Abuzz about bees

San Francisco Chronicle California’s native bees can lessen farmers’ dependence on the European honey bee – Deborah K. Rich, Special to The Chronicle Saturday, May 21, 2005


When Claire Kremen, assistant professor of ecology and evolutionary biology at Princeton University, discovered that California’s native bees could, given suitable nearby habitat, pollinate Central Valley crops as well or better than trucked-in colonies of non-native European honey bees, she gave farmers one more reason to cultivate diversity on the farm: ensuring crop pollination.

“We certainly knew that certain crops require pollinators and that we often use the honey bee as the main pollinator, but we didn’t know much about the contributions that were naturally flowing from the ecosystem: about the wild unmanaged bee populations and what they were contributing,” Kremen said.

Kremen applies her training as a biologist and zoologist to her larger interest in developing models to understand and place a value upon the benefits, or “ecosystem services,” that healthy landscapes and ecological systems provide.

“When we don’t account for the services that nature is providing, we go ahead and degrade them,” Kremen said. Because the services “were never in the marketplace in the first place, they were never accounted for, and nobody pays attention until, whoops, wait a minute, we have no more water. Or we have no more clean water. All of these things are in fact degradations of the services that we were taking for granted.”

Working in Yolo and Solano counties, Kremen and her team first observed which species of bees were visiting insect-pollinated crops that grow in the area. She discovered that at least 50 different native bee species contributed to crop pollination.

To evaluate the contribution of the native pollinators, Kremen focused upon a single crop, watermelon. Watermelon not only attracts 30 of the 50 species of native bees that Kremen’s team had observed, it is entirely dependent upon insect pollination for reproduction and requires several insect visits to ensure adequate pollination. Watermelon has separate male and female flowers and large sticky pollen grains that cannot travel by wind. Moreover, each female watermelon bloom needs to receive about 500-1,000 grains of pollen to produce a marketable fruit. Kremen felt that if a watermelon could receive adequate pollination from native bees, so too could many other bee-pollinated crops.

Kremen mapped the location of 38 farms, calculating their distance from natural habitats and identifying whether the farms were managed conventionally or organically. She discovered a strong correlation between the proximity of the farms to natural areas and the degree to which watermelons on those farms could be pollinated by native bees. She also found a weaker correlation between how a farm was managed — organically or not — and the rate of pollination by local bees.

“We found that the wild bees can be sufficient pollinators on their own, without the honey bee, for watermelon. That is only the case, though, on farms where the bee community is diverse and abundant, and those farms are the ones that are near natural habitat and are organic farms,” Kremen said.

Without bees, California agriculture would lose many of its highest value crops. Of California’s 20 most economically important crops (which together account for 74 percent of gross farm income), 10 are either completely or partially dependent upon insects, especially the bee, for pollination. Almonds, strawberries, tomatoes, hay, cotton, broccoli, oranges, carrots, avocados and lemons all benefit when bees, intent upon garnering nectar and pollen, transfer pollen from the male to the female portions of the same or different plants.

The question is less “was a plant pollinated,” than “how well was it pollinated.” The fruits of many plants have the potential to bear more than one seed; a fully pollinated apple, for example, will have 10 seeds. Plants will shunt more resources toward those fruits that have received enough pollen to fertilize the fruit’s maximum number of seeds, and fully pollinated fruits will be relatively larger and better formed. And when bees carry pollen between different plants of the same species, stirring the plants’ gene pool, seedlings grow more vigorously in a wider range of crop conditions.

Since at least the early 1900s, farmers have employed the non-native European honey bee (Apis mellifera) for crop pollination. The European honey bee arrived in North America in the 1600s when some of the earliest settlers stowed beehives in the holds of westbound ships, bringing their honey producers with them to the New World. (Of North America’s 4,000 species of native bees, only the bumble bee produces honey, and then only in very small quantities.) When the development around 1900 of motorized farm equipment enabled farmers to manage larger acreages, they began to cart hives of European honey bees out into the middle of their fields to ensure adequate pollination.

By the middle of the 20th century, the ready availability of farm chemicals meant that farmers could grow hundreds of acres of a single crop and stave off, at least temporarily, the declines in soil fertility and pest control that occur without crop rotation and diversity. But modern technology didn’t change the evolved dependence of many plants upon insect pollinators. “In the ’50s, the use of pesticides was going way up,” says Mace Vaughan, conservation director of the Xerces Society for Invertebrate Conservation. “We were developing all sorts of chemical fertilizers that were allowing us, as a country, to farm even larger areas even more intensively. At that point, honey bees became that much more important, yet again.”

