Can strip cropping reduce pest activity density and damage while increasing yield? A case study of organic cabbage and faba bean

This study assessed how strip cropping white cabbage (Brassica oleracea) and faba bean (Vicia faba) affects the activity density of pests – diamondback moth (Plutella xylostella) and flea beetles (Phyllotreta spp.) – and how this affects crop yields. The setup consisted of three 270m2 plots representing monocrops of faba bean and cabbage, and a plot of alternating strips of the two. The setup was established in 2018 and 2019. Activity density of P. xylostella was determined through yellow sticky trapping and Phyllotreta spp. activity density was determined by pitfall trapping. Crop yields were determined at harvest, and foliar damage to cabbage leaves was also assessed. The results showed that strip cropping reduced the activity density of P. xylostella, but no suppression effect was observed for Phyllotreta spp. Average foliar damage to cabbage was higher in the strip crop plot, but in 2018 the yield was also higher. Average faba bean yield was lower in the strip crop plot in both years. Our results indicate potential for cabbage-faba bean strip cropping to reduce the activity density of P. xylostella but not Phyllotreta spp. and to increase cabbage yield when overall pest pressure is moderate, but at the cost of faba bean yield. The case study shows that a Brassica vegetable – legume strip cropping system has potential for pest suppression in northern conditions, but agronomic compatibility of crop plants needs attention to ensure similar or higher productivity than a monocrop system

Samspill mellom kornbladlus (Sitobion avenae), havrebladlus (Rhopalosiphum padi) og deres naturlige fiender i korn

