Abstract:
Pollination is an essential ecosystem service, increasing reproductive success of many crops, which can be provided by managed pollinators, wild bees (including honeybees) and other insect pollinators. However, the pollination services and the economic value of wild pollinators are often underestimated. Better understanding of the factors that influence honeybee foraging behaviour and pollination efficiency can contribute to the improvement of management practices that aim to enhance crop pollination and ecosystem services. The objectives of this study were to investigate the importance of managed honeybees and wild honeybees to sunflower pollination as well as to evaluate the response of honeybees to different levels of floral rewards and to behavioural interactions with wild flower visitors. The study was conducted in 16 commercial sunflower farms and one experimental farm of South Africa during the 2009 sunflower flowering season. The results showed that insects, particularly honeybees, were efficient pollinators, improving sunflower production in all self-fertile sunflower cultivars used in this study. Furthermore, wild honeybee colonies were found to be as efficient as managed honeybee colonies in sunflower pollination near to natural habitat. Both sunflower yield and the abundance of pollinators decreased with distance from natural habitat, suggesting that sunflower yield is directly correlated with the abundance of pollinators. The amount of nectar present in the florets of sunflower significantly affected pollinator behaviour, influencing honeybee visitation length and foraging rate which prefer to exploit floral rewards from the same source if they find the higher amount per foraging trip, possibly having a negative impact on cross-pollination. Moreover, the concentration of nectar collected from honeybees was significantly lower than the nectar concentration from florets, suggesting that honeybees diluted highly concentrated sunflower nectar with their saliva to their optimum concentration level. Interspecific exploitative competition between honeybees and wild pollinators (wild bees, butterflies and moths) significantly increased the movement of honeybees among sunflower heads, which enhances cross-pollination. Furthermore, behavioural interactions influenced the length of foraging time spent by individual honeybees per sunflower head. Butterflies were the most influential in enhancing honeybee foraging movement, followed by wild bees and then moths. The importance of a given flower visitor species to honeybee movement is likely related to the size of the visitor, as the bigger size of butterflies and movement of their wings increases the chance of disturbing a neighbouring honeybee. Conservation of natural habitat is important to maintain the diversity of flower visitors which indirectly contribute to crop production by enhancing honeybee foraging activity and consequent direct pollination service. Furthermore, the pollination effectiveness of wild pollinators, density of wild honeybees surrounding sunflower fields and effects of human activities on pollination disruption are suggested as topics for future research.