Aude Vialatte

Limited genetic exchanges between populations of an insect pest living on uncultivated and related cultivated host plants

Habitats in agroecosystems are ephemeral, and are characterized by frequent disturbances forcing pest species to successively colonize various hosts belonging either to the cultivated or to the uncultivated part of the agricultural landscape. The role of wild habitats as reservoirs or refuges for the aphid Sitobion avenae that colonize cultivated fields was assessed by investigating the genetic structure of populations collected on both cereal crops (wheat, barley and oat) and uncultivated hosts (Yorkshire fog, cocksfoot, bulbous oatgrass and tall oatgrass) in western France. Classical genetic analyses and Bayesian clustering algorithms indicate that genetic differentiation is high between populations collected on uncultivated hosts and on crops, revealing a relatively limited gene flow between the uncultivated margins and the cultivated part of the agroecosystem. A closer genetic relatedness was observed between populations living on plants belonging to the same tribe (Triticeae, Poeae and Aveneae tribes) where aphid genotypes appeared not to be specialized on a single host, but rather using a group of related plant species. Causes of this ecological differentiation and its implications for integrated pest management of S. avenae as cereals pest are discussed.

Tree Diversity Limits the Impact of an Invasive Forest Pest.

The impact of invasive herbivore species may be lower in more diverse plant communities due to mechanisms of associational resistance. According to the “resource concentration hypothesis” the amount and accessibility of host plants is reduced in diverse plant communities, thus limiting the exploitation of resources by consumers. In addition, the “natural enemy hypothesis” suggests that richer plant assemblages provide natural enemies with more complementary resources and habitats, thus promoting top down regulation of herbivores. We tested these two hypotheses by comparing crown damage by the invasive Asian chestnut gall wasp (Dryocosmus kuriphilus) on chestnut trees (Castanea sativa) in pure and mixed stands in Italy. We estimated the defoliation on 70 chestnut trees in 15 mature stands sampled in the same region along a gradient of tree species richness ranging from one species (chestnut monocultures) to four species (mixtures of chestnut and three broadleaved species). Chestnut defoliation was significantly lower in stands with higher tree diversity. Damage on individual chestnut trees decreased with increasing height of neighboring, heterospecific trees. These results suggest that conservation biological control method based on tree species mixtures might help to reduce the impact of the Asian chestnut gall.

Tree diversity reduces pest damage in mature forests across Europe

Forest pest damage is expected to increase with global change. Tree diversity could mitigate this impact, but unambiguous demonstration of the diversity–resistance relationship is lacking in semi-natural mature forests. We used a network of 208 forest plots sampled along two orthogonal gradients of increasing tree species richness and latitudes to assess total tree defoliation in Europe. We found a positive relationship between tree species richness and resistance to insect herbivores: overall damage to broadleaved species significantly decreased with the number of tree species in mature forests. This pattern of associational resistance was frequently observed across tree species and countries, irrespective of their climate. These findings confirm the greater potential of mixed forests to face future biotic disturbances in a changing world.

Migration and dispersal may drive to high genetic variation and significant genetic mixing: the case of two agriculturally important, continental hoverflies (Episyrphus balteatus and Sphaerophoria scripta)

Population structure of pests and beneficial species is an important issue when designing management strategies to optimize ecosystem services. In this study, we investigated for the first time the population structure at a continental scale of two migratory species of hoverflies providing both pest regulation and pollination services [Episyrphus balteatus and Sphaerophoria scripta (Diptera: Syrphidae)]. To achieve this objective, we used two sets of 12 species‐specific microsatellite markers on a large‐scale sampling from all over Europe. Our findings showed a high level of genetic mixing resulting in a lack of genetic differentiation at a continental scale and a great genetic diversity in the two species. All the pairwise FST values between European localities were less 0.05 in the two species. These low values reflect a large‐scale genetic mixing probably caused by the existence of frequent migratory movements in the two species. Mantel tests revealed isolation‐by‐distance pattern on the East–West axis, but not on the North–South axis. This isolation‐by‐distance pattern confirms the existence of North–South migratory movements in both directions and suggests an important step by step dispersal. Population features shown by this study are common in invasive species and pests, but are not often observed in beneficial species. They reflect great colonization abilities and a high adaptive potential when dealing with a changing environment. Our results highlight the two studied species as particularly interesting beneficial insects for pollination and pest predation in the current context of global change.