In agronomy, efficient plant protection strategies are required to reduce pest pressure and increase crop yield. Pesticide application has been the dominant method for plant protection for almost a century. However, the over-reliance on pesticides has several detrimental consequences. Therefore, alternative plant protection strategies have been developed and promoted to reduce pesticide use. An overall approach to using alternative strategies is Integrated Pest Management (IPM), defined as a sustainable approach to manage pests by combining eight principles (according to the EU regulation that Norway implemented in 2015) in a way that minimizes economic, environmental and health risks. These include the use of natural enemies and the manipulation of their trophic interactions with pests in order to protect the crop yield (i.e. biological control). In Europe, two main aphid species cause damage in cereals such as winter wheat: the English grain aphid Sitobion avenae and the bird cherry-oat aphid Rhopalosiphum padi. Both species have many natural enemies, among which are entomopathogenic fungi in the sub-phylum Entomophthoromycotina. In this fungal group, the most interesting species for biological control of aphids are Pandora neoaphidis and Entomophthora planchoniana. As for any host-pathogen interactions, three important groups of factors are important potential drivers for an epidemic development: host population, pathogen population and environment. Together they are called the disease triangle. The aim of this PhD thesis was to identify important drivers of the disease triangle influencing (1) the success of fungal infection of aphids in cereals and (2) the capacity of the fungus to spread in these aphid populations. In this thesis I first reviewed, the factors driving the aphid host susceptibility or resistance to fungal pathogens by considering the model system composed of S. avenae, R. padi and P. neoaphidis, E. planchoniana. Aphid behaviour and ecological niche preferences, host origin of the fungal isolate (from which host species has it been collected), aphid morph and presence of endosymbiotic bacteria are among the principal potential factors influencing the success of the fungal infection. Finally, I hypothesize that these aphid pathogenic fungi follow their host during their entire life cycle and therefore follow their spatial distribution. I tested this hypothesis in a study on R. padi. Rhopalosiphum padi overwinter on the bird cherry tree Prunus padus, where it lays overwintering eggs. Fungus-killed cadavers, filled with overwintering structures, were found in the same microhabitat as R. padi eggs. Zoophthora sp. overwintered as resting spores, while E. planchoniana overwintered as modified hyphal bodies. There was a significant negative correlation between number of overwintering eggs and cadavers per branch. Number of both eggs and cadavers varied greatly between years and geographical locations. I discussed the potential role of P. padus as a reservoir for fungi infecting aphids in cereals. In a laboratory study, I studied the potential cross-infection of three P. neoaphidis isolates (from one S. avenae population in Norway) between S. avenae and R. padi. Moreover, the effect of the fungal isolates on aphid mortality and fecundity at three different temperatures relevant for Norwegian conditions were studied. Our results showed that cross-infection is possible but potentially asymmetric. In effect, P. neoaphidis kills more S. avenae than R. padi and also kills S. avenae faster. A significant variability was found between the three fungal isolates in virulence and sub-lethal effect on aphid fecundity. The higher the temperature, the higher the mortality of fungal infected aphids. However, temperature did not consistently affect the time needed to kill the host or the effect on fecundity. Our findings are important for understanding and modelling P. neoaphidis epizootiology in aphid pests of cereals. Finally, a modelling approach was used to investigate the epizootiology of P. neoaphidis infecting S. avenae on winter wheat. A mechanistic tri-trophic model was built that includes a high aphid population in order to overcome any potential host density threshold. Twelve parameters related to the fungus' biology and climatic conditions were allowed to vary in order to identify those most important for aphid and fungus populations and potential biological control. Three parameters were identified as crucial: (1) fungus transmission efficiency, (2) humidity threshold level that triggers fungal sporulation and (3) the weather (temperature and humidity). The longevity of fungus-killed cadavers (how long they may represent an inoculation source) was very important for the fungus population dynamic in this model. Interestingly, the proportion of infected aphids colonising the wheat field was the most important parameter to reduce the yield loss due to the biological control.For å redusere skader på planter og for å øke avlingene i landbruket trenger vi effektive plantevernstrategier. Bruk av kjemiske