Luca Carisio: How much nectar is produced when the effect of flower-visiting insects is considered?

About the paper

Our paper describes how to estimate the nutritional contribution of plants to pollinators and to other flower-visiting insects. The nutritional contribution of plants to pollinators is usually estimated by measuring—using a mesh bag—the nectar volume produced by flowers isolated for a 24 h period from flower-visiting insects. Many studies adopted this 24 h measure as a proxy of plant nectar production.

A fennel inflorescence isolated from flower-visiting insects with a mesh beg (credit: Lucie Schurr)

When the 24 h measure is used, there is the implicit assumption that plant nectar production is independent from insect foraging activity, although many studies have provided evidence to the contrary of this assumption. For instance, some studies demonstrated that flowers visited by foraging animals several times produce either more or less nectar than flowers visited a single time. Thus, important questions on the reliability of the 24-h measure as a proxy of plant nectar production require our attention. For example, does nectar production respond to different nectar removal rates? And, how much the 24-h measure may underestimate nectar production in an environment where nectar removal is frequent?

In our study area—the ‘Plateau de Valensole’ (Alpes-de-Haute-Provence, South of France)—cultivated lavender (Lavandula hybrida) and fennel (Foeniculum vulgare) plants are intensively visited by honey bees (Apis mellifera) brought by beekeepers for lavender honey production. This made ‘Plateau de Valensole’ a perfect place to assess the effect of insect nectar removal on nectar production.

Plateau de Valensole landscape with flourished fennel (yellow) and lavender (purple) (credit: Luca Carisio)

In this paper, we developed a simulation model to identify different scenarios where insect foraging activity can alter the estimated 24 h volume of nectar (‘estimated 24 h volume’ hereafter). In addition, we tested whether the estimated 24 h volume differed from the measured 24 h volume for both crops. High nectar removal rates increased the estimated 24 h volume of lavender, while we found the opposite for fennel. Importantly, the simulation indicated that estimated 24 h volume was always greater than the measured 24 h volume.

The idea of this paper came up during my PhD at the University of Turin, Italy. In this period, I was hosted by the Prof. Benoît Geslin’s lab (Université of Aix Marseille Université, France) and involved in a project which aimed to evaluate the importance of pollination for production of plant essential oils. Preliminary results on nectar production dynamics of lavender and fennel showed that the nectar produced in 24-h measure were not in line with the nectar production in shorter time spans. Therefore, we started to discuss whether the measured 24 h volume was a good proxy of nectar production. My colleague Lucie Schurr had at their disposal a big amount of data on nectar production of the two crops together with data on insect foraging activity. With a little more field sampling, we had the perfect recipe to develop a simulation model.

The simulation suggested that measured 24 h nectar volume may lead to inaccurate estimates of nectar resources produced by a given landscape. This result can be considered a baseline to raise awareness on the limitations of currently used methods to assess nectar resources. Therefore, our findings suggest that it can be fruitful to look for alternative methods that consider nectar production as a trait that interacts dynamically with insect foraging activity. Alternative methods would lead to a better estimation of the number of flower visitors that a given environment can support.

About the research

Poor nutrition has been indicated as a major driver of pollinator decline. Therefore, it is vital to develop pollinator conservation strategies that consider which areas provide substantial nectar resources. Our research highlights how limited our current understanding is in the field of pollinator resources.

According to our results, we should move forward to new estimation methods that consider the effect of insects on nectar production. The shift to a new estimation method would have practical implications; for example, it would help to better assess the beehive load that can be installed in a landscape, while preserving the native flower-visiting fauna.

In the middle of a fennel field (credit: Luca Carisio)

During the research, we were very surprised by the difference between the estimated 24 h volume and the measured 24 h volume. There is an extensive body of literature assessing nectar resources through a measuring method that might be based on an unfounded assumption, at least in some plant-insect interaction contexts.

Different factors affect nectar production such as plant genetics, plant-insect interactions, and abiotic factors. Given the complexity of nectar production, many questions still require an answer. For example, is the dynamic of nectar production a heritable trait?

About The Author

The author, Luca Carisio

While studying at the university of Padua, Italy, I was involved in a practical activity in which we had to test, via simulated data, whether the Torimus synesis—a parasitoid used for the biocontrol of the chestnut gall wasp (Dryocosmus kuriphilus)—had an impact on native oak gall wasps. I was totally fascinated by the huge potential of some quantitative data in the understanding of biology of these extremely small wasps. The excitement that I felt in understanding ecological systems led me to enrol for a Phd at the University of Turin, focussing on pollination ecology. I studied different aspects of pollination as ecosystem service. During my reading on ecosystem services, I was inspired by the concept of ecological intensification—replacement of anthropogenic inputs and/or enhancement of crop productivity, by including regulating and supporting ecosystem services management—which was developed by Prof. Riccardo Bommarco.

Currently, I am a postdoctoral researcher at Prof. Simone Tosi’s lab at the University of Turin, Italy. My current research is aimed at disentangling how natural and anthropogenic factors influence social and solitary bees alone and in combination. Specifically, we are studying risk assessments to assist human activities concerned with the protection of bees and the environment.

The feeling of participating in widening our scientific knowledge on bee ecology drives me to keep on in science. However, research is deeply underfunded in Italy, especially when compared with most of the other European countries. The actual situation puts at risk the economic sustainability of research in Italy. The low availability of funding has led to competition between researchers, finally inducing less collaborative behaviours. My piece of advice would be to keep on being collaborative and to exchange ideas with your colleagues. The best discoveries start with researchers exchanging ideas in front of a pint!

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