the flower constancy of honeybees

The Flower Constancy of HoneybeesAuthor(s): J. B. FreeSource: Journal of Animal Ecology, Vol. 32, No. 1 (Feb., 1963), pp. 119-131Published by: British Ecological SocietyStable URL: http://www.jstor.org/stable/2521 .

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119

THE FLOWER CONSTANCY OF HONEYBEES

BY J. B. FREE

Rothamsted Experimental Station, Harpenden, Herts.

Knowledge of the flower constancy of honeybees is important in helping to solve many pollination problems. Most honeybees visit only one flower species during one foraging trip (e.g. Betts 1935, Maurizio 1953). Little is known of their flower constancy on successive trips and days; the present work investigates this and the effect on it of moving their colonies to new sites.

METHOD

Pollen-gatherers were captured entering their hives, chilled until immobile, and given individually distinctive paint marks. Each had one pollen pellet removed and was then

Table 1. Constancy to kinds of pollen collected on 2 consecutive days by bees originally collecting pure loads

Expt. 1* Expt. 2 Expt. 3 14 and 15 May 1957 14 and 15 May 1959 6 and 7 July 1959

No. of bees collecting (a) their original pollen only 43 34 28 (b) their original pollen and

another pollen 0 2 1 (c) other pollen only 5 6 11

% of bees collecting their original pollen only 89-6 81V0 70-0

* Predominant pollens identified were: Expt. 1: Cruciferae spp., Pyrus Malus L., Syringa vulgaris L. Expt. 2: Pyrus Malus, Cruciferae spp., Acer campestre L. Expt. 3: Trifolium repens L., Castanea sativa Mill., Ranunculus spp.

released; this was repeated each time a marked forager was seen entering its hive with pollen. Pollen was identified microscopically to its species when possible, but often only to a group of related species; knowledge of the local flora sometimes helped establish the species. A pollen could sometimes be distinguished from the rest but not identified. A few pollen loads that contained traces only of a second species are recorded as 'pure' loads because they might have been contaminated by pollen collected on previous trips.

Bees returning home without pollen have been recorded as nectar-gatherers; some may have collected water and some may have been unsuccessful foragers.

RESULTS

Constancy for consecutive days at the same site

In Experiments 1-3 the first pollen loads collected by bees the day after marking were compared with those they had when marked the previous morning. No record was kept of the marked bees that subsequently foraged without collecting pollen. A mean of 80-8 % collected the same pollen on both days (Table 1). It is not possible from the results to calculate the daily decline in the percentage of bees visiting their original species, as this



120 Flower constancy of honeybees

will also depend on the proportion of bees that return to their original species after temporarily deserting them. Hence it was necessary to observe colonies for longer periods.

In Experiment 4, pollen-gatherers were marked on 27 May 1959 and their hive entrance observed on 28, 29 and 30 May and 2 June. Those subsequently seen with pollen originally collected: Acer campestre, seventeen bees; Cruciferae spp., ten bees; Taraxacum officinale Weber, four bees; Aesculus hippocastanum L., four bees; Pyrus Malus three bees. Bees were rarely captured more than once on successive days. In general the proportion

Table 2. Constancy to kinds of pollen collected by bees originally collecting pure loads in Experiment 4

May June A

27 28 29 30 2 No. of bees collecting

(a) their original pollen only 9 18 3 5 6 (b) their original pollen and

another pollen 0 0 1 0 0 (c) other pollen only 5 5 1 6 7

% of bees collecting their original pollen only 64-3 78-8 60-0 45 5 461

visiting their original species decreased with time (Table 2); the relative amounts of the different types of pollen collected did not change greatly during the experiment. Table 3 shows the mean numbers of pollen types collected by bees foraging for pollen on different numbers of days in this and later experiments. In general the number of species collected increased with the number of days spent foraging.

In Experiment 5, pollen-gatherers entering a two-comb observation hive were marked on 9 May 1961 and the hive entrance watched daily until 17 May whenever the bees were flying between 09.00 and 17.30 hours. No bees flew on 15 May. Twenty-six pollens were

Table 3. Mean No. of types of pollen collected by bees collecting pollen for a different No. of days (No. of bees in brackets)

No. of days collected pollen

Expt. No. 2 3 4 5 >5

4 1-5(24) 1.5(8) - -

5 (9-17 May) 1 4(40) 17(41) 2-5(15) 2'8(5) 2 0(4) 5 (13-17 May) 1 5(25) 1-7(7) 1-5(2) 6 1 3(64) 1-6(46) 2-0(18) 1.8(16) 1-8(8) All expts. 1.4(153) 1-6(102) 2-2(35) 2-0(21) 1l9(12)

collected subsequently but only the following six frequently: Pyrus Malus, 181 trips; Cruciferae spp., forty-two trips; Cotoneaster spp., thirty-nine trips; Aesculus hippocastanum, eighteen trips; Fragaria spp., fourteen trips; Endymion nonscriptus L. ten trips.