plantevernmidler har vært den plantevernmetoden som har vært mest brukt i nesten ett århundre. Den avhengigheten av kjemiske plantevernmidler har hatt flere uheldige effekter. Alternative plantevernstrategier er derfor blitt utviklet og det oppfordres også til å bruke disse. Integrert plantevern (IPV) som nå brukes i mange land og defineres som en bærekraftig strategi for å håndtere planteskadegjørere ved å følge åtte prinsipper (ifølge EU regelverket som Norge implementerte i 2018) på en måte som reduserer risiko for økonomi, miljø og helse. Disse åtte prinsippene inkluderer blant annet bruken av naturlige fiender og manipuleringen av deres trofiske samspill med skadegjørere for å beskytte plantene (biologisk kontroll). De to viktigste bladlusartene som opptrer som skadedyr på høsthvete og andre kornarter i Europa er kornbladlusa, Sitobion avenae, og havrebladlusa, Rhopalosiphum padi. Begge artene har mange naturlige fiender og blant disse hører insektpatogene sopp i underrekke Entomophthoromycotina. I denne gruppen er Pandora neoaphidis og Entomophthora planchoniana blant de mest lovede artene for biologisk kontroll. For alle vert-patogensamspill er det tre hovedgrupper av faktorer som er viktige drivere for en epidemisk utvikling: vertpopulasjonen, patogen populasjonen og miljøet. Dette kalles sykdomstriangelet. Målet med denne PhD oppgaven var å identifisere viktige drivere i sykdomstriangelet og som påvirker (1) hvor vellykket disse insektpatogene soppene kan infisere bladlus i korn og (2) soppens evne til å spre seg i bladluspopulasjonene. I denne oppgaven gjennomgår jeg først hvilke faktorer som er drivere for bladlusenes (S. avenae og R. padi) og mottakelighet for eller resistens mot de insektpatogene soppene (P. neoaphidis og E. planchoniana). Følgende faktorer ser ut til å være de viktigste for en vellykket soppinfeksjon av bladlusene: Blaldusartenes adferd og valg av økologisk nisje, bladlusas morf, bladlusarten soppisolatet er isolert fra og om endosymbiotiske bakterier er tilstede i bladlusa. Videre setter jeg opp en hypotese om at disse bladluspatogene soppene følger sine verter gjennom hele deres livssyklus og derfor også følger bladlusenes romlige utbredelse. Den hypotesen tester jeg i en studie av havrebladlus (R. padi). Havrebladlusa overvintrer som egg på hegg (Prunus padus). I studiet fant vi soppdrepte bladlus med overvintrende soppstrukturer i det samme mikrohabitatet som vi fant egg av havrebladlus. Nyttesopp tilhørende Zoophthora sp. overvintret som hvilesporer mens soppen Entomophthora planchoniana overvintret som modifiserte hyfelegemer. Det var ingen signifikant negativ sammenheng mellom antall overvintrende havrebladlusegg og soppdrepte havrebladlus per heggkvist. Antall havrebladlusegg og soppdrepte havrebladlus varierte betydelig mellom år og geografisk lokalitet. Jeg diskuterer den potensielle rollen hegg kan ha som reservoar for sopp som dreper bladlus i korn. I et laboratorieforsøk studerer jeg mulig smitte av tre P. neoaphidis isolater (fra en og samme kornbladlus populasjon i Norge) mellom kornbladlus og havrebladlus. Videre studerer jeg effekten av disse soppisolatene på bladlusenes dødelighet og fertilitet ved tre ulike temperaturer som er relevant for norske forhold. Våre resultater viser at smitte fra en bladlusart til en annen er mulig men at den antagelig er asymmetrisk. Dette vil si at P. neoaphidis fra kornbladlus dreper flere kornbladlus enn havrebladlus og at den også dreper kornbladlusa raskere. Det ble funnet en signifikant variasjon i virulens og sub-letal effekt knyttet til bladlusas fertilitet mellom de tre soppisolatene. Videre fant vi at høyere temperaturer ga høyere dødelighet hos soppinfiserte bladlus men vi fant ingen konsistent effekt av temperatur på tid brukt til å drepe bladlusa eller effekt på fertilitet. Disse resultatene er viktige for å kunne forstå og modellere den epidemiologiske utviklingen av P. neoaphidis i ulike bladlusarter i korn. Til slutt i denne PhD oppgaven har jeg brukt modellering for å undersøke epizootiologien til P. neoaphidis som smitter kornbladlus i høsthvete. En mekanistisk tre-trofisk modell ble bygget og denne inkluderer en høy kornbladluspopulasjon for å sørge for at den ligger over en potensiell terskelverdi for vertstetthet. Elleve parameter var knyttet opp mot nyttesoppens biologi og til klimatiske faktorer og disse fikk variere på en slik måte at vi kunne identifisere hvilke som kunne være viktigst for en god epidemisk utvikling av nyttesoppen i bladluspopulasjonen og hvilke som dermed muliggjorde biologisk kontroll. Tre parameters ble identifisert som vesentlige: (1) soppens spredningsevne (2) terskelverdi for fuktighet som skal til for at soppen skal sporulere og (3) værforhold (temperatur og fuktighet). “Levetid” for soppdrepte bladlus (hvor lenge de kan fungere som smittekilde) var veldig viktig for den epidemiske utviklingen av soppen i denne modellen. Andel soppinfiserte bladlus som koloniserte høsthvete var det parameteret som i størst grad førte til mindre avlingstap som følge av økt biologisk kontroll

