Jean Roger-Estrade

Agroecosystem management and biotic interactions: a review

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.

Earthworm community in conventional, organic and direct seeding with living mulch cropping systems

The loss of biodiversity by intensification of agricultural practices is a major environmental issue that calls for the design of new cropping systems. For instance, negative effects of tillage on earthworm populations have been reported. However, few field studies have compared full cropping systems. Here, we assessed diversity, density and biomass of earthworm populations for 3 years. We use a combined method involving a diluted solution of allyl isothiocyanate to expel earthworms followed by hand sorting. In a long-term trial, we compared 3 systems: (1) a conventional system, (2) a direct seeding living mulch-based cropping system, named a living mulch cropping system, and (3) an organic system. These three cropping systems differed in terms of soil tillage, pesticide and nitrogen use, and crop biomass production. The results showed that measured variables, except diversity, varied depending on the year of sampling. Further, anecic and epigeic density was 3.2–7.2 times higher in the living mulch cropping system than in the conventional and organic systems. There were 3.4–12.5 times more anecic and epigeic earthworm biomass in the living mulch cropping system. The conventional and organic systems showed, respectively, 2.8 and 2.2 times more earthworm density, and 1.9 and 1.8 times more endogeic earthworm biomass than in the living mulch cropping system. Shannon-Wiener and equitability indices were superior in the living mulch cropping system compared with the conventional and organic systems. Cropping systems thus modified specific and functional diversity as well as earthworm community biomass. On the other hand, the organic and conventional systems did not differ in their earthworm density, biomass or diversity.

Multi-scale effects of landscape complexity and crop management on pollen beetle parasitism rate

Improving our understanding about how natural enemies respond to semi-natural habitats and crop management scattered in the landscape may contribute to the development of ecologically based pest management strategies maximising biological control services. We investigated how soil tillage and semi-natural habitats influenced the parasitism rates of pollen beetle (Meligethes aeneus F.) larvae at 8 different spatial scales (from 250 to 2000xa0m radius circular sectors) in 42 oilseed rape (OSR) fields. We used multimodel inference approaches to identify and rank the influence of soil tillage and semi-natural habitats on parasitism rates, and to quantify the importance of each scale. Parasitism rates were due to three univoltine parasitoid species (Tersilochus heterocerus, Phradis morionellus and P. interstitialis) and varied from 0 to 98%. We found that both fine and large scales contributed to explain significantly parasitism rates, indicating that biological control of pollen beetle is a multi-scale process. At the 250xa0m scale, parasitism rates of T. heterocerus were positively related to the proportion of semi-natural habitats and the proximity to previous year OSR fields. At large scales (1500 to 2000xa0m), parasitism rates of T. heterocerus were positively related to semi-natural habitats and negatively related to the proportion of previous year OSR fields with conventional soil tillage. Parasitism rates of Phradis spp. were only positively related to the proportion of semi-natural habitats at the 1250 and 1500xa0m scales. These multi-scale effects are discussed in relation to the influence of semi-natural habitats and soil tillage on parasitoid populations and their movement behaviours within the landscape.

Integration of soil structure variations with time and space into models for crop management. A review

Soil structure plays a major role in the design of new crop management systems. For instance, the transition from conventional to no-tillage changes soil structure, which, in turn, has implications on crop yield greenhouse gas emissions, and pesticide and nitrate leaching. Modelling soil structure at field scale faces two main issues: (1) the spatial variability and (2) the temporal variability. Here, we review how spatial variability of soil structure is taken into account in water transfer models at field scale. We discuss the effects of soil structure on hydraulic properties. We present options to model soil structure effects using pedotransfer functions or calculations based on pore network geometry. Then we review studies on water transfer. Here, we show the utility of one-dimensional (1-D) and 2-D models, and the range of soil profile partitions. In the second part, we study a mean to model the temporal variation of soil structure. We propose an indicator of soil structure dynamics based on the proportion of compacted clods in the tilled layer. This indicator was measured from the observation face of soil pits. We studied this indicator in a long-term field experiment involving various risks of compaction. The results showed that this indicator gave a more precise description of the time course changes in soil structure than the mean soil bulk density measured on the same experimental plots. Lastly, we discuss the principles of a model that predicts the evolution of this indicator under different soil tillage and climatic conditions. This model can be used to evaluate the effects of different crop management systems on soil structure and soil water transfer.