The percentage of bees collecting their original pollen decreased greatly on successive days (Table 4). There was no obvious change in preference for any particular species although bees originally visiting Pyrus Malus were more constant than the rest.

In this and later experiments no bee regularly collected different pollen at different times of day. Some bees collected pollen on some trips but not on others. Bees visiting



Table 4. Constancy to kinds of pollen collected by bees originally collecting pure loads in Experiment 5 (Bees originally collecting Pyrus Malus pollen are also shown separately in brackets)

May

10 11 12 13 14 16 17

No. of bees collecting (a) their original pollen

only 57(41) 44(29) 17(11) 8(7) 3(1) 2(2) 0 (b) their original pollen IT1

and another pollen On same trips 6(3) 2(1) 2(2) 2(1) 1 0 0 Sometimes on same trips 4 6(5) 2(1) 1(1) 0 0 0 On different trips 1 1 1 0 0 0 0

(c) other pollen only 4 10(2) 3(2) 9(4) 4(1) 7(5) 2(2)

% of bees collecting their original pollen only 79 2(93 2) 69 8(78&4) 68X0(68&7) 400(53-8) 37X5(50X0) 22X2(28X6) 0(0)




P. Malus, the species most visited, tended to collect most pollen in the first part of the day and nectar later (Table 5).

The percentage of foragers collecting pollen decreased steadily, irrespective of species, throughout the experiment, and few bees collected pollen towards the end, although many still foraged (Table 6). This may have been because the colony had little unsealed brood from soon after the experiment began, and the total pollen income was decreasing; however, other bees may have been changing from nectar- to pollen-gathering, or the

Table 5. No. of trips made by Pyrus Malus pollen-gatherers at different times of day

One hour period beginning

09.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 Bees collecting pollen every

trip 13 19 16 9 13 4 5 2 0 Bees collecting pollen some

trips (a) Trips with pollen 4 10 13 8 9 3 4 5 1 (b) Trips without pollen 5 6 2 8 12 4 9 12 4

tendency of the foragers to collect pollen may have decreased with age, or removal of pollen from them may have discouraged them from collecting it.

On 13 May nectar- and more pollen-gatherers were marked. Pollen was collected by thirty-four of the sixty-nine pollen-gatherers that foraged again; the percentage collecting pollen decreased daily (Table 7) This decrease was not redressed by the nectar-gatherers collecting pollen, as only one of the sixty-eight that foraged after marking did so. The survival rate of the two groups was similar, forty-two original pollen-gatherers and forty-

Table 6. No. of foragers marked on 9 May collecting nectar and pollen (No. of trips in brackets)

May r

'k I

10 11 12 13 14 16 17 Collecting pollen on

every trip 48(74) 42(81) 16(22) 17(20) 5(9) 3(7) 2(2) Collecting pollen and

nectar on separate trips 29(81) 32(114) 13(39) 4(8) 2(5) 5(20) 3(14)

Collecting nectar only 31(52) 30(61) 50(113) 40(76) 41(94) 34(122) 35(140)

% bees collecting pollen 71-3 71-2 36-7 34-4 14-6 19-0 12*5

three original nectar-gatherers foraging on 17 May; therefore, it is unlikely that the tendency to collect pollen was associated with a forager's age.

The number of the pollen-gatherers with pure loads when marked on 13 May that collected their original kind of pollen only, and the number that collected other pollens only, or together with the original pollen was: 14 May, 16: 3, 16 May, 8: 6, 17 May, 3 :3.

