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The following description refers to grapevine, on which symptoms largely depend on the phenological stage of the reproductive organs.
On inflorescences (first generation), neonate larvae firstly penetrate single flower buds. Symptoms are not evident initially, because larvae remain protected by the top bud. Later, when larval size increases, each larva agglomerates several flower buds with silk threads forming glomerules (nests) visible to the naked eye, and the larvae continue feeding while protected inside. Larvae usually make one to three glomerules during their development which provide protection against adverse conditions, i.e., insulation, rain and natural enemies. Despite the hygienic behaviour of larvae, frass may remain adhering to the nests.
On grapes (summer generations), larvae feed externally and penetrate them, boring into the pulp and remaining protected by the berry peel. Larvae secure the pierced berries to surrounding ones by silk threads to avoid falling. Frass may also be visible. Each larva is capable of damaging between 2 and 10 berries, and up to 20-30 larvae per cluster may occur in heavily attacked vineyards (Thiery et al., 2018). If conditions are suitable for fungal or acid rot development, a large number of berries may be also affected by Botrytis cinerea, Aspergillus carbonarius and Aspergillus niger, which result in severe qualitative and quantitative damage (Delbac and Thiery, 2016). Damage is variety-dependent: generally it is more severe on grapevine varieties with dense grapes, because this increases both larval installation and rot development.
Larval damage on growing points, shoots or leaves is unusual (Lucchi et al., 2011).
The abnormal distribution patterns of L. botrana (see Geographical Distribution) emphasize the inherent risk of new, undesired introductions when infested grapes and/or plant material are transported around the world. Phytosanitary control in commercialization channels should be enforced to limit further pest spread, especially in importer countries with favourable climatic conditions for pest development.
Several cultural methods may reduce pest incidence to a highly variable degree. Voukassovitch (1924) listed some direct (pest-killing) and indirect (microclimate-modifying) practices to reduce L. botrana infestation levels, including pruning the vine canopy, leaf stripping, irrigation, earthing-up, weeding and especially harvesting date. However, cultural methods have a limited efficiency by themselves, and are often inapplicable in major vineyards where possibilities of changing cultural schedules are restricted. For example, a systematic advance of harvesting date to reduce larval damage in the 3rd generation is often incompatible with high quality wine production.
Some vine varietal characters may regulate larval damage. For example, it is often observed that compact grapes are more damaged than lax ones because larval thigmotropic behaviour, installation and grape-derived protection are enhanced.
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
Yield loss quantification when larvae damage inflorescences (1st generation) has been carried out using several approaches: comparing naturally damaged and undamaged grapes (as inflorescences) by weighing or counting formed berries; artificial infestations with larvae; and damage simulation by direct ablation of flowers and berries (Roehrich, 1978; Coscollá, 1980b; Gabel, 1989). Most studies show a high compensation capacity of grapevine, variable between vine varieties, supporting the presence of one to four glomerules, or the ablation of 30 flowers per inflorescence, without significant yield losses (Roehrich and Schmid, 1979). In vineyards of eastern Spain, vines can even compensate for the ablation of 50% of flowers (Coscollá, 1980b). Thus it is generally assumed that grapevine is very tolerant to inflorescence damage, and it is usually recommended not to apply treatment in the 1st generation. Exceptions to this general approach are found in varieties having small inflorescences (Basler and Boller, 1976), and in northern vineyards where climatic conditions promote early rot attacks (ACTA-ITV, 1980). Damage thresholds oscillate in a wide range between 10 and 100 larvae per 100 inflorescences.
On grapes (summer generations), indirect damage is usually more important than direct, at least in the event of less severe attacks. Thus global damage may appear of little importance if it is evaluated exclusively as weight loss (direct damage), because greater damage is due to rot-derived reduction in quality (indirect damage). Larval boring in grapes may promote a number of fungal rots including Aspergillus, Alternaria, Rhizopus, Cladosporium, Penicillium and especially the grey rot caused by Botrytis cinerea (Fermaud and Le Menn, 1989; Fermaud, 1990). Grey rot development is greatly affected by both climatic conditions and grape phenological stage, the incidence of rotting being higher on ripening and ripe grapes than on unripe ones due to several morphophysiological and biochemical factors (Bessis, 1972; McClellan and Hewitt, 1973; Hill et al., 1981; Langcake, 1981; Pezet and Pont, 1986, 1988). In wine grapes, rot development causes bad flavours and bouquet, reducing the quality of wine. In table grapes, both larval boring and rotting cause high grape depreciation. Consequently, damage thresholds on grapes are more restricted, oscillating between two and 20 larvae per 100 grapes (ACTA-ITV, 1980) as a function of several variables including wine variety, yield use, risk of grey rot incidence, and control strategy performed.