The wasp Anagrus atomus L. parasitizes eggs of the cicadellid Empoasca vitis (Göthe) on grapevines during the vegetative season. In vineyards, abundant parasitoid emergence occurs in the autumn as a result of a developmental delay in part of the A. atomus population. In this context, physiological times required by the parasitoid to develop in E. vitis eggs are recorded during the summer. In early summer, all individuals develop fully, with physiological times in agreement with those reported in the literature. From early August onwards, an increasing proportion of parasitoids show a delay in development involving pre-imaginal stages. Under transmission microscopy, larvae of A. atomus inside the eggs of E. vitis can be clearly differentiated as ‘light’ and ‘dark’ types, which are associated with normal and delayed development, respectively. The decreasing photoperiod as autumn approaches appears to promote the retardation of growth. This phenomenon can be considered a typical case of risk spreading because, when the availability of E. vitis eggs in vineyards is high (i.e. in early summer), A. atomus shows (i) only a normal developmental pattern that allows a faster population increase, whereas (ii) when E. vitis egg availability begins to be scarce (i.e. from mid summer), an increasing proportion of individuals having delayed development provides better fitness because it allows the parasitoid to synchronize its life history with leafhopper species that lay overwintering eggs on plants other than grapevines.