Glucose and fructose levels on grape skin: interference in Lobesia botrana behaviour

The vineyard produces one of the most important crops in Portugal and is affected by various pests, such as the Lobesia botrana moth, which can be attracted chemically by compounds on the grape skin. In this study, the sugar contents (glucose and fructose) on grape skin were analysed for five Portuguese Vitis vinifera grape varieties and compared in grape juice and whole grapes. Samples were harvested in July, August and September of 2001 and 2002. The quantification was attained employing a new methodology, which comprised the preparation of a methanol solution to extract the compounds from the grape skin and further chromatographic analyses by liquid chromatography (LC) with refractive index (RI) detection. The samples designated by whole grapes were homogenised, centrifuged and filtrated, while the juice was obtained by crushing and filtration. Grape skin has shown higher levels of glucose than fructose, but both compounds increased from green berry (July) to veraison (August) and had a slight decrease from veraison to harvest (September). As expected, the glucose and fructose contents (total sugar) were higher in the whole grape than in the juice and much higher than in the grape skin itself. For Loureiro and Trajadura varieties, the levels of both sugars on grape skin were very similar. This resemblance strengthened the statement of the decisive role of this parameter in female L. botrana oviposition behaviour.