Pollination Efficiency of Bee Species Pollinating Coffea arabica in Kiambu County Kenya

Prior to commencement of this study, a survey to identify common bees pollinating C. arabica in Kenya was done. Patellapis zonalictus, Apis mellifera and Lasioglossum spp. were identified as the common pollinators of C. arabica flowers. Pollination efficiency was analyzed indirectly by assessing pollen deposition on stigmas. Flowers were bagged at bud stage. When the pollinator exclusion bags were removed, the virgin flowers were allowed a single pollinator visit and then the stigmas crushed on a microscope slide and stained using fuchsin dye. The pollen grains deposited on the stigma were counted under a dissecting microscope. The numbers of non coffee pollen grains were also counted to assess pollen purity. Results indicated that P. zonalictus and Lasioglossum spp. were more efficient pollinators of C. arabica than A. mellifera. They deposited significantly higher amounts of C. arabica pollen grains. The pollen grains also had a significantly higher purity ratio when compared with the pollen grains deposited by Apis mellifera (p<0.001).


INTRODUCTION
Wild bees and other insects can pollinate many crops but their value for crop pollination has been overlooked for centuries (Klein et al., 2008(Klein et al., , 2003a)).The importance of wild bees has been overshadowed by the popularity of the well known honey bee A. mellifera due to its economic returns through honey production.It is essential to determine the importance of a pollinator species to the plant under investigation since most organisms foraging on plants may not be necessarily the most important pollinators and may not significantly benefit the plant (Spears Jr., 1983).
Flowers must produce their pollen and nectar at different times of the day or seasons for the forager to become continuously active.The flowers must also balance the amount of sugar and water in the nectar, so that on each foraging visit, the bee gets the required energy and water while foraging (Roubik, 2002a, b;Raju and Rao, 2008).A pollinator must visit a flower in such a way and within such a period, that viable pollen is transferred from the anther to the stigma of the same or a different flower (Kevan, 1999).
Flowering plants that benefit from insect pollination produce energy rich nectar and nutrient rich pollen to attract pollinators.While foraging for floral resources, insects unwittingly transfer pollen grains within individual flowers, among flowers of the same plant and between flowers of different plants (Black et al., 2007).According to Raju and Rao (2008) floral rewards form the criterion of foraging behavior, being either oligolectic (specialists) or polylectic (generalists).
Pollination efficiency of the main bees pollinating Coffea arabica had not previously been investigated in Kenya.If an efficient pollinator of C. arabica is identified, it might be possible to enhance efforts to conserve it and conserve its habitat so as to increase its population to ensure higher rates of pollination success and higher coffee productivity.
A. mellifera has been identified by many studies as the main pollinator of many crops as well as C. arabica and this study attempted to compare the efficiency of A. mellifera with that of other common bees found foraging on coffee flowers (P.zonalictus and Lasioglossum spp.).Both the pollen load (on the body of a bee) and pollen load purity (absence of non coffee pollen grains) of pollen deposited on virgin coffee flowers were calculated in order to investigate which bee deposited a lot of coffee or non coffee pollen on virgin coffee stigmas.

Relative pollinator efficiency tests:
The main pollinators of coffee flowers were recorded and identified in a previous study.In the current study, flowers were bagged before they opened using fine nylon mesh (10 µm).Once the bags were removed; only a single pollinator was allowed to get in.The pollinator was identified in the field when possible or when not sure of the identity captured for later identification.After a single visit to a flower by a pollinator the stigma of the experimental flower was removed with clean forceps and placed in a microscope slide.A drop of basic fuchsin dye was used to stain the pollen grains.Gentle pressure was applied to the top of a cover slip to distribute the pollen grains into a monolayer (Ricketts, 2004).The slides were put in an ice box awaiting transportation to the laboratory (Larsson, 2005).Pollen grains were dyed using fuschin dye and counted under a compound microscope to determine the number of pollen grains deposited on a virgin stigma.The median rather than the mean pollen deposition per species was used for statistical analysis since previous studies have indicated that it provides a more conservative estimate of pollen deposition per visit (Kremen et al., 2002).Data analysis was done using One Way ANOVA and for separation of multiple responses Dunnett T3 post hoc tests were done (Underworld, 2005).

