Naturwissenschaften (2009) 96:827–834 Table 1 Hatching dynamics of the egg populations subjected to different 5 °C durations 0 20 28 36 49 Log rank Gehan–Wilcoxon Log rank Gehan–Wilcoxon Log rank Gehan–Wilcoxon Log rank Gehan–Wilcoxon Log rank Gehan–Wilcoxon χ² P χ² P χ² P χ² P χ² P χ² P χ² P χ² P χ² P χ² P Days at 5°C 20 3.45 <0.001 2.14 0.03 28 2.61 <0.01 4.36 <0.001 4.19 <0.001 5.53 <0.001 36 4.36 <0.001 5.38 <0.001 11.76 <0.001 10.79 <0.001 1.73 0.84 0.79 0.43 49 2.08 0.04 3.08 <0.01 4.14 <0.001 4.04 <0.001 0.16 0.87 0.38 0.70 2.18 0.03 0.86 0.39 831 63 20.90 <0.001 18.35 <0.001 27.59 <0.001 26.14 <0.001 13.76 <0.001 10.32 <0.001 14.18 <0.001 9.81 <0.001 11.40 <0.001 6.85 <0.001 Anthomyidae) from diapausing pupae is correlated with the increasing low temperature durations (Collier and Finch 1983). Such variation in precocity or lateness of diapause termination may be explained by the cold length exposure as shown by our work. As in numerous winter-diapausing insects in temperate zones (Collier and Finch 1983; Wipking 1995; Bosch and Kemp 2004), the time for S. titanus to hatch after being exposed as an egg to warm temperature (20°C) decreases with cold duration during diapause. But, differences in the earliest and latest emergence before diapause do not solely depend on environmental factors. Biron et al. (1998) have demonstrated in D. radicum the presence of early and late phenotypes which induced an asymmetrical bimodal distribution of the post-diapausing emergence. We observed the same pattern of hatching dynamic for eggs exposed to cold or mild winter. The hatchings lengths were similar. Only the beginning and the peak of hatching were different. Either an intrinsic hatching dynamic or an egg-laying dynamic may have contributed to the outcome, but because ages of the egg collected are not accurately known, we cannot separate these two strategies. In the two cases, the length and daily variations of hatching appear to match a risk-spreading strategy. According to Hopper (1999), two population strategies may have occur in our work: (1) two genotypes of females coexist, one which lays eggs adapted to cold winter and another laying mild winter-adapted ones; (2) a single genotype of females produces phenotypes that lay eggs with different temperature hatching requirements. These two strategies would both be advantageous in unpredictable and varying winter conditions. In phytophagous univoltine species, synchrony of the larvae with the targeting host organs reduces starvation periods and minimizes larval mortality. For optimal larval survival, S. titanus eggs should hatch during or closely following bud bursting of V. vinifera when young leaves present the highest nitrogen content (Mooney and Gulmon 1982), which in turn increases the fitness of other leaf- hoppers (Rossi and Strong 1991). Thus, individuals of Drepanosiphum platanoidis (Homoptera: Callaphididae) hatching while host-plant Acer pseudoplatanus (Sapindales: Aceraceae) bud bursting can double their adult weight, and their offspring can also grow up faster as compared to aphids that hatching after bud bursting (Dixon 1976). Moreover, their survival was negatively correlated with the time elapsed between egg hatch and bud bursting (Dixon 1976). Like most temperate-zone deciduous fruit crops, grapes undergo bud dormancy, which is a phase of development that enables these plants to survive unfavorable winter conditions. Bud dormancy is genetically controlled, natu- rally induced by photoperiod and low temperatures, and released after exposure to adequate chilling temperatures. Vernalization is thus essential in many plants, for example to promote flowering (Henderson and Dean 2004) or bud burst. So, time required to initiate bud break, number of