Pollen feeding by syrphids varies across seasons in a Mediterranean landscape dominated by the olive orchard

Abstract Syrphids provide valuable ecosystem services such as pollination, pest control or organic matter decomposition. In many cases, adults exploit pollen resources from the landscape to complete their life cycles. Yet, the knowledge about the most suitable plants for enhancing syrphid populations is still insufficient for most habitats. Also syrphid distribution across the landscape is understudied. In this work we analyzed: (i) syrphids distribution across different land uses and seasons in a Mediterranean landscape dominated by olive orchards, (ii) pollen resources exploited by the most abundant syrphid species and (iii) specific pollen resources relevant for egg production. Syrphids were captured weekly in spring, summer and autumn from April to November 2013 with a sweep net in patches with different land uses (olive orchards, herbaceous and scrubland patches). Nine species, generally with predatory aphidophagous larvae, were identified. The most abundant species was Sphaerophoria scripta (L.) followed by Melanostoma mellinum (L.). Sphaerophoria scripta was associated to olive orchards while M. mellinum was associated to herbaceous/scrubland patches. Pollen selection and consumption and number of eggs within gravid females for the most abundant species were determined through gut dissection and identified with a microscope. Several pollen types were showed a positive selection index by syrphid species. During spring and summer, the most ingested pollen types belonged to abundant plants in the patches of capture indicating that syrphids possibly do not need to fly among patches to feed during those periods. In spring, Asteraceae and Plantago type were particularly consumed by S. scripta and M. mellinum gravid females respectively while in summer feeding habits of males, gravid and non-gravid females did not differ. Females consumed more pollen than males but the number of eggs in gravid females was not related with the amount of ingested pollen grains. This study brings some insights about the seasonal variation of pollen feeding habits shown by adult syrphids across a Mediterranean heterogeneous landscape dominated by the olive orchard. In spring, dense flowering strips maintained within the olive orchards might attract and nourish adult syrphids while in summer syrphids might fly to non-crop areas to feed.