Pollination by Bumble Bees for Enhanced Clover Seed Production
Red clover (Trifolium pratense L., family Fabaceae) is an important forage legume and rotation crop raised for seed in the Willamette Valley of western Oregon. A critical factor in red clover seed production is pollination. Like many other crops, pollen transfer in red clover is done primarily by bees. Currently, there is little information on bumble bee pollination in red clover and in other agricultural systems, as research efforts have largely focused on honey bees. This study seeks to evaluate and enhance native bumble bee pollinator populations in red clover by providing economic benefits to producers through increase seed production.
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.
To evaluate strategies for drawing native bee pollinators to red clover fields, the diversity and abundance of pollinator populations along with seed yield was assessed during the 2007 and 2008 field season. Visual counts and trapping were utilized to determine the diversity and abundance of pollinators. Seed heads from red clover flowers were collected to determine the effect of these strategies on seed yield. Bombus vosnesenskii was chosen as the target native bumble bee species because its distribution and seasonal activity period coincide with the bloom period of red clover.
Fields that contained a grower-initiated border of uncut red clover, clothesline of semitransparent blue vanes, continuous bloom of uncut red clover and no blue vanes or border (control) were surveyed to determine the efficacy of each of these strategies for drawing native bee pollinators to red clover production.
The uncut border of red clover yielded no pollinators in visual observations and moderate abundance with low diversity from trapping (Table 2 and 4). These results could be explained by the absence of irrigation at this site or the size of the border. The clothesline of blue vanes resulted in greater numbers of bees in blue vane traps adjacent to the clothesline compared to bees captured in traps set up at the east and west ends of the field (Table 3). This indicates that the cluster of semitransparent blue cross vanes can be used to draw bees. However it is not known whether the clothesline drew bees to one area or if these were new bees drawn to the field. The field that was left uncut containing an early bloom of red clover had a great abundance and diversity of native pollinators (Table 2 and 4). However, the control treatment contained the greatest abundance and diversity of native pollinators (Table 2 and 4). To evaluate the effect of each of these treatments on the seed yield in red clover production, samples were collected from various locations within each treatment and the surrounding field. Data analysis is currently in progress. Future comparisons will be made during the 2009 season.
The foraging behavior of Bombus vosnesenskii was observed throughout the bloom period of red clover by combining both field and nest site observations. Colonies reared from wild-caught queens were established within wooden boxes and placed within the margin of a red clover field. All individual female bees were marked to determine the foraging activity of workers entering and leaving the nest box. A total of 1244 foraging trips consisting of 363 pollen trips were recorded over 160 hours of observation throughout the bloom period. All colonies experienced a shift in pollen collection from early to mid July, as foragers brought back loads consisting of other plant species in addition to red clover (Figure 1). To determine the floral composition of these pollen loads samples were taken from foragers as well as honey bees located within the field visiting red clover flowers for comparisons. Data analysis is currently in progress.
In examining the flight range of Bombus vosnesenskii, visual surveys were taken during the month of July within the same red clover field as colonies were established. A total of 17 marked bee foragers were observed along transects (Table 1). These results suggest that Bombus vosnesenskii may not be a doorstep forager given its low abundance. However, given the limited number of observed foragers recorded, future work is needed to confirm this assumption to determine the flight range of this species.
Populations of native bumble bees in red clover fields and adjacent native habitats were surveyed to determine the nesting density and degree of relatedness of foragers. During the 2008 season, Bombus vosnesenskii individuals were collected. Samples are currently being processed to determine the nesting density within red clover fields.
Impacts and Contributions/Outcomes
This study provides benefits to red clover producers through sustainable and economic crop production practices as well as opportunities to improve their education and awareness of native pollinating species of the area. Producers are gaining knowledge to optimize on-farm resources with the use of native bumble bees that currently exist in the region. This research also promotes the diversification of crop production practices by allowing growers to be more aware of the value of additional floral resources for increasing bumble bees, which will enhance seed production. Furthermore, this study provides economic benefits to producers in reducing costs of production associated with renting honey bees.
In facilitating producer involvement in this project, presentations are scheduled to be made during the Clover Commission meeting and OSU Hyslop Farm Field Day. Producers will also have the opportunity to learn how to identify native pollinators of the area through participation in a native bee identification workshop and fact sheet on native bee identification. Furthermore, publications will also be produced for inclusion in the OSU Seed Report and a peer-reviewed scientific journal.