Climate change may facilitate shifts in the ranges and the spread of insect pests, but a warming climate may also affect
herbivorous insects adversely if it disrupts the locally adapted synchrony between the phenology of insects and that of their host
plant. The ability of a pest species to colonize new areas depends on its ability to adjust the timing of phenological events in its
life cycle, particularly at high latitudes where there is marked seasonality in temperature and day length. Here we incubated eggs
of three species of geometrid moth, Epirrita autumnata, Operophtera brumata and Erannis defoliaria from different geographical
populations (E. autumnata and O. brumata from Northern Finland, E. autumnata and E. defoliaria from Southern Finland and all
three species from Germany) in a climate chamber at a constant temperature to determine the relative importance of geographic
origin in the timing of egg hatch measured in terms of cumulative temperature sums (degree days above 5°C, DD5); i.e. the relative
importance of local adaptation versus phenotypic plasticity in the timing of egg hatch. In all three species, eggs from northern
populations required a signifi cantly lower temperature sum for hatching than eggs from southern populations, but the differences
between them in temperature sum requirements varied considerably among species, with the differences being largest for the earliest
hatching and northernmost species, E. autumnata, and smallest for the southern, late-hatching E. defoliaria. In addition, the
difference in hatch timing between the E. autumnata eggs from Southern Finland and Germany was many times greater than the
difference between the two Finnish populations of E. autumnata, despite the fact that the geographical distances between these
populations is similar. We discuss how these differences in hatching time may be explained by the differences in hatch-budburst
synchrony and its importance for different moth species and populations. We also briefl y refl ect on the signifi cance of photoperiod,
which is not affected by climate change. It is a controller that works parallel or in addition to temperature sum both for egg hatch
in moths and bud burst of their host plants.