Ways plants communicate with pollinators
Note: This is part two of a two-part series. [Read part one here]
Plants are stationary, but most flowering plants require pollinators for reproduction. Therefore, they must attract pollinators to themselves. Pollinator syndrome describes the way plants have developed over time to attract specific pollinators, including visual cues and bribery with nectar and pollen. Researchers continue to discover additional unusual methods that plants use to tempt pollinators.
A complex mixture of highly volatile compounds and essential oils, scent is a signal to pollinators to locate particular flowers. From over a mile away, a pollinator follows the concentrated chemical producing the scent to the flower.
Bees prefer sweet scents, and beetles choose musty, spicy or fruity odors. Bees and butterflies pollinate specific plants during the day, when their fragrance is high, while moths and bats pollinate night-blooming plants in the evening, when their scent is greatest.
Once a flower has been pollinated, its floral bouquet is less appealing to pollinators.
Other times, scent and color are used in a deceitful way. Some flowers that are dark red or red-purple produce an odor similar to rotting flesh. A pollinator, such as a female blowfly, visits the flower expecting a meal and a carcass on which to lay her eggs.
In a number of flowering plants, especially orchids, a different sort of mimicry entices the insect pollinator to visit the flower and pollinate it. Eastern marsh helleborine, an orchid native to the Middle East, attracts hoverfly pollinators. These plants produce chemicals that mimic aphid alarm pheromones. Since aphids are a food source, female hoverflies are lured to the orchid by these false aphid warning signals. Additionally, the aphid alarm pheromones actually keep aphids away from the orchid as it protects itself.
The copper beard orchid of Australia has a floral structure and scent that imitates the female scoliid wasp. The male wasp attempts to mate with the flower and provides pollination services as he travels from orchid to orchid attempting to mate.
Just as many of us rely on a caffeine buzz, so do bees. The nectar in the flower of a coffee plant contains almost as much caffeine as a cup of instant coffee. Caffeine occurs naturally in the nectar of citrus plants too. Bees remember the caffeine-laced flowers and return to them.
Sacred datura (Datura wrightii), a plant native to the U.S. Southwest, has evolved a unique strategy to ensure cross-pollination. The flower opens at dusk and its fragrance attracts hawkmoths. Its nectar contains a narcotic, ensuring return visits by the addicted hawkmoth.
Once lured to some flowers, pollinators may become trapped within it. Water lilies (Nymphaea odorata) have a broad open flower with numerous petals. On the first day that the flower opens, it does not release pollen; instead, the female part of the flower is covered by a pool of fluid. When a pollinator visits the flower, the insect falls into the fluid, where it dies. Any pollen grains on its body from a previous visit to a pollen-releasing water lily settles to the bottom of the pool, coming in contact with the female part of the flower. The next day, the flower produces no fluid, the male part of the flower (stamens) release pollen, and pollen-covered pollinators leave to continue this fatal cycle.
Protection from predators
Some plants provide chemicals that allow the insect pollinator to repel predators. Monarch butterfly larvae (caterpillars) feed exclusively on milkweeds (Asclepias spp.) The latex sap in milkweed leaves contains a chemical that the caterpillars incorporate into their bodies. This chemical defense protects them from a number of predator species.
Response to sound
A recent study showed that plants respond rapidly to the specific airborne sound frequencies of bee wingbeats to potentially increase their chances of pollination. The flowers themselves serve as sound sensing organs. Within three minutes, flowers increase the concentration of sugar in their nectar following exposure to bee sounds by an average of 20%.
Scientists have learned that plants use electric fields to communicate with bees. Bumblebees are able to locate and decipher weak electric signals emitted by flowers. Tests reveal that bees can distinguish among different floral electric fields as if they were different petal colors. Researchers suspect the electrostatic force might make bee hairs bristle similar to the way static electricity can affect our own hair.
The electric signals may also let insects know if another bee has recently visited a flower. Bees carry a positive electric charge with voltages as high as 200 volts. They build up the charge as they fly. Plants, rooted in the soil, typically carry a negative charge. As a bee approaches and lands on a flower, the bee transfers part of its charge to the plant stem, resulting in the plant’s charge being positive for about two minutes. This interaction of charges may be how the next bee will know which flower is depleted of nourishment, allowing the bee to move to a flower that hasn’t had a recent pollinator visit.
While we do know that native plants communicate with native pollinators, we are still discovering more about these intricate relationships.
Lela Martin is a Master Gardener with the Chesterfield County office of the Virginia Cooperative Extension. She is also co-chair of the Bumblebee Jamboree, which has been reimagined for 2021 as a self-guided Pollinator Path through Maymont’s Children’s Farm from June 21-27. Visit https://linktr.ee/BBJ2021 for more information.