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Monitoring of plant pathogens is usually carried out by analyzing symptomatic plant material. However when plants are frequently asymptomatic, which is the case for Xylella fastidiosa (Xf), this approach is problematic because it limits our ability to early detect the bacterium hence to understand and control its spread. To complement plant survey and facilitate large-scale monitoring, we propose to generalize the use of insect vectors as sentinels. We tested this approach in Corsica using Philaenus spumarius, an ubiquitous insect vector, as sentinel. We will present a high throughput method based on a two-step PCR approach followed by Illumina sequencing. Two genes of the MLST of Xf were targeted to identify the three main subspecies of Xf (multiplex, pauca or fastidiosa). This approach enables larger screening than classic (nested) PCR approaches followed by Sanger sequencing. We used this approach to monitor the spatio-temporal prevalence of Xf within vector populations (27 populations of 30 specimens). Our results show that Xf occurs throughout Corsica with a prevalence in insects up to 43% depending on the region and the year of sampling. Moreover, we show that Xf is present in ecosystems assumed uncontaminated on the basis of plant monitoring. No spatial, temporal or vegetation-related pattern of Xf prevalence in insects seem to emerge from our first data. Our results confirm that Xf introduction in Corsica likely predates 2015, it suggests ecological resilience of ecosystems and argues in favor of using insect vectors for Xf survey at large scale in Europe., FR; PPT; pauline.farigoule@inrae.fr

Philaenus spumarius (Hemiptera: Aphrophoridae) is a major vector of Xylella fastidiosa (Xf) in Europe. However, the drivers influencing its abundance are still poorly known. In Corsica from 2018 to 2020, we monitored the abundance of P. spumarius three times a year in 64 permanent plots distributed along vegetation and altitudinal (0-600m) gradients. The vegetation gradient ranged from shrubland to oak forest and accounted for the density of Cistus monspeliensis on which P. spumarius usually develops and feed in Corsica. Abundance of P. spumarius was estimated through visual counting of foams and sweep netting of adults. Both vegetation composition and climate descriptors were recorded at each sampling location. We used partial least squares path modelling and generalized linear mixed models to unravel the relationships between vegetation / climate descriptors and P. spumarius abundance. Philaenus spumarius abundance significantly decreases with increasing average temperature. Abundance of P. spumarius was significantly and positively correlated with C. monspeliensis density, confirming difference in host plant preference between Corsica and mainland France. A significant effect of vegetation structure was also found with forests hosting lower P. spumarius abundance. The strong association observed between P. spumarius and C. monspeliensis suggests that Xf expansion may have been facilitated by firebreaks or disturbed habitats where C. monspeliensis is abundant as an early colonizer. Managing C. monspeliensis in the vicinity of agricultural areas may constitute an essential component of prophylaxis against Xf in Corsica.

Identification of adult vectors of Xylella fastidiosa (Xf) based on morphological characters can be difficult for non-specialists. The identification of eggs or larvae is even more difficult and sometimes impossible. To circumvent these difficulties and enable large survey of populations of insect vectors in Europe we have developed a web-interface database of COI barcodes. COI barcodes were generated with a high-throughput approach involving two PCR steps to target COI and index samples followed by Illumina sequencing. Multiple quality controls were implemented including i) non-destructive DNA extraction that allows vouchering and morphological re-examination by taxonomists; ii) bioinformatic controls to discards possible contaminants, PCR or sequencing errors, NUMTs and heteroplasms. All barcodes have been uploaded in a web-interface database (Arthemis DB@se: http://arthemisdb.supagro.inra.fr/DefaultInfo.aspx?Page=Home). Metadata associated with all records are available online, as well as biological data on vectors and pictures. Users can identify a query sequence through BLAST comparison against our reference library. Currently, the database hosts 636 barcodes for 72 species of potential vectors of Xf: 490 sequences representing 43 European species and 145 sequences representing 29 non-European species. All frequent species have been barcoded. Missing species are rare or localized and do not represent an important threat for European agriculture. COI allows reliable identification for 80% of the species, but we observed issues for the remaining 20%. The most important issue concerns Philaenus spumarius and its close relative P. tesselatus. These two species are hardly distinguishable even with male genitalia and share the same COI sequence. It was also difficult to cross validate morphological and molecular results for some species within the genera Neophilaenus and Aphrophora., FR; PPT; jean-claude.streito@inrae.fr

In Europe, many potential and confirmed vectors of Xylella fastidiosa (Xf) are polyphagous but appear to aggregate on their preferred host plants at local scales. For example, Philaenus spumarius is found most often on Asteraceae on the Californian coast, or on Apiaceae and Rubiaceae in the meadows of central Italy. However, vectors keep the ability to exploit multiple host-plants, which can be crucial in primary Xf transmission to crops. In this study, we assessed the habitat preferences of spittlebugs on and in the vicinity of Corsican clementine and olive groves, focusing on four habitats: Cistus monspeliensis border, Dittrichia viscosa cover, crop foliage and grove ground vegetation. Spittlebug abundance was assessed three times a year during two years on five to nine clementine and olive organic groves. Nymph and adult abundances were quantified based on spittle mass counts and nymph identification in April and sweep net sampling in June and October. Habitat preferences were inferred based on generalized linear mixed models. Overall, 6647 spittle masses and 1714 adults belonging to four species were found. Philaenus spumarius had a significant preference for Cistus monspeliensis, and to a lesser extent for Dittrichia viscosa. Neophilaenus campestris mostly occurred on ground vegetation of the grove and on Dittrichia viscosa. Lepyronia coleoptrata was less abundant on crop foliage than on ground vegetation and Aphrophora alni showed an even abundance in all habitats. This study demonstrates the specialization of P. spumarius on C. monspeliensis in the diversified Corsican landscape as well as strong habitat preferences for N. campestris. All recorded species were able to colonize clementine or olive foliage, but rarely, and in similar abundancies for all species. Should they be proven equivalent effective vectors, this suggests similar roles of these four species in Xf transmission to crops.

