Table Of ContentPROC. ENTOMOL. SOC. WASH.
101(4), 1999, pp. 880-886
QUEEN AND FORAGER SIZES OF BOMBUS AFFINIS CRESSON
(HYMENOPTERA: APIDAE)
FlORIAN p. SCHIESTL AND EDWARD M. BaRROWS
(FPS) Institut fiir Zoologie, Abteilung fiir Evolutionsbiologie, Althanstrasse 14, A-1090
Vienna, Austria (e-mail: [email protected]); (EMB) Laboratory ofEntomology
and Animal Behavior, Reiss Building Suite 406, Department of Biology, Georgetown
University, Box 571229, Washington, DC 20057-1229 (e-mail: barrows@gunet.
georgetown.edu)
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Abstract. This study explores the relationships of selected body measurements in the
bumble bee Bombus affinis, and reviews the literature regarding measuring bumble bees.
We found that in foraging workers of this species, collected over 3 sequential years from
the same forest sites, compound-eye length, head length, head width, radial-cell length,
scapus length, tibia length, and wing length all predicted dry weight, a measure ofoverall
size. These same variables, except for head length, also predicted queen weight. Workers
had a higher variance in their radial-cell length than queens, and queens had higher var-
iances in their glossa length and weight than workers. In all 13 regression analyses be-
tween body size measurements, worker correlation coefficients (r- values) were higher
than those for queens. Worker size, measured as head width (a strong predictor of body
weight), increased with time during 2 out of 3 flight seasons.
Key Words: Apidae, bumble bee, forager size, seasonal size changes, queen size
Previous studies of Bombus size found and Johansen 1978). In some species, larger
that the relationships between linear mea- foragers fly faster, forage more efficiently
surements and between linear measure- on certain plants, gather more food during
ments and weights vary in strength depend- cool weather, obtain food from deeper flow-
ing on caste, sex, species, and measurement ers, and thieve less nectar than smaller for-
method (Knee and Medler 1965, Morse agers (Morse 1978, Pyke 1978, Heinrich
1978, Harder 1985, and many references 1979, Harder 1985). Because males and
therein). This study on B. affinis Cresson queens also vary in size within species
(Owen 1989), and they are frequent flower
examines samples ofthis bee from the same
visitors, they are also likely to have size-
collection sites for 3 sequential flight sea-
related effects on pollination; however, we
sons, documents many previously unreport-
found no published studies on this subject.
ed intra-individual and forager-queen size Bombus affinis is a short-tongued bumble
relationships in the genus, and discovers an bee, found from Ontario to New Brunswick
increase in forager size during its flight sea- and south to North Carolina (Mitchell
son. 1962). It nests underground and flies from
Sizes of individual Bombus affect the April through October (Laverty and Harder
pollination ecology of this genus which 1988). Wisconsin colonies commonly con-
contains major pollinators in forests, mead- tain at least 200 workers, and can have up
ows, tundras, and other habitats (Robinson to 350 workers (Medler and Carney 1963).
VOLUME 101, NUMBER 4 881
Materials and Methods end of her median plate to the distal tip of
We obtained a total of 475 foragers and her radial cell; and WL3, the maximum
length of her entire forewing.
100 queens of B. ajfinis from 29 April
Before measuring glossae, tibiae, and
through 27 September 1991-1993 with 20
wings, we straightened glossae by extend-
Townes-style Malaise traps in four water-
ing them into individual capillary tubes
sheds in the Femow Experimental Forest,
(Harder 1982), removed tibiae from their
Tucker County, West Virginia (described in
adjoining femora, and put detached fore-
detail in Barrows et al. 1994, Barrows wings between microscope slides. We used
1995). We emptied the traps every 10 days
glossae that were either relaxed in a humid-
otrnaptshecolslaemcetedJutlhieanbedeasyionfheeaadcsh wyieatrh.9T5h%e ittoytchehiarmhbeeardsorintraeat1e0d%w-hKilOeHstsilolluattitoanchfeodr
ethanol. We removed bees from alcoholic 12 hr at 25°C. Wing lengths were measured
samples, air dried them for at least 1 yr, and from images on a microfiche screen (Harder
randomly selected specimens for measure- 1982, Owen 1988). Because 11 foragers
ments. We then relaxed them in humidity and four queens had frayed wing tips, we
chambers and dissected them to measure did not attempt to measure their WL3s.
their parts.— To investigate the change of forager size
Weight. We took the weight-1 (Wl) during the flight season, we measured head
and weight-2 (W2) of each bee using an widths (HWs) of from one to eight speci-
electronic balance (Mettler AE 50). Weight- mens for each sampling period, using a
1 was the weight of a bee's head, mesoso- maximum of eight bees when they were
imtas,heaandd maentdasmoemsao;soWm2a,. tWhee wteoiogkhtWo2fsonbley- tavhaeillaibnleea.rWmeeamseuarseumreendt HwiWthbetchaeusheighitesits
cause food and organs within Bombus me- correlation with weight-2 (W2).
