Effects of increasing honeybee densities on sunflower yield components

Domesticated honeybee management (Apis mellifera) could be a useful tool to supplement wild pollinators and increase crop productivity. However, it is necessary to know the optimal hive density for each particular crop. Sunflower (Helianthus annuus) is the third most produced oil seed crop in the world and is dependent on insect pollination to increase the number of seeds and oil content. We proposed quantifying the H. annuus dependency to pollinators and assessing the optimal hive density of honeybees to increase sunflower productivity in 2 ha plots immersed in an extensive soybean matrix in Argentina. We set up a field experiment of five plots with a different number of hives each one (0, 5, 10, 15, 20), assessed fruit set and crop yield of flower heads open and excluded from pollinators and insect abundance. The set and crop yield increased in the heads of open flowers. The crop yield did not present a linear response to the density of the hive, although there could be a positive effect of the intermediate densities. Crop yields were not explained by the abundance of the two most frequently pollinator species, Astylus atromaculatus (Coleoptera) and Apis mellifera, suggesting that sunflower productivity was limited by resources other than pollen (e.g., the low abundance of rains). Drought conditions could have limited the response of the crop to hive addition and also influenced the high A. atromaculatus abundance, which could play a pollinator role in this system. More repetitions (plots and years) are needed to get these results more robust.