If you examine plants that are still flowering this late in the season (such as asters, goldenrod and late-blooming turtlehead) early in the morning when it’s still quite cool or late in the day, many of the pollinators you see will be bumblebees, not honey bees. One reason for this is that they have different temperature tolerances for flight. You rarely see a honey bee when the temperature is below 57°F as they cannot fly when it is this cool. Bumblebees, however, are capable of flight when the air temperature is as low as 50°F.
Even so, bumblebees cannot take off unless their flight muscles are above 86°F; they maintain the temperature of their thorax (where wings and wing muscles are located) between 86°F and 104°F regardless of the ambient temperature. The way in which they raise the temperature of their thorax involves uncoupling their wing muscles so that the wings themselves do not move. They then use their wing muscles to shiver and raise the temperature of their thorax until it’s sufficiently warm enough for them to fly.
At rest a bumblebee’s body temperature will fall to that of its surroundings. If it is cool out, and the bumblebee wants to take flight, you can actually see its abdomen pumping to ventilate the flight muscles. An entomologist studying this phenomenon discovered that the rate of pumping can give an indication of the temperature of the bee. It ranges from around 1 pump per second when the bee is 86°F, to 6 pumps per second when it reaches 95°F.
At a time of year when nectar and pollen sources are few and far between, New England Aster provides many species of bees with food. This composite seems designed specifically for easy pollination. Its open, wide flower shape provides a flat surface for insects to land on, and because the nectar and pollen are not hidden deep inside the flowers, both long- and short-tongue bee species can easily access them. Unlike honeybees, bumblebees do not have a large store of honey in their nests, so they need pollen and nectar throughout the season. Thus, the few flowers such as New England Aster that blossom as late as October are visited frequently and in large numbers. (Only the queen bumblebee overwinters, but the workers continue collecting nectar and pollen up until they die in late fall.)
New England Aster flowers close at night, when there are fewer pollinating insects flying. If an unusually cool period arrives during the time when New England Aster is blooming, the blossoms also close. Although it may seem that the aster is losing pollination opportunities during a cold day, bees are not very active in cool weather.
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