Can deuterium stable isotope values be used to assign the geographic origin of an auxiliary hoverfly in south-western France?†

Deuterium δD isotopic analysis is increasingly being used to trace wildlife movement, and undoubtedly has much to offer in this respect, but questions still remain as to the feasibility and practicality of the method in ecology. Here we report our attempt to determine the geographic origin of an auxiliary hoverfly, Episyrphus balteatus, in south-western France. We used quantile regression to calculate the minimum separation distance, based on the International Atomic Energy Agency/World Meteorological Organization (IAEA/WMO) data, at which two insects could be said to originate from different latitudes with a given degree of confidence. We collected larvae in spring 2007 and 2009 to obtain the δD signal of indigenous hoverflies and we trapped adults during one complete year (from Dec. 2006 to Nov. 2007). The smallest separation distance calculated was about 1400 km in western Europe. Our results revealed greater variability in δD of adults in autumn than in spring. From this we infer an autumnal migration. Because of the presence of mountains and the Mediterranean Sea, the δD gradient in precipitation in western Europe is less clear than on the American continent, where it has been used successfully to infer geographical origins of animals under certain conditions. Despite the complications encountered in Europe, the minimum separation distance model proved a useful first step to obtain a first range of possible origins of E. balteatus and the application of the model to other arthropod species in Europe warrants investigation.

Spatiotemporal variations in aphid‐parasitoid relative abundance patterns and food webs in agricultural ecosystems

Understanding the stability of communities is fundamental in theoretical and applied ecology. Organisms atop trophic chains are particularly sensitive to disturbance, especially when they are dependent on a specific trophic resource subject to strong fluctuations in density and quality, which is the case of parasitoids. We investigated the (1) variability in spatiotemporal relative abundance patterns of a cereal aphid parasitoid community, determining at what scales such patterns vary in agrosystems. We also investigated whether (2) parasitoid relative abundances are strongly influenced by host relative abundances and if (3) different host species are exploited at distinct rates. Aphid parasitoid populations were monitored in three remote agricultural regions in France between 2010 and 2012. Five parasitoid species (Hymenoptera: Braconidae: Aphidiinae) and three aphid species (Hemiptera: Aphididae) were identified in each of those three regions. Fields sampled in one region in a single year exhibited similar relative abundance patterns, with aphid parasitoid communities varying across regions and years. All parasitoid species were able to exploit each monitored host species. Metopolophium dirhodum Wlk was consistently a more frequent species among parasitized aphids than among living aphids, indicating that this aphid species was exploited at a higher rate than the other two aphid species found (Sitobion avenae F and Rhopalosiphum padi L). Those findings suggest the cereal aphid-parasitoid network is not strictly determined by the intrinsic permanent environmental characteristics but partially varies from one year to another. The similarity in abundance patterns in different fields of a region in a given year suggests the existence of a mechanism allowing a rapid synchronisation of the relative abundance patterns at an intra-regional scale. This phenomenon could be useful in predicting host-parasitoid communities and bear important consequences for the ecosystem service provided by parasitoids.

Combination of morphometric and isotopic tools for studying spring migration dynamics in Episyrphus balteatus

