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Case Studies

Pollination Case Studies and Initiatives

Case Studies


Pollination Services to Crops in Yolo County, California

The contribution of native bees to crop pollination has generally been overlooked and greatly undervalued. With the problems facing European honey bees, a new emphasis is being put on native insects, especially native bees, as crop pollinators. Claire Kremen led a team of researchers that studied the contributions of wild bee pollinators to pollination of four crops—watermelon, sunflower, almond, and tomato—in the Central Valley of California, one of the most important agricultural regions in North America. The Xerces Society for Invertebrate Conservation subsequently developed and launched an outreach project to share the findings of this work with farmers and agency staff.


The studies were conducted along a gradient of agricultural intensification, from intensively managed farms in a primarily agricultural landscape, to less intensively managed farms in a primarily natural landscape. The main objectives of these studies were to: 1) define the role of wild bee populations in crop pollination and identify the most important contributing species; 2) investigate the influence of agricultural intensification on the crop pollinating species and on the pollination services they provide; and 3) identify floral and nesting resources across the landscape to develop protocols for restoring and promoting the wild bee populations and their services.


Claire Kremen’s team discovered that over 60 native bee species visit crops in this area, but that the diversity and abundance of these, and consequently the pollination services they provide, declined significantly as agriculture became more intensive. Those farms that contained or were close to areas of floral and nesting habitat had greater numbers and diversity of native bees. Pollination was provided by a suite of bee species that fluctuated in composition between years. However, the most important pollinating species were also most sensitive to agricultural intensification, and when these species declined other species did not compensate by increasing in abundance.


The pollination contribution of native bees varies from complete pollination to increasing the efficiency of managed honey bees. On farms with adequate habitat, native bees can provide sufficient pollination for plant species with large pollination requirements such as watermelon. Watermelon flowers open for only a few hours and require the transfer of up to 1,000 grains of pollen per flower to form well-shaped marketable fruit. The primary pollinator is a small halictid bee that may only transfer 10 or so grains of pollen per visit, but whose abundance ensures pollination. In contrast, hybrid sunflower seed production is mainly reliant on managed honey bees. The presence of native bees foraging on the sunflowers apparently makes honey bees move more quickly from flower to flower, doubling the honey bee’s value, on average, as pollinators.


The research team had identified a suite of wild plants that provide foraging resources for the most important native crop pollinators. The bees are able to maintain robust populations for pollination of agricultural crops because of the resources these wild plants provide before and after the target crops bloom. Working closely with the researchers, The Xerces Society for Invertebrate Conservation launched an active outreach program to inform growers and agency staff of techniques to improve pollinator habitat in this heavily agricultural landscape. The program has included a series of workshops organized with the Community Alliance With Family Farms and the Center for Land-Based Learning. Presentations have also been given at farming conferences and a series of fact sheets on specific crops and detailed guidelines on providing habitat for bees have been published. Training seminars such as for Natural Resources Conservation Service farm-support staff and California Department of Fish and Game land managers from adjacent reserves and wildlands are taking place.



Vaughan, M., M. Shepherd, C. Kremen, and S.H. Black. 2004. Farming for Bees. Guidelines for Providing Native Bee Habitat on Farms. The Xerces Society, Portland, OR. 34 pp.

Claire Kremen, University of California, Berkeley, personal communication, (2007)

Mace Vaughan, The Xerces Society for Invertebrate Conservation, personal communication, (2007)



Oak Savanna Restoration - Jefferson Farm, Oregon

Over 99 percent of the prairies and oak savanna in Oregon’s Willamette Valley have disappeared. An inspirational project at Jefferson Farm just south of Salem is restoring more than 200 acres of prairie, oak, and riparian habitat. The farm is owed by Mark and Jolly Krautmann, who own Heritage Seedlings, and the project is managed by botanist Lynda Boyer.


