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

James Cane

doi:10.3390/insects12050457

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.

Highlights

Native bee faunas and wildflower communities continue to suffer from diverse human impacts, in temperate zones of the Northern Hemisphere
This primitively eusocial bee was tracked for annual nest initiation by wintering gynes occupying several nesting aggregations on the campus of Utah State
The bees’ first arrival was poorly predicted by several seemingly relevant seasonal measures recorded at the nearby campus weather station

Summary

Introduction

Records for abundantly collected species can bracket the general calendar dates of seasonal activity, revealing latitudinal trends [7] This has been a popular means to assess responses of bee communities to global warming (refs in [7]). These are averages, the bulk of the specimens coming from the height of the nesting season, not its onset. Annual emergence schedules, which requires multidecadal field data at fixed locations The purpose of this long-term study was to monitor annual emergence schedules of four species of resident wild ground-nesting bees (Figure S1). The bees’ fluctuating emergence dates are compared with available air and soil temperatures drawn from compiled data recorded at nearby weather stations

Materials and Methods

Results

Halictus rubicundus

Annual

Cumulative growing degree

Schedules were only only coarsely