Getting inside the flower of Bottle Gentian or Closed Gentian (Gentiana clausa), one of our latest flowering plants, in order to collect nectar and pollen is a monumental task that few insects, other than fairly large species of bumble bees, attempt. The petals are closed so tightly it takes even bumblebees several seconds of pushing, shoving and cramming to push the petals aside and get through the miniscule opening at the top of the blossom.
Pollen is the primary bumblebee attractant, as the sugar concentration of Bottle Gentian’s nectar is fairly low. Some bumble bees take a short cut – they chew a hole to gain access to the reproductive parts of the flower. The hole is often two-thirds of the way up the blossom, directly opposite the pollen-laden anthers within the flower. Look closely at the hole in the lefthand blossom in the photograph and the adjacent, dissected blossom, and you will see that the bee’s aim was dead on. You can even detect a portion of the anther through the hole.
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.
Pollinators of the plant known as Boneset (Eupatorium perfoliatum) are too many to list. The nectar and pollen of its fragrant flowers which bloom in late summer and fall attract many kinds of insects, including bees, flies, wasps, butterflies, and beetles. Being a member of the Composite family, Boneset’s flower structure is such that the nectar is very accessible and therefore a popular feeding site, especially for Honey Bees (see photo) which are reliant this time of year upon the flat-topped clusters of small, white flowers for nectar which they convert to honey and store for their winter supply of food.
The Pruinose Squash Bee (Peponapis pruinosa) is most often noticed when it’s gathering nectar or pollen from squash, pumpkin, watermelon or gourd blossoms. (Squash bees have been shown to be excellent pollinators of zucchini and butternut squashes, among others. If numerous, they thoroughly pollinate all available flowers, rendering later visits of honeybees superfluous. Before Europeans brought honeybees to the New World, squash bees were busy aiding the adoption, domestication, spread, and production of squashes and gourds by indigenous peoples throughout the Americas.) The bee’s black and white striped abdomen is easy to recognize.
While female squash bees are busy foraging for pollen in the flowers of plants in the Cucurbitae family, male squash bees can be seen darting between flowers, searching for mates. By noon, they are fast asleep in the withered flowers.
Pruinose Squash Bees are solitary bees, with every female digging her own nest in the ground. These consist of vertical tunnels that end with a number of individual chambers that are a foot or two deep in the soil. Each chamber is provided with an egg and a lump of pollen so that when the egg hatches, food is readily available. (Photo: five Pruinose Squash Bees packed into a single Bindweed flower)
If your home, shed or barn has weathered, unpainted wood and is riddled with ½”-diameter, perfectly round holes, there is a chance that carpenter bees are hibernating in them. Carpenter bees resemble bumblebees in both size and appearance (a carpenter bee has very few hairs on the top of its abdomen, which appears black and shiny, whereas bumble bee abdomens are often yellow and hairy), but they are not social insects. Instead of having a common nest in which they live and raise their young, carpenter bees drill holes in wooden structures or trees inside of which they chew tunnels that contain six to eight brood chambers for their young. After creating the chambers, the female carpenter bee places a portion of “bee bread” (a mixture of pollen and regurgitated nectar) in each one. On top of each pile of food she lays an egg and then seals off the chamber. The larvae eat and grow, pupate and emerge as adult bees in late summer. At this point they feed on nectar, pollinating a wide variety of flowers before they return to their tunnels to over-winter.
Having known since childhood that most insects have only one pair of antennae, imagine my surprise when I came upon a hornet on Queen Anne’s Lace that appeared to have two: a pair of slender, black antennae, and between them, a shorter pair of white ones. A bit of research revealed to me that in fact, these white “antennae” were actually the pollen sacs (pollinia) of an introduced and somewhat invasive orchid, Broad-leaved Helleborine (Epipactis helleborine).
Broad-leaved Helleborine is entirely dependent on insects to spread its pollen, especially wasps. It attracts them with nectar, which is said to have an alcoholic and narcotic effect which may help with the spreading of pollen, as an inebriated wasp is less likely to clean pollen off its body before leaving. Helleborine also produces a chemical which other plants produce and use to signal that they are being attacked by insects. It is used purely as a ruse by Helleborine, in order to attract wasps, Helleborine’s primary pollinators, who arrive to fend off other insects, and end up inadvertently collecting Helleborine’s pollinia.
Unlike the pollen of most plants, Helleborine’s pollen grains are so sticky that they cannot separate – thus, the entire package of pollen remains intact and is removed at one time. Wasps are capable of reaching the plant’s nectar without disturbing the pollinia, but cannot crawl out of the flower without striking against and detaching them and in so doing, getting them stuck to their heads. Can you find the pollinia in the insert photograph of a Broad-leaved Helleborine flower (which has not been visited by a wasp yet)?
Due to computer issues, Naturally Curious will resume posts next Tuesday, August 16.
Butterflies pollinate during the day while most flowers are open and they have better color perception than bees or even humans, but they are less efficient than bees at moving pollen between plants. Their legs and proboscis are longer and farther away from the flower’s pollen so they do not pick up as much pollen on their bodies. They also lack specialized structures for collecting pollen. Nevertheless, it is hard to imagine that some of the Daylily pollen that has collected on this Eastern Tiger Swallowtail’s wings might not fall onto or be brushed against the stigma of the next Daylily it visits.