Pollinators on the Edge? The Spatio-Temporal Distribution of Diurnal and Nocturnal Floral Visitors in Mango Orchards and its Effect on Fruit Set

Wild insects are efficient pollinators of many crops, but the stability of the pollination services they provide depends on their spatio-temporal variability, an issue which has received little study. We studied spatio-temporal variation in wild floral visitor assemblages and its effect on mango (Mangifera indica L.) fruit set in the Darwin and Katherine regions of the Northern Territory, Australia. We conducted diurnal and nocturnal floral visitor surveys and recorded fruit set on cv. Kensington Pride mango trees located at increasing distances (up to 400m) from orchard edges adjacent to native vegetation. Floral visitor assemblages varied considerably between farms, survey times and with distance from the crop border. As predicted, floral visitor abundance and diversity declined with increasing distance from the crop border. Visits from the most abundant diurnal visitor – the native stingless bee, Tetragonula mellipes, as well as nocturnal visitors such as moths, declined with increasing distance from the crop border. In contrast, visits from hoverflies (Mesembrius bengalensis) and blowflies (Chrysomya spp.), the second and third most abundant diurnal visitor taxa, were evenly spread across the orchard. We also found that fruit count (at both early and harvest stage) increased with total insect visitation, suggesting a major role for insects in mango pollination. Our results show that some wild insects, particularly T. mellipes, are consistent and abundant floral visitors to mango in these regions and likely to be effective pollinators. Meanwhile, we recorded negligible honey bee visitation (<0.1% total) to mango flowers, illustrating the importance and stability of pollination services from wild insects in these regions. Tetragonula mellipes is a social species that can be kept in hives, like honey bees, and could be managed for crop pollination. Placing T. mellipes hives in the centre of mango orchards may increase pollination services and potentially farm productivity. Similarly, maintaining areas of nearby native vegetation and limiting orchard block size may facilitate pollination in new orchards.