A colony occupying ten combs was used in Experiment 6; it had plenty of unsealed



J. B. FREE 123

brood throughout the experiment. Pollen-gatherers were marked on 10 July 1961 and the colony observed until 20 July. Some bees were chilled before marking and others anaesthetized with chloroform. When they subsequently returned home with pollen, it was removed from one corbicula of each of the bees anaesthetized with chloroform, and most

Table 7. No. offoragers marked on 13 May collecting nectar and pollen (No. of trips in brackets)

May

14 16 17 Collecting pollen when marked

Collecting pollen on every trip 10(14) 6(11) 3(4) Collecting pollen and nectar on

separate trips 13(38) 10(33) 4(14) Collecting nectar only 30(63) 38(116) 35(153) % bees collecting pollen 43-4 29-6 16-7

Collecting nectar when marked Collecting pollen on every trip 0 0 1(1) Collecting nectar only 57(195) 55(274) 42(217)

chilled bees, but pollen was not removed from some chilled bees until 19 July. No bees foraged on 12 July.

Anaesthetizing pollen-gatherers with chloroform does not affect their tendency to collect pollen (Ribbands 1950). The proportion of chilled bees, whose pollen was removed, which continued to collect pollen was as great as that of bees anaesthetized with

Table 8. Foraging of bees given different treatments

July

Treatment* 11 13 14 15 16 17 18 19 20

fa 98 57 43 59 54 67 61 35 38 No. foraged b 24 6 7 10 12 12 12 4 7

Lc 27 24 17 18 15 19 26 21 16

% foragers collecting a 90 60 56 66 26 60 41 19 37 o oragers coectmg . b 96 33 43 30 17 50 9 50 14 pollen Lc 93 71 100 78 60 79 88 81 44

ra 143 124 119 114 101 90 71 48 38 No. of bees present . b 32 24 22 22 19 15 13 9 7

Lc 46 46 43 40 36 34 31 23 17

va - 87 83 80 71 63 50 33 27 % surv1val

of bees b - 75 69 69 59 47 41 28 22 present on II July c _ 100 93 87 78 74 67 50 37

* Treatment a: chilled when marked and pollen removed each trip. b: anaesthetized with chloroform when marked and pollen removed each trip. c: chilled when marked and pollen not removed until 19 July.

chloroform (Table 8). Therefore, chilling does not decrease pollen-gathering. However the proportion of chilled bees, whose pollen was not removed until 19 May, which collected pollen was greater than that of the other two groups, except on 20 May. Hence, removing pollen decreases a bee's tendency to collect it. Care was taken to avoid



Table 9. Constancy to kinds of pollen collected by bees originally collecting pure loads only in Experiment 6 (No. of trips in brackets)

July

11 13 14 15 16 17 18 19 20 0 No. of bees collecting

(a) their original pollen 0

only 87(139) 24(30) 17(26) 31(35) 10(15) 29(36) 15(24) 6(6) 5(6) >

(b) their original pollen and another pollen On same trips 2(3) 0 1(1) 1(1) 0 5(5) 1(1) 0 0

Sometimes on same trips 1(2) 0 0 1(2) 0 1(2) 1(3) 0 0 On different trips 3(8) 0 1(3) 0 0 1(3) 1(3) 0 0

(c) other pollen only 11(16) 9(13) 9(11) 6(11) 5(6) 13(16) 7(7) 2(3) 7(14) i,

% bees collecting their l original pollen only 83 7 72-7 60-7 795 66&6 59-2 60-0 750 41P7



J. B. FREE 125

damaging the corbicula hairs while removing pollen, and it seems unlikely that damage occurred often, as no bees were seen returning home with a larger load in one corbicula than the other.

The number of marked bees alive each day was calculated by assuming that a bee was dead on the day following that on which it was last seen to forage. The survival of bees whose pollen loads were not removed was better than that of other chilled bees (Table 8). Presumably, handling bees when removing their pollen sometimes injured them. Bees anaesthetized with chloroform had the highest death rate.

Fewer bees per day made some trips for pollen and others for nectar only than in the previous experiment (11.4%: 18*2 %); probably because they made fewer trips per day (1 8: 2 4). No difference could be shown between the times of day that bees, constant to a particular pollen, returned home with and without pollen.

