Control of <i>Hyalesthes obsoletus</i> nymphs based on chemical weeding and insecticides applied on <i>Urtica dioica</i>
Bois noir is a grapevine yellows disease associated with Candidatus Phytoplasma solani and transmitted to grapevines by means of the planthopper Hyalesthes obsoletus Signoret (Homoptera, Cixiidae). The overwintering nymphs of the vector acquire the phytoplasma feeding on roots of herbaceous plants, including Urtica dioica L. (stinging nettle). In German and Italian vineyards the possibility to control the H. obsoletus nymphs feeding on stinging nettle roots using chemical weeding and insecticides was investigated. In particular, the effect of herbicides, applied in autumn and in different spring timings, and neonicotinoid insecticides on vector adult emergence was evaluated. Trials conducted to control nettle with glyphosate or a mixture of glyphosate+flazasulfuron significantly reduced the density of emerging adult vectors. The efficacy of herbicides was highest when they were applied in autumn or in early spring with the nymphs not older than the fourth instar. Herbicides applied too close to the beginning of the emergence of adults reduced numbers only during the late part of the planthopper flight-period. Although neonicotinoid insecticides applied in early spring gave efficacy comparable to herbicides, their use is not advisable for the negative side effects on non-target arthropods (e.g. honeybees). Overall, the combination of cultural practices and accurately timed applications of selective herbicides might help to refine the current Integrated Pest Management recommendations for controlling nettle, H. obsoletus and consequently bois noir.
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