The guard honey bee: ontogeny and behavioural variability of workers performing a specialized task

Anirn. Behav., 1987, 35, 1159 1167

The guard honey bee: ontogeny and behavioural variability of workers performing a specialized task A L L E N J. MOORE*~', M I C H A E L D. B R E E D * t ~ & M A R Y JO M O O R *

* Department of Environmental, Population and Organismic Biology, The University of Colorado, Boulder, Colorado 80309-0334, U.S.A. "~Institute for Behavioral Genetics, The University of Colorado, Boulder, Colorado 80309-0447, U.S.A.

Abstraet. Guarding is a relatively unstudied aspect of honey bee, Apis mellifera L., worker behaviour. The

aim of this study was to characterize quantitatively the ontogeny and individual variability of guarding behaviour, the allocation of workers to the guard population in a colony, and the intercolonial variability of guarding behaviour. Guarding is a discrete task performed by a distinct group of workers that are younger than foragers and older than house bees. Workers that guarded initiated the behaviour between the ages of 7 and 22 days. The mean age of the onset of guarding varied; the minimum mean age of guards for a colony was 13.6 days and the maximum was 16.0 days. Workers varied in the length of time they spent as a guard. Most bees guarded for less than 1 day; however, some guarded up to 6 consecutive days. The more time a bee spent guarding during a day the more likely that bee was to guard for more than 1 day. Bees that guarded for more than 1 day also had longer and more frequent individual guarding bouts. All colonies that were studied had guard populations, but not all workers guarded. A relatively small proportion of any age cohort was observed to guard. The percentage of an age cohort that guarded varied among colonies, as did the size of the guard population. Guarding is a specialized task in that few bees guard, but guarding does not appear to require experience because so few bees remained as guards for very long. There was intercolonial variation in all aspects of the ontogeny of guarding and in allocation of workers to guarding. This variation is discussed in the light of other studies of variation in worker behaviour.

In this study we addressed three central questions concerning honey bee, Apis mellifera, guard behaviour. First, what role does guarding play in the behavioural ontogeny of a worker and how does this vary among individuals? Second, how is the guarding behaviour allocated among workers in a colony? Finally, is there intercolonial variation in the pattern of guarding behaviour? These questions were stimulated by the concept that guarding may be a specialized task (Kolmes 1985), because guards may function in nestmate recognition (Breed 1983) and nest defence (Wilson 1971). Specialized tasks may take longer to learn and be more efficiently performed by individuals that are experienced in the task (Seeley 1985). If guarding is a complex behaviour, then perhaps a few specialist bees perform this task for longer periods of time. This type of specialization is supported by studies of other task specialists, such as undertaker bees (Visscher 1983) and scout bees (Seeley 1983). ~:Towhom all correspondence should be addressed.

The pattern of behavioural changes that corresponds to the ageing of worker bees has been termed temporal polyethism. With the exception of egg laying, honey bee workers (sterile females) perform all the tasks necessary for the maintenance of the colony. It is well known that the duty that a given worker performs is determined primarily by her age. Younger workers are found inside the hive, and they clean cells, take care of brood, build comb, or store food while older workers guard the entrance and forage outside the hive (Lindauer 1952; Free 1965; Seeley 1982; Winston & Punnett 1982; Kolmes 1985). Although various other environmental factors may also influence the task that a worker performs (Oster & Wilson 1978; Winston & Punnett 1982; Winston & Fergusson 1985), temporal polyethism is a common determinant of the development of worker behaviour in social insects (Wilson 1971). In the honey bee, guarding has mainly been noted by researchers during investigations of temporal polyethism (Lindauer 1952; Seeley 1982,

1159

1160

Animal Behaviour, 35, 4

1985; Winston & Punnett 1982). By using focal animal techniques (Altmann 1974), Lindauer (1952) showed guarding to be a relatively minor worker activity in terms of the amount of time spent in this activity relative to other activities. Other researchers concluded that guarding is a minor task, as most bees are never seen guarding (Butler & Free 1952; Lindauer 1952; Free 1965; Winston & Punnett 1982). Studies of temporal polyethism in honey bees have focused on behaviour inside the hive or on foraging (Lindauer 1952; Free 1965; Seeley 1982; Winston & Punnett 1982; Kolmes 1985; Winston & Fergusson 1985). Alternatively, researchers have focused on general activity at the hive entrance (Butler & Free 1952; Kalmus & Ribbands 1952) or defensive behaviour (Collins et al. 1982; Seeley et al. 1982). Guarding as a discrete behaviour in honey bees has not previously been studied in any detail.