From 11 to 20 July, twenty pollens were collected, the five main ones being: Trifolium repens, 159 trips; Rubus caesius L., 138 trips; Epilobium spp., fifty-two trips; Bryonia dioica Jacq., twenty-one trips; Rosa canina L., twelve trips. In general, the bees' constancy to their original pollen decreased on successive days but less regularly than in the previous experiment (Table 9). This was probably because availability of the different pollens varied more from day to day. Thus the numbers of bees visiting Trifolium repens and Rubus caesius on different days were:

July

1 1 13 14 15 16 17 18 19 20

Trifolium repens 49 5 2 20 8 31 9 2 4 Rubus caesius 30 14 18 9 1 13 9 4 6

Fifty-seven bees, originally collecting Trifolium repens pollen, foraged after 14 July, thirty-four of them for T. repens pollen. However, only five of them collected T. repens pollen on 13 and 14 July; only five collected other pollens during these two days (two of these collected T. repens later) and nine collected nectar only; the rest did not forage.

Thirty-two bees originally collecting Rubus caesius foraged after 16 July, twelve for R. caesius pollen. On 16 July, only one collected R. caesius pollen, two collected other pollens on 16 July and subsequent days, and fifteen collected nectar only (six of these collected R. caesius pollen on subsequent days), fourteen did not forage. Therefore, it seems that, when a particular pollen is temporarily scarce, most bees remain at home or collect nectar, and few change to another pollen.

On 19 May, seven of the seventeen bees (41 %) whose pollen had not been removed since marking, collected their original pollen and, on 20 May, two of nine (22 %) did so. Because this constancy is lower than that, on the same days, of bees whose pollen had been removed, it seems unlikely that removing pollen from the bees had encouraged them to change the species they collected.

In Experiments 7 and 8 pollen-gatherers were marked and not recaptured until a week later (Table 10). The proportions of bees collecting their original pollens after 1 week in these two experiments are similar to that in Experiment 6 (Table 9) in which the bees' pollen loads were removed after every trip; this is further evidence that removing pollen did not make them change to another species.

One hundred and fifteen (6-1 %) of 1847 pollen loads examined in all the above experiments each contained more than one type of pollen (two types, 103 loads; three types,




nine loads; four types, two loads; five types, one load). 655 bees with pure loads when marked subsequently collected 1032 pure loads and seventy-three mixed; thirty-one bees with mixed loads when marked subsequently collected forty-five pure loads and eleven mixed; thus bees originally with mixed loads tended more to collect mixed loads later (P<0 001).

Six of the thirty-one bees with mixed loads when marked later collected nectar only, eighteen later collected pure loads only (fourteen of them collected one of their original types of pollen), six collected mixed loads only (four collected their original pollens and two collected one of their original pollens and another), and the remaining bee sometimes collected mixed loads containing one of its original pollens and sometimes collected pure loads of another pollen.

A load containing two species did not necessarily indicate that the bee concerned was transferring its foraging from one of the species to the other. Thus in Experiment 5, two bees originally collecting Pyrus Malus only, each collected three mixed loads of P. Malus and Cotoneaster sp. and afterwards Pyrus Malus only. Another originally collecting P. Malus only, collected a mixed load of P. Malus and Fragaria sp. on 11 May,

Table 10. Constancy to types of pollen collected after I week by bees originally collecting pure loads

(Bees originally collecting Ranunculaceae spp. pollen are also shown in brackets)

Expt. 7* Expt. 8* Date bees marked 18 May 1961 21 July 1961

No. bees collecting (a) their original pollen only 15(11) 10 (b) their original pollen and another pollen 2(0) 1 (c) other pollen only 8(1) 5

% bees collecting their original pollen only 60-0(91-7) 62-5

* Predominant pollens were: Expt. 7: Ranunculaceae spp., Cruciferae spp. Expt. 8: Umbellifereae spp., Cirsium spp., Trifolium repens.

a mixed load of Pyrus Malus and Cotoneaster sp. and pure loads of Pyrus Malus only on 12 May, Acer campestre only on 13 May, and a mixed load of Fragaria sp. and Cruciferae sp. and another Cruciferae sp. only on 14 May. Another bee originally collecting Aesculus hippocastanum only, collected a mixed load of Pyrus Malus and Aesculus hippocastanum on 11 May, a mixed load of A. hippocastanum, Pyrus Malus, Spergula arvensis L. and Ribes sanguineum L. on 12 May, and Compositae sp. only on 13 May.

Two bees had propolis only when marked but collected only pollen and nectar afterwards. Another bee collected pollen on four days but propolis on the last day of observation, and another collected propolis and pollen on one day and pollen only before and afterwards.

