248 B. Svensson et al. Agriculture, Ecosystems and Environment 77 2000 247–255
queens, workers and males in order to gain a proper understanding of their use of resources over time and
space Svensson and Lundberg, 1977. Only a few studies have been made on bumble bee nesting-site
preferences and characteristics of the nesting sites Skovgaard, 1936; Svensson and Lundberg, 1977;
Richards, 1978; Fussell and Corbet, 1992b.
Nest-site preferences of bumble bees have been in- vestigated by setting out artificial nests in the field.
However, only a very small percentage of such nests are ever used Richards, 1978; Fussell and Corbet,
1992b. It can, therefore, be advantageous to observe nest-seeking queens in spring, because a larger amount
of data can be gathered in this way. This method was mentioned by Skovgaard 1936, and used by Richards
1973 and Svensson and Lundberg 1977.
The present investigation aimed at estimating the abundance of nest-seeking bumble bee queens in dif-
ferent types of agricultural landscapes and habitats, with special emphasis on uncultivated areas.
2. Material and methods
2.1. Study area The study was carried out between 13 April and 12
June 1991, within a 30 km
2
area near Funbo-Lövsta, about 20 km south-east of Uppsala, Sweden 59
◦
51
′
N, 17
◦
41
′
E. The region is dominated by plains at an altitude of less than 50 m above sea level. Intensive
large-scale agriculture is carried out on clay soil, and mixed broad-leaved and coniferous forest grows on
sand and gravely glacial till, esker material and shal- low soils over granite bedrock.
Within this area, the occurrence of nest-seeking bumble bee queens Bombus spp. was quantified in
12 smaller study areas quadrats, 1 km × 1 km, repre- senting four different landscape types:
1. Open O, 0–15 forest; large agricultural fields, surrounded only by fields and roads. Three
quadrats were investigated. 2. Relatively open RO, 15–50 forest; large agri-
cultural fields, mostly surrounded by other agri- cultural fields and roads, but also by ditches,
bushes, small woods and pastures. Four quadrats were investigated.
3. Relatively wooded RW, 50–85 forest; small agricultural fields surrounded by forests and roads,
but also by several small-habitat types, such as ditches, pastures and woods. Two quadrats were
investigated.
4. Wooded landscape W, 85–100 forest; small agricultural fields, almost always surrounded by
forest. Three quadrats were investigated. The quadrats were randomly chosen from a map
covering the main study area. The longest distance between quadrats was 9 km. Within each quadrat, a
500-metre-long transect was laid out. Each transect included several intermingled habitats, and was made
as representative as possible for the area. However, arable fields were only occasionally included as a
transport way between two other habitats, because they were presumed not to be possible nesting sites.
The transects were not straight, but somewhat curved, in order to cover most habitats of interest within
the quadrat. Table 1 gives details concerning the transects.
Eight habitat types were represented in this study: 1. Field F. Agricultural field with cereals, grass ley
or fallow with a vegetation of weeds. 2. Field boundary Fb. A grass-dominated strip be-
tween two agricultural fields or between an agri- cultural field and other open ground, pasture or
clearing. 3. Pasture P. Natural pasture, partly covered with
broad-leaved trees and bushes, and grazed by horses, cattle or sheep.
4. Other open ground Op. Mainly uncultivated open ground with abundant grass, tussocks and
stones. 5. Clearing C. An area with coniferous forest that
had been clear cut 3–4 years earlier and then replanted. Tall herbs and bushes dominate the
vegetation. 6. Road boundary Rb. A grass-dominated strip be-
tween a trafficable road and an agricultural field, other open ground or clearing.
7. Forest boundary Fob. The edge between a mixed forest and agricultural field, pasture or clearing.
The vegetation mainly consists of grass, herbs and bushes.
8. Forest Fo. Mixed, well-grown forest. The un- dergrowth mainly consists of dwarf-shrubs and
herbs.
