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Bumblebees: Cold tolerance and impacts of diet A study of Buff-tailed Bumblebees (Bombus terrestris audax) Emily Owen, Jeff Bale and Scott Hayward School of Biosciences, University of Birmingham, B15 2TT Email: exo763@bham.ac.uk. Bumblebees: Cold tolerance and impacts of diet

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Bumblebees: Cold tolerance and impacts of diet

A study of Buff-tailed Bumblebees (Bombus terrestris audax)

Emily Owen, Jeff Bale and Scott Hayward

School of Biosciences, University of Birmingham, B15 2TT Email: exo763@bham.ac.uk

Bumblebees: Cold tolerance and impacts of diet

A study of Buff-tailed Bumblebees (Bombus terrestris audax)

Emily Owen, Jeff Bale and Scott Hayward

School of Biosciences, University of Birmingham, B15 2TT Email: exo763@bham.ac.uk

COLD TOLERANCE

DIET AND SUPERCOOLING POINT

COLD TOLERANCE

DIET AND SUPERCOOLING POINT

INTRODUCTION

Bumblebees are economically important crop pollinators with extensive commercial usage

Despite this, their thermal biology is largely unknown

To computer

INTRODUCTION

Bumblebees are economically important crop pollinators with extensive commercial usage

Despite this, their thermal biology is largely unknown

Method

Method

Method

Method

Test tube

Bees were taken from their rearing temperature (20⁰C) and transferred to test tubes which were then plunged into a pre-programmed alcohol bath set at a range of temperatures (30 bees per treatment, figure 2). Survival was assessed 72h after exposure. Preliminary experiments involved measuring the supercooling points of 30 bees and using this data to construct a profile of the expected freezing temperatures of the bumblebee population.

Bees were fed a standard diet of pollen and nectar. Pollen was then removed for a period of 0, 3, 7 or 14 days respectively. Individuals were then transferred to test tubes and each attached to a thermocouple. Test tubes were then added to an alcohol bath, programmed to cool from 20°C to -20°C at a rate of 0.2min-1. The supercooling point (SCP) of each bee was detected by the exotherm on freezing.

Bees were fed a standard diet of pollen and nectar. Pollen was then removed for a period of 0, 3, 7 or 14 days respectively. Individuals were then transferred to test tubes and each attached to a thermocouple. Test tubes were then added to an alcohol bath, programmed to cool from 20°C to -20°C at a rate of 0.2min-1. The supercooling point (SCP) of each bee was detected by the exotherm on freezing.

Bees were taken from their rearing temperature (20⁰C) and transferred to test tubes which were then plunged into a pre-programmed alcohol bath set at a range of temperatures (30 bees per treatment). Survival was assessed 72h after exposure. Preliminary experiments involved measuring the supercooling points of 30 bees and using this data to construct a profile of the expected freezing temperatures of the bumblebee population.

Type K exposed wire thermocouple

Figure 2.

Foam bung

Bombus terrestris audax

  • Bombus terrestris audax
  • UK native subspecies, 8 European conspecifics1
  • Diet of pollen and nectar2
  • Can generate their own heat → early spring emergence3
  • Bombus terrestris audax
  • UK native subspecies, 8 European conspecifics1
  • Diet of pollen and nectar2
  • Can generate their own heat → early spring emergence3 (figure 1)

Figure 2. Apparatus used in the investigation of cold tolerance in Bombus terrestris audax.

Results

Results

Results

Results

SUMMER

SUMMER

Males and new queens produced, which mate

Males and new queens produced, which mate

Colony development (production of workers)

Colony development (production of workers)

Colony death

Colony death

SPRING

SPRING

AUTUMN

AUTUMN

Males die, fertilised queens enter diapause

Males die, fertilised queens enter diapause

Colony initiation

Colony initiation

Queens emerge from diapause

Queens emerge from diapause

Figure 3. Supercooling points of bumblebees (Bombus terrestris audax) that were fed either pollen and nectar or nectar alone for 3, 7 or 14 days respectively, n=30 for each treatment. Corresponding letters indicate significance ≤0.01.

Figure 2. Percentage survival of bumblebees (Bombus terrestris audax) after exposure to a range of sub-zero temperatures for 2h, 4h, 6h, 8h or 10h (±SE). Survival was assessed 72h after each exposure, n=30 for each value.