International audience; Ground dwelling predators provide regulation services of several insect pests. Enhancing these services may be a step toward integrated crop protection. Many studies have shown that soil tillage is deleterious to ground dwelling predators but pest regulation processes and services have rarely been measured. We performed an experiment to study whether simplifying soil tillage before the establishment of spring broccoli enhanced ground dwelling predator populations and the control they provide on Delia radicum. The direct effect of tillage on arthropods was assessed by comparing their emergence rates in plots differing in soil tillage management. The natural regulation service was assessed by comparing a control and an exclusion treatment in which predators were removed. The effect of soil tillage on carabids, spiders and staphylinids did not match the gradient of disturbance induced by tillage treatments. Tillage did not appear to affect the predators that likely contribute to D. radicum regulation. Consistently, the number of pests suppressed and the root injuries were unaffected by tillage treatments. The main deleterious effect of soil tillage was on the emergence of those carabid species that overwinter partly as larvae, suggesting that spring tillage could affect pest control in the following crops.

International audience; For many crops, we have poor knowledge about the relationship between pest density and damage. However, investigating pest harmfulness is particularly relevant currently in the search for alternative crop protection strategies that are unlikely to totally suppress pest populations. Here, we assessed the harmfulness of Delia radicum (L.) (Diptera: Anthomyiidae) on broccoli (Brassica oleracea var. italica Plenck). We worked inside insect-proof cages set up in the field with additional pitfall traps to remove ground-dwelling predators. Plants were manually infested with 10 levels of pest density ranging 0–100 individuals per plant, following a natural infestation pattern. Surprisingly, no plants died but almost 100% of the pests introduced died over the course of the experiment. However, all broccoli development and growth traits were negatively correlated with pest density and broccoli head mass at harvest decreased linearly with pest density. The observation over time of development and growth traits showed evidence of plant compensation, suggesting that the head mass of individual plants may have reached similar values if allowed to fully mature. The relationship between pest density and damage, together with forecast models of pest population dynamics could be used to develop decision support tools assessing the relevance of preventative treatments.

In Europe, many potential and confirmed vectors of Xylella fastidiosa (Xf) are polyphagous but appear to aggregate on their preferred host plants at local scales. For example, Philaenus spumarius is found most often on Asteraceae on the Californian coast, or on Apiaceae and Rubiaceae in the meadows of central Italy. However, vectors keep the ability to exploit multiple host-plants, which can be crucial in primary Xf transmission to crops. In this study, we assessed the habitat preferences of spittlebugs on and in the vicinity of Corsican clementine and olive groves, focusing on four habitats: Cistus monspeliensis border, Dittrichia viscosa cover, crop foliage and grove ground vegetation. Spittlebug abundance was assessed three times a year during two years on five to nine clementine and olive organic groves. Nymph and adult abundances were quantified based on spittle mass counts and nymph identification in April and sweep net sampling in June and October. Habitat preferences were inferred based on generalized linear mixed models. Overall, 6647 spittle masses and 1714 adults belonging to four species were found. Philaenus spumarius had a significant preference for Cistus monspeliensis, and to a lesser extent for Dittrichia viscosa. Neophilaenus campestris mostly occurred on ground vegetation of the grove and on Dittrichia viscosa. Lepyronia coleoptrata was less abundant on crop foliage than on ground vegetation and Aphrophora alni showed an even abundance in all habitats. This study demonstrates the specialization of P. spumarius on C. monspeliensis in the diversified Corsican landscape as well as strong habitat preferences for N. campestris. All recorded species were able to colonize clementine or olive foliage, but rarely, and in similar abundancies for all species. Should they be proven equivalent effective vectors, this suggests similar roles of these four species in Xf transmission to crops., FR; PPT; xavier.mesmin@inrae.fr

Philaenus spumarius (Hemiptera: Aphrophoridae) is a major vector of Xylella fastidiosa (Xf) in Europe. However, the drivers influencing its abundance are still poorly known. In Corsica from 2018 to 2020, we monitored the abundance of P. spumarius three times a year in 64 permanent plots distributed along vegetation and altitudinal (0-600m) gradients. The vegetation gradient ranged from shrubland to oak forest and accounted for the density of Cistus monspeliensis on which P. spumarius usually develops and feed in Corsica. Abundance of P. spumarius was estimated through visual counting of foams and sweep netting of adults. Both vegetation composition and climate descriptors were recorded at each sampling location. We used partial least squares path modelling and generalized linear mixed models to unravel the relationships between vegetation / climate descriptors and P. spumarius abundance. Philaenus spumarius abundance significantly decreases with increasing average temperature. Abundance of P. spumarius was significantly and positively correlated with C. monspeliensis density, confirming difference in host plant preference between Corsica and mainland France. A significant effect of vegetation structure was also found with forests hosting lower P. spumarius abundance. The strong association observed between P. spumarius and C. monspeliensis suggests that Xf expansion may have been facilitated by firebreaks or disturbed habitats where C. monspeliensis is abundant as an early colonizer. Managing C. monspeliensis in the vicinity of agricultural areas may constitute an essential component of prophylaxis against Xf in Corsica., FR; PPT; marguerite.chartois@inrae.fr