Increasing the use of synthetic fertilisers and pesticides in agroecosystems has led to higher crop yields, accompanied by a decline in biodiversity at the levels of field, cropping system and farm. Biodiversity decline has been favoured by changes at landscape level such as regional farm specialisation, increases in field size, and the removal of hedgerows and woodlots. The loss of biodiversity in agroecosystems has increased the need for external inputs because beneficial functions are no longer provided by beneficial species as natural enemies of crop pests and ecosystem engineers. This trend has led to a strong reliance on petrochemicals in agroecosystems. However, many scientists have been arguing for more than two decades that this reliance on petrochemicals could be considerably reduced by a better use of biotic interactions. This article reviews options to increase beneficial biotic interactions in agroecosystems and to improve pest management and crop nutrition whilst decreasing petrochemical use. Four agronomic options are presented. First, it has been shown that the choice of cultivar, the sowing date and nitrogen fertilisation practices can be manipulated to prevent interactions between pests and crop, in either time or space. Nevertheless, the efficacy of these manipulations may be limited by pest adaptation. Second, beneficial biotic interactions may result from appropriate changes to the habitats of natural enemies and ecosystem engineers, mediated by soil and weed management. Here, knowledge is scarce, and indirect and complex effects are poorly understood. Third, changes achieved by crop diversification and, fourth, by landscape adaptation are promising. However, these practices also present drawbacks that may not necessarily be outweighed by beneficial effects. Overall, these four management approaches provide a powerful framework to develop sustainable agronomic practices. (Résumé d'auteur

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The persistence of control of primary infections caused by two soilborne fungal plant pathogens, Rhizoctonia solani and Gaeumannomyces graminis var. tritici , following the incorporation of above-ground parts (AP), below-ground parts (BP) or both (AP + BP) of Brassica juncea into soil was examined through an experiment in controlled conditions. Control was quantified by measuring disease incidence in bioassays where inoculum was introduced at different dates after the incorporation of plant residues. All types of residue showed an unexpected long-term persistence that lasted at least 13 days, while the predominant glucosinolates contained in AP (20·9 μ mol sinigrin g − 1 dry matter) and BP (2·3 μ mol gluconasturtiin g − 1 dry matter) were hydrolysed in less than 3 days. Temporal trends in the efficacy of the residues behaved mostly in a quadratic manner, suggesting that the noxious effect of residues may be attributable to the release of isothiocyanates during the first days following incorporation, but that other mechanisms are most likely to contribute to lasting persistence. Across all treatments, AP and AP + BP suppressed R. solani by 54 and 63%, respectively, and G. graminis var. tritici by 40 and 40%, respectively, compared with controls. While BP did not cause any additional detectable effect when combined with AP, they had a significant effect when incorporated alone (approximately 20% suppression of both species), suggesting the existence of a complex interaction between these two types of residue.

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or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. I n a famous special issue of the Agronomy Journal published in 1996, several renowned authors proposed their views on "Use and Abuse of Crop Simulation Models," aft er more than 30 yr of development of these tools in support of system approaches in agronomy. Th ese authors agreed, with varied arguments, to end with the myth of the "universal model," that is, a model expected to predict the behavior of any crop at any location under any climate and any technical management. Hence the need arose to search for compromises between at one side the mathematical and numerical properties of crop models (parameterization, uncertainty, computing time), and at the other side the scope of application, wished as broad as possible. In other words, a search for the best trade-off between the heuristic role of a model and its utility for decision-making emerged. Sinclair and Seligman proposed in their position paper (Sinclair and Seligman, 1996) which became a reference paper for the crop modeling community, several guidelines to improve "returns on the investment" in crop modeling. One of these guidelines was particularly going against the common practice at that time as they recommended "to tailor models to specifi c problems by building new models or extensively modifying old ones, instead of systematically using existing models." Such "ad hoc models" as defi ned by Sinclair and Seligman (1996), would not be overloaded by "the redundancy inherent to supposedly universal or generic crop models," and could be kept "as simple as the nature of the objective allows." Sinclair and Seligman also added that the construction of models "customized to specifi c problems" was doable thanks to the availability, already at that time, of soft ware tools designed for this purpose. Th e objective of this article is to review the fate of the idea of ad hoc modeling during the past 15 yr, and to discuss two important issues typically arising from ad hoc modeling, that is, the conceptual modeling and the computer programming tasks: (i) how to identify which components and processes of the modeled system have to be taken into account or neglected in the model, as depending on the question to be addressed using the model (model conceptualization) and (ii) how to minimize the eff orts and skills required in soft ware development for the construction of ad hoc crop models (model computerization).We fi rst recall the emergence of the concept of ad hoc modeling through revisiting the literature on crop modeling produced from the late 1980s until the special issue of the Agronomy Journal in 1996. We then review all journal articles in which Sinclair and Seligman (1996) is cited, so as to evaluate whether and how theses citations are referring to their ad hoc modeling idea. We fi nally review a broader set of recent studies tackling model conceptualization and computerization issues, with the aim of provi...