Local and landscape determinants of pollen beetle abundance in overwintering habitats

1 The development of integrated pest management strategies requires that the semi‐natural habitats scattered across the landscape are taken into account. Particular determinants of insect pest abundance in overwintering habitats just before they migrate onto crops appear to be poorly known and of crucial importance for understanding patterns of crop colonization and pest population dynamics at the landscape scale. 2 The emergence of pollen beetle Meligethes aeneus F. was studied in grassland, woodland edge and woodland interior over a 3‐year survey in France using macro‐emergence traps. A suite of variables at the local and the landscape scale was assessed for each trap, aiming to identify potential relevant habitat indicators. The effects of habitat characteristics were evaluated using partial least square regressions. 3 It was found that M. aeneus can overwinter in all types of habitat but that particular habitat characteristics at the local and landscape scales may explain their abundance in overwintering sites more than the types of habitat: relative altitude, litter thickness, soil moisture and proximity to the previous years oilseed rape fields appear to be positively correlated with abundance of adults over the 3 years. 4 Hence, the abundance of emerged pollen beetles depends on both the landscape configuration of the previous years oilseed rape fields around overwintering sites and local habitat characteristics. Landscape configuration may determine population flow towards overwintering sites in the late summer, and local habitat characteristics may influence survival rates during the winter. The findings of the present study provide valuable insight into the role of semi‐natural habitats as a source of pests, patterns of crop colonization in the spring, and the influence of landscape on pollen beetle abundance.

Undersowing wheat with different living mulches in a no-till system. I. Yield analysis

The classical management of no-till wheat has several environmental and economic drawbacks such as the use and cost of herbicides, and the degradation of soil physical quality. Recent investigations suggest that undersowing crops with a living mulch could be a sustainable alternative. Therefore, we studied during three growing seasons the effect of undersowing wheat with living mulches on wheat grain yield. Treatments were wheat grown on a conventionally-tilled soil, on a no-till soil, and on a no-till soil with various living mulches. The living mulches were red fescue, sheep’s fescue, alfalfa, bird’s-foot-trefoil, black medic and white clover. Our results show that the use of living muches during wheat cropping decreased wheat yield of 19–81% by comparison with wheat cropped alone. This decrease is linked to the biomass of living mulches and weeds at the time of flowering. As a consequence, the control of living mulch and weed biomass is a major issue. Our findings also reveal that the wheat yield decrease is mainly the consequence of a decrease in grain number from 37 to 32 grains per spike on average, and of a decrease in spike number from 0.7 to 0.4 spikes per stem. We thus conclude that stem elongation, flowering and fecundation are the major stages of stress for wheat grown with living mulches. Our findings will help to design innovative crop management systems that take into account the biological interactions in agro-ecosystems better.

Undersowing wheat with different living mulches in a no-till system. II. Competition for light and nitrogen

No-till wheat management systems with a living mulch is a possible means to improve agricultural sustainability. Nonetheless living mulches may affect wheat production by competition for light and nutrients. Therefore, here we studied competition for light and nitrogen between wheat and different living mulches under no-till. We grew wheat using three different practices: (1) conventionally-tilled wheat, (2) no-till wheat and (3) wheat undersown with various living mulches. Living mulches were: red fescue, sheep’s fescue, alfalfa, bird’s-foot-trefoil, black medic, and white clover. We measured: leaf area index and height of wheat and living mulch, and radiation partitioning between species; above-ground biomass of wheat and living mulch; nitrogen uptake of wheat and living mulch; and wheat nitrogen status using a nitrogen nutrition index. Our results showed that at flowering, competition for light between mixed species occurred in sixteen out of the eighteen situations, i.e. in three experiments times six no-till/living mulch treatments. Further, the biomass of wheat grown with living mulches was 24–84% lower than no-till wheat alone. At flowering, competition for nitrogen only occurred in seven out of the eighteen situations. Our findings showed that competition for light was due to light partitioning between mixed species. Furthermore, we found that the light competitive ability of wheat undersown with a living mulch was the highest when wheat was much taller than living mulch, and also when wheat leaf area in the mixed canopy layer was greater than that of living mulch. We conclude that the negative effects of living mulches on wheat yield should be solved by a careful choice of living mulch species and the control of living mulches by mechanical or chemical means.

