Influence of water application rates on visitation by the South African honeybee (Apis mellifera capensis) and seed yield of Texas Grano onions

Abstract

South Africa is one of the major producers of onion seed in the world, with onions being the most valuable vegetable seed crop in the country. Grown mostly in the Klein Karoo, onion seed production in South Africa has suffered from historic and periodic problems in crop pollination. That results in honey bees being used for commercial pollination sometimes not working on the flowering onions, leading to poor pollination and poor seed set. Losses can be as much as 44% of annual production. Problems in onion pollination are a global problem with no clear explanation, why honey bees find onion flowers unattractiveness. Best supported theory is that irrigation resulting in high levels of repellent phytochemicals and /or lowered levels of nectar sugars best-supported,In this study the relationship between water irrigation levels, visitation rates of Cape honey bees (Apis mellifera capensis) and other insects, and seed production was studied using Texas Grano onions in Ladismith in the Klein Karoo. A mini-plot design was used with a field of onions being divided into 56 mini-plots, comprising of seven treatments and eight replicates of each treatment. Plots were provided with variable amounts of water during the flowering period, with insect visitation rates being recorded, together with resultant seed production. Water application rates during flowering were found to have no effect on the foraging rates of honey bees during the morning, or on the presence of other insects, and a small but significant effect on the afternoon and overall honey bee foraging rates. Seed production was highly correlated with insect visitation rates, as expected. It was concluded that water irrigation levels during the flowering of onions is not responsible for the periodic unattractiveness of seed producing onions, and water application rates during the earlier growth phases of the onion plants should be assessed in terms of their impact on the attractiveness of the onions to insects during the flowering period. More research must be done to determine the attractiveness of different onion cultivars, with regard to the nectar and pollen compositions. When a baseline of cultivar attractiveness has been established, it could be possible to breed cultivars with more attractive pollen and nectar to honeybees. Additionally, it is extremely important to establish pollen compatibility among hybrid cultivars, and the emphasis must be put on the investigation of non-viable pollen in Hybrid onion cultivars. Lastly, it is crucial to assess the foraging behaviour of honeybees with regard to the natural factors, namely temperature, humidity, rainfall, hive activity and insect activity. This data will enable the industry to use it as an indication of possible complications regarding honeybee pollination on onion cultivars.