Hoverflies (Diptera : Syrphidae) as pollinators of sunflowers in the Waterberg region of Limpopo province, South Africa

Abstract

Pressure is being placed on the agricultural sector to supply an ever-growing demand for food as the population continues to increase globally. This is further complicated by a continues decrease in pollinators needed for fruit or seed yield due to human activity. One such crop is the economically important sunflower (Helianthus annuus L.), prised for its oil and use in animal feed production. Sunflowers are self-pollinated, however, result in a reduced yield without the interaction of pollinators. Most sunflower farms in South Africa rely on wild pollinators or managed honey bees for this service, but the dynamics of these pollinators is poorly known. This study aimed to create a better understanding of the role hover flies have in the pollination of sunflowers, focussing the study on the Lehau, South Africa, sunflower producing region. The two main objectives were (i) to determine hover fly assemblage in and around sunflowers fields during the growing season and (ii) to evaluate their sunflower visitation based on pollen load as well as to quantify their contribution to seed set. Hover fly assemblage was determined sweep net sampling on five sunflower fields and species identification based on taxonomic guides. This showed eleven species present in the area with Ischiodon aegyptius being the most abundant species throughout the growing season. Hover flies followed an edge effect with most of the sampled individuals being close to edge of the fields. Pollen load on the hover fly bodies was determined by comparing them to a pre-created pollen atlas from wildflowers found within a 100m radius of sample fields. This revealed that Asteraceae and Solanaceae were the two most prominent wildflowers interactions, with the sunflower containing Asteraceae pollen group being present on almost all of the sampled species. Contribution to seed set was established be enclosing pollinators (Syrphinae, Eristalinae, Muscidae and Apis mellifera) in voile bags on sunflower heads for 24 hours and comparing that to heads were pollinators were excluded and heads that was open to all pollinators for 24 hours. This resulted in Muscidae interaction showing the largest contribution to the proportion of pollinated seeds with Syrphinae interaction showing the second highest contribution. This showed that not only do a diverse assemblage of hover flies interact with sunflowers, but they do also contribute to the seed set. Ischiodon aegyptius is the most likely candidate for mass rearing as it is a generalist pollinator present throughout the growing season and can interact with a diverse assemblage of wildflowers as alternative food source when sunflower heads are not yet present.