Migration and population dynamics are important in organisms that provide ecosystem services as they determine the occurrence of individuals in a given place at a given time, which, in turn, determines the efficiency of the service provided. To design appropriate landscape management strategies to improve the efficiency of biological control, it is essential to know the origin of beneficial insects that colonize agricultural fields. However, studying migration dynamics in insects is complicated by their small size and short life span which largely prevent the use of conventional techniques based on capture-mark-recapture, or remote sensing. Stable isotopes and morphological characters are intrinsic markers that can be used to infer the geographical origin of many organisms including insects. In this study, we first determined whether the hydrogen isotopic ratio and the wing morphometrics are appropriate markers to study the spring migration dynamics of a major aphidophagous hoverfly species (Episyrphus balteatus). To this end, we assessed the magnitude of variation of these indicators in seven populations along a north-south gradient in Western Europe. Second, we used the two markers in E. balteatus individuals collected in a French agricultural landscape over the course of the spring to assess the proportion of immigrants from southern regions and the period of immigration. Our results revealed uncertainties associated with the use of the hydrogen isotopic ratio and wing morphometrics, but showed that the spring population of E. balteatus in Western France very likely comprises mainly local individuals, with the probable arrival of some immigrants from nearby regions located further south. The low proportion of immigrants in the spring population has important implications for the biological control provided by E. balteatus in terms of landscape management strategies.

Lack of Genetic Differentiation between Contrasted Overwintering Strategies of a Major Pest Predator Episyrphus balteatus (Diptera: Syrphidae): Implications for Biocontrol

Winter ecology of natural enemies has a great influence on the level and efficiency of biological control at spring. The hoverfly Episyrphus balteatus (DeGeer) (Diptera: Syrphidae) is one of the most important natural predators of crop aphids in Europe. Three different overwintering strategies coexist in this species which makes it a good model in order to study ecologically-based speciation processes. The purpose of this study was to determine whether E. balteatus populations with alternative overwintering strategies are genetically differentiated. To that aim, we developed 12 specific microsatellite markers and evaluated the level of neutral genetic differentiation between E. balteatus field populations that overwinter in the three different ways described in this species (i.e. migration, local overwintering at a pre-imaginal stage, and local overwintering at adult stage). Results showed a lack of neutral genetic differentiation between individuals with different overwintering strategies although there are strong ecological differences between them. All pair-wise FST values are below 0.025 and non-significant, and Bayesian clustering showed K = 1 was the most likely number of genetic clusters throughout our sample. The three overwintering strategies form one unique panmictic population. This suggests that all the individuals may have genetic material for the expression of different overwintering phenotypes, and that their commitment in one particular overwintering strategy may depend on environmental and individual factors. Consequently, the prevalence of the different overwintering strategies would be potentially modified by landscape engineering and habitat management which could have major implications for biological control.

Misconceptions of Fragmentation's Effects on Ecosystem Services: A Response to Mitchell et al.

In a recent opinion article, Mitchell et al. [1] propose to reframe fragmentations effects on ecosystem services (ESs) by explicitly considering the effect of fragmentation not only on ES supply but also on ES flow. This new conceptual framework obviously represents a valuable first step towards a more robust theory linking landscape structure with ESs. However, we are concerned by three problems included in this framework on the effect of habitat fragmentation on ESs. We believe they undermine the framework currently proposed by Mitchell et al.

Ecosystem services, social interdependencies, and collective action: a conceptual framework

The governance of ecosystem services (ES) has been predominantly thought of in terms of market or state-based instruments. Comparatively, collective action mechanisms have rarely been considered. This paper addresses this gap by proposing a conceptual framework that brings together ES, social interdependencies, and collective action thinking. We use an ES conceptual lens to highlight social interdependencies among people so as to reflect on existing or potential collective actions among them. This framework can also contribute to increasing people’s awareness of their mutual interdependencies and thereby fostering, framing, or enriching collective action, in ways that take into account the diversity and complexity of ecological processes underlying human activities. Our approach can contribute in particular to agroecological transitions that require landscape level innovations and coordination mechanisms among land users and managers. The framework distinguishes three types of social interdependencies: (i) between ES beneficiaries and ES providers, (ii) among beneficiaries, and (iii) among providers. These social interdependencies are in turn analyzed according to four main dimensions that are critical for collective action: (i) cognitive framing of interdependencies, (ii) levels of organization, (iii) formal and informal institutions, and (iv) power relations. Finally, we propose a strategy to turn this framework into action in contexts of participatory action research, a strategy grounded on a number of methodological principles and tools that convey complexity and increase people’s awareness of interdependencies in agrarian social-ecological systems.