Bottom-up impact of fertilisation on biocontrol agents

President of the jury :    Thibaut Malausa                   

Rapporteurs :                     

• François Verheggen
• Lucia Zappala      

Examiners

• Christine Becker

Thesis Director :

• Nicolas Desneux
• Anne-Violette Lavoir

Abstract :

In agroecosystems, arthropod communities may be influenced by bottom-up forces induced by environmental variations (e.g., fertilization) through the modification of plant traits. The way bottom-up forces affect the second trophic level is well documented: Fertilization is considered a “bottom-up driver” as it influences the first trophic level (“bottom”) by modifying plant tissues macronutrient and defense content, affecting the plant nutritional quality for herbivores. This, in turn, leads to changes in the second trophic level ("up"). But less is known on their ability to affect the third trophic level. Plant-mediated bottom-up forces may affect natural enemies directly (plant-mediate) or indirectly (plant-herbivore-mediated). Furthermore, as bottom-up effect extend to the third trophic level, they could mediate the intensity of top-down regulation, i.e., biological control of pests. In this context, the objective of the PhD was to elucidate how the fertilization influence the interaction among plant, herbivores and their natural enemies at both individual (life-history traits and biocontrol efficiency) and population (population dynamics) levels.
We studied the plant-mediated bottom-up effect of reduced fertilization on the third trophic level (natural enemies), to determine whether diversity in the natural enemy diet regime (carnivorous vs omnivorous predators) or functional guild (predator vs parasitoid) modifies the intensity of bottom-up effects. We evaluated the tri-trophic system “tomato plant – pest - natural enemies” following three different approaches: (a) meta-analysis to overview of the current state of the art; (b) experimental analysis at individual level under controlled conditions (lab experiments) to look for mechanisms and (c) population dynamic level experiment in greenhouses.
Our findings suggest that reducing fertilization does not consistently lead to positive outcomes in terms of reducing pest population dynamics, with efficacy varying depending on the herbivore species. Specifically, we observed that aphid populations increased to a lesser extent under medium fertilization compared to high fertilization, although this trend was not observed for whitefly. Our results also indicate evidence of diluted bottom-up forces through trophic levels, depending on the herbivore types. There was variable but overall marginally positive effects of reducing fertilization from high to intermediate levels on host quality and biocontrol efficiency. In lab experiments, reducing fertilization to 50% of standard (optimal) application levels had an overall positive, albeit limited, impact on the biocontrol efficiency of natural enemies across all functional guilds, with parasitoids being less affected than predators. Parasitoids showed a positive response in biocontrol efficiency at the population level, influencing the temporal dynamics of natural enemy-pest interactions. Furthermore, this study highlights how manipulating these forces could support the management of specific pest species within the framework of Integrated Pest Management.

Keywords :

Predator, parasitoid, tri-trophic interaction, biocontrol efficiency, biocontrol agents, agricultural transition, input reduction, inorganic fertilisers, plant-insect interaction

The thesis can also be followed remotely via Zoom:

https://univ-cotedazur.zoom.us/j/84149633096?pwd=ejYwSVQxYm1UZGdXZTQ5SE10S2szUT09