2 (2.6) 6 (1.8) 4 (5.2) 2 (2.2) 4 (5. 0) 1 (1.3) 11 (3. 4) 3 (3.9) 7 (9.0) 10 (3. 1) 7 (9.1) 4 (4,4) 6 (7.5) 10(12.9) 27 (8.3)

18(4I. 9) 21(53.8) 7 (17.9)

18(41.9) 28(71.7)

6(14.0) ] (2.6) 11(25. 6) 8(20. 5) 5(11.6) 1 (2.6)

22(51.2) 10(25.7)

22 4 6 12 5 5 12

23 5 6 12 6 5 12

1 (1.3)

1 ( I . 3)

1 (0.3) 1 (2.3)

1 (0.3) 1 (2.3)

1 (2.3) 1 (2. 3) 2 (4.6)

Total 77 90 80 78 325 f 43 39 Mean duration 20. 1 20. 0 20. 0 20. 1 20. 0 ! 20. 7 20. 2

two func t iona l ly di f ferent s tages , w i th and wi thou t ene rgy input .

Fou r combs, N1, N2, N , , and N~, in t roduced into the no rma l posi t ion, t ha t is, in

the cen te r of brood r ea r i ng area, a re s imi l a r to each o ther in hav ing h igh survival ,

85. 1% in mean, and a s tab le to ta l l ength of i m m a t u r e s tages, 20.0 days mean. The

mos t f requent combina t ions of du ra t ions of egg, unsealed and sealed brood was 3-5-12

days . I t is in t e res t ing tha t N~ shows a h igh survival , in spi te of the r e l a t ive ly pe r iphe ra l

pos i t ion of the observa t ion area. Th i s resul t a p p a r e n t l y depends on the dense cover

of bees upon the brood area. Seasonal ly the brood deve lopmen t is op t imal in Augus t ,

whi le pess imal in Sep tember , judged f rom both su rv iva l and du ra t ion of i m m a t u r e

s tages . Th i s m a y be pa r t l y caused b y the rma l condi t ions in the env i ronmen t (mean

a i r t e m p e r a t u r e in three per iods measu red on the Unive r s i ty Ca mpus : Per iod I, 16. 7~

II, 20.3~ III, 14.7~ But the difference in brood deve lopmen t is not conspicuous,

obvious ly because the ex t e rna l fac tors affect the brood only a f te r sc reen ing by the

colony organiza t ion .

36

Brood development is clearly disturbed in three combs introduced peripherally.

In P1, the survival is distinctly lower and duration of the immature stages greater

than N1 in normal position. In Ps, all brood disappeared early in the larval stage and

in Pa, observed under pessimal conditions, no egg reached the larval stage. I t must

be stressed that many seemingly healthy eggs and small larvae in P2 and P3 had

disappeared by the next day. This means a positive influence by adult workers on

brood mortali ty, that is, population control by the colony, a feature not expected in

soli tary insects.

Comb N~ offers an interesting instance. Although the observation area was at

the center of the comb, the later contraction of dense bee cover resulted in the exposure

of the lower par t of the area. Most of the brood in this part succumbed, as shown

in Fig. 1. The survival is fairly high, 91.1% ( 4 1 / 4 5 ) , if calculated for the upper two

thirds of the area (upper seven lines in Fig. 2). As already noted, 33 cells of the

observation area of N~ received two eggs in each. This condition is common in

queenless colonies developing laying workers but rare in queenright colonies. In all

these cells, one of the two eggs or larvae disappeared by the third day of the unsealed

brood stage. The survival in these cells was slightly lower, 76.0% (19/25) at the upper

two thirds of the area than in cells with one egg each. In cells with two eggs, the

total duration of immature stages was 20 days for 10 individuals, 21 days for 8 and

23 days for one, showing no marked deviation from the normal.

! �9 ; �9 , �9 : ~ " : - . : ~

: : 1,I. />...

"h.'"~.~,"'(" "T", ":'" "..'." . . . . "':" "v" ".:":"'..."" ""'.'"'" :~.. """" """'{"'. :."( ........ " ). ,.,i

%.... ..: ... ,..~. ,.-" '..f.- .,...y.. .....

""y'"~; . . . . . . . . ~" ""~'" ?" "T" �9 ;. ). . ..;. .,..k...;~ ..,..

"( ":'':"'~'".r "" ""'"? '"~" .'f'"'';""

:, i i ~ ..~ . . . . �9 t "'r ..k..

"".,r ~ ' r " "~" :. "'~ : . : ", : : : i ~ ~ ~*" " " 4..,,:'...,.'.....-"-...':"..J~ ....... ' ........ . ......... , .................. ~.."%: ............... %.'%,-~., ~..

