Why are bees important? Do flies pollinate? Explore these questions and more as well as a list of pollinator insects that keep seed-bearing plants flourishing.
The fact that more than two-thirds of the world’s crop species are pollinator-dependent is important to vegetable gardeners who grow species cultivated for their edible fruit, such as squash and melons. But the role performed by pollinators is even more significant for seed savers, because without successful pollination and subsequent fertilization, plants will not set seeds. While the work of many pollinators goes on with little or no intervention by a gardener, a knowledge of some of the key players can help a gardener appreciate the work pollinators perform and may even provide an understanding of what levels of cross-pollination to expect.
Butterflies, flies, wasps, some beetles, bats, and hummingbirds visit flowers in search of nectar and brush against the anthers of a flower, getting pollen grains on their bodies in the process. In this manner, they carry pollen from flower to flower and may transfer it between flowers on the same plant, to another plant of the same species, or even onto flowers of an unrelated plant. But among pollinators, bees are considered the most important group, primarily because they are the only group of insects that actively collect pollen and, in the process, transfer large amounts of pollen from flower to flower. Bees also exhibit a behavior called “floral constancy,” which means that they visit flowers of one species repeatedly over a period of time. This is important because it means that most of the pollen they collect will be transferred to flowers of the same species, allowing fertilization to proceed provided other conditions are suitable. In addition, bees are also one of the few groups of insects that actively construct a home for their young. This place-based behavior limits their foraging range; bees tend to visit the same groups of plants over and over again, making them ideal garden pollinators.
Why are Bees Important?
North America is home to roughly 4,000 species of bees, the vast majority of which live solitary lives, whereby each female constructs and provisions a nest, lays a small number of eggs, and dies before her offspring emerge. Solitary bees often go unnoticed because of their small size and drab appearance, but they are the bees most commonly found in vegetable gardens. They contribute enormously to crop pollination – in many cases providing all of the pollination a farmer or gardener needs. At the other end of the bee diversity spectrum are the large bumble bees, which are social, forming an annual colony of a single queen and dozens of her worker-daughters. Whether small or large, solitary or social, many species of bees are effective pollinators. Different types of bees prefer different crops.
European honey bees (Apis mellifera) were introduced to North America in the late 1600s. Colonies live for multiple years and have a caste system consisting of a single egg-laying queen, a number of non-reproductive worker-daughters, and a lesser number of male drones who exist only to mate with newly hatched virgin queens. Over the past few decades, honey bee populations have dwindled due to disease, parasites, pesticides, and other factors, but there are still a significant number of honey bees, in part due to the resurgence of backyard beekeeping. Given that honey bees can forage several miles from their hive, they are effective pollinators, although their foraging range increases the likelihood of unwanted cross-pollination between varieties within a species.
Because the honey bee is adapted to a Mediterranean climate, it tends not to forage during cool and wet weather. In general, honey bees are very good pollinators of sunflowers, cucurbits, buckwheat, and many brassicas. They tend to be less frequent visitors of umbelliferous crops, such as carrots and fennel. However, honey bees are not needed to produce a good crop. Healthy and diverse organic gardens should already support large numbers of native pollinators.
Bumble bees (Bombus spp.), like honey bees, live in a social unit led by a single egg-laying queen, but unlike honey bees, bumble bee colonies live for only a single season – the old queen, workers, and drones all die at the end of the growing season. Only new queens, born late in the season, survive to the following year – usually by digging down into loose soil to hibernate. Unless their nests are disturbed, most bumble bee species are extremely gentle and unlikely to sting. Bumble bees usually nest in underground cavities, such as old rodent burrows, or in empty spaces in compost piles as well as brush piles and hay bales. They are very efficient pollinators, working in both cooler and wetter weather than other bee species, and excel at pollinating sunflowers, cucurbits, some brassicas, and solanaceous crops, such as tomatoes and eggplants. The foraging range of bumble bees is thought to be a mile or less. More than 40 species of bumble bees are native to North America.
Sweat bees (Agapostemon spp., Halictus spp., Lasioglossum spp.) get their common name from their occasional attraction to human perspiration. Ranging in color from black to brown, gray, and metallic green, most species are ground nesting, relatively small (from one-quarter to more than one-half inch long), and extremely gentle, stinging only when squeezed or trapped in clothing. Although often overlooked because of their size and solitary nature, sweat bees are probably the most common garden pollinators. Sweat bees are also common visitors to sunflowers and cucurbits and are probably among the most important pollinators of brassicas.
Squash bees (Peponapis spp., Xenoglossa spp.) are a small group of specialist bees that nest in underground tunnels at the base of squash plants and visit the flowers early in the morning, often before sunrise. They are often among the first bees to appear in new gardens – even at inner-city sites – when squash is planted. They are fairly common in most of North America with the exception of extreme northern regions and the maritime Northwest.
