Pollination by Bumble Bees for Enhanced Clover Seed Production

Project Overview

Project Type: Graduate Student
Funds awarded in 2008: $19,977.00
Projected End Date: 12/31/2010
Grant Recipient: Oregon State University
Region: Western
State: Oregon
Graduate Student:
Major Professor:
Dr. Sujaya Rao
Oregon State University

Annual Reports


  • Agronomic: general silage crops
  • Additional Plants: native plants


  • Crop Production: intercropping
  • Education and Training: demonstration, extension, on-farm/ranch research, participatory research
  • Natural Resources/Environment: habitat enhancement, hedgerows, wildlife
  • Pest Management: field monitoring/scouting, traps
  • Production Systems: agroecosystems
  • Sustainable Communities: public participation, sustainability measures, urban agriculture


    Red clover is an important forage legume and rotation crop that is raised for seed in western Oregon. Red clover growers rent hives for pollination, but honey bees are not efficient in this crop due to their preference for flowers in which nectar is more accessible. In contrast, bumble bees are better pollinators of red clover, but currently there is little information on their performance in the Pacific Northwest. This study was conducted to develop strategies for enhancing native bumble bee pollinator populations in red clover seed crops to provide economic benefits to producers through increased seed production.


    Red clover (Trifolium pratense) is an important forage legume grown for seed in the Willamette Valley of western Oregon. Pollination is a critical factor in the production of red clover. Given the self-incompatibility of flowers, plants rely on cross-pollination for adequate seed set (Smith, Taylor and Bowley, 1985). Bees serve as the primary pollinating agents in red clover. Traditionally, growers have rented honey bees for pollination by incorporating one to two hives per acre. However, given the increasing difficulties in hive management attributed to parasites and diseases, honey bees have declined significantly in abundance, resulting in low availability and consequently increased costs to growers (Winfree et al., 2007). Furthermore, honey bees were found to be inefficient pollinators of red clover when compared to alternative bee species such as bumble bees (Rao and Stephen, 2006; Rao et al., 2007). Bumble bees exhibit a stronger preference for red clover over alternative flowering species, visiting two to three times more flowers per day than honey bees (Holm, 1966). In a recent cage study conducted by Rao et al. (2007) in a red clover field, bumble bees produced seed set comparable to that of honey bees.

    Growers of the Willamette Valley have expressed interest in evaluating bumble bees as alternative pollinators in red clover. Since bumble bees are commercially unavailable in Oregon, growers must rely on feral populations of bumble bees for pollination. Unfortunately, the abundance and foraging behavior of the majority of feral bumble bee species are unknown in the Pacific Northwest (Stephen, 1957). These data are vital to evaluating bumble bees as viable alternative pollinators for red clover seed production.

    Project objectives:

    1. Evaluate strategies for drawing native bumble bees to red clover fields.

    2. Compare the pollen loads on honey bees and bumble bees visiting red clover fields.

    3. Compare bumble bee populations in red clover fields and adjacent native habitats.

    Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.