[Un article de The Conversation écrit par Guy Richard – Directeur de l”expertise scientifique collective, de la prospective et des études à l’Institut national de la recherche pour l’agriculture, l’alimentation et l’environnement (INRAE), Inrae – Alexandre Gohin – Directeur de recherche, INRAE, Inrae – Anne Laperche – Professeure en génétique et amélioration des plantes, Institut Agro Rennes-Angers – Bernard Bodson – Docteur en sciences agronomiques, Professeur honoraire, Gembloux Agro-Bio Tech, Université de Liège – Christophe David – Associate Dean, Senior scientist, Institut supérieur d’agriculture Rhône-Alpes – Eugénie Hebrard Directrice de recherche, Institut de recherche pour le développement (IRD) – Marianne Sellam – Directrice scientifique et technique, Acta – Philippe Reignault – Directeur de la santé des végétaux, Agence nationale de sécurité sanitaire de l’alimentation, de l’environnement et du travail (Anses) & Virginie Ravigné – Chercheuse en écologie des communautés microbiennes et virales, Cirad]
The 1er September 2018, France banned the agricultural use of neonicotinoids (NNI), insecticides which act on the nervous system of insects. These molecules, recognized as one of the causes of the decline of bee colonies and biodiversity, have been used since the 1990s to control aphids which infest crops, such as sugar beet.
Present in seed coatings, different NNIs were able to spread through plants and into the soil where they could reach soil organisms, including seeds of subsequent crops and seeds of pollen and nectar-producing plants, which can then be visited by pollinators.
The ban on NNIs led to an average yield drop of 28% in 2020 across the entire area cultivated with French sugar beets, due to viral yellows spread by high populations of aphids.
Agricultural professionals then mobilized to:
- obtain an exemption from the use of NNIs subject to the annual opinion of a supervisory board bringing together parliamentarians, an exemption which will itself be prohibited in 2023 following a ruling by the European Court of Justice;
- support and accelerate the search for alternatives to NNI on beet.
Since 2021, the ministry in charge of agriculture has supported a national research and innovation program (PNRI) initially allocated 7.2 million euros over three years – and since renewed – to explore short-term operational solutions. The program is co-led by the National Institute for Research on Agriculture, Food and the Environment (Inrae) and the Technical Institute of Beet (ITB). It relies on a steering committee made up of representatives from the sector and a scientific council bringing together several institutes and chaired by INRAE.
As members of this scientific council, we wish to take stock of the PNRI four years after its establishment, at a time when an exemption from the use of acetamiprid (NNI applied by spraying on plant leaves) was proposed, provoking numerous reactions in the agricultural world and among citizens.
The PNRI supported 25 research projects carried out by scientists from diverse origins: public research, higher education, agricultural education, technical institutes, sugar mills, seed and biocontrol companies, insurers.
Several projects directly involved farmers for field experiments. The PNRI thus made it possible to better understand and predict the development of the disease, to test different agronomic means to manage it and to study the conditions for their deployment.
But to understand what solutions have been adopted today to combat jaundice without NNI, let's start by looking at what this disease consists of.
Jaundice disease
Beet yellows is caused by four plant viruses, sometimes in multiple infections (the Beet Yellow Virus, or BYV, being the most damaging virus). These viruses are transmitted to plants by insect vectors, mainly the green peach aphid (Myzus persicae), but also by the black bean aphid (Aphis fabae), when these aphids bite plants to feed on their sap.
The virus then causes an alteration in photosynthesis and the transfer of sugars from the leaves to the roots: the plant turns yellow and loses its ability to accumulate sugar in its root.
However, the viruses cannot be transmitted to the offspring of the carrying aphids. It is only by biting an already infected plant that the aphid picks up the virus and can then transmit it to other beet plants. Controlling virus reservoirs therefore appears to be an important element in disease prevention and management.
Control of virus reservoirs
Beet cultivation is the main reservoir of yellows virus via :
- the regrowth of beet plants left in the plots following the harvest, or at the edge of the plots following the temporary storage of the tuberous roots before their transport to the sugar factory;
- plants intended for seed production, which spend the winter in plots sown in the fall and harvested the following summer.
In both cases, the continued presence of beetroot allows aphids to load with viruses throughout the year. Prevention consists of burying or destroying regrowth and geographically separating (at least one kilometer) the plots intended for seed production and those intended for sugar production. The proximity of the two types of beet plots is probably the cause of particularly marked viral symptoms in the Center and Île-de-France regions.
Disruption of aphid movement
Unlike the yellows virus, aphids have multiple host plants (rapeseed, mustard, etc.). Having a winged stage, their movement distances reach several tens, or even hundreds, of kilometers.
These two elements compromise the control of their dissemination. On the other hand, it is possible to predict the arrival dates of aphid flights from winter temperature data with models that are improving. Furthermore, the presence of so-called companion plants between the rows of beet (oats, barley in particular) disrupts the identification of beet plants by aphids.