tasomas, e.g., crops, fat bodies, and ovaries, To discover possible correlations be-
can cause their weights to vary appreciably tween sizes of body parts, we used least-
(Alford 1969, Heinrich 1979). Because square regression (SPSS for Windows, No-
some specimens lost parts oftheir antennae, rusis 1993). To look for possible differences
total antennae, and parts of their legs, we between measurement variances, we used
removed their entire antennae and legs dis- PROC TTEST (SAS Institute, Inc. 1979).
tal to their trochanters to control for missing
parts, before ascertaining their dry weights. Results and Discussion
— Queens of B. afftnis showed a greater
Linear measurements. We measured variation in measurement ranges than for-
left compound-eye length (CL), glossa aging workers in all measurements, except
length (GL), head length (HL), head width radial-cell length (WLl) and tibia length
(HW), scapus length (SL), and tibia length (TL) (Table 1). Further, queens had greater
(TL) using a dissecting microscope with an variances in glossa length (GL) and weight
ocular grid, and we measured forewing (Wl and W2) than workers, and the latter
lengths (WLls, WL2s, and WL3s) with a showed a greater variance in tibia length
microfiche reader (Minolta RP 605Z). Head (TL) than the former (Table 2). Because
width was measured as the greatest distance queens are larger than workers, they would
across a bee's compound eyes; HL, the dis- be expected to show greater variations and
tance from the vertex to the distal end of variances in all body-part sizes than work-
her clypeus; GL, the distance from the pos- ers. The possible biological significance of
terior end of her basiglossal sclerite to the these three surprising exceptions to this ex-
tip of her labellum; WLl, her radial-cell pectation awaits discovery.
length; WL2, distance from the proximal Workers showed significant correlations
882 PROCEEDINGS OF THE ENTOMOLOGICAL SOCIETY OF WASHINGTON
Table 1. Measurements of Boinhns (iffini.s'
Measurement
VOLUME 101, NUMBER 4 883
Table 2. Comparisons of measurement variances ofqueens and workers in Bomhus afftnis".
CL
884 PROCEEDINGS OF THE ENTOMOLOGICAL SOCIETY OF WASHINGTON
Table 3. Measurement regressions of Bomhiis afftnis"
Regression
VOLUME 101, NUMBER 4 885
I 6.0 1991 1992 1993
i =•'
9
^
5.0
Q
< 2 8 8 2
I 4.5 8 8 8 8 8 * 8 « * 8 8
CC 4.0
LU
-, 3.0
170 200 230 260 170 200 230 260 170 200 230 260
^ June July Aug. S«pt. June July Aug. Sept. June July Aug. Sept.
2 JULIAN DAY
Fig. I. Mean worker head width versus sampling period, Fernow Experimental Forest, WV, 1991-1993.
this increase in B. fenndus (Fabricius), B. petitive interactions involving conspecific
griseocollis (De Geer), B. nevadensis auri- bees and other animals at flowers, foraging
comus (Robertson), and B. perplexus Cres- efficiency, frequency of flower robbing and
son (Knee and Medler 1965, Plowright and thieving, kinds of flowers used, and other
Jay 1968). These increases occurred in phenomena still remain unstudied, or only
healthy bumble-bee colonies, but the size of meagerly studied, in most Bombus species.
workers decreased when colonies were par-
Acknowledgments
asitized (Knee and Medler 1965). Our sam-
ples from Malaise traps may have contained Many people kindly helped with this
bees from both healthy and parasitized col- study and are acknowledged in Barrows et
onies, and colonies in different develop- al. (1994). In addition, we are pleased to
mental stages, which might have lowered thank S. W. T. Batra (U.S. Department of
our size-season correlation coefficients. Agriculture, Beltsville, MD) for critically
In conclusion, we found that many body reading a draft of this paper and C. C. Ed-
measurements are significantly correlated gerton (Georgetown University), H. L. Ne-
with body weight and other body measure- meschkal (University of Vienna, Austria),
ments in both queens and foraging workers and R. J. Skarr (Smithsonian Institution,
ofB. afftnis. Tibia length is most correlated Washington, DC) for their help. George-
with weight in queens, and head width is town University and the Appalachian Inte-
most correlated with weight in workers. Ra- grated Pest Management Gypsy Moth Pro-
dial-cell length is highly correlated with to- ject ofthe Northeastern Area, State and Pri-
tal wing length in both castes. Laboratory- vate Forestry, USDA Forest Service, fund-
preparation methods influenced glossa ed this research.
length, and, therefore, whether it correlated
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—
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