Inoculation of pear flowers with Metschnikowia reukaufii and Acinetobacter nectaris enhances attraction of honeybees and hoverflies, but does not increase fruit and seed set

Abstract

Currently, one of the most important challenges is to provide sufficient and affordable food and energy for a fast-growing world population, alongside preserving natural habitats and maintaining biodiversity. About 35% of the global food production depends on animals for pollination. In recent years, an alarming worldwide decline in pollinators has been reported, putting our food production under additional pressure. Therefore, there is an urgent need to find sustainable ways to ensure this crucial ecosystem service. Recent studies have shown that floral nectar is generally colonized by microorganisms, specifically yeasts and bacteria, which may alter nectar chemistry and enhance attraction of pollinators. In this study, we investigated changes in pollinator foraging behavior and pollination success in European pear (Pyrus communis L.) cultivars ‘Regal Red’ and ‘Sweet Sensation’ (red sports of ‘Doyenné de Comice’) after flower inoculation with the typical nectar-inhabiting microorganisms Metschnikowia reukaufii and Acinetobacter nectaris, and a combination of both. Pollination success was monitored by measuring the number of flower visits, fruit set and seed set in two consecutive years, 2019 and 2020. Results revealed that application of a mixture of M. reukaufii and A. nectaris resulted in significantly higher visitation rates of honeybees and hoverflies. By contrast, no effects on flower visits were found when yeasts and bacteria were applied separately. Fruit set and seed set were not significantly affected by any of the inoculation treatments. The only factors affecting fruit set were initial number of flower clusters on the trees and the year. The absence of treatment effects can most likely be attributed to the fact that pollination was not a limiting factor for fruit set in our experiments. Altogether, our results show that inoculation of flowers with nectar microbes can modify pollinator foraging patterns, but did not lead to increased pollination success under the conditions tested.

The Floral Microbiome and Its Management in Agroecosystems: A Perspective

“…Despite this, no effect on fruit set was observed in two consecutive years of treatment. The authors hypothesized however that this lack of an effect was likely due to the hosts not being pollen-limited, and further, that under such conditions, treating flowers with attractive microbes could potentially boost pollination . In another study involving almond, Schaeffer et al also observed a neutral effect of nectar-inhabiting microbes on pollination, as measured through pollen tube number.…”

“…Recent work in wildflower systems has revealed that floral microbes can affect plant reproductive outcomes; however, evidence for such an effect in agricultural systems is limited. In a recent study conducted with pear, Colda et al found that application of a mixture of M. reukaufii and Acinetobacter nectaris at bloom can increase hoverfly and honey bee visitation to blossoms. Despite this, no effect on fruit set was observed in two consecutive years of treatment.…”

Potential effects of nectar microbes on pollinator health

“…However, to our knowledge, little work has been done to address the influence of nectar microbes on non-bee flower visitors. Two studies on Dipteran species have revealed an attraction to nectar yeasts and bacteria: hoverflies and mosquitoes ( Culex pipiens ) are attracted to the bacterium Acinetobacter nectaris and nectar yeasts M. reukaufii and Lachancea thermotolerans , respectively [94,133]. Nectar sugars were not addressed in either of these studies though, with VOCs instead noted to play an important role in attraction.…”

“…This possibility suggests potential for both additive and non-additive effects of nectar microbe assemblages on nectar chemistry and pollinator health. Indeed, recent studies have revealed that yeast and bacteria consortia can increase nectar scent in an additive manner and can have positive effects on honeybee and hoverfly visitation [44,94], yet effects on bumblebee reproduction and colony development are no better than exposure to individual species alone [88]. Considerable work remains, however, as to the effect of nectar microbe consortia on pollinator attraction, nectar feeding, and health [95].…”

Honey Bee Foraging Decisions Influenced by Pear Volatiles

“…However, it was demonstrated that pear flowers are not as attractive as the flowers of other crops, such as apple, peach, and rape, due to the low volume and sugar concentration of their nectar [13,14,17]. Microorganism inoculation, bee attractants, pollen trapping, and syrup feeding have been explored to promote honey bee foraging and pear pollination in orchards [18][19][20].…”