Constancy of bees when their colonies are moved to new sites

At dusk on 29 July 1956, a colony was moved to a place 3 miles away, where the flora was similar to that at the old site. Fifty-five pollen-gatherers marked on 29 July collected pollen at the new site on 30 July; forty-three (78 %) collected the same pollen on both days. The types of pollen, and the number of bees collecting them at the old and new sites were:



J. B. FREE 127

Cruciferae spp., 12, 13; Rubus fruticosus L., 19, 12; Trifolium repens, 17, 25; Conium maculatum L. 1, 1; Epilobium spp., 2, 2; Bryonia dioica, 1, 1; Ligustrum vulgare L., 2, 0; Unidentified pollen, 1, 0; Convolvulus sp., 0, 1. There were no mixed loads.

Most changes were probably because there was less Rubusfruticosus and more Trifolium repens at the new site. Thus, all bees collecting T. repens at the old site did so at the new, and eight bees changed to it; five of the twelve bees collecting Rubusfruticosus at the old site did not do so at the new, and no bee changed to it.

In the evening of 16 May 1957 a colony was moved to within 600 yd (549 m) of a 2 acre crop of mustard (Brassica nigra L.). Before the move thirty-one bees collected Cruciferae spp. (only two of them probably collected Brassica nigra); after the move, twenty-nine of the thirty-one bees collected Cruciferae spp. (twenty-six of them had changed to Brassica nigra). Only nine of twenty bees collecting pure loads of other species changed to B. nigra, and one to another Cruciferae sp.

The main species other than Cruciferae, and the number of bees collecting them on 16 and 17 May, were Prunus avium L., 8, 6; and P. domestica L., 5, 3. Four bees collecting mixed loads of Cruciferae spp. and another type of pollen before moving, all collected Brassica nigra only afterwards; and one bee collected Taraxacum officinale only before

Table 11. Foraging behaviour of bees taken to mustardfield

31 May (Original site) 2 June (Mustard field) 3 June (Original site) No. of bees collected No. of bees collected

B. nigra and S. nigra

Original pollen B. nigra S. nigra ondifferent Nectar Original Another Nectar collected trips pollen pollen

Brassica nigra 17 2 0 7 0 1 12 Sambucus nigra 2 16 4 3 8 1 9 Other species 28 8 3 33 7 2 43

moving, and T. officinale and Cruciferae sp. (not Brassica nigra) on the same trip afterwards.

On 31 March 1962, pollen-gatherers were marked and their colony moved to the SE corner of a mustard field early on 1 June. Because of bad weather none flew on 1 June, but 123 did so on 2 June and forty-seven of them collected pollen, all of which was B. nigra or Sambucus nigra L. (Table 11).

Most bees collecting Brassica nigra or Sambucus nigra on 31 May, collected the same species on 2 June. Four bees originally with S. nigra pollen, collected Brassica nigra at the beginning of 2 June but changed to Sambucus nigra later, presumably as they foraged farther afield. A large proportion of bees collecting other species on 31 May (seventeen species altogether, three bees collected loads of two species each), collected nectar only on 2 June; most of the remainder collected Brassica nigra pollen.

Four colonies were put beside the mustard crop on 29 May when it had begun flowering, the flowers at the north end being more advanced. On 2 June 100 pollen-gatherers were captured entering each colony and their pollen loads removed. Pollen loads from the same colony were mixed in water, and samples of the pollen grains counted and identified. These colonies collected much more Sambucus nigra than Brassica nigra pollen (Table 12).

The colony with marked pollen-gatherers was moved to its original position in the evening of 2 June. On 3 June few bees collected pollen and none collected mustard pollen,

J.A.E. I




presumably because conditions were unsuitable for dehiscence of the mustard anthers (Table 1 1). Six of the eight bees collecting Sambucus nigra on 3 June had collected it on 2 June, one had collected Brassica nigra on 2 June, and the other Sambucus nigra and Brassica nigra on separate trips on 2 June. Seven bees collected their original pollen, other than Sambucus nigra, on 3 June; four of them collected Brassica nigra, one Sambucus nigra, and two nectar only on 2 June. One bee had that changed to Brassica nigra then Sambucus nigra on 2 June, collected S. nigra on 3 June.

In the three final experiments, made between 31 July and 9 August 1961, colonies were moved from the same apiary to red clover (Trifolium pratense L.) crops. Altogether fifty-four of 130 bees that collected T. pratense at the old site collected it at the new, one bee collected T. pratense and another species at the new site, and the remaining seventy- five collected nectar only; one of four bees collecting T. pratense and another species at the old site collected T. pratense only at the new, the remaining three collecting nectar only; seven of twenty-one bees collecting species other than T. pratense at the old site collected T. pratense only at the new, three collected their original pollen at the new and eleven collected nectar only.

