Thermal biology and nest-site selection in the bee 'Halictus rubicundus' (Hymenoptera : Halictidae)
Abstract
Aggregations of the ground-nesting bee Halictus rubicundus were found at
several locations across the UK. The phenology, social organisation, nest
architecture and foraging behaviour of this bee were described for the largest
of these aggregations (Invergowrie, Scotland). This site was unusual in having
an extremely high brood mortality due to the impact of an anthomyiid fly.
Nest parasitism was found to be directly density-dependent and it lead to a
marked decline in nest numbers over the period of this study, indicating the
possible forthcoming extinction of the aggregation.
The other sites contained smaller nesting aggregations and the level of
parasitism was considerably less. There was a marked variation in size across
UK populations and this was explained by a temperature rather than a
latitudinal cline.
There was no evidence from field or laboratory studies to suggest that this
species is endothermic; H. rubicundus is purely a behavioural thermoregulator.
The effect of size upon various rates of heat exchange were examined in the
laboratory for both sexes, and related to behaviours observed at the nest-site.
Thus the microclimatic windows for different activities were established. The
abundance of flying individuals at the nest-site was highly predictable from
ambient temperature and light intensity; with predictions during a single day
being more accurate than those combining several days throughout the season.
Furthermore the usefulness of standard operative temperatures in predicting
flight activity was assessed. The thoracic temperature of both sexes depended
on the prevailing ambient temperature and also on the size of the individual
while either basking or flying. Body temperatures increased with both ambient
temperature and head width. However when both these predictors were
combined into a single model, then size was only a strong predictor at lower
temperatures. These findings were used to explain many of the behavioural
patterns observed at Invergowrie.
The nest site selection behaviour of females was examined both within and
across sites. H. rubicundus was able to utilise a range of edaphic and
microclimatic conditions when choosing a site to excavate a nest. There were
some factors with broad tolerances such as slope and hardness, and others
with much narrower limits such as aspect, soil humidity and particle
composition. The thermal advantages of having a warm nest meant that the
most suitable areas were those with a southern aspect and a slope which
maximised the absorption of solar radiation. Limited areas of substrate with
the most desirable characteristics resulted in gregarious nesting ('limited
substrate' hypothesis). There was a preference for softer soils, that were easier
to dig, within a site with a low overall density; but in much denser
aggregations problems of maintaining the structural integrity of a nest lead to
the favouring of harder soils. The tendency to nest in close proximity to the
natal nest (philopatry) complemented the 'limited substrate' hypothesis in
producing an aggregation of nests. The spatial distributions of nests within
aggregations were examined using nearest neighbour distance analyses; at low
densities, microscale variation in substrate quality produced clumped patterns,
whereas at high densities the risk of adjacent nests collapsing into one another
forced nest spacing to be more regular.
Findings concerning temperature dependence of nesting and foraging
activities, and broader environmental controls on nest-site selection, are
considered in relation to key aspects of bee biology: the origins and function of
social behaviours, the conservation of, or provision of, nest-sites, and the utility
of solitary bees as crop pollinators.
Type
Thesis, PhD Doctor of Philosophy
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