Squash bees primarily visit crops in the genus Cucurbita (squash and pumpkins) and usually do not visit Citrullus or Cucumis crops (watermelons, cucumbers, and melons), although these three genera are in the same family. Because squash bees are active early in the day, many farmers and gardeners don’t realize that the bees have already fully pollinated their crops before honey bees have even left the hive.
Leafcutter bees (Megachile spp.) use their scissor-like mandibles to snip sections out of leaves (often in perfectly circular pieces). They bring the cut sections back to their nests, where they fold the pieces into cylindrical, origami-like packages to surround their eggs. Like most native bees, leafcutter bees are solitary, with each female constructing and provisioning her own nest. While not the most common garden visitors, leafcutter bees are occasional pollinators of onions, carrots, beans, and sunflowers.
Long-horned bees (Melissodes spp.) get their common name from their most obvious characteristic, their long antennae. These solitary ground-nesting species are also recognizable by the thick tufts of pollen-collecting hairs on their hind legs, which resemble chaps. One species found in the eastern United States, the melon bee (Melissodes bimaculata), deserves specific mention as a conspicuous and common garden visitor, often gathering pollen from sunflowers as well as melons. Melon bee nests are often found in the ground in gardens or sparse turf areas nearby.
What’s with Wasps
The closest relatives of bees, wasps have life cycles that closely mirror those of bees. The primary difference between the two groups is that bees feed on pollen during their larval stage, but wasp larvae feed on meat (usually other insects). Because of this carnivorous life stage, wasps contribute enormously to the control of garden pests, including aphids, stink bugs, caterpillars, and many other types of insects. Wasps also may play a role in pollination.
Like bees, wasp species are either solitary or social and include exotic and often aggressive social species, such as the German yellowjacket and the European paper wasp. The vast majority of North America’s thousands of wasp species are gentle, solitary hunters that avoid people and do not sting. Wasps come in a wider range of sizes and shapes than bees – from only a few millimeters in length to larger than a bumble bee – but most are relatively hairless. Like bees, they have two pairs of wings and sometimes have striped abdomens. A few of the wasp families common to vegetable gardens are the hornets, yellowjackets, and paper wasps (Vespidae); the digger wasps (Sphecidae); and parasitoid wasps (Chalcididae, Braconidae, and Ichneumonidae). Adult wasps visit flowers for nectar; because they have short tongues, they prefer shallow flowers with readily accessible nectar droplets. In the process of feeding on nectar, they help move pollen between flowers. Wasps are occasional visitors to various brassicas and very common visitors to alliums and umbelliferous crops.
A Focus on Flies
With nearly 120,000 identified species, flies are one of the largest insect groups on earth. Of these, perhaps half have been documented to visit flowers, primarily to sip nectar. They are less significant pollinators than bees, but flies do move some pollen between flowers of certain vegetable crops, and three families of flies can be important pollinators. Flies tend to prefer shallow, open flowers with readily accessible nectar droplets. Common vegetable crops pollinated by flies include various brassicas and alliums. Flies are also often the primary pollinators of crops, such as carrots, cilantro, and fennel.
Hover flies, or flower flies (Syrphidae), are valuable to gardeners because they prey upon other small insects (especially aphids) during their larval stage. As adults, these flies spread occasional pollen grains between flowers. The striped and sometimes hairy bodies of many flower flies can make them hard to distinguish from bees at first glance, but, like all flies, a flower fly has only one pair of wings, short stubby antennae, and very large round eyes that almost press against each other on the top of its head. By contrast, bees have two pairs of wings that are usually hooked together in the middle, longer antennae than those of a fly, and smaller eyes that are more oval in shape.
Tachinid flies (Tachinidae) control many garden pests and contribute to pollination. Tachinids usually lay their eggs on the bodies of other insects. When the fly larvae hatch, they burrow inside the host insect, eating it from the inside out and eventually killing it. They can be important for controlling garden pests, such as tomato hornworm and cabbage loopers. Tachinids are usually recognized by their rounded shape and the bristly hairs on their abdomens.
Blue Bottle flies (Calliphoridae) have an unpleasant reputation for eating carrion, detritus, and feces during their larval (maggot) stage. While this doesn’t endear them to many people, they plan an important role in cycling nutrients and keeping the world clean. As adults they visit flowers for nectar and are easy to recognize by their metallic blue abdomens and somewhat hairy bodies. Blue bottle flies are commercially available and are often used as introduced pollinators in isolation cages for the production of carrot seed.
For more on seed saving, see our Seed Saving Guide.
Reprinted with permission from The Seed Garden, by Micaela Colley & Jared Zystro, edited by Lee Buttala & Shanyn Siegel and published by Seed Savers Exchange, 2015. Buy this book from our store: The Seed Garden.