Dynamique saisonnière d’une population de carabes auxiliaires de cultures (Pterostichus melanarius) et régulation biologique dans une mosaïque de cultures et d’éléments non cultivés.

il s'agit d'un type de produit dont les métadonnées ne correspondent pas aux métadonnées attendues dans les autres types de produit : DISSERTATIONMasterThe goal of this study was to analyse (i) the distribution and seasonal dynamics of a carabid species Pterostichus melanarius (Illiger) in a changing mosaic of crops and semi natural elements, and (ii) the influence of this distribution on the biological control of aphids in cereals and pea by this carabid species. Pitfall traps were used to characterise emergences, activity-density and movements of carabid beetles between crops and semi-natural elements. The results show that winter crops and grassy strips are characterised by high emergences of young adults. The activity-density of carabid bettles is similar in crops and semi-natural elements. Activity-density in crops varies according to temperature and crop surroundings. Carabid beetles move from crops in semi-natural elements. They also move from crops with no or sparse vegetation cover to crops with dense vegetation. The high activity-density of carabid beetles observed in several fields does not lead to a reduced infestation of crops by aphids. The methods used in this study are not appropriate to demonstrate the role of P. melanarius in controlling aphid populations.Les objectifs de cette étude étaient d’étudier (i) la distribution et l’évolution saisonnière d’un coléoptère carabique Pterostichus melanarius (Illiger) dans une mosaïque changeante de cultures et d’éléments non productifs, (ii) et l’influence de cette distribution sur le contrôle des pucerons des céréales et du pois par ce carabe. Les émergences, l’activité-densité et les déplacements des carabes entre éléments du paysage ont été caractérisés par piégeage. Les résultats montrent que les cultures d’hiver et les bandes enherbées sont très productives en jeunes carabes adultes. L’activité-densité des carabes est comparable dans les éléments non productifs et les cultures. Dans les cultures, l’activité-densité varie avec la température au sol et l’environnement des parcelles, mais ne varie pas selon l’état des couverts végétaux. Les carabes se déplacent des cultures dans les éléments non productifs. Ils se déplacent également des cultures à couvert végétal peu favorable vers les cultures à couvert dense. L’activité-densité élevée des carabes dans certaines parcelles n’est pas associée à une réduction des pucerons. Les méthodes utilisées dans cette étude ne permettent pas de montrer le rôle de P. melanarius pour le contrôle biologique des pucerons

Interactions between the cereal aphid pests Sitobion avenae and Rhopalosiphum padi and their fungal natural enemies

In agronomy, efficient plant protection strategies are required to reduce pest pressure and increase crop yield. Pesticide application has been the dominant method for plant protection for almost a century. However, the over-reliance on pesticides has several detrimental consequences. Therefore, alternative plant protection strategies have been developed and promoted to reduce pesticide use. An overall approach to using alternative strategies is Integrated Pest Management (IPM), defined as a sustainable approach to manage pests by combining eight principles (according to the EU regulation that Norway implemented in 2015) in a way that minimizes economic, environmental and health risks. These include the use of natural enemies and the manipulation of their trophic interactions with pests in order to protect the crop yield (i.e. biological control). In Europe, two main aphid species cause damage in cereals such as winter wheat: the English grain aphid Sitobion avenae and the bird cherry-oat aphid Rhopalosiphum padi. Both species have many natural enemies, among which are entomopathogenic fungi in the sub-phylum Entomophthoromycotina. In this fungal group, the most interesting species for biological control of aphids are Pandora neoaphidis and Entomophthora planchoniana. As for any host-pathogen interactions, three important groups of factors are important potential drivers for an epidemic development: host population, pathogen population and environment. Together they are called the disease triangle. The aim of this PhD thesis was to identify important drivers of the disease triangle influencing (1) the success of fungal infection of aphids in cereals and (2) the capacity of the fungus to spread in these aphid populations. In this thesis I first reviewed, the factors driving the aphid host susceptibility or resistance to fungal pathogens by considering the model system composed of S. avenae, R. padi and P. neoaphidis, E. planchoniana. Aphid behaviour and ecological niche preferences, host origin of the fungal isolate (from which host species has it been collected), aphid morph and presence of endosymbiotic bacteria are among the principal potential factors influencing the success of the fungal infection. Finally, I hypothesize that these aphid pathogenic fungi follow their host during their entire life cycle and therefore follow their spatial distribution. I tested this hypothesis in a study on R. padi. Rhopalosiphum padi overwinter on the bird cherry tree Prunus padus, where it lays overwintering eggs. Fungus-killed cadavers, filled with overwintering structures, were found in the same microhabitat as R. padi eggs. Zoophthora sp. overwintered as resting spores, while E. planchoniana overwintered as modified hyphal bodies. There was a significant negative correlation between number of overwintering eggs and cadavers per branch. Number of both eggs and cadavers varied greatly between years and geographical locations. I discussed the potential role of P. padus as a reservoir for fungi infecting aphids in cereals. In a laboratory study, I studied the potential cross-infection of three P. neoaphidis isolates (from one S. avenae population in Norway) between S. avenae and R. padi. Moreover, the effect of the fungal isolates on aphid mortality and fecundity at three different temperatures relevant for Norwegian conditions were studied. Our results showed that cross-infection is possible but potentially asymmetric. In effect, P. neoaphidis kills more S. avenae than R. padi and also kills S. avenae faster. A significant variability was found between the three fungal isolates in virulence and sub-lethal effect on aphid fecundity. The higher the temperature, the higher the mortality of fungal infected aphids. However, temperature did not consistently affect the time needed to kill the host or the effect on fecundity. Our findings are important for understanding and modelling P. neoaphidis epizootiology in aphid pests of cereals. Finally, a modelling approach was used to investigate the epizootiology of P. neoaphidis infecting S. avenae on winter wheat. A mechanistic tri-trophic model was built that includes a high aphid population in order to overcome any potential host density threshold. Twelve parameters related to the fungus' biology and climatic conditions were allowed to vary in order to identify those most important for aphid and fungus populations and potential biological control. Three parameters were identified as crucial: (1) fungus transmission efficiency, (2) humidity threshold level that triggers fungal sporulation and (3) the weather (temperature and humidity). The longevity of fungus-killed cadavers (how long they may represent an inoculation source) was very important for the fungus population dynamic in this model. Interestingly, the proportion of infected aphids colonising the wheat field was the most important parameter to reduce the yield loss due to the biological control

