Identification and phenology of Hyalesthes obsoletus (Hemiptera: Auchenorrhyncha: Cixiidae) nymphal instars

Relationships between Hyalesthes obsoletus, Its Herbaceous Hosts and Bois Noir Epidemiology in Northern Italian Vineyards

Hyalesthes obsoletus is the vector of "Candidatus Phytoplasma (Ca. P.) solani," the causal agent of grapevine yellows Bois noir (BN). The relationships among the planthopper, its main herbaceous hosts as phytoplasma reservoirs (Convolvolus arvensis and Urtica dioica) and BN spreading were studied in northern Italy. In two areas the relationship between host plants and the phenology and survival of planthopper adults was investigated in potted plants and in field conditions. Moreover, H. obsoletus ecology, newly symptomatic grapevine occurrence and "Ca. P. solani" tuf-types' presence were studied in two vineyards (2014-2019). An earlier occurrence of H. obsoletus adults on C. arvensis than U. dioica and better adult survival of the originating host were observed. When U. dioica was prevalent, the vector occurred almost exclusively along the ditch outside the vineyard. Hyalesthes obsoletus amount varied widely from year to year and nymphal mortality due to late frosts was supposed. In one vineyard, the amount of newly symptomatic grapevines was significantly correlated with vector abundance in the previous year. The "Ca. P. solani" tuf-type was influenced by vector population levels on the two hosts. Since the abundance of H. obsoletus populations on the two hosts influences BN epidemiology and dynamics and the "Ca. P. solani" tuf-type, this must be considered in BN control strategies.

Widespread plant specialization in the polyphagous planthopper Hyalesthes obsoletus (Cixiidae), a major vector of stolbur phytoplasma: Evidence of cryptic speciation

The stolbur phytoplasma vector Hyalesthes obsoletus is generally considered as a polyphagous species associated with numerous wild and cultivated plants. However, recent research in southeastern Europe, the distribution centre of H. obsoletus and the area of most stolbur-inflicted crop diseases, points toward specific host-plant associations of the vector, indicating specific vector-based transmission routes. Here, we study the specificity of populations associated with four host-plants using mitochondrial and nuclear genetic markers, and we evaluate the evolution of host-shifts in H. obsoletus. Host-plant use was confirmed for Convolvulus arvensis, Urtica dioica, Vitex agnus-castus and Crepis foetida. Mitochondrial genetic analysis showed sympatric occurrence of three phylogenetic lineages that were ecologically delineated by host-plant preference, but were morphologically inseparable. Nuclear data supported the existence of three genetic groups (Evanno’s ΔK(3) = 803.72) with average genetic membership probabilities > 90%. While populations associated with C. arvensis and U. dioica form a homogenous group, populations affiliated with V. agnus-castus and C. foetida constitute two independent plant-associated lineages. The geographical signal permeating the surveyed populations indicated complex diversification processes associated with host-plant selection and likely derived from post-glacial refugia in the eastern Mediterranean. This study provides evidence for cryptic species diversification within H. obsoletus sensu lato: i) consistent mitochondrial differentiation (1.1–1.5%) among host-associated populations in syntopy and in geographically distant areas, ii) nuclear genetic variance supporting mitochondrial data, and iii) average mitochondrial genetic distances among host-associated meta-populations are comparable to the most closely related, morphologically distinguishable species, i.e., Hyalesthes thracicus (2.1–3.3%).

