phenModel: A temperature-dependent phenology/voltinism model for a herbivorous insect incorporating facultative diapause and budburst

A comprehensive phenology/voltinism model was developed for Phratora vulgatissima, an important pest of bioenergy crops. The model, phenModel, was developed based on development times of different life cycle stages (eggs, larvae, pupae, pre-oviposition, oviposition, sexual maturation) obtained from constant temperature laboratory experiments. As part of this study, a number of linear and non-linear models which describe the temperature-dependent development rate (inverse of development time) for each of the different life cycle stages were fitted. Based on the criteria of model parsimony and model fit, the non-linear Lactin-2 model was chosen as the optimum model to describe temperature-driven development in P. vulgatissima. To account for the variation in development times between individuals, an important but often ignored aspect in phenology models, a number of stochastic models (2- and 3- parameter Weibull and logistic models) were evaluated, based on the assumption that normalised development times conform to a similar shaped ('same shape') distribution. Novel aspects of the phenology model include the incorporation of a biologically relevant biofix, based on a budburst model for Salix viminalis, and a photoperiod threshold to induce facultative diapause. The model, which is written in R for accessibility, requires inputs of daily minimum and maximum temperature and site latitude and produces outputs describing the timing of completion of developmental stages for specified proportions of the population. It was evaluated against available field data and found to largely reproduce the observations providing a measure of its potential utility. A key component of the model allows for a sensitivity analysis of the model parameters. The model is structured so that it can easily be adapted for other leaf-feeding beetles which display a facultative reproductive diapause cued by photoperiod, and where the onset of oviposition is dependent on budburst, assuming relevant life cycle stage parameters are available

Cold winter temperatures condition the egg-hatching dynamics of a grape disease vector

The leafhopper Scaphoideus titanus is the vector of a major phytoplasma grapevine disease, Flavescence dorée. The vector’s distribution is in Eastern and Northern Europe, and its population dynamics varies as a function of vineyard latitude. We tested the hypothesis that hatching dynamics are cued by cold temperatures observed in winter. We exposed eggs from a natural population to simulated “cold” and “mild” winters and varied the exposure time at 5 °C from 0 to 63 days. We show that temperature cooling mainly affected the onset of hatching and is negatively correlated to the cold time exposure. The majority of hatchings occurred more quickly in cold rather than in mild winter simulated conditions, but there was no significant difference between the duration of hatching of eggs whatever the cold time exposure. In agreement with the Northern American origin of the vector, the diapause termination and thus the timing regulation of egg hatching require cold winters

Scaphoideus titanus behaviour, spatial and demographic consequences

La Flavescence dorée est une maladie à phytoplasme incurable qui affecte la vigne. Cette pathologie est uniquement transmise de plante à plante par la cicadelle vectrice Scaphoideus titanus. Malgré l'importance de cette maladie, peu d'études ont été menées sur le vecteur. Ce travail de thèse vise à apporter des connaissances sur la biologie et l'écologie de S. titanus mais aussi de manière plus générale sur l'écologie des insectes phytophages. Trois grands thèmes ont été abordés dans ce mémoire : i) les relations entre l'insecte et son environnement à travers l'influence des températures hivernales sur les œufs en diapause, ii) les relations de l'insecte avec sa plante hôte en examinant l'attractivité de cette dernière et le comportement alimentaire du vecteur, iii) les interactions entre congénères par l'étude du comportement d'agrégation des larves. Les résultats obtenus indiquent que 1) des températures froides ne sont pas indispensables à la levée de diapause mais affectent la protandrie en régulant le sex-ratio opérationnel, 2) l'attractivité de la plante hôte est en grande partie liée à sa couleur et il existe des différences de comportement en fonction du stade de développement, 3) les mâles et les femelles n'ont pas le même comportement alimentaire, ce qui pourrait expliquer leur différentes capacités de vection, 4) l'agrégation des larves ne semble pas due à un recrutement actif de leur part et pourrait contribuer à un transfert horizontal de phytoplasme d'insecte à insecte via la plante. A terme, ces connaissances pourront contribuer à l'élaboration de pistes alternatives à la lutte chimique actuelle contre cette cicadelle dans le cadre d'une réduction des intrants.The Flavescence dorée is an incurable phytoplasma disease of grapevine. This pathology is exclusively transmitted from plant to plant by the leafhopper vector Scaphoideus titanus. Despite the importance of this disease, few studies have been conducted on the vector. This thesis aims to provide knowledge on the biology and ecology of S. titanus but also more broadly on the ecology of phytophagous insects. Three major themes were addressed in this thesis: i) the relationships between the insect and its environment through the influence of winter temperatures on diapausing eggs, ii) the relationships of the insect with its host plant by examining the attractiveness of the plant and the feeding behaviour of the vector, iii) interactions between congeners in the study of the aggregative behaviour of larvae. Our results indicate that 1) cold temperatures are not essential to break the diapause but contribute to protandry through operational sex ratio regulation, 2) the attractiveness of the host plant is largely due to its colour and different instars exhibit differences in behavioural choices, 3) males and females have different feeding behaviour which may explain their different efficiencies at vection, 4) the aggregation of larvae did not seem to result from active recruitment and could contribute to horizontal insect to insect transfer of phytoplasma through the plant. Such knowledge can contribute to the development of alternative S. titanus pest management techniques

