BUMBLEBEE FORAGING PREFERENCES: DIFFERENCES BETWEEN SPECIES AND INDIVIDUALS

3.6 TONGUE AND HEAD MEASUREMENTS OF EACH SPECIES
The tongue and head measurements made in August were used to test the null hypothesis that there were no significant differences in these dimensions between the different species. The data for each species were compared using ANOVA, then Tukey's Honestly Significant Difference Test to compare between pairs of species, as is shown in FIGURE 8. All of the mean head lengths were significantly different from other species (P < 0.05), except for Bombus lapidarius and B. pratorum. For the mean tongue lengths B. hortorum was significantly different (P < 0.05) from all other species, and B. pascuorum was significantly different (P < 0.05) from all except B. pratorum. None of the mean head widths were found to be significantly different (P = 0.06). The tongue lengths measured were not the maximum lengths, which can really only be found after dissection, but the length that the bees were willing to extend in order to reach a reward during foraging.

FIGURE 8. Mean head and tongue length of bumblebees marked in August 1995.
Columns designated with the same letter were not significantly different from each other at P < 0.05, all others were significantly different.

3.7 RELATIONSHIP BETWEEN TONGUE LENGTH AND FLOWER PREFERENCE
There appears to be some relationship between tongue length and flower preference, as the four shorter tongued bee species forage on Compositae for nectar, but Bombus hortorum does not, nor does it forage on the open flowers of Cotoneaster simonsii. However, the four shorter tongued bees have two distinct foraging patterns, which cannot be explained by tongue length. B. pratorum strongly prefers the Labiatae and C. simonsii, while B. pascuorum shares the preferences of B. hortorum and B. pratorum. B. lapidarius and B. lucorum prefer the Compositae, and the more accessible flowers of Allium schoenoprasum; these flowers require large numbers of probes for small rewards.

In tongue and head length B. pratorum and B. lapidarius are not statistically different, yet their preferred flower choices are very different, all of B. pratorum's preferred flowers for all three sessions were situated within a 3-4 m patch.

In August B. lucorum and B. lapidarius, with shorter tongues showed a preference for the Compositae and other "easy" flowers, requiring large amounts of probing for small rewards; this cannot be wholly explained by tongue length, as B. pratorum, with a similar tongue length did not show this preference, but strongly preferred the two clumped resources of Stachys lanata and Lavandula angustifolia, situated within 1.5 m of each other.

3.8 FLOWER PREFERENCES OF INDIVIDUAL BEES
Data gathered on marked bees during the bee walk were used to investigate the flower preferences of individual bees. For all species except Bombus lapidarius, data on individuals recorded five or more times were used; for B. lapidarius, the most common bee, only bees recorded eleven or more times were used. The individual preferences are shown in FIGURES 9, 10, 11 and 12. The data were gathered during the bee walk from 2nd-6th August, and also on 15th August, when the bee walk was done for one extra day, to record marked bees only. None of the bees recorded in the four graphs were male; although many males were re-sighted, they tended to be re-sighted less often than the workers.

On FIGURE 9 (B. lucorum) the most noticeable thing is the constancy of the bees; of the ten bees, eight were recorded on one flower species only, but four different flower species were used overall. No hint of such constancy is apparent in FIGURE 6 showing species flower preferences. Another striking thing is the visits of B41 to Lavandula angustifolia. In FIGURE 6 visits to L. angustifolia make up less than 6% of the total visits. In all there were twelve recorded visits of B. lucorum to L. angustifolia during August, B41 made eleven of these.

FIGURE 9. Bombus lucorum preferences of individual workers on bee walk in August 1995

B. lapidarius in FIGURE 10 shows a strong preference for Centaurea nigra, as is shown for the species preference in FIGURE 6, however B35 and B51 show a strong preference for L. angustifolia and do not pay even one visit to C. nigra.

The preferences shown by individual B. pratorum on FIGURE 11 are the same as the species preference in FIGURE 6, i.e., for the two morphologically similar species, Stachys lanata and Lavandula angustifolia, although the preference ranges from 100% to almost 50-50% for individual bees.

In August (FIGURE 6) B. hortorum as a species showed a strong preference for Digitalis purpurea, but the preferences of frequently seen individuals, (as shown in FIGURE 12), do not indicate any preference for D. purpurea. Stachys lanata and Lavandula angustifolia are the preferred flowers of the three individuals concerned.

Only one B. pascuorum individual showed a strong preference in FIGURE 12, the other two visited three flowers each, a high number compared with individuals of other species.

The dataset available was too small, and the range of flowers too narrow, to indicate whether the morphology and/or colour of the major flower used by each species and/or individual influenced the choice of the minor flower(s).

3.8.1 CONSTANCY
Individual bees were followed for extended periods on two days, 27th June and 7th August. On each day five bees were followed, and on each occasion one bee was followed twice, as it was re-sighted again, as another bee was lost. FIGURE 13 shows the consecutive visits of these bees to flowers. Most of the bees followed stayed within the bee walk area, but one bee, the Bombus lapidarius queen, foraged entirely outside the bee walk area on an area of lawn and topsoil on which weeds had grown. In this area there were 2500 heads of Trifolium repens and 3000 Lamium purpureum. The T. repens was a clumped resource but the L. purpureum was spread fairly evenly throughout the area covered by topsoil, and on part of the lawn. there was not a clear boundary between the lawn and topsoil.

The most striking thing about FIGURE 13 is the almost complete constancy of the bees on a single foraging trip. Only one bee changed its flower use during a foraging episode. The foraging episodes varied in length from only 6-62 minutes. B41 (B. lucorum), is also shown in FIGURE 9, where it was recorded eleven times on Lavandula angustifolia and just once on Cirsium arvense. B49 is recorded twelve times on L angustifolia and ten times on Stachys lanata in FIGURE 11, whereas in FIGURE 13 it was followed twice and on both occasions it was 100% constant on L. angustifolia. P48 (B. pascuorum) is constant over a long run to L. angustifolia in FIGURE 13, but in FIGURE 12 it was recorded only twice on L. angustifolia.

FIGURE 13. Constancy of bumblebees followed on 27th June and 7th Augst 1995.

27th June 1995

7th August 1995

Too few bees were followed to calculate whether there is a statistical difference in constancy between individuals of different castes or species, but of the ten individuals from four species followed in June and August the constancy to a single species of flower was almost universal.

3.9 ENVIRONMENTAL VARIABLES AND THEIR EFFECTS ON FORAGING
The environmental variables appeared to have little effect on flower preference, there were no very cold periods during observations, and only a little light rain. During the bee walk sessions the temperature ranged from 9^oC and 28^oC, and the wind speed was between 0.3 and 7.9 m s.

(C) Copyright 1999 L. Smith