Fig. 2. The observation area of comb _]Vd (outlined with solid line). White and black circles mean respectively' the eggs which gave adult workers or not. Two circles in one and the same cell indicate the presence of two eggs. Four large dotted circles indicate the nails used as the points of reference.

DISCUSSION

The high brood survival in healthy honeybee colonies has empirical ly been recognized

in apiculture. BODENHEIMER (1937) developed his method to est imate the seasonal

population trend in bee colonies upon the assumption of no brood mortali ty. The high

37

oviposition rate and high brood survival, which are balanced by the reproductive

incapacity of the majority of offsprings produced, are certainly key characters for

understanding the mode of life of advanced social insects. But high survival does not

mean the absence of brood mortality. Already MERRILL (1924) pointed out that the

amount of sealed brood was less than that of unsealded (not much over 83% even under

favorable weather, nearly 50% under adversed condition), and warned against basing

estimates of the number of eggs laid upon numbers of sealed brood. Information

obtained for other social insects is summarized by BRIAN (1965), who gave the survival/

stage function for Bombus agrorum as 100 eggs, 71 larvae, 52 prepupae and 34 adults,

and for Myrmica ru~ra as 100 eggs, 50 larvae, 42 prepupae, 37 pupae and 33 worker

adults, that is, about one egg in three produced an adult worker. Our result in the

honeybee gives a higher survival, 100 eggs, 94 larvae, 86 sealed brood and 85 adults

under normal conditions. 1 Further detailed studies are needed on brood survival of

various social insects, so far empirically regarded as remarkably high, under different

conditions.

Various factors may contribute to brood mortality in the honeybee. For the combs

normally introduced in our observations, no detectable food deficiency and no interference

by macroscopic natural enemies or highly infectious diseases were confirmed. Under

such circumstance, the mortality is mainly attributable to physiologically conditioned

weaknesses, which normally kill only a fraction of the brood, provided food supply

and microclimatic regulation are sufficiently controled by adult workers. The latter

factor is of utmost importance to understand the colony life of honeybee. The colonies

maintain an unusually stable thermal condition in brood rearing area, according to

HIMMER (1927) fluctuating only 2-3~ during weeks. Correspondingly, bee young are

extremely stenothermal, with the survival range of 32 36~ and the range of normal

emergence only 34-36~ Increased mortality due to adverse microclimatic conditions

is vividly demonstrated in combs PI, P2, Pa, and N., cited above. The different survival

in two subareas of Na and increased mortality in P2 on the second day, on which the

comb was transferred from central to peripheral position, especially show the importance

of worker cover. Probably oviposition by the queen and brood rearing by workers

reciprocally coact at a given density of adult workers. Under normal circumstances,

the queen seldom extends her oviposition beyond the area densely covered by workers.

The latter in turn increases its consistency where there are eggs. Consequently the

extent of brood area within which high brood survival is maintained is mainly determined

by the size of the worker population. Various external factors operate only indirectly

through worker density. In this connection, it must be stressed that no other social

insects have developed traits releasing such extensive bodily contacts as in honeybees,

1 Under favorable condition, the brood survival in bumblebee colonies may be higher. The high brood mortality in B. agrorum reported by BR~AN (1951), based upon which BRIAN (1965) prepared the data cited above, was not observed in a Neotropical species, B. stratus, observed by one of us (S. F. S.) under favorable conditions (SAK 6̂AM~, AKA~IRA and Zucc~, 1967).

38

which ultimately lead to the formation of compact clusters, observed especially in

swarming and wintering.

Increased mortal i ty is expected from some other factors not represented in the

present study. There are no sufficient quantitative data concerning brood mortali ty

from infectious diseases. This is principally due to the all or none character of many

bee diseases from the apicultural point of view. Infected colonies, especially those

attacked by the worst one, American Foulbrood, are often detected in a catastrophic

stage and destroyed soon after discovery. Indubitably brood mortal i ty may attain a

very high rate in such colonies.

HACHINOE and JIMBU (1958) reported increased egg inviability in queen produced

by sibling mating, reaching 40-50% of total eggs laid. Recently this problem was

extensively studied by WOYKE (1962) who recorded marked differences between the

eggs produced by normal and sibling-mating queens.

Another aspect worthy of mention is the removal of brood placed out of the brood

rearing area. Many of them would be removed after their deaths. But it is very

probable that some were removed before death. One of us (S. F. S.) repeatedly observed

in nuclei kept in the observation hives the eating of eggs, usually laid at the periphery

of the brood area, by workers. MYSER (1952) reported the same observation, citing,

as an extreme case, disappearance of 103 out of 117 eggs within 24 hours after oviposition.