A key objective of this project is to establish large, genetically diverse populations of native plants, including threatened and endangered species. Insects, especially pollinators, are considered an integral component of the habitat they are restoring. Although there are still pockets of the original grassland flora, most of the site has been intensively grazed or in the process of disappearing under scrub and dense oak woodland. To reduce and control the invasive and nonnative grasses, forbs, shrubs, and trees in the project area, a series of actions have been taken. These include a combination of broadcast-sprayed and spot-sprayed herbicides, mechanical tree shearing, mowing, hand-cutting, and other techniques.


More than 45 species of endemic forbs and 5 species of endemic grasses will be reestablished on over 100 acres of restored grassland by no till seeding, hydroseeding, hand broadcasting, and planting of plugs. In addition to retaining oak trees, snags are being created, and bare ground left for bee nest sites.


The project is funded by the Krautmanns, the Partners for Fish and Wildlife Program of the U.S. Fish and Wildlife Service, the Oregon Department of Fish and Wildlife Landowners Incentive Program, and the Oregon Watershed Enhancement Board.

An informational document based on the Jefferson Farm experience has been prepared by Lynda Boyer and is available to the public through Heritage Seedlings.



Heritage Seedlings, Inc., accessed May 5, 2008.

Lynda Boyer, Heritage Seedlings, personal communication,  (2000)


Pepco Rights-of-Way Integrated Vegetation Management Program 

Pepco, the Potomac Electric Power Company, manages a network of transmission lines on 330 miles of rights-of-way (ROW), which covers approximately 10,000 acres in five Maryland counties and the District of Columbia. The ROWs traverse farmland and natural areas, include diverse ecosystems and habitat, and form fingers through many suburban and urban communities. The fundamental objective of Pepco’s ROW management is to provide safe and reliable transmission of electricity, but while doing so Pepco strives to maximize the habitat value of ROWs.


In 1998, a single tree which was growing into transmission wires resulted in a major power outage. Subsequent surveys discovered hundreds of other trees that were growing dangerously close to wires. These events lead to a review of ROW maintenance and the development of the Integrated Vegetation Management Program. A significant outcome of this program has been the creation of open, sunny habitats of low-growing shrubs, grasses, and flowers, perfect conditions for many butterflies and other pollinator insects.

This program is both beneficial to wildlife habitat and compatible with providing safe and reliable electric service. “We (at Pepco) have found that practicing environmental stewardship of our rights-of-way encourages partnering with wildlife support groups, environmental government agencies and other ecological interest groups to achieve our corporate goals,” said Genua.

One of these beneficial partnerships formed based on rights-of-way stewardship is the collaboration between PHI, the Patuxent Wildlife Research Center and WHC.

The meadow management program allows dormant annual and perennial wildflower seeds to germinate and grow. These wildflowers provide food and nectar for butterflies and caterpillars. Butterfly populations are an integral part of the wildlife habitat on the ROWs because they pollinate flowers, provide an essential food source for wildlife and naturally beautify surroundings.

Land management for butterflies also often adds benefits by creating high quality habitats for plants, birds and other insects. The Washington Area Butterfly Club and the International Butterfly Breeders Association, along with WHC, are integral partners with the Butterfly Enhancement Project, which has repopulated more than 300 acres of land with butterflies.

During WHC’s 17th Annual Symposium in November 2005, Pepco was distinguished by winning the 2005 Wild Turkey Management Award. An added honor, Pepco was also recognized by the North American Pollinator Protection Campaign and WHC for establishing pollinator friendly practices along the rights-of-way. The Pepco Rights-of-Way has been certified since 2000.

Excerpted from:

Wildlife Habitat Council. No Date. Rights-of-Way. Online at, accessed May 5, 2008.



