Multi-target biocontrol plants: the case of the tomato agrosystem

President of the jury :                       

• Dr. Thibault MALAUSA, Directeur de Recherche, Institut Sophia Agrobiotech, France

Rapporteurs :                     

• Dr. Matthew BACK, Enseignant-chercheur, Harper Adams University, UK
• Dr. Emilie DELETRE, Chercheure, CIRAD UR HortSys Dept PerSyst Campus de Baillarguet, France

Examiners

• Dr. Estelle FOREY, Professeure des universités, Université de Rouen, France

Thesis Directors :

• Dr. Caroline DJIAN-CAPORALINO, Ingenieure de recherche, Institut Sophia Agrobiotech, France
• Dr. Anne-Violette LAVOIR, Maître de Conférences, Université Côte d’Azur, France

Abstract :

Agricultural systems face multi-pest attacks that threaten crop productivity and call for integrated agroecological approaches that reduce synthetic pesticides for pest management. Multi-target biocontrol plants (MBPs) offer a promising strategy by leveraging plant traits to simultaneously manage multiple pests (e.g. above- and below-ground). This thesis proposes and explores the MBP concept, application, and demonstration through bibliographic and experimental approaches.

First, a systematic review defined the MBP concept and proposed a method to identify potential MBPs, taking the tomato agrosystem as a model cropping system. Findings reveal a paucity of documented MBPs, with only one such plant tested for the management of both above and below-ground pests described in a single experiment. The literature highlights the predominance of studies focusing on a single biocontrol plant against a single pest species or pest type (mainly aerial), with multi-trophic and multi-pest interactions not being studied. Yet, applying our method, allowed us to identify 9 potential MBPs for the tomato agrosystems, among them 3 Tagetes species: T. erecta, T. patula, and T. minuta.

The potential of the Tagetes genus was therefore tested on root-knot nematodes (Meloidogyne spp.) and aerial pests, specifically the whitefly (Bemisia tabaci). We performed various experiments in controlled conditions to test MBP candidates and clarified their mode of action on each targeted pest separately. The evaluation in a climatic chamber of the 3 marigolds for their potential nematicide properties confirmed their quality as non-hosts for 3 important species of root-knot nematodes of the genus Meloidogyne. In the greenhouse, the selected nematicidal plants (T. erecta and T. patula) in co-culture with tomato were found to be significantly effective in controlling M. incognita during its first development cycle, especially at a density of 1 nematicidal plant per 1 tomato, but their effectiveness decreased after the 3rd cycle of nematode multiplication. The thesis then explores the deterrent effects of MBP candidates to the whitefly, B. tabaci using free dual-choice set-ups in the climate chamber. The three marigolds showed deterrent effects by reducing B. tabaci settlement and oviposition on tomato plants. The use of fake plants demonstrated a physical barrier effect and the positioning of marigolds behind tomato plants suggests a repellent olfactory message. The odour of marigold leaves was sampled and analysed using GC-MS to formulate hypotheses on this repellent effect.

The thesis then explores the MBP potential of T. minuta when intercropped with tomato, targeting both above-ground (B. tabaci) and below-ground (M. incognita) pests in the greenhouse. Results showed that this marigold effectively reduced whitefly populations but had no significant impact on RKN infestation after 3 cycles, confirming previous results with the other marigold species and highlighting the complexity of multi-pest interactions. Plant growth measurements revealed a competitive effect of T. minuta co-culture on tomato aboveground biomass but a positive effect on fruit production, highlighting the complexity of the system and the need for a multi-criteria assessment of the use of an MBP in a cropping system.

Our research advances the concept of MBPs, proposing potential candidates for multi-pest management. Overall, the findings emphasise the challenges in identifying and validating MBPs, as plant-pest interactions are often complex and context-dependent. They highlight the need for further field trials under production conditions, exploring alternative implementation strategies and new MBP candidates, and integrating MBPs into complementary agroecological practices (IPM) combinations. This work will thus contribute to developing sustainable pest management strategies, reducing reliance on synthetic pesticides, and supporting agroecological transitions.

Keywords :

Multi-target biocontrol plants, multi-pest species attacks, pest management, plant functional diversity, agroecology.

In person or via Zoom: 

https://univ-cotedazur.zoom.us/j/82715541933?pwd=2bcNWuBY4bSUHXW4rjRGbZHYcrdQiF.1