WOYKE (1962, 1963) confirmed that eggs laid by queens produced by sibling mat ing

were less viable than those laid by the normal queens, but these eggs could hatch

or even develop when kept away from interference by workers. He revealed that the

intake of still alive eggs and larvae by workers was an important factor in producing

the lower brood viability in these instances. In this way, the organic mat ter of brood

is returned to the adult population, literally by feedback. This positive elimination

of excess or unwelcome brood, well known at nest foundation by the queens of higher

ants, presents a particular factor affecting the brood mortality.

Finally a few words are given concerning the duration of the immature stages.

JAY (1963) reviewed previous information by various authors. There are more records

of the mean total length of 21 days than 20 days. This difference largely depends on

the duration of the unsealed brood stage, either five or six days. It is conceivable

that the real mean length lies between 20.0 and 21.0 days, and the fraction has been

counted as one day by some authors. Most authors, except MtLLUM (1930), did not

give precise data on the variation of the duration of immature stages. MILLUM gives

the range of 19. 57 to more than 24 days, with the mean 20. 5 days, which does not

deviate much from our results.

SUMMARY

The brood survival in honeybee workers was measured in order to obtain the

data basic to the preparation of life tables. Under normal condition, that is, at the

center of brood area, the survival is high. The survival /s tage function runs 100.0

39

eggs, 94.2 unsealed brood (= feed ing larvae) , 86.4 sealed brood (=pos t - feed ing larvae

and pupae) and 85. 1 adults. The total dura t ion of immatu re stages is 20 days in 87. 1%,

21 days in 8.3% and 19 days or more than 22 days in the residual fract ion of successfully

emerg ing workers.

The surviva l r emarkab ly decreases at per ipheral areas wi th in the hive. Various

factors affecting the brood survival are discussed and the impor tance of a dense

worker cover on the brood area is stressed in relat ion to the ma in tenance of thermal

condit ions opt imal to the brood. The occurrence of one factor, which is not expected

in sol i tary animals , the self-control of populat ion by egg eating, is pointed out.

REFERENCES

BODENHEIMER, F.S. (1937) Studies in animal population. ]I. Seasonal population trends of the honey bee. Quart. Rev. Biol. 12: 406-425.

BRIAN, A.D. (1951) Brood development in Bombus agrorum. Ent. Mort. ll4ag. 87: 207-212.

BRIAN, J~41. V. (1965) Social insect populations, viii+135pp., Academic Press, London and New York.

FUKU1)A, H. and K. SRKmUCHI (1966) Seasonal change of the honeybee worker longevity in Sapporo, North Japan, with notes on some factors affecting the life-span, lap. ]. Ecol. 16: 206-212.

HACltINOE, Y. and M. ]~MBU (1958) Occurrence of the lethal eggs ~n the honeybee. Bull. Nat. Inst. Agric. Sci. Set. G (Animal Husbandry) 14:123-130 (In Japanese with English summary).

HIMMZR, A. (1927) Der soziale Wtirmehaushalt der Honigbiene. II. Die Wtirme der Bienenbrut. Erlangener lb. Bienenkd. 5: 1-32.

JAY, S.C. (1963) The development of honeybees in their cells. ]. Apic. Res. 2: 117-134.

MERRILL, J.H. (1924) Sealed and unsealed brood. Amer. Bee, ]. 64: 424-425.

MILLUM, V.C, (1930) Variations in time of development of the honey bee. ].Econ. Ent. 23: 441-447.

MvszR, W.C. (1952) Ingestion of eggs by honey bee workers. Amer. t~ee ]. 92: 67.

SAKAGAMI, S.F., AKAttIRA, Y. and R. Zuccnl (1967) Nest architecture and brood development in a Neotropical bumblebee, Bombus atratus. Insectes Sociaux 14: 389-414.

SEKmuc~, K. and S.F. SAKAaAM~ (1966) Structure of foraging population and related problems in the honeybee, with considerations on the division of labour in bee colonies. Hokkaido Nat. Agric. Exp. Sta. Rep, 69: 65pp.

WOYKE, J. (1962) The hatchability of 'lethal' eggs in a two sex-allele fraternity of honeybees. ].Apic. Res. 1: 6-13.

Wo~ze, J. (1963) What happens to diploid drone larvae in a honeybee colony. Ibid. 2: 73-75.

~ k ~ ) 94.2, ~ ( ~ j ~ + k ~ ) 86.4, ~_~, 88. ~o i ~ F ~ @ ~ , ~c~'.]~l~u~= s d) 87. 1~ ~ 20 H, 8. 3f~ ]9~ 21 H, z~ ~ ' U 19 H ~!./Z ~. 22 [] ~j,_b.-e a~ -~ ~: o ~}~/~ ~ { 0 ) ~ : ) ~ ~ < ~ t:[E