828 (e.g., climate, altitude) factors are always linked, and it may be difficult to determine the influence of each individual component. The leafhopper Scaphoideus titanus (Homoptera: Cica- dellidae) is a harmful vector of a phytoplasma disease of grape, the Flavescence dorée, and in that respect is one of the current major threats in European viticulture. It originates from North America, in the Great Lakes region (Vidano 1966), and was accidentally introduced in Europe during the 1950s (Bonfils and Schvester 1960). This species is considered monophagous, though it may feed on few plants not belonging to Vitaceae in laboratory as Cineraria maritima (Syn. Senecio cineraria; Asteracea), Vicia faba (Fabaceae; Caudwell et al. 1970) and in nature on Ulmus americana (Ulmaceae; Gibson 1973). In France, females oviposit on grapevine (Vitis vinifera) woody canes from summer ending to autumn, eggs enter diapause, and then larvae hatch from May to July of the following year (Boudon-Padieu 2000a). The ecology of this insect is still poorly understood, even if few recent studies document phytoplasma transmission (Bressan et al. 2005a; 2006), spatial distribution (Bosco et al. 1997; Lessio and Alma 2006), fitness variation (Bressan et al. 2005b), or popula- tion genetics (Papura et al. 2006, 2007; Bertin et al. 2007). As S. titanus is a recently introduced monophagous species whose host occurs from ca. 35 to 50 ° N and which suffers little predation or parasitism in Europe, its main biotic factors are constant. However, the leafhopper does not occur in all the vineyards, and so far, only climatic factors can explain this (Fig. 1). In fact, S. titanus is found in vineyard areas under different climatic conditions, but it is more abundant in the Eastern and Northern parts of Naturwissenschaften (2009) 96:827–834 Europe rather than in the South and the Mediterranean vineyards (Boudon-Padieu 2000b). Furthermore, the syn- chronization between phytoplasma and insect occurrence may be influenced by climatic factors. Along the latitudinal European gradient of grapes, climatic factors are thus good candidates to explain the vector geographical distribution. One hypothesis is that S. titanus is well adapted to cold winters and requires sufficient cooling to initiate diapause termination with consequences on synchrony between egg hatching and host plant and therefore reproductive success. A field survey in Corsica (warm winter) documented important overlapping between the different S. titanus larval instars, where young larvae and adults may occur simultaneously (Doncarli 1990). This may illustrate the link between cold winters and peaked egg hatching dynamics, while warm winters provoke the spreading of the hatchings over time. Here, we test whether the diapause termination of a natural population of eggs depends upon the exposure to cold temperatures in winter in two experiments: (1) Eggs were subjected to different climatic conditions in order to reproduce winter conditions of two different French vineyards, Burgundy and Bordeaux; and (2) eggs were exposed to different cold time exposure. In this study, we used the cold definition by Salt (1961) where cold encompasses temperatures too low to support normal development of the insect. For S. titanus, we considered temperatures equal to or lower than 5 °C as cold. Because the eggs are inserted into the bark, they are supposed to be poorly exposed to light. Therefore, photoperiod influence or interaction with thermal factors has been neglected in this study. Fig. 1 European vineyards with S. titanus occurrence. Arrows represent directions and impor- tance of expansion of the insect since its first discovery in Southern France. Asterisks Occurrence reported in Croatia by Gabrijel in 1987