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Plantes et Système de cultures Horticoles

Zone de texte éditable et éditée et rééditée

CBC : Biological control by Conservation

The general framework for the CBC team is that of conservation biological control in orchards in a context of pesticide reduction. Conservation biological control aims to promote the control of pests by the community of naturally occurring pest enemies in agro-ecosystems, in combination with other biological control practices. To this end, we are interested in the dynamics of pest and pest enemy populations throughout their life cycles, including when the species use resources outside the orchards. This implies considering sometimes a scale greater than that of the plot. Our work focuses mainly on two main apple pests, the codling moth (Cydia pomonella, Lepidoptera) and the rosy apple aphid (Dysaphis plantaginea, Hemiptera) and on arthropods and birds as pest enemies. Our objective is to increase knowledge on conditions that are favorable to the natural control of pests to contribute to the design of sustainable horticultural systems (together with team 4). Our approach is to make a back and forth between theoretical questions, methodological developments and their specific applications to our study systems, using modeling when relevant.
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The research of the CBC team focus more specifically on the impacts of agricultural practices, in particular phytosanitary practices, and semi-natural elements, at the level of plots (hedges, flowering strips) and landscapes (wooded areas, hedges, grasslands ...) on natural pest control. Numerous studies have shown the adverse effect of chemical pesticides on biodiversity in plots. Contrarily, the presence of semi-natural elements at the edges of plots tends to increase it. For example, hedgerows can be barriers to movement, provide shelter or provide alternative resources to species that are active in the orchards. Beyond the plot, the complexity of the landscape and the intensity of agricultural practices affect the dynamics of the pest and pest enemy populations, and their interactions. In addition, populations of pests and pest enemies are likely to adapt to the most intensive agricultural practices, which raises the issue of the sustainability of control methods. Research is also being carried out in our team on both chemical and biological insecticide resistance and on the effects of these resistances on the life history traits of pests or pest enemies.

To address these topics, we rely on experimental orchard systems from INRA Avignon and Gotheron (Drôme), as well as on a network of orchards from producers in our study area in the lower valley of the Durance (Bouches-du-Rhône). The tools and skills present at the PSH insect rearing platform (insectarium) and within the regional molecular biology platform (LBM, Avignon) also support methodological developments (eg molecular analyzes of gut contents, mass protein labeling, sentinel prey…) to better characterize the ecology of pests and pest enemies and their trophic relationships. We are also interested in innovative cropping systems that are potentially more favorable to conservation biological control based on plant diversification within plots, including mixing trees and annual crops (on the Durette farm or within the Safir project at Gotheron).

Three lines of investigation structure our research questions:

Understanding how the dynamics and genetics of pest and pest enemy populations depend on landscape structures and farming practices.

Characterizing insecticide resistance and understanding the causes of their distributions in agricultural landscapes.

Understanding the structure of trophic networks around pests and the impacts of pest enemies on predation and parasitism rates.

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Understanding how the dynamics and genetics of pest and pest enemy populations depend on landscape structures and farming practices.

Using landscape ecology approaches, we have shown that, in combination with local practices, the landscape surrounding the orchards may impact pests and pest enemies. This effect may be due to landscape scale phytosanitary practices: we found a negative effect of conventional plant protection practices within 250m of study plots on levels of predation and parasitism of the codling moth, a positive effect of organic farming practices within 50 m on the abundance of the predatory spider Cheiracanthium mildei. The effect of the landscape may also be due to the presence of semi-natural elements: we found that the presence of hedges bordering orchards impacted the composition of the codling moth parasitoid community, the abundance of C. mildei and the abundance of the codling moth. These results are consistent with the results of a review of the literature highlighting the role of semi-natural habitats for pest enemies. Nevertheless, this type of studies that relate a biological variable (abundance, predation rates…) with variables describing agricultural practices or landscape characteristics pose methodological problems (numerous explanatory variables often correlated) and are not easy to interpret from an ecological point of view. By a modeling approach we have shown, for example, that relations between biological responses and landscape variables are impacted both by the structure of the landscape and by the characteristics of the dynamics of the studied species.

At the same time, part of our projects thus aimed to better understand the dynamics of pests and pest enemies. Kinship analyses in codling moth larvae showed a low dispersal of females between two egg laying events, mostly within a single orchard and rarely between orchards. Movements between orchards were independent of distance and host plants. Most dispersal occurred in the last yearly generation. In order to take better account of the heterogeneity of landscapes in this type of analysis, methodological developments are being made in collaboration with statisticians. These studies are continuing at the landscape level in the lower Durance valley. We also studied the movements of predatory arthropods between the orchard and hedgerows by mass marking of arthropods in hedgerows. The results obtained show daily movements between the two habitats but also high disparities between the taxa.

