El Niño and Maine’s Bumble Bee Populations

As I write this on a cold January morning with snow falling outside my office window, the mated bumble bee queens that emerged last fall as their parent colonies were ending are now hibernating under the soil waiting for spring. Their bodies are able to produce glycerol, the same ingredient in anti-freeze, which keeps them from freezing completely. Technically, the queens are in a state called diapause, during which their metabolisms have slowed as they consume the fat and nectar reserves they collected in the fall. This stage will last for six to nine months depending on location. Temperature and day length are thought to potentially control the onset and termination of diapause.

temperature map with key

Temperature Anomaly on December 25th, 2015. The red/orange in the graphic indicates areas of North America where there were unusually warm temperatures on Christmas Day.

We have had unusual weather for the Northeast in the past few months as a particularly strong El Niño event has affected our climate. El Niño (“little boy” in Spanish) is a large-scale ocean-atmosphere climate interaction linked to warming sea surface temperatures in the Pacific Ocean near the equator. Occurring on average every two to seven years, an El Niño creates a very active jet stream across the southern US which keeps much of the cold air in Canada. This results in warmer-than-average temperatures for the Northeast and colder-than-average temperatures across southern states. This past fall, Maine experienced one of the strongest El Niño events since the late 1990s, with temperatures well above normal. The National Weather Service reported temperatures in northern and eastern Maine averaging 1.5 to 4 degrees Fahrenheit above normal, and we saw Bangor tie its previous Christmas Day record of 54º F, well above the 31º F average and Portland reach 62º F, leaving the old record of 54º F in the dust.

Will these unusually warm fall and winter temperatures have an effect on the metabolism of bumble bee queens in diapause? The rate of all biological mechanisms increases with temperature, especially in animals whose internal temperatures vary with external temperatures, such as insects. Because the queens’ metabolisms will be processing at a faster rate, they may consume more of their fat and nectar stores during warmer temperatures. This could potentially result in a depletion of resources such that a queen might not survive until spring.

Another possible effect of a warm winter is that important early season sources of pollen and nectar, such as willows and rhododendrons, may bloom earlier than usual. Bumble bee queens may emerge after the high point of this bloom, resulting in less availability of these resources, which are often staples for the establishment of new colonies. Indeed, the National Weather Service Climate Prediction Center does predict that temperatures in April – June of this year will be above normal.

The bottom line is that we cannot be certain as to how the 2015-2016 El Niño will affect our bumble bee populations for 2016. An important thing to keep in mind is that bumble bee population dynamics are not static from year to year. The relative abundances of bumble bee species can vary each year with weather and climate conditions and complicated interactions between species and resources occur with these changes. Some bumble bee species do tend to be more tolerant of a wider range of climates, such as the broadly dispersed Eastern Bumble Bee (Bombus impatiens), and may have a competitive advantage in times of climate uncertainty.


For more information:

Bumblebee mating, death and hibernation; Stage 4 in the lifecycle of the bumblebee colony. Available from http://www.bumblebee.org/lifeMate.htm

Fall 2015 Climate Summary for northern and eastern Maine. Available from http://www.weather.gov/car/Fall2015ClimateSummary

Three Month Outlooks. Available from http://www.cpc.ncep.noaa.gov/products/predictions/long_range/seasonal.php?lead=4.

What are El Niño and La Niña? Available from http://oceanservice.noaa.gov/facts/ninonina.html.

Map Image obtained using Climate Reanalyzer (http://cci-reanalyzer.org), Climate Change Institute, University of Maine, USA.



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