1 Essentials of Pollinators and Pollination
Pollination is something we take for granted, but it’s essential to life on Earth. It is the means by which most flowering plants reproduce. As you’ll recall from biology class, pollination involves the physical movement of pollen from the male part of a plant to the female part of a plant. Without pollination, a plant can’t be fertilized so that seeds can form. Pollination can happen in several ways:
• by animals (including insects)
• by wind
• by water
Most flowering plants are animal pollinated, with insects being the largest group of pollinators. Plants such as grasses, conifers, and oaks depend on wind pollination. Water pollination typically occurs with flowering aquatic plants.
The Importance of Animal Pollinators
Pollinators and most flowering plants have a mutually beneficial relationship. Pollinators visit flowers to drink nectar or eat pollen, depending on the type of pollinator. As they feed, they move grains of pollen from flower to flower on the same plant or from flower to flower on different plants. Pollination is not an intentional act by pollinators but usually a consequence of feeding. In some cases, pollen is the only food source of the pollinator; not all pollinators eat nectar. Depending on the plant species, its flowers may be visited by a number of different pollinator types or it may attract a very narrow group of pollinators.
Native plants, such as butterfly weed (Asclepias tuberosa), attract many different pollinators.
At least 80 percent of all flowering plants on Earth depend on pollinators for reproduction. Pollinators are vitally important to most ecosystems, with some pollinators even functioning as a keystone species—that is, a species upon which other species depend in an ecosystem. If a keystone species becomes extinct or vanishes from an ecosystem in which it has evolved, that ecosystem fundamentally changes, and usually not for the better.
In addition to providing pollination services, pollinators are part of food webs (interdependent food chains) and can be a nutritious protein meal for creatures farther up a food chain. While it may be painful to watch a bird eat a butterfly or bee, every creature must eat. Food webs are the basis of ecosystems, and pollinators are part of them. When you garden, consider that you are really planting for food webs, not only for specific species. Specialist pollinators do need extra help in our gardens though. These specialists have evolved with certain plants they depend upon and vice versa. Include those plants in your garden—for example, the hibiscus bee specializes on hibiscus.
The world’s food supply is heavily dependent on pollinators—mainly bees. While not all food crops require pollination, according to the United Nations, 84 percent of crops grown for human consumption need pollinators to increase their yields and enhance their quality. While various insects pollinate food crops, bees—including but not limited to honey bees—are the most important pollinators for food crops. The United States Department of Agriculture estimates that 80 percent of insect-based crop pollination is performed by bees. Marla Spivak, a renowned entomologist and bee expert, explains if bees don’t have enough to eat, we won’t have enough to eat. Because many of our food crops are grown as giant monocultures and pollinators need a variety and abundance of food to eat, the more plant diversity we can bring into our gardens, the better.
Native squash bees pollinating a squash plant
Threats to Pollinators
Pollinators face a magnitude of threats to their survival that seems disproportionate to their ecological value as creatures that are vital to life on Earth. Many of these challenges to pollinators result from human activity and decisions made without consideration of ecological consequences. Building and development takes its toll on pollinators as their habitat, nesting sites, and floral resources vanish. One of the worst threats is the fragmentation of pollinator habitat, which occurs when buildings, roads, or clear-cutting split up natural areas, dramatically reducing pollinator survival.
The lawn—a pollinator desert
A decrease in biodiversity of plants, an emphasis on lawns, the negligible use of native plants, and the explosion of invasive plant species are all taking their toll on pollinators. Pesticide use in agriculture and in commercial and residential landscapes further stacks the deck against pollinator survival. Already weakened by these myriad challenges, pollinators become more susceptible to new pests and diseases that plague them. Research on pollinators from Penn State shows that the combination of exposure to pesticides and reduced nutrition may make bees more susceptible to the other major suspects in bee decline: parasites and pathogens. By decreasing the human-driven threats to pollinators, we can surely help them rebound.
Removing Threats
When creating your Pollinator Victory Garden, focus on increasing the resources pollinators need and improving their habitats. You can start quite simply by reducing your lawn (a pollinator wasteland) and replacing it with a variety of flowering native plants that you maintain free of pesticides. The accompanying chart details some of the impediments to pollinators (minuses) and the improvements you can make to help them (pluses).