Scaphoideus titanus behaviour, spatial and demographic consequences

La Flavescence dorée est une maladie à phytoplasme incurable qui affecte la vigne. Cette pathologie est uniquement transmise de plante à plante par la cicadelle vectrice Scaphoideus titanus. Malgré l'importance de cette maladie, peu d'études ont été menées sur le vecteur. Ce travail de thèse vise à apporter des connaissances sur la biologie et l'écologie de S. titanus mais aussi de manière plus générale sur l'écologie des insectes phytophages. Trois grands thèmes ont été abordés dans ce mémoire : i) les relations entre l'insecte et son environnement à travers l'influence des températures hivernales sur les œufs en diapause, ii) les relations de l'insecte avec sa plante hôte en examinant l'attractivité de cette dernière et le comportement alimentaire du vecteur, iii) les interactions entre congénères par l'étude du comportement d'agrégation des larves. Les résultats obtenus indiquent que 1) des températures froides ne sont pas indispensables à la levée de diapause mais affectent la protandrie en régulant le sex-ratio opérationnel, 2) l'attractivité de la plante hôte est en grande partie liée à sa couleur et il existe des différences de comportement en fonction du stade de développement, 3) les mâles et les femelles n'ont pas le même comportement alimentaire, ce qui pourrait expliquer leur différentes capacités de vection, 4) l'agrégation des larves ne semble pas due à un recrutement actif de leur part et pourrait contribuer à un transfert horizontal de phytoplasme d'insecte à insecte via la plante. A terme, ces connaissances pourront contribuer à l'élaboration de pistes alternatives à la lutte chimique actuelle contre cette cicadelle dans le cadre d'une réduction des intrants.The Flavescence dorée is an incurable phytoplasma disease of grapevine. This pathology is exclusively transmitted from plant to plant by the leafhopper vector Scaphoideus titanus. Despite the importance of this disease, few studies have been conducted on the vector. This thesis aims to provide knowledge on the biology and ecology of S. titanus but also more broadly on the ecology of phytophagous insects. Three major themes were addressed in this thesis: i) the relationships between the insect and its environment through the influence of winter temperatures on diapausing eggs, ii) the relationships of the insect with its host plant by examining the attractiveness of the plant and the feeding behaviour of the vector, iii) interactions between congeners in the study of the aggregative behaviour of larvae. Our results indicate that 1) cold temperatures are not essential to break the diapause but contribute to protandry through operational sex ratio regulation, 2) the attractiveness of the host plant is largely due to its colour and different instars exhibit differences in behavioural choices, 3) males and females have different feeding behaviour which may explain their different efficiencies at vection, 4) the aggregation of larvae did not seem to result from active recruitment and could contribute to horizontal insect to insect transfer of phytoplasma through the plant. Such knowledge can contribute to the development of alternative S. titanus pest management techniques

Réflexion sur le devenir d'insectes du vignoble dans le contexte d'un réchauffement climatique global

National audienc

The vineyard under threat: esca and flavescence dorée

Worldwide viticulture is confronted to global change and should face to new phytosanitary issues. Some ones are present in the vineyard for centuries but are skyrocketing, another ones are more recent. The former category is represented by Esca, while the latter include Flavescence dorée (FD). Esca was described as a grapevine disease since the Antiquity, while FD began to cause damages less than one century. Despite their differences, these two diseases have common features and represent the more threatening sanitary problem for European viticulture. One of the common characteristic between ESCA and FD is the use of pesticide, but for antagonist reasons. Esca problems result to the withdrawal of the use of a fungicide while this is the occurrence of FD that implies the use of insecticides. In the first case, growers deplore the lack of pesticide while in the second case, some growers and people complain against the use of insecticides. Both diseases lead to uprooting, offset by new plants, which lead to an high heterogeneity grape level in the plots, with consequences on wine quality. So, contrary to downy and powdery mildew, ESCA and FD occurrence have a long-time effect on vineyard. As they represent a threat for the economic viability of the entire sector, ESCA and FD have been identified as a priority by winegrowers, scientists and decision-makers. Thus, European wine industry leaders are urging the European Union and national governments to provide funding to step up research to find a cure or a preventive treatment

Winter temperature and Flavescence dorée vector ecology

The invasive leafhopper Scaphoideus titanus is spreading in Europe from ca. 35 to 50°N. According to the origin area of this insect, we hypothesized that its Southern limit of distribution could partly be the consequence of the lack of cold temperatures that are essential to break egg diapause. We investigated the effects of winter incubation temperature on hatching and post hatching development. Here, we conclude that winter temperatures could be one from other key factors explaining the weak colonization of Southern vineyards in Europe by S. titanus

Egg incubation temperature differently affects female and male hatching dynamics and larval fitness in a leafhopper

Temperature effects on ectotherms are widely studied particularly in insects. However, the life-history effects of temperature experienced during a window of embryonic development, that is egg stage, have rarely been considered. We simulated fluctuating temperatures and examined how this affects the operational sex ratio (OSR) of hatching as well as nymph and adult fitness in a leafhopper, Scaphoideus titanus. Specifically, after a warm or cold incubation we compared males and females hatching dynamics with their consequences on the sex ratio in the course of time, body size, weight, and developmental rate of the two populations, all reared on the same posthatching temperature. Males and females eggs respond differently, with females more sensitive to variation in incubation temperature. The different responses of both sexes have consequences on the sex ratio dynamic of hatchings with a weaker protandry after warm incubation. Temperatures experienced by eggs have more complex consequences on posthatching development. Later nymphal instars that hatched from eggs exposed to warm temperature were larger and bigger but developmental rate of the two populations was not affected. Our study demonstrates how incubation temperature could affect operational sex ratio and posthatching development in an insect and how this may be critical for population growth