Inter-species horizontal toxicity: Experimental control of Argentine ants and their citrophilus mealybug mutualists in a viticultural system

<p>The invasive Argentine ant, Linepithema humile, is known to form a trophobiotic association with honeydew excreting homopterans Pseudococcus sp. providing protection from natural enemies in exchange for the honeydew they excrete. The vine mealybug Pseudococcus calceolariae, can transmit Grapevine leafroll- associated virus 3 (GLRaV-3) between vines as it travels and feeds with the ensuing leafroll disease negatively impacting on vine health and wine quality. Therefore, if an effective chemical control method targeting incursions of Argentine ants in vineyards contributes to the dissociation of this invasive ant species with its citrophilus mealybug mutualist, then in theory the spread of GLRaV-3 in vineyards by its mealybug vector can be stemmed. Three insecticidal treatments targeting Argentine ants in the canopy of potted Pinot Noir grapevines inoculated with citrophilus mealybugs were trialled at a field site established in Nelson during the summer of 2016/2017. Bifenthrin (1200ppm) was sprayed on vine trunks and the low- toxicity baits, thiamethoxam (0.0006%) or boric acid (0.5%) carried in polyacrylamide gel with 25% sucrose and 0.15% citric acid solution, were placed at the base of vines. A significant decline in ant activity (p < 0.001) and citrophilus mealybugs was observed for the bifenthrin treatment. A follow-on bioassay was conducted at Mt. Albert Plant and Food Research, in the absence of P. calceolariae’s natural enemies to test the hypothesis that the decline in citrophilus mealybugs in response to vines treated with bifenthrin, could in fact be due to inter-species horizontal toxicity because of Argentine ants transferring the toxicant bifenthrin to citrophilus mealybugs while tending them or contaminating the substrate that they fed on. The significant decrease in average citrophilus mealybug activity rate (p < 0.001) for bifenthrin treatments compared with the controls provides evidence for inter-species horizontal toxicity. Bifenthrin sprayed on grapevine trunks may be suitable to control Argentine ants in the vine canopy and indirectly control P. calceolariae, a known vector of GLRaV-3 between grapevine hosts. The concept of inter-species horizontal toxicity could become a model for targeted pest management by exploiting different insect mutualisms in various horticultural cropping systems.</p>

Inter-species horizontal toxicity: Experimental control of Argentine ants and their citrophilus mealybug mutualists in a viticultural system

<p>The invasive Argentine ant, Linepithema humile, is known to form a trophobiotic association with honeydew excreting homopterans Pseudococcus sp. providing protection from natural enemies in exchange for the honeydew they excrete. The vine mealybug Pseudococcus calceolariae, can transmit Grapevine leafroll- associated virus 3 (GLRaV-3) between vines as it travels and feeds with the ensuing leafroll disease negatively impacting on vine health and wine quality. Therefore, if an effective chemical control method targeting incursions of Argentine ants in vineyards contributes to the dissociation of this invasive ant species with its citrophilus mealybug mutualist, then in theory the spread of GLRaV-3 in vineyards by its mealybug vector can be stemmed. Three insecticidal treatments targeting Argentine ants in the canopy of potted Pinot Noir grapevines inoculated with citrophilus mealybugs were trialled at a field site established in Nelson during the summer of 2016/2017. Bifenthrin (1200ppm) was sprayed on vine trunks and the low- toxicity baits, thiamethoxam (0.0006%) or boric acid (0.5%) carried in polyacrylamide gel with 25% sucrose and 0.15% citric acid solution, were placed at the base of vines. A significant decline in ant activity (p < 0.001) and citrophilus mealybugs was observed for the bifenthrin treatment. A follow-on bioassay was conducted at Mt. Albert Plant and Food Research, in the absence of P. calceolariae’s natural enemies to test the hypothesis that the decline in citrophilus mealybugs in response to vines treated with bifenthrin, could in fact be due to inter-species horizontal toxicity because of Argentine ants transferring the toxicant bifenthrin to citrophilus mealybugs while tending them or contaminating the substrate that they fed on. The significant decrease in average citrophilus mealybug activity rate (p < 0.001) for bifenthrin treatments compared with the controls provides evidence for inter-species horizontal toxicity. Bifenthrin sprayed on grapevine trunks may be suitable to control Argentine ants in the vine canopy and indirectly control P. calceolariae, a known vector of GLRaV-3 between grapevine hosts. The concept of inter-species horizontal toxicity could become a model for targeted pest management by exploiting different insect mutualisms in various horticultural cropping systems.</p>

Influence of Grapevine Leafroll-associated Virus-3 in Mature Plants of Vitis vinifera L. cv Albariño on 110R and 196.17C Rootstocks

