The Impact of Distinct Insect Species on Pollination and Gene Flow in Alfalfa

Authors: Brunet, Johanne; Stewart, Christy

Submitted to: Psyche
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/30/2009
Publication Date: 2/1/2010
Citation: Brunet, J., Stewart, C.M. 2010. The Impact of Distinct Insect Species on Pollination and Gene Flow in Alfalfa. Psyche. Available: http://www.hindawi.com/journals/psyche/2010/201858.html.

Interpretive Summary: We determined that different species of insect pollinators visited different number of flowers per plant and differentially tripped alfalfa flowers. Therefore different insect pollinators have distinct impact on pollination of alfalfa flowers. Leaf cutter bees and honeybees, the two species used as managed pollinators for alfalfa seed production, visited the lowest number of flowers per cluster. In addition, in the current study, honeybees and leaf cutter bees tripped the lowest number of flowers per raceme relative to other solitary bees or bumble bees. Patch density influenced the number of flowers tripped and did so differentially for distinct pollinator species. Therefore the efficacy of a pollinator at tripping flowers varied with plant density. Finally, distinct pollinator species visited different number of flowers in a patch during a foraging bout and this value has been shown to affect the potential for gene flow. The number of flowers visited in a patch was also affected by patch density for all pollinator species except bumble bees. Pollinator species and patch density affected both the pollination process and the potential for gene flow in alfalfa. Our data indicate that native pollinators can be very efficient at pollinating alfalfa flowers and that plant density can affect pollinator efficiency. Finally, distinct pollinators, including native pollinators have different potential impact on gene flow, also affected by plant density. These data can be useful to alfalfa seed producers as indicator of the various species of insect visitors that can efficiently pollinate alfalfa flowers. It also indicates that the potential for gene flow varies between these different visitors, an aspect that should be considered especially when we start releasing transgenic alfalfa.

Technical Abstract: Pollinator species and plant density can influence pollination and gene flow. The efficacy of different floral visitors at tripping alfalfa flowers was examined in both low and high density patches to determine their potential role in pollination. In addition, for a subset of the floral visitors, the number of flowers visited in a patch during one foraging bout was determined in low and high density patches as indicators of the potential for gene flow. Plots of 121 alfalfa plants were planted at two densities in a linear array in the summer of 2008: 1 plant every 0.3m (3m*3m) or every 0.9m (9m*9m) with 4 replicates per density and 10 m between plots. One hive of bumblebees (Bombus impatiens) and one of honeybees (Apis mellifera) were brought to the field in the summer of 2009. Alfalfa leaf cutter bees (Megachile rotundata) were released periodically throughout the study period. Alfalfa flowers were also visited regularly by two other native bumble bees and several solitary bees during our observation periods. The different pollinator species were not affected by flower color (P= 0.35) or patch density (P = 0.50) and all tended to visit racemes of similar sizes (p = 0.06). When visiting a raceme, bumble bees visited more flowers relative to honey bees and solitary bees and more flowers were visited in high relative to low density patches. Honeybees and leaf cutter bees, the two species used as managed pollinators for alfalfa seed production, tripped the lowest number of flowers per raceme relative to other solitary bees or bumble bees. In addition, the number of flowers visited per patch during a foraging bout was affected by both pollinator species and patch density. Only honeybees visited the same number of flowers in low and high density patches; all bumble bee species visited more flowers in lower density patches. Pollinator species and patch density both affected the pollination process and the potential for gene flow in alfalfa.