Floral nectar yeasts enhance parasitoid foraging and maintenance

Floral nectar is a sugar-rich, nutritional reward offered by many plant species to recruit flower-visiting insects for their ecological services such as pollination and plant protection. Owing to its high concentration of sugars, floral nectar is a typical habitat for many nectarivorous microbes, most often nectar-inhabiting yeasts (NIYs) and bacteria. NIYs alter nectar chemistry and change its odor by releasing a specific blend of microbial volatile organic compounds (MVOCs). These changes by NIYs may impact the insect attraction and the fitness of nectar-consuming insects. In this study, we tested the hypothesis that different NIYs affect insect attraction, nectar intake and adult life span disparately, by altering the chemistry of nectar and/or producing distinct aroma-active compounds. We used the generalist aphid parasitoid Aphidius ervi (Haliday) as a study organism. Adult parasitoids were provided with a synthetic nectar solution that was fermented with one of five different NIYs that commonly occur in floral nectar. Overall, NIYs significantly impacted nectar chemistry by changing its acidity, sugar and amino acid profiles. Moreover, the NIYs produced a species-specific blend of MVOCs that was substantially different from yeast-free (mock) nectar. Olfactometer bioassays revealed that A. ervi females showed a strong preference for nectar fermented with Metschnikowia reukauffii, followed by Metschnikowia gruessii and Aureobasidium pullulans, but did not distinguish between mock nectar and nectar fermented with Hanseniaspora uvarum. Nectar fermented with Sporobolomyces roseus was clearly repellent to female parasitoids. Parasitoids showed a significant difference in intake among the various fermented nectars. Furthermore, adults of A. ervi survived longest when fed with nectar fermented with M. reukauffii and M. gruessii. The outcomes of our study provide further insight into the ecological implications of NIYs in tritrophic interactions.status: publishe