Intra-annual heterogeneity of the cultivated mosaic: impact on carabid beetle populations

 Spatio-temporal heterogeneity is recognised as a key factor for biodiversity. In agricultural landscapes, landscape heterogeneity is mostly described by the amount of semi-natural elements. In addition, the heterogeneity of cover types in the cultivated matrix might also influence biodiversity (Fahrig et al. 2011). Within a year, this matrix is highly heterogeneous in space and time, due to the diversity of crop covers, to crop phenology and cultural practices. Thus, for species that use crops during their life cycle, the cultivated matrix can be viewed as a mosaic of asynchronous habitats with fast-changing suitability, which is likely to affect species distribution and survival (Vasseur et al. 2008). In this study, we tested whether intra-annual heterogeneity of the cultivated mosaic affects population distribution and dynamics of a beneficial carabid species (Pterostichus melanarius). We hypothesize that distribution of carabid populations is driven in space and time by seasonal changes in habitat suitability in crops, and by insect movements from unsuitable to suitable crops during the season. The study was conducted in a 30ha mosaic of five to six adjacent crop fields (winter cereals, maize) and their boundaries in Western France. Winter cereals and maize exhibit dense vegetation cover at different periods (May-July and July-September respectively) and were supposed to have asynchronous suitability for carabid beetles. From May to September 2009 and 2010, trapping was used to describe: 1) densities of post-emerged adults in crop fields (enclosed emergence arenas), 2) carabid activity-density in crop fields and field boundaries (open pitfall traps) and 3) carabid movements between adjacent crop fields, and between crop fields and their boundaries (directional barrier traps). In the early season (May-July), carabid activity-density was higher in cereals than in maize fields. This was partly explained by lower densities of emergent adults in maize fields due to destructive effects of spring soil tillage on larvae and pupae in maize. On the other hand, suitable vegetation cover and microclimatic conditions in cereals might also have result in higher carabid activity-density. During this period, carabid beetles moved preferentially from maize fields with bare soil, toward cereal fields with dense vegetation cover. Later in the season, cereal harvest and vegetation growth in maize were followed by a drastic increase in carabid activity-density in maize fields due to massive colonization of these crops by insects. The results suggest that the distribution and dynamics of carabid populations is driven by asynchronous suitability of cereals and maize fields, which act as complementary habitats during the season. The spatio-temporal heterogeneity of the cultivated mosaic might therefore play a crucial role for the persistence of beneficial carabid beetles

Intra-annual heterogeneity of the cultivated mosaic: impact on carabid beetle populations

