Oviposition by Lobesia botrana is stimulated by sugars detected by contact chemoreceptors

Abstract.  The influence of glucose, fructose and sucrose on oviposition site selection by Lobesia botrana is studied by combining behavioural and electrophysiological experiments. Oviposition choice assays, using surrogate grapes treated with grape berry surface extracts of Vitis vinifera cv. Merlot at different development stages, show that L. botrana females are most stimulated by extracts of mature berries containing the highest concentrations of glucose and fructose. Choice assays reveal that the oviposition response to these sugars is dose-dependant (with a threshold of the applied solution = 10 m m and a maximum stimulation at 1  m ) and that females are more sensitive to fructose than to glucose. Tarsal contact-chemoreceptor sensilla are unresponsive to stimulation with sugars but the ovipositor sensilla contain at least one neurone most sensitive to fructose and sucrose with a threshold of approximately 0.5 m m . Corresponding to the behavioural data, glucose is significantly less stimulatory to sensilla than fructose or sucrose. It is argued that fructose may be of special importance for herbivorous insects exploiting fruit as an oviposition site.