  • Figure 4. Supercooling points of bumblebees (Bombus terrestris audax) that were fed either pollen and nectar or nectar alone for 3, 7 or 14 days respectively (±SE), n=30 for each treatment. Corresponding letters indicate significance ≤0.01.

WINTER

WINTER

Figure 3. Percentage survival of bumblebees (Bombus terrestris audax) after exposure to a range of sub-zero temperatures for 2h, 4h, 6h, 8h or 10h (±SE). Survival was assessed 72h after each exposure, n=30 for each value.

Figure 1. A summary of the lifecycle of Bombus terrestrs audax

Figure 1. A summary of the lifecycle of Bombus terrestrs audax

Discussion

Discussion

Discussion

Discussion

Research aims

Investigate the cold tolerance of B. t. audax

Discover if this is affected by diet

Why is this important?

Optimising commercial usage

Conservation

  • Bumblebees were able to tolerate temperatures as low as -5°C for 10h without significant mortality
  • Below -5°C, survival deviated from the predicted values if freezing was the sole cause of death; with longer durations producing lower survival. This mortality can be attributed to cold shock
  • In other insects, there is usually a temperature window between death due to cold shock and death due to freezing. In B. t. audax, this window is comparatively small
  • Potentially, this means bumblebees can survive at temperatures close to their freezing temperature
  • When fed the standard diet of pollen and nectar, the SCP ranged from -5°C to -10.9°C (mean -7.1 ± 0.22°C)
  • After removal of pollen from the diet, the SCP decreased to a minimum of -17.9°C (mean of 14 day nectar-feeding -12.5 ± 0.52°C)
  • Pollen is a known ice nucleator and causes freezing at higher temperatures. These data suggest that pollen appears to contribute to the increase in SCP for pollen-feeding bumblebees

Research aims

Investigate the cold tolerance of B. t. audax

Discover if this is affected by diet

Why is this important?

Optimising commercial usage

Conservation

  • Bumblebees were able to tolerate temperatures as low as -5°C for 10h without significant mortality (figure 3)
  • Below -5°C, survival deviated from the predicted values if freezing was the sole cause of death; with longer durations producing lower survival. This mortality can be attributed to cold shock
  • In other insects, there is usually a temperature window between death due to cold shock and death due to freezing. In B. t. audax, this window is comparatively small
  • Potentially, this means bumblebees can survive at temperatures close to their freezing temperature
  • When fed the standard diet of pollen and nectar, the SCP ranged from -5°C to -10.9°C (mean -7.1 ± 0.22°C, figure 4)
  • After removal of pollen from the diet, the SCP decreased to a minimum of -17.9°C after 14 days of nectar-feeding (mean -12.5 ± 0.52°C)
  • Pollen is a known ice nucleator and causes freezing at higher temperatures.
  • These data suggest that pollen appears to contribute to the increase in SCP for pollen-feeding bumblebees

CONCLUSIONS

Bumblebees tolerate temperatures down to -5°C without a significant decrease in survivorship. Below this temperature, death was caused by a combination of freezing and cold shock

Pollen in the diet increased bumblebees’ freezing temperature, making them susceptible to freezing at higher temperatures

References

1. Rasmont et al. (2008) Annales de la Société Entomologique de France. 44(1):243-250

2. Raine et al. (2006) Entomologia Generalis. 28(4):241-256

3. Dafini et al. (2010) Population Ecology. 46(3):243-251

Acknowledgements

Thanks to BBSRC for funding the project, Biobest for providing the bees and to Jeff Bale and Scott Hayward for their continued support

References

1. Rasmont et al. (2008) Annales de la Société Entomologique de France. 44(1):243-250

2. Raine et al. (2006) Entomologia Generalis. 28(4):241-256

3. Dafini et al. (2010) Population Ecology. 46(3):243-251

Acknowledgements

Thanks to BBSRC for funding the project, Biobest for providing the bees and to Jeff Bale and Scott Hayward for their continued support

CONCLUSIONS

Bumblebees tolerate temperatures down to -5°C without a significant decrease in survivorship. Below this temperature, death was caused by a combination of freezing and cold shock

Pollen in the diet increased bumblebees’ freezing temperature, making them susceptible to freezing at higher temperatures