B. Svensson et al. Agriculture, Ecosystems and Environment 77 2000 247–255 249
Table 1 Description of the study area near Uppsala, Sweden. The proportional distribution of habitat types in each landscape type LS where
nest-seeking bumble bees were counted during April–June 1991.Each of the twelve transects was 500 m long. LS
a
Habitat type
b
Transect total m
c
LSmps
d
No
e
F Fb
P Op
C Rb
Fob Fo
O 10
41 13
36 1 500
27 000 17, 18, 19
RO 4
33 16
5 25
16 1
2 000 35 000
17, 17, 17, 19 RW
13 10
23 43
11 1 000
19 000 19, 19
W 5
6 29
16 15
29 1 500
26 000 17, 17, 18
Habitat Tot m
f
360 1 365
430 360
425 1 510
980 570
6 000 –
– Hmps
g
6 610 24 165
7 870 6 405
7 375 26 795
17 550 10 230
– 107 000
–
a
O, open; RO, relatively open; RW, relatively wooded; and W, wooded landscape.
b
F, field; Fb, field boundary; P, pasture; Op, other open ground; C, clearing; Rb, road boundary; Fob, forest boundary; and Fo, forest.
c
Total length of transects in each LS in the total study area.
d
Total length inspected in each LS during the season in metres per season.
e
Number of inspections in each transect during the season.
f
Total length of transects of each habitat in the total study area.
g
Total length inspected of each habitat during the season in metres per season.
2.2. Queen bee studies Bumble bee queens searching for a nesting site char-
acteristically fly in a zigzag pattern close to the ground within a small area termed a ‘patch’, occasionally
going down to investigate the ground surface Lund- berg and Svensson, 1975. The size of each patch
characterised was about 1 m × 1 m, which was chosen from the behaviour of the queens at each nest-seeking
event. To characterise each patch, the occurrences of withered grass, new grass, tussocks, stones and moss
within it, and in the area immediately surrounding it was classified as low 10, medium 10–50 or
high 50.
Bumble bee nests are difficult to find in numbers large enough to allow statistical analysis. Therefore,
the number of queens performing nest-seeking be- haviour in an area of a given type was used as an
indication of their relative preference for nesting in that type of area compared with other types of areas
cf. Richards, 1973; Svensson and Lundberg, 1977. Queens were counted using the transect method ‘belt
method’, Banaszak, 1980, with all observations of nest-seeking bumble bee queens in an area extend-
ing 5 m on either side of the transect being counted. For each observation, the habitat type was noted and
the characteristics of the environment adjoining the patch were described. Once detected, a queen was
observed continuously until she stopped performing nest-seeking behaviour, i.e., flew away from the patch.
The observation time varied from 1–10 min. A queen could visit more than one patch, and each visit to a
particular patch was counted as one observation. Most queens visited only one patch.
In most cases, each transect was inspected twice a week occasionally, only once. Temperature and wind
speed during the inspections varied within the range of 2–19
◦
C and 1–4 ms, respectively. No inspections were made in rainy weather. Walking speed during a
transect inspection was 20 mmin 1.2 kmh. The time of day of the inspection was varied randomly between
0900 and 1900 hours, Swedish Summer Time +one hour GMT.
Bumble bees were identified according to Løken 1973. Bumble bee species differ in the mean date on
which they break hibernation Løken, 1973, so that nest-seeking observations need to be made throughout
the spring season in order to be able to record all species. During April 1991, the mean temperature was
near normal, whereas the weather was very cold, with a mean temperature 2
◦
C lower than normal for the months of May and June. This may have delayed the
emergence of some species. Under normal conditions, the late-appearing species, B. hortorum, in particular,
could be expected to appear more frequently.
The study period was limited to early and mid-season to minimise the possible effects of the
nematode Sphaerularia bombi on the bumble bee
250 B. Svensson et al. Agriculture, Ecosystems and Environment 77 2000 247–255
queens’ nest-seeking behaviour. This parasite ap- parently disturbs a queen’s orientation capacity. As
a result, infected queens search and dig in unusual places and continue nest-seeking until they die in the
later part of summer Lundberg and Svensson, 1975.
2.3. Statistical analysis To compare the activities of bumble bees in dif-
ferent landscape types and habitats, all results were transformed to total number of observations per 100 m
during the season
No. of observations 100 m
: No. of observations
mps ×
100 where mps metres per season for landscape types
= length of transect × number of inspections during
the period Table 1, and mps for habitat =length of each habitat in the total study area × number of
inspections during the period Table 1. The data were log transformed before the statisti-
cal analysis. One-factor ANOVA the SAS procedure GLM; SAS Institute Inc., 1982 was used to anal-
yse the data on landscape type n = 12 and habitat n = 96, respectively. To compare the landscape types
and habitats pairwise, a PDIFF command was used.
To analyse the data on bumble bee species, a split-plot design was used the SAS procedure GLM;
SAS Institute Inc., 1982, with the bumble bee species as the dependent variable. The independent variables
were landscape type quadrats as error term and habitat n = 96.
3. Results and discussion