Table 12. Pollen collected by colonies taken to mustardfield on 29 May

Percentage of pollen collected Colony No. Part of field ,- - -

located Sambucus nigra Brassica nigra Other species

I South 79.4 1-0 19-6 2 West 70 5 22-7 6-8 3 North 54 9 16-7 28-3 4 East 73-5 21-3 5 2

DISCUSSION AND CONCLUSIONS

The proportion of mixed loads was similar to that found by Betts (1920, 1935) and Mau- rizio (1953), indicating that during one foraging trip a bee usually collects pollen from one flower species only.

Many workers (e.g. Minderhoud 1931, Butler, Jeffree & Kalmus 1943, Singh 1950) have marked bees foraging on a crop and found that some returned to it, and often to a small part of it, for successive trips and days. However, these observations gave little information on constancy in general, because bees that did not return could have been visiting the same species elsewhere, visiting another species, inside their hives or dead. Also, most of the observations were made on crops commonly visited by bees.

The time of day at which different flower species present their pollen differs (e.g. Synge 1947, Maurizio 1949, Percival 1950, 1955); Park (1949) claimed that bees rarely leave their hives when the nectar or pollen they are accustomed to gather from a particular species is unavailable. Ribbands (1949) watched five bees foraging on a plot of Papaver rhoeas L., Eschscholtzia californica (Champ), Tropaeolum majus (L.), Limnanthes douglasii R. Br. and Nemophila insignis (L.) for 2 or 3 days each and found that two of them visited two or more species; however, the behaviour of these two bees might have been exceptional because of the mixed crop; Brittain & Newton (1933) found that the percentage of bees with pure pollen loads was greater when one species predominated.

My results show that the constancy from day to day was less than the constancy within trips and the proportion of bees collecting their original pollen decreased with each



J. B. FREE 129

successive day of foraging. Bees collecting the most common pollen tended to be the most constant. Presumably changes from one species to another reflected changes in the availability and attractiveness of their pollens, and the differences in constancy in different experiments reflected differences in foraging conditions. However, when pollen was temporarily unobtainable from the particular species they had previously visited most bees stayed at home or collected nectar only, rather than collect pollen from another species. Thus temporary fixation, together with adaptability over a longer period, seems to explain the results obtained.

Ribbands (1949) watched a bee collecting pollen from Eschscholtzia californica and nectar and pollen from Limnanthes douglasii during the same foraging trips for 2 consecutive days. Few of my bees with mixed pollen loads when marked, collected the same mixture again, but most had collected from one of the same species when next they were captured. Bees with mixed loads had probably become temporarily 'dissatisfied' with their original crop and were sampling another (see Ribbands 1949, Singh 1950); they were not necessarily transferring from one of the crops to the other, and later they often returned to the original crop, or visited this and another.

Park (1949) claimed that only a 'few' of 'hundreds' of marked water-gatherers, nectar- gatherers and pollen-gatherers changed their occupation during the 5-10 days he observed them. However, many bees in my experiments collected pollen on some trips but not on others on the same day, and days on which one type of trip only was made, often came between days of making the other type. It is not known whether the bees visited the same species for nectar only as for pollen. Ribbands (1949) watched a bee which on 3 consecutive days visited Papaver rhoeas for pollen in the morning and Limnanthes douglasii for nectar only in the afternoon, and Free (1962) found that when Viciafaba pollen was unavailable, most pollen-gatherers deserted the crop and did not collect nectar from it. Therefore, perhaps, when pollen is not collected incidently while nectar-gathering many bees become conditioned to one type of foraging behaviour on some species. If so their overall flower constancy will be much less than that when they are collecting pollen only. The absence of a definite sequence of pollen or nectar collection with age agrees with the conclusions of Rosch (1925) and Ribbands (1952).

Removing pollen from the corbiculae of foragers decreased their tendency to collect it, but did not affect the constancy of those that continued to do so. Because nectar- gatherers often do not touch the anthers and stigmas of many flowers (e.g. field beans, fruit, lucerne, red clover) they are less valuable as pollinators than pollen-gatherers. It has been suggested that removing pollen from homecoming bees with pollen traps might induce the bees to collect more (see Butler, Free & Simpson 1956), but this now seems unlikely.

