Trying to attract pollinators to your garden? Keep in mind that flower features can influence what pollinators visit most frequently.
Plants and animals have evolved together over time by adapting morphological features to ensure pollination (plants) and to better harvest and increase the supply of food sources (pollinators). The combinations of attributes that relate to what pollinator is likely to visit are called pollinator syndromes. These traits include flower shape, color, scent, nectar amount, pollen amount and presence of nectar guides. Flower shapes can create landing pads or perches for pollinators to land on, or can be more open and bowl-shaped for pollinators that hover or are not graceful at landing. Long tubular flowers can exclude insects that are not good pollinators. Beetles often pollinate more open flowers while butterflies visit narrow tubular flowers with landing pads. Bees pollinate shallow tubular flowers with landing platforms. Birds pollinate funnel shaped flowers or cup-shaped flowers that have a strong perch support. Flies pollinate shallow flowers that are funnel-like or flowers with floral features that trap the flies. Moths pollinate tubular flowers without a lip.
The quantity of nectar and ease of access differs between flowers. Deeply hidden nectar helps ensure that it is not stolen by non-pollinating insects and increases pollen contact with the pollinator. Pollinators that have other food sources such as flies and beetles visit flowers with little or no nectar. The amount of pollen that contacts and sticks to different pollinators varies as does the amount of pollen produced by a flower. Some flowers have two types of pollen, one that is a food source and a second for pollination. Nectar guides, which are areas of ultraviolet reflectance that can’t be seen by humans, are generally present on brightly colored flowers to help direct the pollinator towards the center of the flower. Flowers that attract with scent are usually dull in color, relying on scent which can attract pollinators up to 1 km away. These plants are usually white to dull green or dark purple to dull red to brown. Night blooming plants are pollinated by nocturnal pollinators such as moths and bats and are commonly strongly scented. Bees are generally more attracted to purple, blue and yellow flowers while birds and butterflies are attracted to red, orange, and yellow. Dull white, green, purple, and brown flowers are generally pollinated by flies, beetles, moths, and bats.
The flowering of plants at different times of the year is also an evolutionary process to be able to attract different pollinators, or to attract pollinators during a time when there is less competition.
Interesting Pollinator Strategies:
Some plants have adapted unique strategies to ensure pollination. Mimicry and entrapment are two of these interesting strategies.
Mimicry can be used by plants to attract pollinators either by scent or by shape. Some flowers will produce a foul scent that mimics the smell of rotten flesh. Pollinators, especially flies, visit these flowers in order to eat the rotting flesh or to lay their eggs so there will be a food source for their young. The pollinators will unintentionally pollinate the flower while laying its eggs or looking for food. Another type of mimicry combines both scent and shape. For example, some orchids have floral features that resemble female insects and can produce a scent similar to female pheromones. Males will then visit the flower, attempting to mate with the female, and in the process often pollinates the flower or transports pollen.
Entrapment uses a mix of other pollination strategies to attract pollinators. However, after the pollinator visits the flower, the plant traps the pollinator within the flower to ensure pollination. There are different ways the plant accomplishes this: pools of fluid, closing flowers, or movement of one flower part is response to pollinator presence on another. By trapping the pollinator, the plant ensures that either it is pollinated or that the pollinator becomes covered in pollen when looking to escape.
Lesser Thought of Pollinators
Bees and butterflies receive much attention regarding pollination and pollinator health; however, there are also a number of lesser thought of pollinators that are important to pollination. Beetles are one of the oldest pollinators, having pollinated some of the earliest angiosperms over 120 million years ago. Although beetle pollination is more common in tropical areas, there are a number of common temperate ornamental plants that are beetle pollinated. Flowers commonly visited by beetles are generally more primitive and include white to green colored, bowl-shaped flowers open during the day. Some of these plants that grow in New England include Magnolia, Asimina (paw-paw), Sassafras, and Calycanthus (sweetshrub). Many pond lilies are also beetle pollinated. Beetles can be attracted by a variety of scents including spicy, sweet, or fermented. It is common for beetles to eat through petals and other flower parts in order to get to nectar, giving them the nickname of “mess and soil pollinators”. The sheer number of beetle species also makes them an important pollinator worldwide.
Flies are another large group of lesser thought of pollinators, and may have been the first pollinator. Pollination by flies is second to bee pollination in increasing flower diversity throughout evolution. Flies are generally attracted to foul-smelling pale, dull purple to brown flowers. These flowers are often funnels or trap the insects inside to ensure pollination. Some common flowers that use bad smells to attract flies and insects for pollination include Arisaema triphyllum (Jack-in-the-pulpit), Asimina (paw-paw), and Trillium erectum (red trillium). Flies (specifically tropical midges) are the only pollinators of Theobroma cacao (chocolate)! Other less common pollinators include wasps, ants, moths, and birds. Bats are also common pollinators for various cacti species in the southwestern US.
Plant Spotlight: Lotus and Water lily
Many lotus and water lilies use entrapment for pollination. When flowers open on the first day, the stamen are not yet producing pollen. Pollinators, commonly beetles, will visit the flower and become trapped when the flower closes. Lotus and water lily attract the pollinators in different ways. Lotus flowers produce heat which helps the flower scent spread as well as provide a warm environment for the pollinators overnight, which encourages them to move around and feed and in the process pollinate the flower. Water lilies have pools of fluid in the flower on the first day. When pollinators visit the flower, they frequently fall into the fluid where pollen is washed off their body and fertilizes the flower. For both lotus and water lily, on the second day the flower opens, releasing the trapped pollinator (usually). Pollen is also produced on the second day, so that as the pollinator leaves the flower, it becomes covered in pollen which it can transport to the next flower.
Shepherd, M., S.L. Buchmann, M. Vaughan, and S. Hoffman Black. 2003. Pollinator Conservation Handbook. The Xerces Society. Portland, OR.
Committee on the Status of Pollinators in North America. 2007. Status of Pollinators in North America. The National Academies Press. Washington, D.C.
USDA Forest Service. Plant Pollinator Strategies. http://www.fs.fed.us/wildflowers/pollinators/Plant_Strategies/index.shtml
USDA Forest Service. Pollinators. https://www.fs.fed.us/wildflowers/pollinators/animals/