Pesticide Poisoning of Bees

In the U.S., pesticides must carry a warning if their use will be a hazard to managed crop pollinators. However, these label warnings are inadequate to protect honey bees or native bees. For example, a pesticide that is restricted for use on a flowering crop has no restrictions when used on rangeland for grasshopper suppression. Restrictions on labels that are designed to protect honey bees, such as closing hives, not spraying over apiaries, or preventing the bees from foraging on the crop for a period after application, offer limited protection to feral honey bees or native bees for two reasons. First, exposure to insecticides while foraging can be more hazardous to bees than having the outside of the nest sprayed (Delaplane and Mayer 2000), as bee poisonings occur from contact between treated vegetation and the bee (Johansen et al. 1983). Second, smaller bees are more susceptible to poisoning from pesticide residues (Johansen et al. 1983), and thus they are still affected by “safe” levels of pesticides.


Bees are poisoned either by absorption of the pesticide through their outer skin or by eating contaminated nectar or pollen. Absorption occurs if they are foraging as the spray is applied or if they walk across plants that have spray residues. The majority of research on bee poisoning has been done on honey bees. A smaller amount of research has been conducted on two other bees used as alfalfa pollinators, the native alkali bee, Nomia melanderi, and the European imported alfalfa leafcutting bee, Megachile rotundata. Because smaller bees have a higher surface to volume ratio they will absorb a relatively higher dose of pesticides and thus are more susceptible to field-weathered residues (Johansen et al. 1983). This greater sensitivity also means that the residues remain toxic longer for small bees than for larger species (Johansen and Mayer 1990). For example, the residual toxicity of malathion ULV is 5.5 days for honey bees and 7 days for the smaller alfalfa leafcutting bees (Riedl et al. 2006). In bee terms, the honey bee is a large animal (12-14 mm; ½”); even the alfalfa leafcutting bee is relatively large (10 mm; 2/5”). Most native bees are significantly smaller, in some cases only a fraction of the size: bees in the genus Perdita may be only 2 mm (<1/10”) in length.


Other factors that make populations of native bees more susceptible to poisoning include their nesting habits, foraging strategies, and whether they are social or solitary nesters. The great majority of native bees are solitary nesters (Michener 2000). Solitary bees are more vulnerable to long-term impacts when compared with social species, such as native bumble bees or the nonnative honey bee. In social nests, it is the workers that forage and have direct contact with the pesticide. The egg-laying female (the queen) remains in the nest and is isolated from the direct impact of poisoning, although once back in the nest workers jostle against her and feed her, so she may be poisoned indirectly. In these nests, the queens can keep laying eggs while the workers suffer. In contrast, each solitary nesting bee has to complete all the nest preparation and foraging tasks, and thus the egg-laying female is directly exposed to pesticide residues on flowers and leaves (Johansen and Mayer 1990). Since the egg layers are directly impacted, populations of solitary bees can take longer to recover.


Bees are central place foragers. They collect nectar and pollen for the brood cells in which the young are laid and fed. Contaminated nectar and pollen left as a food supply for the larvae can remain hazardous for many weeks (Johansen and Mayer 1990, Delaplane and Mayer 2000).


Nest-building behaviors can expose bee offspring to residues. Most of the species in the subfamily Megachilinae carry materials from outside the nest to make brood cells or nest partitions (Michener 2000). In particular, the genera Megachile and Osmia use leaf pieces or masticated plant materials to make brood cells within their nests. Collecting nesting materials from sprayed plants introduces insecticide residues into the structure of the nest, impacting the larvae.


Approximately 70 percent of native bees nest in the ground, burrowing into areas of bare or partially vegetated soil (Michener 2000). Most of the remaining 30 percent nest in abandoned beetle galleries (where eggs are laid) in snags or soft-centered and hollow twigs and plant stems. The small number of remaining solitary bees nest in a variety of other locations, such as empty snail shells or in cells constructed on the outside of twigs. Bumble bees nest in cavities in the ground or under grass tussocks. Unlike managed honey bee hives, it is not possible to prevent native bees from leaving their nests for foraging during or immediately after spraying operations. Leaving a buffer zone around honey bee hives will not have any benefit for native bees, unless they happen to be nesting in the same area.



Delaplane, K.S., and D.F. Mayer. 2000. Crop Pollination by Bees. Wallingford, U.K: CAB International.