MATERIALS AND METHODS

We define a guard bee as any bee that patrols the landing board in front of the hive and greets bees that land near the hive entrance. A single act of greeting a bee was sufficient to define a worker as a guard. Greeting involves approach and antennation (1 3 s; Herman & Blum 1981; Seeley 1985) of the newly arrived bee. In contrast to most bees seen at the hive entrance, guards are very active and quickly approach bees that are landing. Guard bees also adopt a characteristic stance when they are not moving about; they stand with their forelegs off the ground and their antennae pointing forward (Butler & Free 1952; Herman & Blum 1981; Seeley 1985). Guards typically hold their wings away from their bodies, as if preparing for flight (Butler & Free 1952); other bees on the landing board lay their wings flat against their bodies. Guards often fly up to inspect disturbances around the hive (such as the approach of a human observer). They also expel intruders. The definition of guarding that we employed is based on personal observations and on the descriptions of guarding published previously (Butler & Free 1952; Herman & Blum 1981; Seeley 1985). This definition allows quick and easy identification of guards in front of the hive and has the advantage of not requiring disruption or agitation of the hive, unlike previously published research on guards (Butler & Free 1952). Not all guards are seen continuously on the

landing board (below; also Butler & Free 1952). The time that a bee was unambiguously seen greeting other bees was considered to be a single guarding bout. While hives were being observed, bees could be seen guarding once, several times, or continuously. They might also be seen during several different observation periods. Each time a bee was seen guarding it was considered to have engaged in one guarding bout. Bouts ended when the guard bee entered the hive and presumably rested, or when the guard entered a group of fanning bees on the landing board and began fanning. Experiments were conducted between 1 June and 31 August 1984, and between 1 June and 19 July 1985. All studies were conducted using standard 10-frame commercial hives maintained on the East Campus of the University of Colorado, Boulder. Different hives were used each year, and a total of seven hives were used in both years. All colonies were matched for size and were exposed to environments that were as identical as possible in a natural setting. All hives were maintained using standard beekeeping techniques. Individual bees were marked to facilitate the study of guarding in specific age cohorts and to allow recognition of the onset of guarding. Combs containing sealed worker pupae were removed from each of the hives and maintained in an incubator in the laboratory. Newly emerged bees less than 24 h old were marked with a drop of enamel paint, colour-specific for the day of emergence, and were returned to their natal colony. In each colony an age cohort of approximately 1000 newly emerged workers from a single frame was marked and returned to its respective colony; typically the emergence of this number of bees was spread over 2 or 3 days. In 1984, the ontogeny of guarding was documented by observing each colony for 1 h at the same time of day, beginning when marked workers were added to the colony (day 0). When a marked bee was first observed guarding, it was captured with forceps and marked with a second colour of paint, again colour-specific for that day. The second mark allowed us to identify bees that had previously guarded. These bees were held in a container until the end of the observation period, when they were returned to the hive. Bees treated in this manner could be followed for more than 1 day and guards could be distinguished from other workers.