Lastly, we sought to better characterize the life-history traits of great tits nesting in orchards. Great tits are potentially predators of larvae of lepidopteran pests but are also affected by the use of pesticides. We found an impact of pesticide applications in apple orchards on the sex ratio of young fledglings, with a bias against males in conventional orchards (23% of males produced only) compared to orchards in IPM and in organic agriculture (about 60% of males produced). In addition, from a methodological point of view, we contributed to a study assessing the importance of the nest boxes characteristics on the reproduction success of tits.

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Characterizing insecticide resistance and understanding the causes of their distributions in agricultural landscapes.

Pests develop resistances in response to high insecticide selection pressures. Two types of mechanisms are frequent: target mutation and detoxification. Understanding the mechanisms of these resistances helps: (i) to implement detection tools to monitor resistances over large scales and to locally manage problematic situations, (ii) to anticipate cross-resistance to different or new insecticides, often linked to detoxification, and (iii) to improve strategies for managing these resistances.

In apple orchards of south-eastern France, which are often heavily treated (FTI – frequency treatment index of approx. 35, among which approx. 9 insecticides in conventional orchards), codling moth has developed many resistances to chemical and / or biological insecticides. Analyzes of resistance mechanisms to chemical insecticides at local scales confirmed their complex interweaving. Indeed, detoxification enzymes and target mutations are sometimes found within a single individual in French populations. More conventionally, the activities of the cytochrome P450 oxygenases as well as the glutathione-S-transferases confer cross-resistance to several insecticides in Greek populations. The resistance of the codling moth to the granulosis virus is particularly interesting because this is the first case of resistance to a virus in a pest. Our work has shown that the resistance of the codling moth to the granulosis virus is multigenic, with a major heterosomal gene explaining mostly the resistance phenotype. Another autosomal minor gene is also involved in this resistance. No cost of resistance has been demonstrated, resistant individuals even have shorter developmental periods than susceptible ones.

We have also carried out studies on insecticide resistances of the oriental fruit moth (Grapholita molesta), on Tuta absoluta a major pest of tomato, and on the European corn borer (Ostrinia nubilalis).

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Understanding the structure of trophic networks around pests and the impacts of pest enemies on predation and parasitism rates.

The pest – pest enemy interactions and the resulting parasitism or predation of the pests are key for conservation biological control. We enhanced our investigations on these questions thanks to methodological developments, in particular the development of PCR primers specific of the main apple tree pest species (leafrollers, aphids, anthonoma, sawfly) making it possible to detect them in the guts of potential predators. We also developed primers specific of the main predators or parasitoids of these pests.

Our studies on parasitoids of codling moth larvae showed that larvae are predominantly attacked by an ovo-larval parasitoid specializing on the tortricidae: Ascogaster quadridentata (Braconidae). This species represents 50 to 90% of the parasitoid community in non- treated orchards. This species is widespread throughout Europe. A second parasitoid (Pristomerus vulnerator, Ichneumonidae), more generalist, was more rarely present in southern France (5 to 15% of the community). Finally, Perilampus tristis (Perilampidae) is the third species present in larvae. The molecular detection of parasitoids in their host larva enabled us to show that P. tristis is an almost exclusively an hyperparasitoid species (98%), developing on A. quadridentata and P. vulnerator. The three species are present on all generations of the codling moth during the season in untreated orchards. Parasitism is low in commercial apple orchards, either organic (about 5%) or conventional (about 2%). It is also lower when orchards are surrounded by conventional orchards.

Codling moth larvae are prey of ground beetles and spiders in the fall. Aphids, including the rosy apple aphid, are prey of canopy dwelling spiders in the spring. The list of arthropods predators of the rosy apple aphid and the codling moth in orchards is quite large (at least 10 families of predators identified by PCR). An advantage of generalist predators such as spiders is that they can be present early in the season and therefore control the pest before it becomes abundant. On the other hand, generalist predators are likely to cause intra-guild predation that may be detrimental to pest control. PCR analyzes of gut contents or Petri dish observations make it possible to study the predator community of pests but do not provide direct evidence of their control of pests. We thus combine these approaches with two other approaches. The first is the exposure of sentinel prey to measure potential predation rates. The use of codling moth eggs as sentinel prey showed the important role of farmers' phytosanitary practices in the study plots and in their surrounding plots on predation, with predation rates ranging from 18% to 63% in August. It also showed an unexpected negative effect of an abundant grassy ground cover (untreated orchard). The second approach, applicable when it is possible to estimate finely the abundances of pests and pest enemies, consists in correlating these abundances. Through monitoring in commercial apple orchards, we have shown that the presence of pest enemies generally reduces the growth of aphid colonies. Early in the season, canopy dwelling spiders (particularly those of the genus Philodromus) may reduce the number of aphid colonies in organic or untreated orchards. However, spiders are also predators of pest enemies such as syrphid larvae, including in the presence of aphids, which may impair the effectiveness of syrphids later in the season.

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