Drifting of workers in nest aggregations of the giant honeybee Apis dorsata
Apidologie
Drifting of workers in nest aggregations of the giant honeybee Apis dorsata
Jürgen PAAR 0 2
Benjamin P. OLDROYD 2
Ernst HUETTINGER 1
KASTBERGER 0
0 Institute of Zoology, Karl-Franzens University Graz , Universitätsplatz 2 8010 Graz , Austria
1 BFL-Institute of Apiculture , 3293 Lunz , Austria
2 School of Biological Sciences, A12, University of Sydney , 2006 NSW , Australia
- The extent of worker drifting between nests in aggregations of colonies of the giant Asian honeybee Apis dorsata was studied using DNA microsatellites. Four aggregations with three, six, seven and eight colonies were sampled. 1537 workers were genotyped using four loci. Maternity testing was used to separate drifted and natal workers, and to assign drifted individuals to their actual maternal colony. The proportion of drifted workers ranged from 0 to 6.25% with an average of 1.27% (sd = 0.245). No significant differences in rates of drifting were found between the four aggregations. There was also no correlation between the direction of the drift and the position of the nests relative to each other. These results show that in A. dorsata, a bee species that frequently nests in dense aggregations, the extent of forager drifting between colonies can be very low.
1. INTRODUCTION
D r i f t i n g o f f o r a g e r s a n d d r o n e s i s a
well-known phenomenon in Apis mellifera L.
(e.g. Betts, 1932; Free, 1958)
. During drifting
events, foragers return to the wrong colony due
to orientation errors (Rauschmayer, 1928),
where they must then be accepted by the guard
bees. Acceptance rates vary. When colonies are
nutritionally stressed, and robber bees are
about, very few foreign bees are admitted to
colonies, but when conditions are favourable,
drifted bees are usually accepted
(Downs and
Ratnieks, 2000)
.
In apiaries of Apis mellifera the degree of
drifting depends on a variety of environmental
and apiary layout factors. Hive orientation with
respect to the sun and prevailing wind can
influence direction and level of drift
(Jay, 1965,
1971; Moritz and Neumann, 1996)
. Placing
colonies in long rows with little space between
and few markings on or between colonies can
cause confusion among returning foragers, a
large proportion of which return to the wrong
colony. Under these extreme circumstances,
50–90% of foragers can move from their natal
nest to neighbouring nests, depleting the
foraging populations from colonies situated in the
middle of the rows, and greatly increasing
those of the colonies located at the ends
(Free
and Spencer-Booth, 1961; Jay, 1965, 1966;
Pfeiffer and Crailsheim, 1998)
. However, if
colonies are arranged in small clusters around
conspicuous land marks, with random entrance
orientations, or other appropriate apiary
layouts, rates of drifting are much lower – around
5% (Neumann et al., 2000).
Drifting of foragers between natural
colonies is not expected because it is likely to
reduce colony fitness
(Jay, 1969a, b; Robinson,
1979; Goodwin et al., 1994)
. First, colonies
that receive foragers from other colonies may
become exposed to contagions from other
colonies
(Jay, 1968; Matheson, 1984; Sakofski,
1990; Boylan et al., 1991; Rath et al., 1991;
Goodwin et al., 1993, 1994)
, though they may
benefit from an increased forager force.
Second, kin selection arguments suggest that it is
mal-adaptive for workers to forage for
unrelated colonies. Finally, honey bees frequently
rob the colonies of their con-specifics. Thus
there is a strong chance that a forager arriving
from another colony will be mistaken for a
robber and expelled or killed (Downs et al., 2001).
Taken together, it is somewhat surprising
that A.mellifera foragers are so prone to
drifting between colonies. This suggests that
drifting is an artefact of artificial aggregations in
apiaries. In the wild, colonies of A. mellifera
are either found singly, or in loose aggregations
in which colonies are separated by many
meters both vertically and horizontally
(Oldroyd
et al., 1995; McNally and Schneider, 1996)
,
suggesting that in the wild, foragers would
rarely drift.
Unlike the rather loose aggregations
sometimes found in A. mellifera the giant Asian
honeybee Apis dorsata Fabr. regularly forms
extremely dense aggregations of colonies. Up
to 200 colonies may occur in a single tree
(Oldroyd et al., 2000)
and the combs are often
separated by only a few centimetres. Despite
observations of short range swarming –
‘budding’ – (Lindauer, 1956) and home site fidelity
of swarms after their seasonal migration
(Neumann et al., 2000; Paar et al., 2000)
colonies within aggregations do not seem to be
closely related
(Oldroyd et al., 2000; Paar et al.,
unpublished data)
.
The single combs of A. dorsata are attached
to tree branches, cliff overhangs or buildings
and so are completely exposed on all sides.
Thus A. dorsata colonies would appear to be
particularly vulnerable to conspecific robbing,
and to the spread of pathogens and parasites
amon (...truncated)