The honeybee (Apis mellifera) is in rapid decline worldwide. While it is important to understand the reasons behind this decline in order to try and restore populations, the fall in numbers has led to recent and growing interest in the relative value of wild vs. managed insects for pollinating crops. However, surprisingly few studies have directly compared the effectiveness of managed honeybees with wild alternative pollinators on cultivated land.
Effects of different pollinators were compared using the crop Brassica rapa var. chinensis or 'pak choi'. Flower-visiting insects were observed in 11 commercial pak choi fields in New Zealand for a three month period. Two components of pollination were assessed: the efficiency of pollen transfer, which describes the effectiveness at which pollinators remove and transport pollen, and visitation frequency, which considers both the abundance of the pollinator and of the flowers.
31 species in total were observed attending the pak choi flowers, but only seven insects other than the honeybee were considered frequent enough visitors to be analysed. These were three other bees and four flies. There was a large variation amongst the species in the amount of pollen deposited on the flower and the number of occasions in which the stigma was contacted. Three species were similar to the honeybee on these measures: the bees Bombus terrestris and Leioproctus sp., and the fly Eristalis tenax. However, results indicated that honeybees visited flowers at a significantly higher rate than all other insects.
When both efficiency and visitation frequency were combined, the honeybee was the most effective pollinator and deposited an estimated 7879 pollen grains per hour. This rate was three times greater than the next highest pollinator, B. terrestris, which transferred an estimated 2247 pollen grains per hour. E. tenax deposited 968 grains per hour and Leioproctus sp. approximately 300 grains per hour.
The results indicate that wild insects are capable of providing pollination services similar to those currently performed by honeybees. Honeybees visit flowers at a significantly higher rate than the wild pollinators, but they do not differ in the number of flowers visited per minute. This suggests that the alternative pollinators are simply not common enough. To make them as effective, the population sizes of wild pollinators needs to increase. This requires a change in land management practices to ensure year round feeding, nesting and other resources are provided.
In order to understand how to maintain stable populations of wild pollinators, further research is needed on the biology of the potential pollinators and their current role in the ecosystem. It is also important to carefully consider any negative implications of increasing wild pollinator populations, to ensure that there are no adverse effects on the local ecosystem.
Source: Rader, R., Howlett, B.G., Cunningham, S.A. et al. (2009). Alternative pollinator taxa are equally efficient but not as effective as the honeybee in a mass flowering crop. Journal of Applied Ecology. 46(5):1080-1087.