Moore et al.: Guard honey bees The variation in behaviour among individual guards was documented in 1984 by marking 30 guard bees with numbered plastic discs at 0800 hours. Observations lasting 30 min were made at intervals of 2 h. The number, the duration and the times of day each marked bee initiated and terminated bouts of guarding were recorded. Observations were made until 1800 hours, when activity at the hive entrance (foragers returning and leaving) began to diminish. Colonies were observed daily until no more marked workers were seen guarding. The number of bees guarding at any one time was calculated using the Jolly (1965) c a p t u r ~ recapture method. Guard bees were randomly captured at the same hour daily and marked with different colours of paint. Random selection of guards was ensured by capturing any guard that entered a 5 x 5 cm area of the landing board of the hive. The paint colour was changed each day so that bees could be captured more than once and scored for the last day of capture. The size of guard bee populations were then calculated using the Jolly (1965) technique. This capture-recapture method of estimating population size is similar to the Lincoln Index (population size=number of marked individuals x total second sample+total recaptured) but also takes into account death and emigration, and birth and immigration (Jolly 1965). All-day studies were conducted in the summer of 1985 to verify the results obtained the previous summer and to gather more detailed information about individual guards. Marked age cohorts of 1000 bees were returned to the hives when the bees were newly emerged. To allow us to follow individuals, paint-marked workers were captured when they first guarded. A unique number tag was then applied to the thorax and the guard bees were immediately returned to the hive. This manipulation apparently did not interfere with guard behaviour, as bees that were returned to the hive often resumed guarding immediately. Colonies with these numbered individuals were observed from 0800 hours to 1900 hours for 30-rain intervals each hour. The number of times that a bee was seen guarding during an observation period and the length of each guarding bout were recorded. The onset of foraging by the marked bees was also noted. Statistical tests were conducted according to methods outlined in Sokal & Rohlf (1981). When appropriate, log transformations were performed

1161

to correct for heterogeneity of variances or nonnormality. Non-parametric tests were used when assumptions of analysis of variance could not be met. Appropriate a priori ANOVA tests (df= 1) were performed by contrast analysis using orthogonal contrast codes (Rosenthal & Rosnow 1985). Means are presented + 1 SE unless otherwise indicated.

RESULTS In all cases, the results obtained in different years and by using different methods of assaying guarding were similar. A bias resulting from the more labour intensive all-day observations conducted in 1985 would have increased the probability of a worker being observed as a guard. This was not the case; the two colonies observed in 1985 yielded intermediate values of the number of guards when compared to values obtained for colonies observed in 1984. We have therefore included all of the colonies in our analyses of guarding, except for the analysis of individual differences in guarding.

Ontogeny of Guarding The youngest age at which bees guarded varied among colonies (Fig. 1). In colony 1 the youngest guards were 7 days old and in colonies 6 and 7 they were 10 days old. The latest age of initiation of guarding was 21 or 22 days in all of the colonies. These distributions were affected somewhat by environmentalchanges such as cold weather or rain (e.g. on day 17 it rained on colonies 6 and 7). By examining the mean age of guards we found that there was significant intercolonial variation (Table I; F=4.89, dr= 6,741, P < 0-001). The mean age of workers that guard was significantly younger in colony 5 (F= 12-89, df= 1,741, P < 0.05; Scheffe's a posteriori test) and significantly older in colony 4 ( F = 12"82, df= 1,741, P<0.05; Scheffe's a posteriori test). There was no significant difference in the mean age of guards among the other colonies. Workers invariably became foragers after cessation of guarding. A bee that was no longer guarding was always seen returning from foraging trips within 24 h of the last time she had been seen guarding. This was observed on both individually numbered and distinctively paint-marked bees. Once workers had been observed foraging they were never observed guarding again.

Animal Behaviour, 35, 4

1162 25

F u r t h e r s u p p o r t for the n o n - o v e r l a p p i n g n a t u r e of guarding with foraging can be d e m o n s t r a t e d by c o m p a r i n g the distributions of the age o f the onset of guarding b e h a v i o u r a n d the age of onset of the foraging behaviour. We did this for colony 2 a n d colony 3 where foragers were being studied simultaneously; the distributions were highly significantly different in b o t h (colony 2 age of foragers, ~? = 19.145 • 0.26; A N O V A ; F = 136.38, dr= 1,290, P < 0 . 0 0 1 ; colony 3 age o f foragers, Y = 19.029+0.44, F = 65.66, df= 1,214, P < 0 . 0 0 1 ) .

Colony 7

_add]la]lllh

15 10 5 25 20

Colony 6

15 10 5 25

.~

28 15 10

..

2015

s

_.!1111.11. Colony 5

- - - " I I . I . . .

Colony 4

Allocation of Workers to Guarding l0

s 25" Z

20

20 i5 10 5

'~

.

