Pollination syndrome of the African custard apple ( Annona senegalensis Pers.) reveals reliance on specialized brood-site weevil pollinators in Annonaceae

The pollination syndrome of the African custard apple ( Annona senegalensis Pers.; Annonaceae) was investigated to gain insights into the reproductive strategy of this multipurpose wild shrub. Flower visitors were collected and frequently surveyed in the morning on 30 flowers of 30 plants per site, at four sites across the Sudanian and Sudano-Sahelian climatic zones of Burkina Faso during the flowering season, from May to June. A total of nine species of insects including Coleoptera (Curculionidae, Scarabeidae), Hymenoptera (Formicidae) and Diptera (Tephritidae) were observed visiting flowers. Among the recorded insects, two sympatric species of weevil (Curculioninae, Ochyromerini, Endaeus castus Schoenherr and Endaeus floralis Marshall) were by far the most abundant (97% of specimens) and were observed on all the 120 flowers sampled. Both species were observed on the fleshy petals of the cantharophilous flowers of Annona senegalensis that they use as substrate for the development of larval stages. These observations echo with the ever-growing number of researches highlighting the major role of weevils in the pollination of the pantropical Annonaceae. Our study provides first evidence of brood-site mutualism between an Annonaceae species and its specialized weevils pollinators.