The detrimental effects of grapevine leafroll disease (GLD) have been documented mainly in young plantsand little data is available on infected grapevines when they reach maturity. This study examined theinfluence of the rootstock on the effect of GLRaV-3 in a 20-year-old Vitis vinifera cv Albariño vineyard inwhich the virus has been spread by Planococcus ficus. Plants grafted on Richter 110 or Castell 196.17 withsimilar development were analysed for grapevine leafroll-associated viruses. In particular, 25 GLRaV-3-infected and 25 leafroll-free and asymptomatic vines were selected and monitored during a period of threeyears to determine grape yield and must components. Although the virus infection affected plants on bothrootstocks, it caused a greater effect on plants grafted onto 110R, with average accumulated yield losses ofup to 33%, relative to losses of 16% for plants on 196.17C. The sugar content was lower in the must frominfected plants on 110R (-2.1ºBrix) than in the must from plants on 196.17C (-1.5ºBrix). The presence ofthe virus was associated with decreased concentrations of primary amino nitrogen (PAN) and ammonium(NH4+), with the greatest effect for 110R (-35%). As 110R is the most used rootstock in Spain, particularcare must be taken to prevent the transmission of GLRaV-3 to certified virus-free plants on 110R in newvineyards. In sensitive varieties such as Albariño, particularly in areas with an active spread of the virusby mealybugs, grafting on 196.17C seems to minimise the effect that GLD has in a medium to long term.

Identification of mealybugs, soft scale insects and their predators in vineyards across the savannah agro-ecological region of Nigeria

Insect-vectored viruses are a major threat to grapevine production but there is a dearth of information on the occurrence and distribution of key grapevine pests in Nigeria. The recent detection of grapevine leafroll associated virus-1 (GLRaV-1), a known insect-vectored ampelovirus, in Nigeria elevates the importance of the identification of its potential vectors as a precursor to assessing the risk of grapevine leafroll disease spread. This study was conducted to determine the occurrence and diversity of potential vectors of grapevine viruses and their natural enemies in vineyards across the savannah agro-ecological region of Nigeria. Forty vineyard and nursery locations were surveyed during 2016 and 45 arthropod samples were collected. The samples were first morphologically identified, and DNA barcoding was conducted on a subset of 16 representative samples using universal primers specific to the Mitochondrial Cytochrome Oxidase subunit I (mtCOI) gene of most insects. The results indicated the presence of two species of scale insects (Parasaissetia nigra and Saissetia coffeae) and two mealybug species (Maconellicoccus hirsutus and Ferrisia virgata), some ofwhich are potential grapevine virus vectors, in Nigerian vineyards. In addition, the natural enemies of these insect species were detected which includes three species of parasitoids (Anagyrus kamali, Anagyrus pseudococci and Encarsia inaron) and one predator (Hyperaspidius mimus). While the detection of mealybugs and scale insects underscore the risk of vector-mediated virus spread in Nigerian vineyards, the identification of their natural enemies indicates presence of natural biological control agents to facilitate an integrated management of economically important grapevine virus diseases in the country. Keywords: Mealybugs; scale insects; parasitoids and predators; insect vectors; grapevine viruses.

Probing into the Effects of Grapevine Leafroll-Associated Viruses on the Physiology, Fruit Quality and Gene Expression of Grapes

Grapevine leafroll is one of the most widespread and highly destructive grapevine diseases that is responsible for great economic losses to the grape and wine industries throughout the world. Six distinct viruses have been implicated in this disease complex. They belong to three genera, all in the family Closteroviridae. For the sake of convenience, these viruses are named as grapevine leafroll-associated viruses (GLRaV-1, -2, -3, -4, -7, and -13). However, their etiological role in the disease has yet to be established. Furthermore, how infections with each GLRaV induce the characteristic disease symptoms remains unresolved. Here, we first provide a brief overview on each of these GLRaVs with a focus on genome structure, expression strategies and gene functions, where available. We then provide a review on the effects of GLRaV infection on the physiology, fruit quality, fruit chemical composition, and gene expression of grapevine based on the limited information so far reported in the literature. We outline key methodologies that have been used to study how GLRaV infections alter gene expression in the grapevine host at the transcriptomic level. Finally, we present a working model as an initial attempt to explain how infections with GLRaVs lead to the characteristic symptoms of grapevine leafroll disease: leaf discoloration and downward rolling. It is our hope that this review will serve as a starting point for grapevine virology and the related research community to tackle this vastly important and yet virtually uncharted territory in virus-host interactions involving woody and perennial fruit crops.

