Discussion
Bumble bees were generally less flower constant during pollen collection
than honey bees, supporting our prediction that trapline foraging
results in a greater proportion of bees visiting more than one plant
taxon for pollen collection during single foraging bouts. Likewise, the
observation that honey bees tended to forage on a single plant type
during a foraging bout supports the hypothesis that dance communication
generates cohorts of individuals that forage on a single profitable
patch (Visscher & Seeley, 1982). At the colony level, the diversity of
pollen collected by bumble bees was greater than that of honey bees,
however the richness of pollen collected was temporally variable for
both bee species (Fig.3). Similar differences in flower constancy and
diversity were also observed between honey bees and bumble bees in
agricultural systems in Germany (Leonhardt & Blüthgen, 2012).
Interestingly, during period three, we observed an increase in the rate
of flower constancy, which was associated with a decrease in the
richness of pollen collected by bumble bee colonies, with the opposite
pattern observed in honey bees, supporting a link between individual and
colony level foraging patterns. These results support the idea that
foraging strategies, such as trapline foraging and dance communication
help predict differences in pollen foraging patterns by different bee
species. Such knowledge not only highlights interspecific differences in
foraging patterns, but also informs questions concerning the dynamic
responses of each bee species to spatiotemporal ecological challenges
present in human altered landscapes. This type of information can be
used to facilitate the development of pollinator friendly habitats that
also support pollination of bee-dependent crops.
During period three the number of foraging bumble bees declined, while
honey bees were regularly foraging for pollen at sites two and three,
but not at site one (Appendix Table S1). We might expect changes in
resource availability, which are known to mediate distinct responses
among bee species, to contribute to the observed temporal foraging
dynamics of honey bees and bumble bees (Jha & Vandermeer, 2009).
However, at the spatial scale of 500m around each hive site we did not
detect any temporal differences in the richness or diversity of plant
resources available to bees (Appendix Fig. S2). Interestingly, in period
3, both bee species gathered pollen from plant families found outside
the survey areas, such as Brassicaceae by bumble bees or Geraniaceae by
honey bees (Table 2; Appendix Fig. S3). It has been suggested that
summer is a challenging time for bees, possibly as a result of
competition for shared resources (Couvillon, Schürch, & Ratnieks, 2014;
Minahan & Brunet, 2018). In July, Bumble bee individuals foraged more
frequently from single pollen sources, while more honey bee individuals
visited multiple resources, and both bee species exploited new plant
families for pollen. For honey bees, it would be interesting to
determine if fewer foragers followed the dance communication during this
period, which if true would suggest more bees scouting for resources, or
visiting previously learned locations (Biesmeijer & Seeley, 2005).
Future studies should further examine life history and ecological
factors associated with the temporal dynamics of pollen and nectar
foraging, and how these are linked to distinct foraging strategies.
Understanding these factors will improve the ability of conservation
planners to select resource types and plan their spatial and temporal
distributions according to the dynamic needs of pollinator communities.
As predicted based on the trapline foraging strategy, bumble bees
demonstrated a preference for Fabaceae_Tricolporate but an avoidance of
Apiaceae pollen, while honey bees foraged randomly, based on the floral
abundance of plant families in the area. The preference for certain
plant types when foraging for pollen could depend on differences in the
amount of pollen made available by plants, with bumble bees being able
to detect such differences (Brunet, Thairu, Henss, Link, & Kluever,
2015). Preference could also be influenced by the ability of a bee
species to handle and exploit particular flower morphologies (Raine and
Chittka, 2007), or the nutritional complement of pollen relative to
other available resources (Somme et al. 2015). The observation of
preference by bumble bees but not honey bees suggests a potentially
greater ability of bumble bees to respond to floral traits such as fatty
acid content of pollen (Ruedenauer et al., 2020) relative to honey bees.
This may be necessary when using trapline foraging as opposed to dance
communication as a foraging strategy.
There exist some challenges to our approach of determining preference. A
first challenge is selecting an appropriate spatial scale and survey
method to estimate floral resource availability. Our surveys were
limited to 500m, which is likely to encompass many foraging trips, but
not all, as bees can forage beyond this range (Beil, Horn, & Schwabe,
2008; Danner, Molitor, Schiele, Härtel, & Steffan-Dewenter, 2016;
Nagamitsu, Tsukuba, Ushirokita, & Konno, 2012). A second challenge is
that pollen types must be present in both pollen pellets and resource
surveys for our analyses. However, it can also be informative when
pollen is found in resources but not bee collected pollen, or vice
versa. For example, the absence of Linaria vulgaris ,Oxalis, and Phlox pollen in pollen pellets suggests that
bees are avoiding these available resources. Plants only present in the
pollen pellets, such as Geraniaceae, most likely came from home gardens,
which can provide valuable resources for bees (Goulson, Lye, & Darvill,
2008). These results highlight the challenge associated with analyzing
preferences in complex human altered landscapes. However, future
experiments should explicitly consider the diversity of habitat types
present when studying pollinator foraging patterns and their
relationship with foraging strategies.