0

7 " 8

.

. 9

. I0

. 11 1 2

nnnnnnn ._ 13

14

Day

15

First

16 I7

18

19 20

21

22

23

Seen

Figure 1. The distributions of the age of onset of guarding for seven colonies. Each colony was sampled for the presence of guarding individuals that had never been seen guarding before, In colonies 1-5 the sampling period was 1 h, once a day. In colonies 6 and 7 the sampling periods were for 30 min every hour throughout the day. The number of individuals observed guarding on day 15 in colonies 1, 2 and 3 and day 17 in colonies 6 and 7 is low due to rain.

In each of the colonies a r e m a r k a b l y small percentage of the m a r k e d bees was observed to g u a r d (Table I). T h e percentage of an age c o h o r t that guarded differed between colonies; these differences were highly significant (G-test with William's correction; G = 7 1 . 7 6 8 , df=6, P < 0 . 0 0 1 ) . W h e n colony 5, a colony t h a t was clearly unusual, was omitted from the analysis there was still highly significant heterogeneity a m o n g colonies in the percentage of a c o h o r t t h a t was seen guarding ( G = 15.70, df=5, P < 0 . 0 1 ) . In all colonies, the majority of guards were present for only I day (Table II). This was true regardless of the technique used to observe guarding. The m o s t persistent bees guarded 6 days. Some bees were seen o n consecutive days; bees seen guarding o n any day after their first day of g u a r d i n g were a subset of those g u a r d i n g on each of the preceding days. The n u m b e r of guards within a colony at any one time was calculated for three of the five colonies

Table 1. Mean age of workers and the percentage of the marked age cohort which was eventually observed guarding for the different colonies No. marked bees Total no. introduced back marked bees % Age Colony to the colony seen guarding cohort seen 1 2 3 4 5 6 7

1161 926 1094 1020 960 1004 1083

116 119 146 104 39 90 127

9.7% 12.9% 13.3% 10.2~ 4.1% 9.0% 11.7%

Mean age of initiation of guarding* 15.19_+5.20 14.57_+3-02 15.27_+2.96 16.03_+4.227 13.59_+4.297 15.57_+2.91 15.54_+2.65

* Mean age of guards given in days post emergence_+ 1 SD. ? Mean age of guards significantly different; colony 4 is significantly greater than all other means, colony 5 is significantly smaller than all other means (Scheffe's a posteriori test, P < 0.05).

Moore et al.: Guard honey bees

1163

Table II. Number of worker bees that were seen guarding a specific number of days

Colony 5

Total number of days seen guarding* Colony

0

1

2

3~

3 4 5 6

Colony 4

25

1 2 3 4 5 6 7

1045 99 2 12 807 99 14 4 948 103 25 10 916 88 9 5 921 36 3 0 914 68 14 7 956 85 25 10

0 1 5 2 0 0 4

2 1 2 0 0 0 2

1 0 1 0 0 1 1

20

9; t0

~5

3o 2s

* These categories represent the maximum number of days an individual bee was seen guarding. Each category does not include bees that were seen on subsequent days.

~_ 2o Z

15 10 5 30 25 20

Table II1. Estimates of the number of guards at colony entrances using the Jolly capture recapture method

t5 to s 0

Day Colony 1 2 3

2

3

4

5

69.8 42.3 -88.6 24.3 46.2 - 91.2 50.6 135.7 40.2

6 36.4 78.8 160.0

observed in 1984 (Table III). The Jolly (1965) m e t h o d gave highly variable estimates o f the g u a r d population. T h e size range into which all o f the colonies fell was 24.3-160.0. The m e a n estimated n u m b e r of guards over the three colonies was 72.01 ( S E = 12'00).

