Host plant preferences of <i>Hyalesthes obsoletus</i>, the vector of the grapevine yellows disease ‘bois noir’, in Switzerland

Bois noir is an important grapevine yellows disease in Europe that can cause serious economic losses in grapevine production. It is caused by stolbur phytoplasma strains of the taxonomic group 16Sr-XII-A. <i>Hyalesthes obsoletus</i> Signoret (Hemiptera: Cixiidae) is the most important vector of bois noir in Europe. This polyphagous planthopper is assumed to mainly use stinging nettle [<i>Urtica dioica</i> L. (Urticaceae)] and field bindweed [<i>Convolvulus arvensis</i> L. (Convolvulaceae)] as its host plants. For a better understanding of the epidemiology of bois noir in Switzerland, host plant preferences of <i>H. obsoletus</i> were studied in the field and in the laboratory. In vineyards of Western Switzerland, adults of <i>H. obsoletus</i> were primarily captured on <i>U. dioica</i>, but a few specimens were also caught on <i>C. arvensis</i>, hedge bindweed [<i>Calystegia sepium</i> (L.) R. Brown (Convolvulaceae)], and five other dicotyledons [i.e., <i>Clematis vitalba</i> L. (Ranunculaceae), <i>Lepidium draba</i> L. (Brassicaceae), <i>Plantago lanceolata</i> L. (Plantaginaceae), <i>Polygonum aviculare</i> L. (Polygonaceae), and <i>Taraxacum officinale</i> Weber (Asteraceae)]. The preference of the vector for <i>U. dioica</i> compared to <i>C. arvensis</i> was confirmed by a second, more targeted field study and by the positioning of emergence traps above the two plant species. Two-choice experiments in the laboratory showed that <i>H. obsoletus</i> adults originating from <i>U. dioica</i> preferred to feed and to oviposit on <i>U. dioica</i> compared to <i>C. arvensis</i>. However, <i>H. obsoletus</i> nymphs showed no host plant preference, even though they developed much better on <i>U. dioica</i> than on <i>C. arvensis</i>. Similarly, adults survived significantly longer on <i>U. dioica</i> than on <i>C. arvensis</i> or any other plant species tested [i.e., <i>L. draba</i> and <i>Lavandula angustifolia</i> Mill. (Lamiaceae)]. In conclusion, although nymphs of <i>H. obsoletus</i> had no inherent host plant preference, adults tested preferred to feed and oviposit on <i>U. dioica</i>, which is in agreement with the observed superior performance of both nymphal and adult stages on this plant species. <i>Urtica dioica</i> appears to be the principal host plant of <i>H. obsoletus</i> in Switzerland and plays therefore an important role in the epidemiology of the bois noir disease in Swiss vineyards.