Spatio-temporal heterogeneity is recognised as a key factor for biodiversity. In agricultural landscapes, landscape heterogeneity is mostly described by the amount of semi-natural elements. In addition, the heterogeneity of cover types in the cultivated matrix might also influence biodiversity (Fahrig et al. 2011). Within a year, this matrix is highly heterogeneous in space and time, due to the diversity of crop covers, to crop phenology and cultural practices. Thus, for species that use crops during their life cycle, the cultivated matrix can be viewed as a mosaic of asynchronous habitats with fast-changing suitability, which is likely to affect species distribution and survival (Vasseur et al. 2008). In this study, we tested whether intra-annual heterogeneity of the cultivated mosaic affects population distribution and dynamics of a beneficial carabid species (Pterostichus melanarius). We hypothesize that distribution of carabid populations is driven in space and time by seasonal changes in habitat suitability in crops, and by insect movements from unsuitable to suitable crops during the season. The study was conducted in a 30ha mosaic of five to six adjacent crop fields (winter cereals, maize) and their boundaries in Western France. Winter cereals and maize exhibit dense vegetation cover at different periods (May-July and July-September respectively) and were supposed to have asynchronous suitability for carabid beetles. From May to September 2009 and 2010, trapping was used to describe: 1) densities of post-emerged adults in crop fields (enclosed emergence arenas), 2) carabid activity-density in crop fields and field boundaries (open pitfall traps) and 3) carabid movements between adjacent crop fields, and between crop fields and their boundaries (directional barrier traps). In the early season (May-July), carabid activity-density was higher in cereals than in maize fields. This was partly explained by lower densities of emergent adults in maize fields due to destructive effects of spring soil tillage on larvae and pupae in maize. On the other hand, suitable vegetation cover and microclimatic conditions in cereals might also have result in higher carabid activity-density. During this period, carabid beetles moved preferentially from maize fields with bare soil, toward cereal fields with dense vegetation cover. Later in the season, cereal harvest and vegetation growth in maize were followed by a drastic increase in carabid activity-density in maize fields due to massive colonization of these crops by insects. The results suggest that the distribution and dynamics of carabid populations is driven by asynchronous suitability of cereals and maize fields, which act as complementary habitats during the season. The spatio-temporal heterogeneity of the cultivated mosaic might therefore play a crucial role for the persistence of beneficial carabid beetles

Management of wireworm damage in maize fields using new, landscape-scale strategies

International audienceCrop-damaging wireworms—the soil-dwelling larvae of click beetles—have resurged in Europe over the past 15 years, particularly in French maize crops. There is currently no curative treatment available to control wireworms, and preventive treatments are mainly chemical. We therefore need to better understand factors that rule damage for developing agroecological control strategies. In this investigation, we tested the effect of agricultural practices and local landscape on wireworm damage in maize crops. We surveyed wireworm damage in 341 fields under various conditions in western France in 2011 and 2012. We used in particular a random forest algorithm to impute missing values and an automated model selection routine to select the best beta regression model. Our results show that the occurrence of grassland in the rotation increases wireworm damage. Tillage also shows a high influence, though varying with season and year. Wireworm damage is decreased by the presence of hedges or cultivated crops at the field border, whereas it is increased by the presence of grassland at the field border. Overall, our findings provide some insights to develop preventive solutions for the sustainable control of wireworms, as well as a framework for data processing to analyze a wide range of similar situations involving other crops and pests

Can aphids be controlled by fungus? A mathematical model

The control of insect pests in agriculture is essential for food security. Chemical controls typically damage the environment and harm beneficial insects such as pollinators, so it is advantageous to identify targetted biological controls. Since predators are often generalists, pathogens or parasitoids are more likely to serve the purpose. Here, we model a fungal pathogen of aphids as a potential means to control of these important pests in cereal crops. Typical plant herbivore pathogen models are set up on two trophic levels, with dynamic variables the plant biomass and the uninfected and infected herbivore populations. Our model is unusual in that (i) it has to be set up on three trophic levels to take account of fungal spores in the environment, but (ii) the aphid feeding mechanism leads to the plant biomass equation becoming uncoupled from the system. The dynamical variables are therefore the uninfected and infected aphid population and the environmental fungal concentration. We carry out an analysis of the dynamics of the system. Assuming that the aphid population can survive in the absence of disease, the fungus can only persist (and control is only possible) if (i) the host grows sufficiently strongly in the absence of infection, and (ii) the pathogen transmission parameters are sufficiently large. If it does persist the fungus does not drive the aphid population to extinction, but controls it below its disease-free steady state value, either at a new coexistence steady state or through oscillations. Whether this control is sufficient for agricultural purposes will depend on the detailed parameter values for the system.publishedVersio