Summer but not spring resource supplementation improves bumblebee colony survival

Global agricultural expansion has led to the deterioration of many key
foraging habitats for bees, but the impacts of nutritional stress caused
by temporal gaps in food availability can be hard to isolate from other
agricultural stressors. For bumblebee colonies (Bombus spp.), which have
an annual lifecycle culminating in a summer reproductive period, this
issue is further complicated by potential variation in vulnerability over
the colony lifecycle. Isolating the effects of nutritional stress on
bumblebee colonies could therefore direct agricultural interventions that
seek to mitigate against further decline. We assayed whether available
food resources are limiting colony productivity and survival by
supplementing post-worker emergence Bombus terrestris colonies with ad
libitum food during mid-spring or early summer on 20 farms across
south-east England (“earlier” vs “later” treatments). In mid-spring,
forage is potentially more abundant, but colonies are at an earlier
developmental stage, while in early summer, colonies are more established,
and forage is potentially sparser. Later-supplemented colonies survived
for significantly longer than both unsupplemented controls and
earlier-supplemented colonies. We found no detectable effects of
supplementary feeding on reproductive output, which was minimal overall.
Colonies in areas dominated by non-flowering arable crops failed soonest
and had fewer workers irrespective of the feeding regime, and colonies
experiencing higher weekly precipitation produced fewer workers. Our
findings suggest that B. terrestris colonies became food-limited towards
the end of their colony cycle in early summer. We suggest ecological
intensification through planting native wildflower species may be more
effective if it targets this flowering period, particularly within heavily
arable areas, and that a staggered patchwork of mowing or grazing could be
important in ensuring seasonal resource continuity.