The behavioural response of Drosophila suzukii to fermentation products

Drosophila suzukii, or the Spotted Wing Drosophila (SWD), is an invasive fly that is a pest of soft fruits and berries. Unlike other Drosophila species, the females lay their eggs in ripening and ripe fruits, causing crop losses. In the last decade SWD has invaded North and South America and Europe and since 2014 the fly can also be found in Sweden. Despite preferring to oviposit in ripe fruit, D. suzukii is, like many other drosophilids, dependent on yeasts as a source of important nutrients. An especially strong connection has previously been seen to the yeast Hanseniaspora uvarum. In this work the behavioural effect of this yeast on D. suzukii was investigated. A larval assay was carried out to identify the most attractive single compounds from the headspace of H. uvarum. A blend of two of the more attractive compounds, ethyl acetate and acetoin, produced similar attraction levels as an H. uvarum liquid culture. Oviposition assays were carried out with real and artificial blueberries where mated female flies were given a choice between laying eggs in berries inoculated with undiluted, diluted or no H. uvarum. While no significant differences could be found between the treatments, it was noted that females were more likely to lay eggs if given access to undiluted yeast. Mating observations to investigate where matings where most likely to take place showed that a majority of the flies aggregated and mated on raspberries inoculated with H. uvarum compared to clean raspberries, fresh raspberry shoots and an H. uvarum culture the flies could smell but not feed on. Finally, a monitoring project was carried out in collaboration with hobby growers to investigate the current distribution of D. suzukii in Sweden. The results showed that D. suzukii is now present in most of southern Sweden, up to and including Stockholm, in both populated and rural areas.