Intercolonial Differences in Guarding Persistence of guarding can be analysed in two ways: persistence t h r o u g h a day a n d persistence for more t h a n 1 day. Relatively few bees guarded t h r o u g h o u t a day (Fig. 2). The distribution o f the n u m b e r of bees exhibiting g u a r d i n g activity t h r o u g h o u t a day differed significantly between colonies 1, 2, 3 a n d 4 (G-test with William's correction, G = 2 6 . 0 0 2 , dr= 12, P < 0 . 0 2 5 ) . In par-

I

3

5

7

9

Hours Figure 2, The persistence of guarding individuals within a single day. Thirty guarding bees were individually numbered at 0800 hours. Four colonies were observed for 30 min every other hour for the presence of numbered bees.

ticular, 30% of the guards from colony 3 persisted an entire day, while less than 10% o f the bees in the other colonies guarded an entire day. T h e n u m b e r o f workers t h a t guarded for more than 1 day also varied greatly a m o n g colonies (Table II). The distribution of bees t h a t guarded for different lengths of time was significantly different between colonies (G-test with William's correction; G = 123.07, df= 36, P < 0 . 0 0 1 ) . The persistence of guards varied depending o n the colony. Colonies with persistent g u a r d populations within a day also tended to have persistent guard p o p u l a t i o n s between days (e.g. colonies 3 a n d 7).

Inter-individual Differences in Guarding In colonies 6 a n d 7, e x a m i n a t i o n of the behaviour o f individually n u m b e r e d bees d u r i n g guarding b o u t s indicated that there were i m p o r t a n t differences between persistent guards and short-

Animal Behaviour, 35, 4

1164

term guards in the nature of their guarding bouts. On the first day that a bee was seen guarding, there was a significant variation in the amount of time that was spent in a single guarding bout dependent upon whether that bee ultimately guarded for 1 day (N=153, Y=0"22_+0.05 min), 2 days (N=39, ~=0-33__0.09 rain), 3 days (N=17, 52=0-67• 0.14 min) or 4 days (N--4, ~ = 0 . 4 4 + 0 . 2 9 rain; bees that guarded 5 and 6 days were not included due to small sample sizes, N = 2 for both; F=4.86, df= 3,212, P < 0.005). There was a significant linear trend in these means: the more days a bee spent guarding the longer the time spent in its initial guarding bout (F = 12"90, df= 1,212, P<0"001). There is no significant deviation from the linear regression ( F = 0.83, df= 2,212, P > 0-25). After the initial day of guarding there were no significant differences in the length of time bees guarded. In colonies 6 and 7 there were also significant differences between the number of guarding bouts individual bees performed during an observation period. Since most bouts were less than 1 rain long, individuals were often seen guarding many times during a 30-min observation period. Bees that guarded more than 1 day (N=63) were seen significantly more often within an observation period than bees that guarded for only 1 day (N--162). Comparisons were made between the mean number of bouts for the first day a bee was seen (Y = 1.520 • 0.478), the last day a bee was seen

(~" = 1.552 • 0.261) and the only day a bee was seen ("~=1-148• A comparison of the three means indicates no significant differences (ANOVA; F=2.418, df= 2,285, P > 0-05). A comparison of the means between the first or last day of guarding for multiple-day guards and the mean for 1-day guards indicates that bees that guard for more than 1 day were seen significantly more times during an observation period than were bees that guard for only 1 day (F=4.827, df=1,285, P<0.05). There was no significant difference between the means for the first day of guarding and the last day of guarding for bees that guarded for more than 1 day (F=0.039, df= 1,285, P>0.75). Thus, bees that guarded for more than 1 day were more likely to be seen guarding more often during an observation period than were bees that guarded for only 1 day. The mean length of a bout and the number of bouts in an observation period can be incorporated into a similar measure, the total amount of time spent by a bee guarding during a day. When the total amount of time on the first day that a bee was seen was compared between bees that ultimately guarded l, 2, 3 or 4 days, we found significant differences between the means (Fig. 3; ANOVA, F = 5.27, c//= 3,209, P < 0.005). There was a significant linear trend between these means; the more days a bee guarded the more time that it spent in its initial day of guarding (F=13.31, df=l,209,

1.8 1.6 1.4 ~"

E

"-~ ~D

1.2 1.0

E~

0.8

~'~

0.6 0.4 0.2 0.0 1

Total

2

number

3

of

4

days guarding

Figure 3. The total amount of time spent guarding on the first day of guarding by bees which ultimately guarded 1, 2, 3 or 4 days in a row. These data were collected on individually marked bees in colonies 6 and 7.

