Egg laying and egg removal by workers are positively correlated in queenright Cape honeybee colonies (Apis mellifera capensis)
Apidologie
Egg laying and egg removal by workers are positively correlated in queenright Cape honeybee colonies (Apis mellifera capensis)
ChristianW.W. PIRK 1
Peter NEUMANN 0
H. Randall HEPBURN 1
0 Martin-Luther-Universität Halle-Wittenberg, Institut für Zoologie / Molekulare Ökologie , Kröllwitzerstr. 44, 06099 Halle/Saale , Germany
1 Rhodes University, Department of Zoology and Entomology , 6140 Grahamstown , South Africa
- Queenright Apis mellifera capensis colonies exhibit egg laying by workers in periods of both low and high egg removal. To reproduce workers should lay in times of low egg removal to increase survival of their eggs. Were this so, a negative correlation between egg laying and removal would be expected. Egg removal rates for queen (N=240) and worker-laid (N=240) eggs and egg laying by workers were tested in queenright colonies. Worker-laid eggs were removed significantly faster than queen-laid eggs; but significant differences in egg laying by workers occurred among colonies. Egg laying and removal are positively correlated and co-dependent. Egg removal appears triggered by the number of worker-laid eggs. Intercolonial variation for laying worker egg number and egg removal rates may explain the phenotypic variation in worker reproduction in queenright Cape honeybee colonies. Apis mellifera capensis / egg removal / laying workers / worker reproduction / worker policing
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1. INTRODUCTION
With few exceptions (Oldroyd et al.,
1994), worker-laid eggs are removed by
other workers in queenright colonies of the
European subspecies of Apis mellifera L.
(Ratnieks and Visscher, 1989)
. This seems
to be based on relatedness grounds
(Ratnieks, 2000)
, because laying workers
usually produce male offspring (Free,
1987). However, laying workers of the
Cape honeybee (Apis mellifera capensis
Eschscholtz) produce female offspring
(Onions, 1912; Neumann et al., 2000;
Hepburn and Radloff, 2002; Radloff et al.,
2002)
, leading to predictions that egg
removal is either not expressed at all, or is
less expressed in this subspecies
(Greeff,
1996)
. Nevertheless, queenright Cape
honeybee colonies exhibit worker policing
(Neumann, Pirk, Ratnieks, unpublished
data), indicating that removal of
workerlaid eggs can also be based on colony
efficiency grounds.
Brood above the queen excluder is more
frequently observed in queenright colonies
of Cape honeybees
(Pettey, 1922; personal
observations)
than in other subspecies of
A. mellifera
(Visscher, 1996)
. It has been
shown that such brood is actually
workerderived
(Moritz et al., 1999)
, indicating
successful worker reproduction despite the
presence of a queen and egg removal.
Indeed, thousands of queenright colonies of
the neighbouring subspecies Apis mellifera
scutellata Lepeletier were taken over by
laying A. m. capensis workers
(Allsopp and
Crewe, 1993; Martin et al., 2002)
, showing
that A. m. capensis workers are facultative
social parasites
(for details see Hepburn and
Allsopp, 1994; Neumann et al., 2001; Calis
et al., 2002; Moritz, 2002; Neumann and
Hepburn, 2002; Reece, 2002; Wossler, 2002)
.
These observations strongly indicate that
laying workers of A. m. capensis are able to
evade worker policing, but what potential
strategies, if any, could these laying
workers use to increase the survival of their eggs?
Earlier observations showed (Pirk,
Neumann, Hepburn, Radloff, unpublished
data) that worker policing is subject to
environmental variation within colonies of
A. m. capensis, thus leading to periods of
low egg removal rates under unfavourable
weather conditions within a colony.
Because worker policing is only exercised
against eggs
(Ratnieks and Visscher, 1989)
,
worker-laid eggs need only survive three
days after oviposition, which might fit well
in a time window of low egg removal. So
one potential strategy of laying A. m.
capensis workers to evade worker policing,
which we designate as hypothesis 1 could
be that they are able to evaluate periods of
low egg removal and lay their eggs during
this time window. In this case one would
expect a negative correlation between worker
egg laying and the removal of worker-laid
eggs by other workers, because workers
should lay more eggs when there is less egg
removal by other workers and vice versa.
But, the same environmental factors
which affect egg removal behaviour may
also reduce egg-laying activity
(hypothesis 2). This might be simply due to the
generally reduced activity of workers during
periods with unfavourable weather
conditions
(Riessberger et al., 1998)
.
Alternatively, periods with unfavourable weather
conditions may also reduce the survival
chances of worker-derived offspring
because nurse bees change from the care of
young larvae to the care of older larvae
during such periods
(Blaschon & Crailsheim,
2001)
. This may easily outweigh the risk of
being removed by other workers. Thus,
unfavourable weather conditions may not
only reduce the activity of egg removal
beha (...truncated)