BUMBLEBEE FORAGING PREFERENCES: DIFFERENCES BETWEEN SPECIES AND
3.5.1 Caste flower preference
3.6 Tongue and head measurements of each species
3.7 Relationship between tongue length and flower
3.8 Flower preferences of individual bees
3.9 Environmental variables and their effects
3.5.1 CASTE FLOWER PREFERENCE
Caste flower preferences were also calculated using the PREFER program
using the same variables as Species Flower Preference, and are shown in
FIGURE 7. Both male and worker Bombus
lapidarius show a strong preference for Centaurea nigra. They
differ slightly in their preferences for minor flowers, though both chose
flowers that are similar morphologically to C. nigra. Male B.
pratorum show a strong preference for Lavandula angustifolia,
while workers show a strong preference for Stachys lanata, both
these flowers could be considered to be morphologically similar. The males
do not visit Hypericum perforatum, but, as H. perforatum was
foraged for its pollen, this is hardly surprising.
FIGURE 7. Male and worker flower
preferences of Bombus lapidarius and B. pratorum foraging in the beewalk
in August 1995.
(n = number of observations)
|B. lapidarius male (n = 159)
||B.lapidarius worker (n = 486)
|B. pratorum male (n = 16)
||B. pratorum worker (n = 117)
3.6 TONGUE AND HEAD MEASUREMENTS OF EACH
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
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.
None of the head width means were found to be significantly
3.7 RELATIONSHIP BETWEEN TONGUE LENGTH AND FLOWER
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.
FIGURE 10. Bombus lapidarius
preferences of individual workers on bee walk in August 1995
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.
FIGURE 11. Bombus pratorum
preferences of individual workers on the bee walk in August 1995
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
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.
FIGURE 12. Bombus pasuorum and
B. hortorum preferences of individual workers on bee walk in August
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
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
3.9 ENVIRONMENTAL VARIABLES AND THEIR EFFECTS ON
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 9oC and 28oC, and the wind speed was between
0.3 and 7.9 m s.
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