Johansen, C.A., D.F. Mayer, J.D. Eves, and C.W. Kious. 1983. Pesticides and bees. Environmental Entomology 12: 1513-1518.

Johansen, C.A., and D.F. Mayer. 1990. Pollinator Protection. Cheshire, CT: Wicwas Press.

Michener, C.D. 2000. The Bees of the World. Baltimore, MD: Johns Hopkins University Press. 914 pp.

Riedl, H., E. Johansen, L. Brewer, and J. Barbour. 2006. How to Reduce Bee Poisoning from Pesticides. Pacific Northwest Extension Publication 591. Corvallis, OR: Oregon State University.



Canadian Blueberry and Native Pollinators in Southern New Brunswick

Until recently, lowbush blueberry farmers in Eastern Canada relied heavily on approximately 70 species of native wild insects for commercial production. Between 1969 and 1978 New Brunswick blueberry fields were sprayed with the organophosphate insecticide, Fenitrothion, used to reduce spruce budworm populations in the adjacent forest. Beginning in 1970 and over the next several years, blueberry crop yields in the affected area, and in New Brunswick in general, were dramatically lower that would have been expected based on the crops in neighboring Nova Scotia and Maine. These lower crop yields coincided with significant reductions in the population of native bumblebees (Kevan 1975; Kevan et al. 1997).


Peter Kevan examined the link between these two trends. He found that the insecticide spray program explained the reduced pollinator populations and consequently the lower blueberry crop yields. This conclusion was based on the observation that reduced populations of native bees were found only after and where Fenitrothion had been sprayed. In addition, dead or debilitated bumblebees collected from fields suspected of contamination did indeed have Fenitrothion in their tissues (Keven and Plowright 1995; Kevan et al. 1997).



Kevan, P.G., C.F. Greco, and S. Belaoussoff. 1997. Log-normality of biodiversity and abundance in diagnosis and measuring of ecosystemic heath: pesticide stress on pollinators on blueberry heaths. Journal of Applied Ecology 34: 1122 – 1136.

Kevan, P.G. and R.C. Plowright. 1995. Impact of pesticides on forest

pollination. In: Armstrong, J.A. & W.G.H. Ives (Eds). Forest Insect Pests in Canada, Ottawa, Canada: Natural Resources Canada. pp. 607-618.

Kevan, P.G. 1975. Pollination and environmental conservation. Environmental Conservation 2(4): 293–297.



Invasion of Africanized Honeybees to North America

In 1956, apiculturist Warwick Kerr introduced the African honey bee (Apis mellifera scutellata) to the southeastern coast of Brazil. He wanted to breed, in a controlled environment, a more productive honey bee better suited than the European honey bee for the Neotropical climate. One year after their introduction, 26 queen bees were accidentally released into the forest. Since then they have spread throughout Latin America, moving northward between 200 and 300 miles each year. In 1990, the first Africanized honey bee was reported in the U.S. in southern Texas. By the mid 90s they were spreading at a pace of 375 miles per year (Kunzmann et al. 1995). Movement east has been slow but by 2005 Africanized honey bees were found in Louisiana, Arkansas, and Florida (USDA 2007).


African and European honey bees are nearly identical. They share similar biochemistry, genetics, diet, and social behaviors. For humans, the most significant trait that separates these two subspecies is the African honey bee’s aggressiveness. African honey bees respond to threats more quickly and in far greater numbers than their European counterpart. Although the numbers are still moderately low, by mid 2004 fourteen fatalities and hundreds of non-fatal stings from Africanized honey bees were reported (Sanford and Glen 2005). These behaviors make the African honey bee, and European honey bee colonies invaded by African queens, more difficult to use as managed pollinators of agricultural crops. In addition to public health risks, the invasion of African honey bees to North America poses serious ecological consequences. Their aggressive nature and foraging strategies have led some scientists to believe that Africanized honey bees will out-compete many native pollinators for valuable forage resources (Kunzmann et al. 1995).