Moore et al.." Guard honey bees P < 0.001). There was no significant deviation from the linear regression ( F = 1-25, df-- 2,209, P > 0-25). There were no significant differences between the means for the amount of time spent guarding on the second day of guarding (day 2, Y ~ 4.81 __.0.13; day 3, r day 4, Y=6.46• F = 0.16, df= 2,57, P > 0.75). The sample sizes were too small and the variation too large to make meaningful comparisons between the means for days 3 and 4 on the third day of guarding.

DISCUSSION

Ontogeny and Inter-individual Differences in Guarding Guarding is a robust behaviour; even in the face of the sometimes disruptive manipulations, guards were always easily identified. The population of guards was extremely well defined regardless of the method used to assay guarding. This is especially evident in the consistency of the ontogenetic aspects of guarding. There was a very clear delineation of the onset and end of guarding behaviour. In all colonies, once bees were seen as foragers they were never again seen guarding. It is possible that bees performed internal hive duties as well as guarding, but we feel this is unlikely, given the nonoverlapping nature of guarding and foraging. A bee guarding and performing hive duties is also inconsistent with Seeley's hypothesis of spatial efficiency (Seeley 1982, 1985) and other studies of temporal polyethism in the honey bee (Winston & Punnett 1982; Visscher 1983; Kolmes 1985). However, guarding would need to be studied in an observation hive to eliminate unambiguously the possibility that guards also perform hive duties other than resting and fanning. In contrast to the report of Butler & Free (1952), our results suggest that guarding is a discrete behaviour. This may be because our definitions of guarding differed. Butler & Free included aggressive behaviour (biting, mauling and attempted stinging of intruders) in their definition of guarding. They also disturbed colonies to incite guarding. We, on the other hand, purposefully excluded overt aggression as a part of our definition and concentrated on behaviours that are normally seen in undisturbed colonies. Aggression is a part of guarding, but individuals other than guards (e.g. foragers that are at the hive entrance) may be

1165

stimulated to become aggressive. Of the five observations we had of bees expelling intruders, four were expulsions by bees that were then guarding or had been seen guarding that day. One expulsion, however, was performed by an individual that had been foraging for several days and that was not actively engaged in patrolling behaviour. This also explains why Butler & Free (1952) observed foragers apparently guarding, as they only mention foragers engaging in aggressive behaviours and not patrolling the front of the hive. Perhaps foragers can be incited by guards to become aggressive. Individuals display differences in the quantity of guarding behaviour that is performed. Workers that were seen guarding frequently within a day were almost always seen guarding for more than 1 day. Most guarding workers were seen for only a brief portion of the observation period. Individuals also differed in the length of time that a guarding bout lasted. The cause of these differences is not known. Why some workers guarded persistently over several days while others guarded very briefly may be due to genetic differences which might influence colony level characteristics of guarding, such as the number of bees that guard and the size of the guard population.

Allocation of Workers to Guarding This study presents the most detailed quantification to date of the allocation of workers to guarding. All of the colonies that we studied had guard populations, but not all workers guarded. In all the colonies, less than 15% of an age cohort guarded. The number of workers guarding at any one time was also very small, typically fewer than 100. Thus, the number of bees allocated to guarding within a colony is very small relative to the total colony population. One of the unexpected results of this study was that not all bees that guarded underwent a long period of specialization in this task. The fact that Very few bees guard supports the idea that guarding is a specialist's task. However, most bees that guarded did so very briefly. Based on Seeley's (1982, 1985) hypothesis that complex tasks that require learning will be performed for longer periods of time by fewer bees, we had predicted that colonies would allocate a small number of bees to guarding but those bees would remain as guards for long periods of time. It may be that guarding is specialized but is not a complex task and does not

1166

Animal Behaviour, 35, 4

require learning. Alternatively, specialized tasks may not require long-term investment in the behaviour by workers in order to learn that task. Although some bees specialized in guarding for up to 6 days, we have no explanation for the existence of these persistent guards. They may differ in some way from other guards. We noticed no immediately obvious behavioural differences between any of the bees we saw guarding.

the colonies. Further investigations into the genetic basis for the differences in guarding should give some insight into the adaptive nature of guarding and the evolution of guarding. There may also be practical applications of such knowledge due to the impending colonization of the North American continent by the Africanized bee.

