Time for a new approach to crop pollination
Deborah K. Rich, Special to The Chronicle
Saturday, May 21, 2005
The parasitic mite that devastated honey bee colonies across the United States this spring
served notice that we are overly reliant upon the honey bee for crop pollination. Beekeepers
report the mite infested 40 to 60 percent of managed beehives. Unless we find alternate
pollinators to cart around, or another means to pollinate our fields, we risk periodic crop
failures due to lack of pollination. And not just of almonds (whose February bloom faced
severe honey bee shortages), but of any of the more than 100 insect-pollinated crops grown in
the United States as well.
The larger our crop fields, and the more intensively we farm, the more we need a bee that we
can propagate off-site and truck to our fields on demand. Conventional “clear cut” agriculture
relegates natural landscapes that support native bee populations to the far edges of productive
regions, and any bees that might make the long trip out to the fields are subject to pesticide
The honey bee is an obvious choice for managed pollination. We have a long association with
honey bees and their honey, and we have learned, over hundreds of years of observation,
their nesting preferences and how to transport their hives. Most importantly for large-scale
agriculture, honey bees nest together and raise their young in colonies whose populations
number in the tens of thousands. Other species nest alone or in much smaller colonies, which
complicates efforts to house and transport them.
But the social tendencies of the honey bee, combined with the size and national scope of
honey bee operations (beekeepers have consolidated, just as other agricultural suppliers
have, and often ship beehives by the semi-truck load), nearly guarantees the rapid
transmission of diseases and parasites when they appear.
Recognizing the very real threat of crop failure that our dependence upon a single species of
bee poses, researchers are coaching pollinator understudies. The blue orchard bee (also
known as the orchard mason bee) is proving a cooperative pollinator of some early blooming
orchard crops, and the bumble bee is helping to pollinate hot-house tomatoes.
Still, it may be time — while there still is time — for another approach entirely. The United
States is home to 4,000 bee species, of which 1, 500 are found in California, to say nothing of
the many moth, fly, wasp and butterfly species that also assist with pollination. (Of the insect
pollinators, the bee is the most important because only the bee actively gathers both pollen
and nectar to transport to her nests; other insects gather pollen only incidentally while they
seek out nectar, similar to the way our socks gather burrs while we pick flowers.) If we have
pushed native bees out of our agricultural regions, not to mention our inner cities, it could be
that we can pull them back in.
Thanks to Claire Kremen’s recent work in Yolo and Solano counties, we know that, when
present, native bee populations are more than equal to the task of fully pollinating many crops.
In a study that began in 1999, Kremen, an assistant professor of ecology and evolutionary
biology at Princeton University, discovered that on organic farms within two kilometers of
riparian forests or hillsides of chaparral and oak, native bees were so numerous as to even
pollinate fields of watermelon. Watermelon provides a good measure of pollination services
because it is entirely dependent upon insects for pollination and requires several insect visits
to transfer enough pollen to set a marketable fruit.
Perhaps, then, the better approach to reducing our reliance on the honey bee for crop and
ornamental plant pollination is to protect and, where necessary, recreate ecosystems that
support a wide range of native bee species; to focus more on being general managers of
diverse farm, park and garden landscapes, and less on micro-managing specific species.
Fortunately, we have reason to believe that even small patches of habitat within landscapes
can greatly boost bee diversity and abundance. “Bees,” wrote Kremen in the December 2002
Proceedings of the National Academy of Sciences, “can seek out patchy resources and persist
within small fragments of habitat… .” And while it would be presumptuous to think that we’ve
deciphered the subtle nuances in habitat preferences among different bee species, we do
understand their basic needs.
All bees require pesticide-free nesting and overwintering sites (for most bee species, they are
one and the same), and forage. The vast majority of our native bees are solitary nesters,
meaning that each female builds and provides for her own nest. More than two-thirds of the
species nest in the ground, while most of the others nest in wood. A few seek out small spaces
between rocks or beneath clumps of grass. The bumble bee, a social-nesting native bee,
needs a modest cavity to nest in and often opts for abandoned rodent burrows.
Ground-nesting bees prefer patches of dry, sloped, bare or partially vegetated dirt. The Xerces
Society, dedicated to the conservation of invertebrates, suggests that, “Some of the best
places around farms for creating habitat for native pollinators are the worst places for growing
crops. For example, areas with the poorest soils may provide some of the best sites for
ground-nesting bees, because these animals often prefer to nest in well- drained, inorganic
sand and silt.”
Where farmers can leave field edges, fence rows, set-aside acres, road and canal berms, and
utility easements uncultivated and unsprayed, they will greatly increase their chances of
hosting native bees. “A lot of times bees will nest right along farm roads. So, if you know that
and start to recognize that resource, then maybe you’ll pull up your plow before coming all the
way up to the road and keep that ground stable,” says Mace Vaughan, conservation director
for the Xerces Society.
If, along with the bees, invasive weeds move into uncultivated fringe areas and threaten to
leap-frog into the crop, planting hedgerows of native trees, shrubs and grasses will help keep
the weeds at bay while allowing strips of land to lie untended (after the hedgerows have been
established). Mowing is another option for weed control. Mowing in the late fall or winter is less
likely to disturb nesting bees. When mowing must be done earlier, waiting to mow until after
the plants flower, but before they set seed, will allow bees time to forage.
When dense root mats or thatch blanket areas designated for bee habitat, farmers must clear
away some of the vegetation; the goal is to give the bees access to the soil while leaving some
grass and shrubs in place to provide forage and to help prevent soil erosion. Bee-conscious
gardeners will want to uncover soil at the back of sunny flowerbeds, pushing aside woodchips
and compost, and they have justification to label persistent bare patches in the lawn “bee
Ideally, habitat areas will encompass slopes of varying degrees. The nesting preferences of
bee species range from vertical banks to nearly flat ground.