The diffusion of volatile organic compounds in the atmosphere or the presence of certain non-host plant species also divert aphids from these plots (grasses) or attract them outside (crucifers). The average effectiveness of these two techniques was estimated at 50% on aphid populations and 40% for yellows symptoms. Companion plants should be destroyed at the right time to avoid any competition with the beet itself and not cause a reduction in yield.
The destruction of aphids
The provision of insects that prey on aphids (lacewings, hoverflies) and the presence of plant cover that is favorable to them (grass strips, hedges) are ways of reducing aphid populations.
Their effectiveness varies greatly, from 0 to 68% depending on the plot, probably linked to climatic conditions that are more or less favorable to the survival and development of the beneficials.
Simply favoring predators does not control the development of aphids. This technique must in fact be associated with other means of control.
The provision of synthetic insecticides remains possible with two active ingredients currently authorized, flonicamid and spirotetramat, relatively specific for aphids and effective. These insecticides have a maximum duration of action of two weeks, which may require several passes depending on the succession of aphid flights. Natural insecticide products have been tested, with disappointing and very variable results in the field, except for an entomopathogenic fungus (Lecanicilium muscarium) with an efficiency of 0 to 41%.
Towards varieties less susceptible to the virus
Reducing plant susceptibility to yellows virus is another course of action. Variations in disease susceptibility were measured for current varieties and for plant populations used in future variety breeding programs. This suggests future more or less resistant varieties with variable production potential, which will ultimately make it possible to better adjust the choice of varieties to the epidemiological context.
The stage of development of the plant is also a factor of sensitivity to the virus, the damage of which is more significant at young stages. We must therefore seek to sow as early as possible to avoid the usual dates of arrival of aphids.
We also know that the quantity of nitrogen in the plant affects the attractiveness of the plant: it is higher when the nitrogen content of the plant is high. A reduction in early nitrogen fertilization therefore remains a possibility to be explored. Stimulators of natural plant defenses have also been tested, without proven effectiveness in the open field.
The combination of means of control
Various levers can therefore be combined to combat yellows at the plot level: choice of variety, management of nitrogen fertilization, provision of plant defense stimulators, sowing of companion plants in inter-rows, sowing of plants sheltering aphid predators, repelling or attracting aphids, provision of aphid predators or parasitoids, provision insecticides, of natural or synthetic origin.
At the scale of the production area, we can play on the destruction of beet regrowth from the previous year, the spatial organization of beet plots for seed or sugar production.
Several combinations were tested in plots of farmers who signed up to implement new practices in return for compensation. Trials are continuing to better evaluate the effectiveness and cost of the practices. We can say today that there is not an alternative to NNI that is as effective when applied alone, but a set of alternatives to combine for maximum effectiveness of around 70% on aphids and 50% on the disease. However, these assessments remain difficult to carry out in relatively low infestation contexts since 2020.
The results of the PNRI program
Apart from the year 2020, the average yield of French agricultural area in beet was little affected by viral yellows. This is probably due to several factors: the climatic characteristics of the years 2019, 2023 and 2024, considered unfavorable for the development of aphids (2021 and 2022 benefited from the NNI exemption), the use of the two synthetic insecticides, the preventive actions with regard to virus reservoirs implemented since 2024.
Even if the PNRI has not completely achieved its objective of deploying, from 2024, operational solutions, it is necessary to underline the quality of the research carried out to understand the disease, the scale and the operationality of the results obtained. Alternatives to the systematic use of chemical insecticides exist, more complicated, more expensive to deploy and less effective than foliar spraying of an NNI, but they do not harm environmental health or human health.
Given these results, it seems entirely possible to us to promote alternatives to NNI, but recognizing that farmers cannot be the only ones to bear the additional cost and associated risk. Two approaches, to be combined, are to be developed:
- on the one hand, an insurance mechanism could be put in place to allow farmers to cope with years of heavy infestation. One of the PNRI projects demonstrated its feasibility, especially since this situation was only observed in 2020 (i.e. one year out of four);
- on the other hand, a mechanism for distributing the cost across the entire producer-processor-distributor-consumer chain and not just on farmers in the spirit of the Egalim 1 and 2 laws for improving the balance of commercial relations in the agricultural and food sector.
To this must be added an intra-European or international non-competition mechanism.
The PNRI therefore appears to be a multidisciplinary program with various actors which has made it possible to develop and define methods for deploying alternatives to NNI. This program benefits from an extension to complete the evaluation of these alternatives. We see here the essential role played by political decisions in giving a credible signal to the stakeholders of a system, thus allowing them to mobilize to find solutions.
With an unwavering passion for local news, Christopher leads our editorial team with integrity and dedication. With over 20 years’ experience, he is the backbone of Wouldsayso, ensuring that we stay true to our mission to inform.