As crops became ever larger, farmers’ dependence upon the European honey bee grew as well. California’s 550,000 acres of almond orchards require the services of approximately 1.1 million colonies of honey bees (with more than 40,000 inhabitants in each colony) each February.

Today 15 to 30 percent of our diet in the United States relies upon the European honey bee for pollination: a tribute to the bees and their keepers, but a dependency which exposes the United States to the threat of large-scale crop failure. Just as planting a single crop across hundreds and thousands of acres creates a perfect habitat for whichever pest thrives in that crop, so too do housing and trucking millions of colonies of a single bee species render the honey bee particularly susceptible to pests and disease infestations.

Early this spring, reports Eric Mussen, Cooperative Extension apiculturist at UC Davis, United States beekeepers lost 40 to 60 percent of their European honey bee colonies to a parasitic mite about the size of a dog or cat flea whose name reflects its impact, varroa destructor. Had the rainy weather not already dampened almond yields, up to 30 percent of California’s almond crop would likely have not been pollinated.

The USDA and others are studying whether other bee species can be shipped upon demand to blooming fields, a challenge because, by and large, native bees do not lend themselves to being directly managed. At least 50 species of native bees nest socially like the European honey bee, but colonies of native social bees range in size from a handful of bees up to a couple hundred individuals: mere outposts compared to the hives of honey bees with their populations of tens of thousands that better match the scale of industrial agriculture. In addition, more than two-thirds of native bees nest in the ground — while most others nest in wood or plants — complicating attempts to create transportable housing for them.

“There are several efforts under way to manage our native bees,” says Vaughan. “They have been quite successful in the case of the blue orchard, or mason, bee. It’s a great pollinator of orchard crops, and we know quite a bit about how to get it to nest in large numbers, basically in holes in wood or in paper tubes. And bumble bees are now being managed for pollinating greenhouses. So, in certain crop situations, with a couple of our native bees, we can actually manage them.”

While Kremen recognizes that managed bees — be they honey bees or another species being groomed by researchers — are necessary for large- scale agriculture, she would argue that we should simultaneously spread our risks by increasing our opportunities to benefit from pollination by native unmanaged bees.

Kremen’s findings about the potential of native bees to help pollinate crops in the Central Valley has given Charlie Rominger one more reason to evaluate whether to farm the more marginal corners and lowlands of his property.

Rominger, together with his brothers Rick and Bruce, farms 2,700 acres 15 miles west of Davis, including approximately 500 acres of sunflowers. Each summer a beekeeper trucks 750 hives of European honey bees to the Romingers’ farm to pollinate the sunflowers.

“Whenever you come across a whole bunch of new information like this, it all goes into your knowledge base,” Charlie Rominger says of Kremen’s findings. “Before I heard this, I didn’t realize that native pollinators were that significant. We’ve been planting insectary hedgerows to lure beneficial insects for a while now. Now the more we learn about the benefits of the bees and how, if you’ve got some pollen and nectar sources early in the spring in these hedgerows, you’ll get bigger populations of these native bees, it’s one more reason to plant hedgerows.” Kremen’s data indicate the need for native bee habitat within 2 kilometers of cultivated fields: good news for farmers whose fields abut foothills, protected open spaces, or streams. Those farmers with fields in the middle of the intensively farmed rich soils of the Salinas and Central valleys will have the most work to do to diversify their pollinators. For every 100 acres isolated from native habitat, says Kremen, a farmer seeking 100 percent pollination from native pollinators must convert 30 to 40 percent of his land to suitable habitat.

Alternatively, suggests Kremen and Vaughan, farmers can achieve up to 40 percent pollination by native bees if 10 percent of their farm is enhanced for bees. Ten percent can often be carved out of total farm acres around irrigation ponds, beneath utility towers and in marginal lowlands. While planting these areas with habitat and nectar sources for native bees will actively encourage them, even simply leaving these areas untilled, so as not to disturb ground-nesting bees, and eliminating pesticide use nearby will improve the chances that native pollinators will move in.

“Ten percent, especially when you get on the better ground out in the valley, that’s going to be difficult,” says Rominger. “But we try to take advantage of the wet corners, all the places where, if we farmed them, we’d lose money anyway. You don’t want too many of those on your farm but now, rather than just beating our heads against the wall on the low corners or the poor soil, we’re trying to do something useful.”