Improvements for Pollinators (Do This) |
|
Large lawn (the green desert) |
Reduce or eliminate the lawn and replace with flowering native plants. |
Landscapes with few flowers |
Plant an abundance of flowering native trees, shrubs, and perennials. |
Time periods without blooms |
Design for a succession of blooming plants throughout the growing season. |
Monoculture plantings |
Include diverse plantings of many different flowering plant species that bloom at different times. |
Mostly nonnative plants |
Emphasize regional native plants that pollinators have evolved with. |
Invasive plants |
Remove invasive plants (even if pollinators like them) and replace with native plants. |
Pesticide use |
Eliminate pesticides; attract beneficial insects to keep insect pests in check. |
Lack of nesting sites |
Provide patches of bare soil, brush piles, dead trees and logs, plants with pithy stems, stone walls with crevices, and man-made bee houses. |
Fragmented habitat |
Join with neighbors to create pollinator corridors that run from one landscape to the next, and the next . . . |
Types of Pollinators
Most animal pollinators are insects. While bees are arguably the best-known animal pollinators, beetles, butterflies, moths, flies, and wasps are other pollinators you will most likely attract and support in a Pollinator Victory Garden. You may also be able to entice nectar-eating birds (nectivores) such as hummingbirds and orioles, depending on your location. Hummingbirds are the largest group of pollinating birds in North America, with an estimated 350 species. Some bird pollinators even specialize in pollinating specific plants, such as the white wing dove, which is an important pollinator of the saguaro cactus in Arizona.
Carpenter bee nectaring on mountain mint (Pycnanthemum species)
Hummingbird moth nectaring on wild bergamot (Monarda fistulosa)
In tropical and desert areas, bats may be important pollinators to support in your landscape. More than 300 types of fruit, including mangoes, bananas, and guavas, depend on bat pollination. Even the saguaro cactus and agave plants are pollinated by bats.
Less common pollinators include certain species of mosquitoes, ants, and even some mammals and lizards, depending on your country and region. Which pollinators are important? All of them. If they have evolved in your locale, they are part of your ecosystem and they have ecological value.
Bees and Beyond
It can be difficult to convince people to garden for pollinators besides bees. Actually, the fact that bees are even being considered in gardens means that we have made much progress in the last decade or so. Not long ago, butterflies, and perhaps hummingbirds, were the only pollinators that were truly welcome in a majority of landscapes. Bees were often viewed as the wards of beekeepers or inconvenient wild creatures to be avoided and feared for their sting, certainly not creatures to be encouraged in managed landscapes. Much of this bee fear was based on misinformation; most bees are quite docile and will usually sting only if they’re threatened around their nesting area.
Now that home gardeners are starting to embrace bees, we need to expand our ecological horizons to garden for all types of pollinators. If we don’t, we will lose even more species. Biodiversity is the key to healthy ecosystems and a critical tool in fighting climate change. A healthy garden ecosystem is one that is full of many living creatures, including pollinators. Plant diversity = animal diversity. In your garden, include a diversity of native plants, such as trees, shrubs, vines, and perennials, that have a multitude of shapes and sizes to attract a greater diversity of pollinators.
Pollinator Variations and Garden Considerations
Pollinators vary in many ways that impact what a pollinator garden must provide. The tongue length of a pollinator is a major factor in determining the flowers it can access. A long-tongued species of bee, butterfly, or hummingbird will be able to reach nectar in a long, tubular flower, while a short-tongued pollinator will need flatter, more open, accessible flowers. By planting flowering plants with various flower forms, you will attract a greater diversity of pollinators.
Open, accessible flower: pollinating fly on a native aster (Symphyotrichum species)
Generalists and Specialists
Generalist pollinators like honey bees and bumble bees can sip nectar from a wide array of flowering plants. And while most pollinators are generalists (approximately 75 percent of bee species), some have evolved as specialists and may depend on a particular plant species or a small group of plants. The specialist spring beauty bee (Andrena erigeniae) only collects pollen from two species of plants: Virginia spring beauty (Claytonia virginica) and Carolina spring beauty (Claytonia caroliniana). Without those two plants, the spring beauty bee may become extinct. Without the spring beauty bee, those plants may not be able to reproduce. Make sure to include plants for both generalist and specialist pollinators in your garden.
Example of a tubular flower: beardtongue (Penstemon species)
Butterfly caterpillars also have specialist relationships with plants, called larval host plants, discussed in chapter 3. A list of common host plants and the caterpillars they feed can be found on the author’s website, www.ecobeneficial.com/PVG. Plants for specialist bees and beetles can be trickier to determine; not much has been written on this subject. Contact your local native nurseries and native plant society for help and refer to the recommended sources in this book.
Pollinator Sociability
When most people think of pollinators, they think of European honey bees, which are communal and live in large colonies, typically man-made beehives. Honey bees are social creatures and very different from the vast majority of pollinators; of the world’s 20,000 bee species, it is estimated that 90 percent are solitary and nest by themselves or in small groups. This solitary behavior has implications for your Pollinator Victory Garden because you need to create or facilitate areas where many individual bees can nest. Read more about providing bee habitat in chapter 2.