The management and financial implications of variable responses to grapevine leafroll disease

© 2021, Società Italiana di Patologia Vegetale (S.I.Pa.V.). Grapevine leafroll-associated virus 3 (GLRaV-3) is a worldwide pathogen of Vitis negatively affecting wine production. In red berry cultivars grown in New Zealand, the foliar changes to GLRaV-3-infected vines inform decisions on vine removal (roguing). However, roguing does not always contain GLRaV-3 spread in the presence of an insect vector like the mealybug, Pseudococcus calceolariae. Virus incidence and mealybug abundance data collected annually over 7 years were modelled under differing scenarios to ascertain support for roguing. In a simulated 1 ha vineyard planted in 2500 mature Merlot vines, simulations over 20 years evaluated the effectiveness of roguing, ‘rogue 1 + 2’ (concurrent roguing of symptomatic plus both within-row neighbouring vines), inefficient roguing (50% of symptomatic vines rogued), and ‘no-action’. The model used variable initial GLRaV-3 incidence (0.4, 5, 10, 15 and 20%), and low, median and high vector densities (6, 26 and 75 mealybugs per 100 vine leaves, respectively). Roguing was the optimal response to GLRaV-3, independent of the initial incidence, but results were vector density dependent. At a low vector density, roguing relative to the other responses tested, sustained the lowest annual GLRaV-3 incidence, the least need to plant replacement vines and the lowest estimated average annual costs plus loss of income. At median and high vector densities, roguing remained the most favourable response but virus control was less effective and the costs incurred were higher. Thus, for vineyards affected by GLRaV-3, achieving economic sustainability relies on integrating efficient roguing with effective vector management.

The management and financial implications of variable responses to grapevine leafroll disease

© 2021, Società Italiana di Patologia Vegetale (S.I.Pa.V.). Grapevine leafroll-associated virus 3 (GLRaV-3) is a worldwide pathogen of Vitis negatively affecting wine production. In red berry cultivars grown in New Zealand, the foliar changes to GLRaV-3-infected vines inform decisions on vine removal (roguing). However, roguing does not always contain GLRaV-3 spread in the presence of an insect vector like the mealybug, Pseudococcus calceolariae. Virus incidence and mealybug abundance data collected annually over 7 years were modelled under differing scenarios to ascertain support for roguing. In a simulated 1 ha vineyard planted in 2500 mature Merlot vines, simulations over 20 years evaluated the effectiveness of roguing, ‘rogue 1 + 2’ (concurrent roguing of symptomatic plus both within-row neighbouring vines), inefficient roguing (50% of symptomatic vines rogued), and ‘no-action’. The model used variable initial GLRaV-3 incidence (0.4, 5, 10, 15 and 20%), and low, median and high vector densities (6, 26 and 75 mealybugs per 100 vine leaves, respectively). Roguing was the optimal response to GLRaV-3, independent of the initial incidence, but results were vector density dependent. At a low vector density, roguing relative to the other responses tested, sustained the lowest annual GLRaV-3 incidence, the least need to plant replacement vines and the lowest estimated average annual costs plus loss of income. At median and high vector densities, roguing remained the most favourable response but virus control was less effective and the costs incurred were higher. Thus, for vineyards affected by GLRaV-3, achieving economic sustainability relies on integrating efficient roguing with effective vector management.

A Diverse Virome of Leafroll-Infected Grapevine Unveiled by dsRNA Sequencing

Quebec is the third-largest wine grape producing province in Canada, and the industry is constantly expanding. Traditionally, 90% of the grapevine cultivars grown in Quebec were winter hardy and largely dominated by interspecific hybrid Vitis sp. cultivars. Over the years, the winter protection techniques adopted by growers and climate changes have offered an opportunity to establish V. vinifera L. cultivars (e.g., Pinot noir). We characterized the virome of leafroll-infected interspecific hybrid cultivar and compared it to the virome of V. vinifera cultivar to support and facilitate the transition of the industry. A dsRNA sequencing method was used to sequence symptomatic and asymptomatic grapevine leaves of different cultivars. The results suggested a complex virome in terms of composition, abundance, richness, and phylogenetic diversity. Three viruses, grapevine Rupestris stem pitting-associated virus, grapevine leafroll-associated virus (GLRaV) 3 and 2 and hop stunt viroid (HSVd) largely dominated the virome. However, their presence and abundance varied among grapevine cultivars. The symptomless grapevine cultivar Vidal was frequently infected by multiple virus and viroid species and different strains of the same virus, including GLRaV-3 and 2. Our data show that viruses and viroids associated with the highest number of grapevines expressing symptoms included HSVd, GLRaV-3 and GLRaV-2, in gradient order. However, co-occurrence analysis revealed that the presence of GLRaV species was randomly associated with the development of virus-like symptoms. These findings and their implications for grapevine leafroll disease management are discussed.