From host plant resistance to agroecosystem resistance : Lessons from a comprehensive case-study on the management of sorghum panicle pests in West and Central Africa

Although it has often been the cornerstone of integrated pest management (IPM) programs, an approach based on crop host-plant resistance (HPR) alone shows limitations, unless it takes into account upper scale levels than the mere plant or field, namely the agroecosystem or the landscape levels, in view of achieving sustainable HPR. On the other hand, agroecology, which is based on the enhancement of bioecological processes in agriculture can help achieve, beyond mere HPR, an actual sustainable agroecosystem resistance to pests. This approach is illustrated with examples taken from the development and implementation of options following this approach for the management of sorghum panicle pests in West and Central Africa. We firstly undertook bioecological studies on mirid panicle-feeding bugs which have recently become major pests of improved short-cycled compact-headed sorghum cultivars in this region. These studies demonstrated the role of these pests as factors increasing grain mould infection, how they were affected by weather factors, and established the existence of alternate hosts. Then, multi-local tests improved our knowledge of the relationships (in terms of damage) between a set of sorghum cultivars and these pests, while laboratory studies put forward a likely cause of sorghum resistance to mirid bugs, namely quicker endosperm hardening. Meanwhile, we investigated the genetic bases of these resistances, both by quantitative genetics and molecular methods. Lastly, crosses were made which ultimately resulted in the release of sorghum cultivar CIRAD 441 which combines resistance to both sorghum midge and panicle-feeding bugs with desirable agronomic traits. On the other hand, the results of a study on on-farm experimental designs suggested that panicle-feeding bug infestation and damage was not only influenced by sorghum genotype, but also field size, and its genotypic environment, namely the vicinity of other farmers' fields of pest-susceptible or -resistant cultivars, and the relative positions of these fields. On-farm trials demonstrated that castor bean was an alternate host and a significant source of sorghum infestation by panicle-feeding bugs. This led to the prospect of reducing pest damage by management of castor beans (by destruction of its spikes before sorghum flowering, and/or using it as a trap crop after sorghum maturity). These results are placed in perspective with observations on other pests of sorghum in the region, and on other crops and/or other tropical regions, which also call for an agroecological approach to crop pest management for sustainable food production. (Résumé d'auteur