Bumble bee queen nectaring on an early-blooming violet
Seasonal Activity
Seasonal activity varies according to the pollinator species. Early-emerging bumble bee queens are active early in the growing season before many other insects are active. Bumble bees can tolerate cooler weather and somewhat rainy conditions in colder climates. When they emerge from their long winter’s nap, they are hungry and looking for flowers. To help bumble bees and other early bees survive, include early-blooming plants in your garden, many of which are flowering trees and shrubs. Depending on where you garden, these early bloomers may include native willows, red maples, spicebush, serviceberries, redbud, witch hazel, trout lily, Dutchman’s breeches, squirrel corn, wild geranium, spring beauty, and violets.
Other pollinator species appear later in the growing season; for instance, leafcutter bees emerge in warmer summer weather. Make sure they have enough to eat in your garden. In the fall, mated queen bumble bees need extra floral food before overwintering. Provide an uninterrupted succession of blooming plants throughout the entire growing season to provision a profusion of pollinator species.
Pollinator Life Span
Pollinator life spans can be quite different from one species to another. Most adult butterflies live for only a week or two, but an adult monarch or a mourning cloak butterfly may survive for nine months or more. Bee species have very different life spans, sometimes with significant differences within the same species. A honey bee worker might live just five to seven weeks, while a honey bee queen may live two to five years. Solitary wild bees may be active as adults for only three to four weeks, although their developmental stage is much longer at about eleven months. Simplify the provisioning process by planting for ongoing bloom, with a diversity of blooms at all times in the growing season, to feed pollinators with different life spans and seasons of activity.
Pollinator Foraging and Nutrition
When pollinators forage (look for food from flowers), nectar and pollen are primarily what they seek. Bees are the main flower-visiting insects that drink nectar and also proactively collect and transport pollen. Pollinating flies and beetles may eat nectar, pollen, or flower parts, depending on the species. Butterflies and moths, with some exceptions, forage only for nectar, and they transfer pollen accidentally. Hummingbirds and bats visit flowers for nectar but may unintentionally digest grains of pollen.
Mining bee on golden Alexanders (Zizia species)
A limited number of bee species collect other plant substances to line their nest cavities. Small resin bees gather plant resin that serves as a structural and waterproofing element with antimicrobial properties. Some other bee species forage for floral oils produced by plants with special floral glands. The bees mix these oils with pollen to line the brood cells in their nests, which also adds a protective, antimicrobial coating.
For the most part, nectar, pollen, and water are the essentials of a pollinator diet (with the exception of the larval stage of butterflies and moths).
Flower Diversity
Like humans, the majority of pollinators benefit from a varied diet. In most cases, the pollen and nectar they eat should come from multiple sources. Not all nectar and pollen have exactly the same composition or nutritional value. Think about providing a pollinator buffet by offering a wide selection of different flowering plants to your numerous pollinator guests. Don’t let them go home hungry!
Floral Constancy and Flower Abundance
Flower diversity is important, but so is flower abundance. Some pollinators practice floral constancy, meaning they visit one plant species on a foraging trip. To forage successfully, these pollinators need a large enough quantity of a given flowering species. Research from the U.C. Berkeley Urban Bee Lab suggests that a 3 foot (0.28 m) square patch of the same plant species makes a good target for pollinators. Planting well for pollinators means striking a balance between plant diversity and plant sufficiency.
A pollinator buffet
Foraging Range
Not all pollinators can travel the same distance to find food. Tiny solitary bees may live within a few hundred feet of their nest; larger, quasi-social bees like bumble bees may venture a mile away, while honey bees can travel even farther. Butterflies and birds may travel even greater distances to get a meal. But travel has a significant cost for pollinators—they burn up calories and expend energy. Help pollinators save their energy by planting your garden intensively and convincing neighbors to create pollinator corridors that connect to your landscape and beyond.
Nectar
Most, but not all, flowering plants produce nectar. Some plants, including the majority of roses, produce little or no nectar but are important sources of pollen. Nectar is a sweet substance composed mostly of water and sugars, primarily glucose, fructose, and sucrose in various proportions. The amount of each type of sugar in nectar varies according to the plant and can vary within a species. Some pollinators seem to show a preference for a particular sugar type. Reportedly, hummingbirds and longer-tongued bees prefer sucrose, possibly due to its viscosity. Flies, beetles, and butterflies are thought to prefer lower levels of sucrose, while bats appear to be more attracted to nectar with glucose and fructose.
