Global Warming, Advancing Bloom and Evidence for Pollinator Plasticity from Long-Term Bee Emergence Monitoring

Abstract

Global warming is extending growing seasons in temperate zones, yielding earlier wildflower blooms. Short-term field experiments with non-social bees showed that adult emergence is responsive to nest substrate temperatures. Nonetheless, some posit that global warming will decouple bee flight and host bloom periods, leading to pollination shortfalls and bee declines. Resolving these competing scenarios requires evidence for bees' natural plasticity in their annual emergence schedules. This study reports direct observations spanning 12-24 years for annual variation in the earliest nesting or foraging activities by 1-4 populations of four native ground-nesting bees: Andrena fulva (Andrenidae), Halictus rubicundus (Halictidae), Habropoda laboriosa and Eucera (Peponapis) pruinosa (Apidae). Calendar dates of earliest annual bee activity ranged across 25 to 45 days, approximating reported multi-decadal ranges for published wildflower bloom dates. Within a given year, the bee H. rubicundus emerged in close synchrony at multiple local aggregations, explicable if meteorological factors cue emergence. Emergence dates were relatable to thermal cues, such as degree day accumulation, soil temperature at nesting depth, and the first pulse of warm spring air temperatures. Similar seasonal flexibilities in bee emergence and wildflower bloom schedules bodes well for bees and bloom to generally retain synchrony despite a warming climate. Future monitoring studies can benefit from several simple methodological improvements.

Keywords: Anthophila; climate change; global warming; ground-nesting; phenology; pollinator; seasonality; weather.

Figure 1

Variation in the annual date of first activity by H. rubicundus at their natal nesting aggregation in Logan Utah USA.

Figure 2

Annual date for first sighting of E. pruinosa at squash flowers (Cucurbita) in a market garden near Logan Utah USA. Missing years have no bar.

Figure 3

Cumulative growing degree days (GDD > 10 °C) for the actual and median dates that H. rubicundus emerged in Logan Utah USA. Temperatures recorded at the campus weather station.

Figure 4

Daily soil temperatures from 20-cm depth at Cache Junction, Utah, across the valley from the market garden with squashes visited by E. pruinosa. The plot highlights the daily average soil temperatures over years (green line), daily maxima (red) and minima (blue), and the particularly cold year of 2011 (in violet) when both bee species were late to emerge.

Figure 5

Annual date for first sightings of male and female H. laboriosa at flowering blueberries (Vaccinium) near Auburn, Alabama, USA.

Figure 6

Annual variation in the emergence date for A. fulva at their nesting aggregation in Poznan Poland (adapted from data in [13]). Data is missing for 1993.