Sweet Scents: Nectar Specialist Yeasts Enhance Nectar Attraction of a Generalist Aphid Parasitoid Without Affecting Survival

Abstract

Floral nectar is commonly inhabited by microorganisms, mostly yeasts and bacteria, which can have a strong impact on nectar chemistry and scent. Yet, little is known about the effects of nectar microbes on the behavior and survival of insects belonging to the third trophic level such as parasitoids. Here, we used five nectar-inhabiting yeast species to test the hypothesis that yeast species that almost solely occur in nectar, and therefore substantially rely on floral visitors for dispersal, produce volatile compounds that enhance insect attraction without compromising insect life history parameters, such as survival. Experiments were performed using two nectar specialist yeasts (Metschnikowia gruessii and M. reukaufii) and three generalist species (Aureobasidium pullulans, Hanseniaspora uvarum, and Sporobolomyces roseus). Saccharomyces cerevisiae was included as a reference yeast. We compared olfactory responses of the generalist aphid parasitoid Aphidius ervi (Haliday) (Hymenoptera: Braconidae) when exposed to these microorganisms inoculated in synthetic nectar. Nectar-inhabiting yeasts had a significant impact on nectar chemistry and produced distinct volatile blends, some of which were attractive, while others were neutral or repellent. Among the different yeast species tested, the nectar specialists M. gruessii and M. reukaufii were the only species that produced a highly attractive nectar to parasitoid females, which simultaneously had no adverse effects on longevity and survival of adults. By contrast, parasitoids that fed on nectars fermented with the reference strain, A. pullulans, H. uvarum or S. roseus showed shortest longevity and lowest survival. Additionally, nectars fermented by A. pullulans or S. roseus were consumed significantly less, suggesting a lack of important nutrients or undesirable changes in the nectar chemical profiles. Altogether our results indicate that nectar-inhabiting yeasts play an important, but so far largely overlooked, role in plant-insect interactions by modulating the chemical composition of nectar, and may have important ecological consequences for plant pollination and biological control of herbivorous insects.

The impact of yeast presence in nectar on bumble bee behavior and fitness

“…, Sobhy et al. ). Saccharomyces , for example, produces isobutanol and acetaldehyde, which are known to directly affect insect behavior and play a role in social wasp attraction (Landolt ).…”

“…Artificial nectar fermented with the yeasts M. reukaufii and M. gruessii is enriched in alcohols (Sobhy et al. ). Because many insects specifically employ microbial VOCs to locate food sources (DeVries , Becher et al.…”

“…One way to avoid cheating involves "honest signaling," meaning that the presence of yeasts in nectar gives a fair indication of its nutritional value or an overall fitness benefit (Douglas 2010, Knauer andSchiestl 2015). Previous research has shown that yeasts change the olfactory bouquet of the nectar (Golonka et al 2014, Rering et al 2018, Sobhy et al 2018). Saccharomyces, for example, produces isobutanol and acetaldehyde, which are known to directly affect insect behavior and play a role in social wasp attraction (Landolt 1998).…”

“…Saccharomyces, for example, produces isobutanol and acetaldehyde, which are known to directly affect insect behavior and play a role in social wasp attraction (Landolt 1998). Artificial nectar fermented with the yeasts M. reukaufii and M. gruessii is enriched in alcohols (Sobhy et al 2018). Because many insects specifically employ microbial VOCs to locate food sources (DeVries 1987, Becher et al 2012, Davis et al 2012, Stamps et al 2012, yeast-mediated changes in olfactory bouquets may improve food location by bumble bees and thus reduce the allocation of resources devoted to find suitable nectar patches (Madden et al 2018).…”

Yeast volatomes differentially effect larval feeding in an insect herbivore

“…A wide overlap in compounds released by plants (18) and microbes (19) suggests furthermore that plant headspace includes volatiles that are produced by plant-associated epiphytic and endophytic microbes. A striking example is floral scent, where bacteria and especially yeasts metabolize pollen, nectar and other floral compounds and thus become a prominent source of volatiles (20-25).…”