ACKNOWLEDGMENTS Intercolonial Differences in Guarding There is variability between colonies in the ontogenetic aspects and the allocation of workers to guarding. Some colonies have larger, older, or more persistent guard bee populations, with workers remaining guards for long periods. Other colonies have slightly smaller, younger, or shorterlived populations of guards. For example, the percentage of an age cohort that guards varied significantly among colonies. Other important worker behaviours vary among colonies (Gonqalves & Stort 1978; Collins 1979); perhaps there are similar influences on the variation in guarding behaviour. Aggressiveness (such as that characterized in the assays of Collins & Kubasek 1982) is a colony-level feature that is variable and may be tied to the persistence of guards. The colonies with the most persistent guards also appeared to be the most aggressive (A. Moore, personal observation). Colony 5, which had few guards at any one time and where guards persisted for less than 1 day, was the least aggressive hive studied. Few or no bees flew up to the approaching observer and no bees from this colony ever stung a human observer. Colonies 3 and 7, which had guard populations with the opposite characteristics, were the most aggressive colonies in the apiary. Bees always flew up to an approaching researcher and the human observer was stung on several different occasions by workers from these colonies. Other studies have noted variability in aggression between colonies, both anecdotally and quantitatively (Collins & Kubasek 1982; Collins et al. 1982). Collins (1979; Collins et al. 1982) presents evidence that colony aggression is highly heritable. Aggression is apparently not due simply to additive genetic influences but is influenced by dominant alleles (Gonqalves & Stort 1978; Collins 1979); this may also be the case with guarding. The variation in guarding among colonies may therefore be due to genetic differences in

We thank Jon Harrison for graciously providing us with the data for the age of onset of foragers in colonies 2 and 3. We also thank Trish Moore for preparing all of the figures. D. Arthur, B. Bennett, J. Coelho, J. Harrison, A. Lewis, T. Stiller and K. Williams provided comments on earlier versions of this manuscript. A. J. M. was supported by a training grant from NIMH (MH16880) in behavioural genetics. This research was funded by a University of Colorado graduate school grant to M. D. B.

REFERENCES Altmann, J. 1974. Observational study of behaviour: sampling methods. Behaviour, 49, 227-267. Breed, M. D. 1983. Nestmate recognition in honey bees. Anita. Behav., 31, 86-91. Butler, C. G. & Free, J. B. 1952.The behaviour of worker honey bees at the hive entrance, Behaviour, 4, 262 292. Collins,A. M. 1979. Geneticsof the response of the honey bee to an alarm chemical, isopentyl acetate. J. apicult. Res., 18, 28~291. Collins, A. M. & Kubasek, K. J. 1982. Field test of honey bee (Hymenoptera: Apidae) colony defensivebehavior. Ann. Entomol. Soc. Am., 75, 383-387. Collins, A. M., Rinderer, T. E., Harbo, J. P. & Bolten, A. B. 1982. Colony defense by Africanized and European honey bees. Science, N. Y,, 218, 72--74. Free, J. B. 1965. The allocation of duties among worker honey bees. Syrup. Zool. Soc. Lond., 14, 39 59. Gonqalves, L. S. & Stort, A. C. 1978. Honey bee improvement through behavioral genetics. A. Rev. Entomol., 31, 197-213. Herman, H. R. & Blum, M, S. 1981. Defensive behavior in the social Hymenoptera. In: Social Insects, Vol. 11 (Ed. by 1-I. R. Herman), pp. 77-197. New York: Academic Press. Jolly, G. M. 1965. Explicit estimates from capturerecapture data with both death and immigration: stochastic model Biometrika, 52, 225 247. Kalmus, H. & Ribbands, C. R. 1952. The origin of the odours by which honeybees distinguish their companions. Proc. R, Soc. Lond. (B), 140, 50-59. Kolmes, S. A. 1985. An information-theory analysis of

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(Received 12 May 1986; revis'ed 14 July 1986; MS. number: A4776)