Wood-nesting bees claim beetle tunnels in dead branches, or burrow into shrubs with soft-
centered twigs, for example: elderberry, sumac and blackberry. Providing habitat for wood-
nesters can be as simple as leaving stand dead or dying trees and planting shrubs with pithy
Alternatively, farmers and gardeners can fabricate wood nests by drilling holes, ranging in size
from 3/32 to 3/8 inch in diameter, in wooden blocks, chunks of firewood, stumps or even fallen
logs. (Holes 1/4 inch or less in diameter should be drilled 3 to 5 inches deep, and holes larger
than 1/4-inch diameter should be 5 to 6 inches deep). Mounting or hanging bundles of paper
straws, bamboo and reed stems, or pithy twigs is another option for providing habitat for wood-
nesting bees. Constructed nests should be cleaned out or replaced every other year to avoid
parasites, fungi and diseases.
Flowering plants that offer bees nectar and pollen will likely already be present on the farm
and in the garden, but several adjustments to the timing and diversity of blooms can help
ensure that bees are present in large populations when the cash crops (or favored landscape
plants) need them. In general, the more food that is available, and the closer it is to nesting
sites, the larger the bee populations. When female bees can find large clusters of flowering
plants near to their nests, they can spend relatively more time provisioning the nests and
laying eggs and less time commuting.
In temperate zones, having plants blooming nearly year-round is key to promoting bee
abundance and diversity. The active periods of bees’ lives vary, and one bee species or
another will be foraging for nectar and pollen from at least February through November.
Orchards often have difficulty sustaining bee populations year-round because, after their flush
of blooms during the spring, they typically offer few foraging opportunities. Planting the
avenues between orchard trees with a cover crop that blooms before or after the orchard -
clover, thyme or short yarrow, for example — will not only help ensure pollinators for the
orchard, but for neighboring crops as well.
The bumble bee, the sweat bee and many other native bees important for crop pollination
produce several generations. The presence of late winter and early spring blooms will facilitate
the foraging and egg-laying of the first generation and help to ensure large pollinator
populations on-site in the summer and fall.
Plant diversity is important too. Different bees seek out different bloom shapes and prefer one
plant’s pollen to another’s. Many non-native plants can be excellent sources of nectar and
pollen, but bees and plants have co-evolved together to achieve the most efficient pollen
transfer. Planting forage plants native to the region (see accompanying list for suggestions for
California) will likely best service native bee populations and accommodate their particular
preferences and body types.
Gardeners can consider whether they can eschew grassy lawns in favor of swaths of clover,
violets and chamomile. Where grass is a must-have, raising the mowing deck to allow any
flowering plants that have moved in to bloom will help eke a little forage out of the grassy
swath. Open space that doesn’t receive much foot traffic can be planted with low-growing
herbs like oregano, thyme and marjoram: all three of which swarm with small bees when in
If we are to sustain native bee populations on-site, we must reduce our use of chemical
pesticides. Pesticides don’t have to be sprayed directly on bees to kill them; because
pesticides dissolve in nectar and settle on pollen, they also poison bees that arrive later to
forage. Even when exposure isn’t lethal, pesticides can affect the bees’ ability to fly and
navigate, or cause spasmodic movements and paralysis, all of which limit foraging and nest
The Xerces Society recommends creating buffer zones around crops, leaving, for example,
the outermost 15 or 20 feet of a field unsprayed. Trying to spray when bees are less active, for
example at night or in the winter months, will help to reduce direct kills.
And, since we apply pesticides every day in our parks, schools, office buildings and homes, it
is not only farmers who will have to reconsider pesticide use.
For more information
The Xerces Society publishes both a short manual (Farming for Bees) and a handbook
(Pollinator Conservation). Both offer guidelines for recognizing and providing native bee
habitat. Contact them at (503) 232-6639, or www.xerces.org.
For an overview of pollinators and conservation issues, see: “The Forgotten Pollinators” by
Stephen L. Buchmann and Gary Paul Nabhan. (Island Press, Washington D.C., 1996)
Knox Cellars supplies artificial nests for native bees and other native bee-related equipment.
Contact them at (425) 898-8802, or www.knoxcellars.com.
Pinnacles National Monument near Monterey: a center of bee diversity
Towering rock spires draw the eye at the Pinnacles National Monument, 14 miles east of
Soledad in Monterey County, but take note of the bees on your next visit as well. In the late
1990s, Olivia Messinger and Terry Griswold of the USDA Bee Biology and Systematics
Laboratory conducted a survey and found that the 25-square-mile preserve hosts almost 400
bee species, perhaps the highest-known bee diversity per unit area of any place on earth.
For comparison, consider that Clark County, Nev., another center of bee diversity, with 598
species, encompasses approximately 7,910 square miles. The Mojave National Preserve, with
305 bee species, is spread over 2,500 square miles.
“Nearly 400 bee species are now known to reside in the monument, representing 52 genera
and all six North American bee families. The bees at Pinnacles range in size from minuscule
(mosquito-sized) to gargantuan (the size of one’s thumb), and come in colors as varied as the
plants they visit –
from coppery greens to steely blues, or glossy black,” Messinger and Griswold wrote in a
report published in Fremontia in 2002.
At least two of the species (Andrena annectans and Ceratina hurdi) appear to be endemic to
the monument and surrounding areas.
Factors contributing to the diversity of bees at the Pinnacles include the relatively open
landscape, dry soils typical of temperate zones, and the presence of nearly 600 species of
For visitor information call (831) 389-4485, or visit www.usparks.com/pinnacles/park_info.html.