Optimizing ecological mechanisms of pest and disease control for sustainable improvement of agroecosystem productivity: major lessons drawn from cirad's omega3 project

CIRAD's Omega3 project, which operated from 2008-2012, aimed at (i) gaining knowledge on ecological pest and disease regulation processes that can be mobilized via plant species diversity (PSD) deployment in agroecosystems, as an alternative to conventional practices based on pesticide use, and (ii) generating tools and methods to design and evaluate innovative pest and disease-resilient cropping systems based on PSD. Some biological models (="pathosystems") representing a range of spatial scales of PSD deployment, across the pest/pathogen life history traits the most amenable to manipulation via by PSD (namely dispersal ability and host specificity), were selected, with a view to robustness and generality of expected results. At a metric scale, we studied the effects of sanitizing plants on soil borne white grubs and parasitic weed Striga affecting upland rice in Madagascar, and on tomato bacterial wilt (TBW) in Martinique. At the field level, we studied the luring effects of trap plants, combined (i) with barrier effects and conservation biological control on tomato fruitworms (TFW) and sap-feeding pests on vegetable crops in Martinique and Niger and, (ii) with a food attractant mixed with a biological insecticide on cucurbit fruit flies in Reunion. We also studied the effect on cocoa plant bugs and black pod rot (BPR) of intercropping cocoa trees with other perennial plants using different spatial designs in Cameroon. At the landscape scale, we studied the effects of the arrangement of various land uses on the incidence of coffee leaf rust (CLR) and the abundance of coffee berry borer (CBB) in Costa Rica. Our examples stressed the need for trade-offs to manage conflicts or exploit synergies in underlying PSD-based processes. For instance, against TBW or upland rice white grubs, the tradeoff between high biomass production for indirect regulation via alteration of microbial communities vs low biomass production but direct regulation via biocidal/allelopathic effect. Or the trade-off between the prevention or encouragement of infestation of the main vegetable crop by early occurring/little damaging sap-feeding pests, in perspective with a positive or adverse effect on regulation of later occurring/highly damaging fruit pests (e.g. TFW), via top-down pathways. Or the trade-off to account for conflicting interactions between cocoa and plant bugs and BPR in relation with shade and natural enemies (entomopathogenic fungi and ants) on the one hand, and between CBB and CLR (and their natural enemies) on coffee in relation with landscape fragmentation/connectivity, on the other hand. Specifically, a spatially-explicit individual-based model including three interacting modules was developed, to be used as a generic tool to improve our understanding of system functioning in our field-level case studies, by assessing relative attractiveness of the commercial vs trap crops, the spatiotemporal planting design of the crops, and the insect behavioural traits. (Texte intégral