When a colony was moved to a new site where no one species predominated, its bees tended to collect the same pollen as before the move when it was available, their constancy being similar to that shown over 2 consecutive days by bees whose colonies were not moved. It is not known whether most bees searched for the plant species themselves or were informed of it by others.

When one pollen species predominated at the new site many of the bees that had not previously collected it did so, although in one experiment the absence of their original pollen at the new site resulted in many bees becoming nectar-gatherers. The species to which a bee changed after being moved was sometimes influenced by the species previously visited. In one experiment more of the bees previously collecting Cruciferae spp. (other than mustard) than of bees collecting other species, collected mustard after




the move. Louveaux (1954) found that colonies, not moved, that collected most mustard pollen also collected most pollen of the other Crucierae available.

The results confirm that colonies should not be taken to crops before the plants flower (Free 1959, Free, Free & Jay 1960) otherwise, many of the bees will continue to visit species to which they were previously conditioned and unless the crop to which they are moved is more attractive they will probably not change to it. Although most bees at first visit the crop nearest to their colony, their adaptability to seek better forage, and their short foraging lives, probably explain why the numbers of bees visiting a relatively unattractive crop often soon decreases (Free et aL 1960, Free & Smith 1961). However, nearness of a crop is an attractive factor in itself (Fran9on 1939, Boch 1956), particularly during unfavourable weather (Butler et al. 1943, Ribbands 1951). Karmo (1961) suggested that colonies should be rotated between different crops of an unattractive species. Pre- sumably, after such a move, bees that had changed to it would continue to visit it, bees that had not foraged on it might do so, and possibly bees that had deserted it might visit it again. The results of Levin & Bohart (1957) suggest that, when a lucerne field is relatively unattractive, it will be visited more by bees from colonies moved to it from another lucerne field than by bees from colonies put beside it at the beginning of flowering, but more information is needed.

ACKNOWLEDGMENT It is a pleasure to thank Miss Yvette Spencer-Booth for helping with the work.

SUMMARY 1. The flower constancy of honeybees on successive days has been studied by removing

and identifying pollen collected by marked pollen-gatherers. Removing pollen from the bees decreased their tendency to collect it later; chilling the bees before marking did not influence their foraging behaviour; neither treatment affected their constancy to the kind of pollen collected. No sequence of nectar or pollen collection with age of bee, was found.

2. In each experiment most of the bees collected only a few of the pollens available to them. Bees collecting the most comnmon pollens tended to be the most constant. In general, the proportion of bees collecting their original pollen decreased as the number of foraging days increased, and only about half were doing so after I week; the rate of decrease differed in different experiments. No bee regularly collected different pollens at different times of the day. When the pollen they were accustomed to collect was unavailable for a day, most foraged for nectar only or remained at home. Most bees that changed to another pollen probably did so when the pollen they had previously collected was scarce or unattractive for longer periods.

3. About 6 % of the loads each contained more than one species of pollen. Bees that collected mixed loads were more inclined to do so later; probably they were dissatisfied with the crops they were working and were sampling others.

4. When a colony was moved to another site with a similar flora, the bees tended to visit the same species as before, but when one species predominated, bees that had not visited it before tended to do so.

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Brittain, W. H. & Newton, D. E. (1933). A study of the relative constancy of hivebees and wild bees in pollen gathering. Canad. J. Res. 9, 334-49.

Butler, C. G., Free, J. B. & Simpson, J. (1956). Some problems of red clover pollination. Ann. Appl. Biol. 44, 664-9.

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Bztg. 2, 320-455. Maurizio, A. (1953). Weitere Untersuchungen an Pollenhoschen. Beih. Schweiz. Bieneztg. 2, 485-556. Minderhoud, A. (1931). Untersuchungen ilber das Betragen der Honigbiene als Bliutenbestauberin.

Gartenbauwiss. 4, 342-62. Park, 0. W. (1949). Activities of Honey Bees (Ed. Grout, R. A.), Chap. 4. Hamilton, Illinois. Percival, M. S. (1950). Pollen presentation and pollen collection. New Phytol. 49, 40-63. Percival, M. S. (1955). The presentation of pollen in certain angiosperms and its collection by Apis

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normalen Bienenstaate und ihre Beziehungen zum Alter der Arbeitsbienen. Z. vergl. Physiol. 2, 571-631.

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the flower constancy of honeybees

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