Multiple interacting factors drive recent declines in wild and managed bees, threatening their pollination services. Widespread and intensive monitoring could lead to more effective management of wild and managed bees. However, tracking their dynamic populations is costly. We tested the effectiveness of an inexpensive, noninvasive and passive acoustic survey technique for monitoring bumble bee behavior and pollination services. First, we assessed the relationship between the first harmonic of the flight buzz (characteristic frequency) and pollinator functional traits that influence pollination success using flight cage experiments and a literature search. We analyzed passive acoustic survey data from three locations on Pennsylvania Mountain, Colorado to estimate bumble bee activity. We developed an algorithm based on Computational Auditory Scene Analysis that identified and quantified the number of buzzes recorded in each location. We then compared visual and acoustic estimates of bumble bee activity. Using pollinator exclusion experiments, we tested the power of buzz density to predict pollination services at the landscape scale for two bumble bee pollinated alpine forbs (Trifolium dasyphyllum and T. parryi). We found that the characteristic frequency was correlated with traits known to affect pollination efficacy, explaining 30-52% of variation in body size and tongue length. Buzz density was highly correlated with visual estimates of bumble bee density (r = 0.97), indicating that acoustic signals are predictive of bumble bee activity. Buzz density predicted seed set in two alpine forbs when bumble bees were permitted access to the flowers, but not when they were excluded from visiting. Our results indicate that acoustic signatures of flight can be deciphered to monitor bee activity and pollination services to bumble bee pollinated plants. We propose that applications of this technique could assist scientists and farmers in rapidly detecting and responding to bee population declines.

Bumble bee morphology and flight buzz frequency - alpine speciesMorphological measurements (tongue length, wing length, intertegular distance) and flight buzz frequency for two alpine bumble bee species (Bombus balteatus and B. sylvicola) as collected by Nicole Miller-Struttmann and colleagues in 2014 at Niwot Ridge Long Term Ecological Research Site in Colorado. COLUMN DETAILS: BeeNumber - a unique identifier given to each individual collected Species - bumble bee species Caste - bumble bee caste BuzzNumber - the buzz replicate (1 through 3) for each individual FlightBuzzFrequency - the characteristic frequency of the buzz in mm TongueLength - tongue length in mm WingLength - wing length in mm IntertegularDistance - the distance between the bases of the wings in mmBumble bee morphology and flight buzz frequency - literature surveyTongue length (mm) and flight buzz frequency (Hz) for queens and workers of 17 bumble bee species as reported in the literature.Automated versus manual bumble bee flight buzz countsBuzz counts quantified by manual (auditory and visual) counts and automated counts produced by the algorithm (modified Computational Auditory Scene Analysis with focal templates) composed by David Heise with data collected in 2015 at Pennsylvania Mountain in Colorado.Bee density, buzz density, and pollination servicesBumble bee density, flight buzz density (Hz) and pollination services (seed set) as collected by Galen and colleagues in 2015 at Pennsylvania Mountain in Colorado. COLUMN DETAILS: Site - the location on Pennsylvania Mountain PlantSpecies - the genus and species name of the focal plant BuzzesPerHour - the number of buzzes recorded per hour via acoustic monitoring BeesPerHour - the number of bees recorded per hour via visual surveys ControlSeedSet - seed set of individuals left open to pollinators ExcludedSeedSet - seed set of individuals covered in mesh netting to exclude bumble bee pollinators.