Nectar also contains amino acids and other components in small quantities. This sweet treat provides pollinators with an important source of energy that enables them to fly, breed, and stay warm. But the concentration of sugar in nectar can range dramatically, from 10 to 75 percent, which can affect a pollinator’s ability to function. This is yet another reason to incorporate plant diversity in your garden so a variety of nectar resources are available.
Flowers secrete fresh nectar at varying rates, and this secretion and availability of nectar can occur over the course of a few hours or persist for a number of days. Some plants can produce nectar only during the day, while others can also produce nectar at night, making them especially valuable to nocturnal pollinators, like some nectaring moth species. Common milkweed (Asclepias syriaca) and evening primrose (Oenothera species) are two examples of plants that can deliver nectar at night, but choose plants appropriate to your region.
Temperature and weather conditions also affect nectar production by plants; very high or very low temperatures result in a decrease in nectar production. Excessive rain coupled with many cloudy days will cause nectar to become diluted. Drought stress significantly decreases the amount of nectar plants produce. Don’t let plants wilt from lack of water, because pollinators will suffer the consequences. The best nectar delivery comes when plants have sufficient water as well as sufficient sun.
Pollen
Pollen is a source of protein for pollinators and provides other nutrients including fats, carbohydrates, minerals, and vitamins. Certain pollinators eat pollen, and some use it to feed their young. Pollen is essential for bee brood development (the development of young bees). Almost all bee species eat pollen, as well as some beetle and fly species, a few butterfly species, and some bird and mammal pollinators. Pollinators that proactively collect pollen for themselves or for their offspring expend a significant amount of energy to do so; be sure to have plenty of nectar plants in your garden to fuel them.
Spicebush swallowtail on common milkweed (Asclepias syriaca), an excellent nectar source.
Not all plants provide the same quantity or quality of components in their pollen, nor is all pollen equally nutritious. Protein levels in pollen can range from 2.5 to 61 percent, yet a honey bee needs a diet with an average of 20 percent protein. If pollinators have access only to low-quality pollen, like that found in wind-pollinated plants, they will suffer; wind-pollinated plants will not get them to their minimum threshold.
The majority of pollen feeders seek pollen from a variety of sources, but there are also pollen specialists that have evolved with specific plants. Although most pollinators are pollen generalists, they may actually show a preference for particular plants.
Bee covered in pollen grains
Bees need a clean source of water to drink.
Water
In addition to nectar and pollen, one resource rarely provided in a garden is water. Pollinators may use nectar as their primary source of hydration, but landscapes with a pond, stream, or river can be good water sources for pollinators, as long as there are shallow banks and the water is reasonably still. Even man-made water features, such as a tiny pond, can offer pollinators a drink, but design it to be pollinator friendly by keeping the same requirements in mind.
In the heat of summer, a honey bee colony can drink one quart (946 ml) of water or more each day. Gardeners can offer some simple water sources, which may be particularly important during droughts or very hot weather when nectar flow is low. Birdbaths are best kept for bird use, but shallow ceramic dishes, such as the saucers for plant pots, can be filled with small uncolored pebbles and topped off with water. This gives insect pollinators a safe place to drink without risk of drowning. Clever beekeepers have learned that automatic pet waterers can be used for bees too; just put small pebbles in the water dish to enable bees to drink safely from a dry perch.
How Plants Attract Pollinators
Flowering plants have developed a number of traits to attract pollinators. Overall plant features such as flowering time, density of flowers, number of flowers, flower height, and spatial pattern of flowers can influence which pollinators will use a given plant.
An individual flower’s color, shape, structure, size, fragrance, and availability of nectar and/or pollen also help determine what type of pollinator will visit. The combined groups of traits that predict the type of pollinator that will be attracted are known as pollination syndromes. Pollination syndromes are helpful guidelines for planting, but exceptions often occur. For more information, see here.
Nectar guides can be one factor in a pollination syndrome. They function as runways—patterns on flowers that direct pollinators, namely bees and butterflies, to the nectar. But not all pollinators are attracted by nectar guides; beetles, moths, flies, birds, and bats don’t need them. Nectar guides are colored differently from the rest of the flower, visible to pollinators but often invisible to humans.
Nectar guides are visual cues to some pollinator species (spring beauty, Claytonia species).
The complication for gardeners when choosing plants is that there is no “one size fits all” for pollinators. Bees favor violet, purple, blue, yellow, and white flowers, but hummingbirds favor red ones. Fragrance can play a tremendous role (or not), depending on the type of pollinator. For example, the intense perfume of a flower is lost on a bird because birds cannot smell, but a nocturnal moth may be enticed by a sweet nighttime bloom. Detailed information about the specific flower traits that attract pollinator types is given in chapter 4.