Vineyard Colonization by Hyalesthes obsoletus (Hemiptera: Cixiidae) Induced by Stinging Nettle Cut Along Surrounding Ditches

Stinging nettle (Urtica dioica L.) is the most important host plant for both phytoplasma associated with Bois noir disease of the grapevine and its vector Hyalesthes obsoletus Signoret (Hemiptera: Cixiidae). Vector abundance in vineyards is favored by stinging nettle growing in surrounding areas. Nettle control by herbicides or cutting can reduce vector population in vineyards. However, chemical weeding can cause environmental problems. Many authors suggest that stinging nettle control applied during H. obsoletus flight could force adults to migrate into vineyards. We evaluate if cutting of nettle growing along ditches during adult flight favors vineyard colonization by H. obsoletus. Three different weed management regimes ("no cuts," "one cut" just before the beginning of adult flight, and "frequent cuts" over the whole vegetative season) were applied to the herbaceous vegetation in ditches bordering two vineyards. The flight dynamics of H. obsoletus were recorded by placing yellow sticky traps on the vegetation along the ditches and at different positions in the vineyards. Frequent stinging nettle cuts (compared with a single cut) in surrounding areas favored the dispersion of vectors inside the vineyards. Stinging nettle control should be based on an integration of a single herbicide application before H. obsoletus emergence followed by frequent cuts to minimize negative side effects of chemical weeding. In organic viticulture, a frequent-cuts strategy should avoid cuts during H. obsoletus flight period, at least in the first year of adoption.

Influence of environment and climate on occurrence of the cixiid planthopper Hyalesthes obsoletus, the vector of the grapevine disease 'bois noir'

Species distribution models (SDMs), which are well established in many fields of biological research, are still uncommon in the agricultural risk analysis of pest insects. To exemplify the use of SDMs, we investigated the influence of environmental factors on the occurrence of Hyalesthes obsoletus Signoret (Hemiptera: Cixiidae). The planthopper is the only known vector of the grapevine yellows disease 'bois noir'. The study was conducted in 145 locations in the Baden region of southwest Germany. The planthopper was surveyed on host plant patches, consisting of stinging nettle and/or bindweeds. We used a stratified modelling framework where (1) species presence-absence data were related to an extensive environmental dataset using logistic regressions; and (2) different types of average models were developed based on an information theoretic method. The results show that the incidence of H. obsoletus is associated to above- as well as below-ground environmental factors, particularly to the amount of fine soil and average annual precipitation. This result was consistent across all average models. The relative importance of other environmental variables was dependent upon the average model under consideration and thus may vary according to their intended use, either the explanation of habitat requirements or the prediction and mapping of occurrence risks. The study showed that SDMs offer a quantification of species' habitat requirements and thus, could represent a valuable tool for pest management purposes. By providing examples of current issues of grapevine pests in viticulture, we discuss the use of SDMs in agricultural risk analysis and highlight their advantages and caveats.

Sympatric diversification vs. immigration: deciphering host-plant specialization in a polyphagous insect, the stolbur phytoplasma vector Hyalesthes obsoletus (Cixiidae)

The epidemiology of vector transmitted plant diseases is highly influenced by dispersal and the host-plant range of the vector. Widening the vector's host range may increase transmission potential, whereas specialization may induce specific disease cycles. The process leading to a vector's host shift and its epidemiological outcome is therefore embedded in the frameworks of sympatric evolution vs. immigration of preadapted populations. In this study, we analyse whether a host shift of the stolbur phytoplasma vector, Hyalesthes obsoletus from field bindweed to stinging nettle in its northern distribution range evolved sympatrically or by immigration. The exploitation of stinging nettle has led to outbreaks of the grapevine disease bois noir caused by a stinging nettle-specific phytoplasma strain. Microsatellite data from populations from northern and ancestral ranges provide strong evidence for sympatric host-race evolution in the northern range: Host-plant associated populations were significantly differentiated among syntopic sites (0.054 < F(HT) < 0.098) and constant over 5 years. While gene flow was asymmetric from the old into the predicted new host race, which had significantly reduced genetic diversity, the genetic identity between syntopic host-race populations in the northern range was higher than between these populations and syntopic populations in ancestral ranges, where there was no evidence for genetic host races. Although immigration was detected in the northern field bindweed population, it cannot explain host-race diversification but suggests the introduction of a stinging nettle-specific phytoplasma strain by plant-unspecific vectors. The evolution of host races in the northern range has led to specific vector-based bois noir disease cycles.