Farmers and agronomists design new biological agricultural practices for organic cropping systems in France

New organic cropping systems are needed to keep pace with the growing demand for organic food. Those systems should ideally give more yield and safe for the environment. Current innovations such as non-inversion tillage with cover crops are promising, but investigations usually do not take farmers view into account. Therefore, research work should include farmer participation to maximize success. We present here a method to help farmers in designing innovative cropping systems. This method involves several design workshops with farmers. The first steps of the method foster creativity by changing ways in which farmers thought and worked. The final steps of the method facilitated learning. Participatory tools are used to exchange views and knowledge. System prototypes were developed. The method was applied using groups of six and seven farmers from two French regions. The farmers generated 14 system prototypes. We found that system prototypes differed radically from current practices because prototypes are based on biological rather than mechanical methods. Indeed, cover crop use was almost four times more frequent in prototypes than in current systems. Moldboard plowing and mechanical weeding frequencies were, respectively, two and eight times lower. The main benefits of our method are (1) the involvement of volunteer farmers in the design process, (2) the combination of farmer knowledge and scientific knowledge, and (3) the use of various methodological supports.

Nutritional state of the pollen beetle parasitoid Tersilochus heterocerus foraging in the field

Many laboratory studies have demonstrated that parasitoids of various species depend on sugar sources such as nectar or honeydew. However, studies about nectar acquisition by parasitoids foraging in the field are scarce. Tersilochus heterocerus Thomson is one of the more abundant and widespread parasitoids of the pollen beetle (Meligethes aeneus F.) but nothing is known about the nutritional ecology of this species. In this study we examined the nutritional state of T. heterocerus at the time of emergence and at various time periods throughout the season while foraging in the field using high-performance anion-exchange chromatography. We found that: (i) T. heterocerus emerge with relatively small amounts of sugar, composed mainly of trehalose, glucose and fructose; (ii) the first parasitoids caught just after they appeared in the field at the beginning of oilseed rape flowering had already consumed significant amounts of sugar reserves; and (iii) the total amount of sugar at the end of flowering was always significantly higher than the total amount of sugar at the beginning of flowering. This study provides valuable insights into the acquisition of sugar in the field by the parasitoid T. heterocerus and suggests that nectar acquisition takes place in the oilseed rape field or in the surrounding landscape.

Integrating Crop and Landscape Management into New Crop Protection Strategies to Enhance Biological Control of Oilseed Rape Insect Pests

The development of sustainable cropping systems is a major challenge for agronomists and crop scientists in many regions of the world. The prophylactic uses of broad spectrum insecticides are actually the main solution for farmers to control insect pests. Therefore, there is a growing need to develop innovative crop protection strategies through an integrated approach which aims at favouring natural enemies and enhancing biological control in agroecosystems. The development of such strategies requires a thorough understanding of agroecosystem functioning. In this chapter, we present the effects of different elements, from the field scale to the landscape scale that are known to enhance biological control in agroecosystems and limit pest damage with particular reference to oilseed rape. Linking integrated pest management and landscape ecology brings a regional perspective to the management of pest populations. Available techniques are often added together rather than combined in an integrated way and are rarely evaluated through environmental and economical criteria. Therefore, we present a methodological framework to design and assess sustainable cropping systems, with a particular emphasis on complementariness between models, systemic trials and more analytical approaches. Implementation of sustainable cropping systems implies the development of new integrated pest management strategies and thereby an increased participation of the different stakeholders from farmers to policy makers.