RESULTS
Among the collected bee species during this study, the bees that were found to deposit high amounts of pollen on the stigma were Patellapis (Zonalictus spp.) followed by Lasioglossum spp.The highest level of deposited pure pollen was from Patellapis (Zonalictus spp.) while A. mellifera had the lowest purity ratio (Fig. 1).

DISCUSSION
This study attempted to identify and document the efficiency of different pollinators of coffee flowers.Results on pollen load analysis from the main bees pollinating C. arabica shed light to the fact that solitary bee species were the most efficient pollinators of coffee contrary to the common notion that A. mellifera is the most important pollinator of C. arabica (Greenleaf and Kremen, 2006;Vergara and Badano, 2008).
This study indicates that Lasioglossum spp.and Patellapis zonalictus were more efficient in pollen transfer than A. mellifera.This is because solitary bees are oligolectic thus deposit purer pollen on flowers than the polylectic A. mellifera which forages on many flower species.Therefore A. mellifera deposits on C. arabica flowers pollen grains that are mixed with pollen from other plants.A higher number of pollen grains were deposited by the solitary bees than the social bees.
Previous studies in Brazil have indicated that solitary bees collect pollen from plants of related species or genera and they are also reputed to be efficient pollinators with no known cases of nectar and pollen thieves (Schlindwein, 2004).
These results agree with those of a study carried out in Central Sulawesi (Indonesia) that indicated that the pollination efficiency of solitary bees was significantly higher than that of the social bee species.Similar results were given on C. canephora in Indonesia where solitary bees led to 86.5% fruit set while social bees led to 70.4% fruit set (Klein et al., 2003b, c).Higher amounts of pure C. arabica pollen deposition, leads to a higher seed set rate (Paschke et al., 2002;Philpott et al., 2006).The results differ with those of Klein et al. (2002) whose results indicated that A. mellifera has the most important influence on pollination of C. arabica and C. canephora.
Inadequate deposition of pollen grains may have implications on fruit set of coffee.A study in Chiapas, Mexico, concluded that pollen load affects the size and number of seeds or fruits and there is a probability that it can lead to seed or fruit abortion (Philpott et al., 2006).Pollen purity is important for pollination since high pollen loads with no foreign pollen contamination have been documented to lead to faster pollen tube growth, earlier fertilisation and therefore a longer maturation period and an increase in fruit and seed weight (Philpott et al., 2006;Marques-Souzal et al., 2007).Larger pollen loads may also provide plants with higher donor diversity and thus a genetic diversity of pollen.Thus, one possibility in coffee, as in other plants, is that increased coffee pollen diversity from other coffee plants lead to pollen competition, increased pollen vigour and subsequent increase in fruit weight and overall quality (Paschke et al., 2002;Philpott et al., 2006).A. mellifera in this study was found to deposit on coffee flowers pollen from other plant species growing within and around the coffee farms.This may be because after foraging from a plant some pollen grains may remain embedded in the corbiculae even as the bee visits another plant.These findings are in agreement with other findings that pollen loads gathered by honey bees are moistened with regurgitated nectar or honey and packaged in form of small pellets in their corbiculae or pollen baskets situated in the hind legs.During the process of collection and packaging of pollen, contamination of pollen from other sources occur (Chauzat et al., 2006).
Pollinator fidelity has been found to be very important for pollination success (Hanley et al., 2008).A. mellifera is a social bee and faced with the pressure to sustain large perennial colonies; these bees are super generalists and cannot afford to specialize on particular plant species (Gikungu, 2006;Danforth, 1999).These findings emphasize the need to enhance other wild bees' population in coffee farms by providing floral resources and favorable nesting sites so as to provide highly efficient pollinators of coffee.

CONCLUSION
The results of this study points out that solitary bees/wild bees are more efficient in pollen collection and deposition than A. mellifera.Therefore, enhancing population of other wild bee species while still maintaining the social bee populations may be very important in coffee fields.This can be done through creation of natural buffers of vegetation that the bees can use when foraging.

Fig. 1 :
Fig. 1: Pollen deposition rate and pollen purity analysis from main bees pollinating coffee