Researchers at USDA Agricultural Research Service are looking at behavior and physiological traits that may impact hybridization and survival of both the African and European honey bees. They report that in the tropics and subtropics Africanized honey bee genetic contribution and behavior dominate when they hybridize with European bees (Harrison et. al 2006). In temperate environments the European honey bee still predominates. Harrison et al. (2006) suggest a relationship between temperature and foraging: “Preferences for pollen vs. nectar may be an important trait mediating these ecological trade-offs, as preference for pollen enhances nutrient intake and brood production for the AHB in the tropics, while a relative preference for nectar enhances honey stores and winter survival for EHB.”


Due to its potential implications for agriculture, the expansion of African honey bees is monitored and researched extensively. At this point the actual effect of the invasion and how far northward the African honey bees will spread are still not known.



Harrison, J.F., J.H. Fewell, K.E. Anderson, and G.M. Loper. 2006. Environmental physiology of the invasion of the Americas by Africanized honeybees. Integrative and Comparative Biology  pp. 1-13. Online at, accessed on May 5, 2008.

Kunzmann, M.R., S.L. Buchman, J.F. Edwards, S.C.Thoenes, and E.H. Erickson. 1995. Africanized Bees in North America. In: LaRoe, E., G.S. Farris, C.E. Puckett, P.D. Doran, and M.J. Mac (Eds). Our Living Resources: a Report to the Nation on the Distribution, Abundance, and Health of U.S. Plants, Animals, and Ecosystems. Washington, DC: U.S. Department of the Interior – National Biological Service. pp. 448-451.

Sanford, M. and H. Glenn. 2005. African honey bee: what you need to know. Fact Sheet ENY-114. Entomology and Nematology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Online at, accessed on May 5, 2008.

USDA Agricultural Research Service map of the spread of Africanized honey bee by year. January 2007. Online at, accessed on May 5, 2008.



Fig Trees and Fig Wasps in Tropical Forest Communities

Figs are a critical resource for both people and animals living in many tropical forest communities. In some areas the figs may constitute as much as seventy percent of the diet of vertebrate species. Most of the world’s 750 fig species rely on wasps for pollination. In return, the wasps depend on developing fig seeds for food in their larval stages (Janzen 1979). Until recently it was believed that most types of figs rely on a single species of wasp as their exclusive pollinator. Exciting recent studies have revealed that a significant number of figs have multiple pollinator species (Molbo et al. 2003).


Figs are often considered a ‘keystone’ species because they are an essential component linking many organisms within an ecosystem. If fig populations are disrupted due to selective logging or other types of habitat fragmentation, the results can be disastrous. Such disruptions could trigger cascading extinctions of the multitude of species that depend on them, such as primates and birds. The results can be equally devastating if the population of a fig tree’s exclusive pollinator is disrupted by, for example, insecticide overspray (Bronstein 1992).


Judith Bronstein and her colleagues conducted a study of fig species and their pollinators in an attempt to determine the effects of fragmentation on sustainability. Their goal was to determine how many fig trees an area must retain to support a wasp population. They concluded that between 95 and 294 trees of one fig species would be necessary to support the survival of a pollinator population for at least four years. In a related study it was determined that a minimum of 300 trees would be required to ensure the long-term sustainability of a population of fig wasps (Bronstein 1992).



Bronstein, J.L. 1992. Seed predators as mutualists: ecology and evolution of the fig-pollinator interaction. In: Insect-Plant Interactions. Vol. IV, E. Bernays E. (Ed). Boca Raton, FL: CRC Press. pp.1–44.

Janzen, D.H. 1979. How to be a fig. Annual Review of Ecology and Systematics 10: 13–51.

Molbo, D., C.A. Machado, J.G. Sevenster, K. Laurent and E.A. Herre. 2003. Cryptic species of fig-pollinating wasps: implications for the evolution of the fig–wasp mutualism, sex allocation, and precision of adaptation. Proceedings of the National Academy of Sciences 100(10): 5867–5872.