Field data on winter moths were collected yearly since 1994 in four forests around Arnhem, the Netherlands, using simple funnel traps to catch adult moths in winter (see [Van Asch et al. 2013, Nat Clim Change] for details). Eggs collected from these wild adults were kept in a field shed at the Netherlands Institute of Ecology. Deposited field data for the period 1994–2021 include per year: number of adult moths collected, with for each moth (individual-based data with individual identifier): number of eggs laid, spring seasonal timing of their eggs kept in our field shed, and spring seasonal timing of budburst of oak trees in the field on which adults were caught. Experimental data were collected in a caterpillar feeding experiment in the Spring of 2021, using eggs from the long-term field monitoring (described above). The experiment consisted of a split-brood design, where the timing of hatching of eggs laid by each female was manipulated to induce staggered hatching. Caterpillars were then divided over different photoperiod treatments (constant photoperiod or naturally changing photoperiod) and different phenological mismatch treatments (hatching before [0–4 days] or after oak budburst [1–5 days], and then fed with oak leaves accordingly). Deposited experimental data include per caterpillar (individual-based data with individual identifier): parent origin (Catch area, tree, and date), hatch date, death date (if died before pupating), pupation date, pupation weight, date of adult emerging, adult weight, and adult sex.

Climate change can severely impact species that depend on temporary resources by inducing phenological mismatches between consumer and resource seasonal timing. In the winter moth, warmer winters caused eggs to hatch before their food source, young oak leaves, became available. This phenological mismatch changed the selection on the temperature sensitivity of egg development rate. However, we know little about the fine-scale fitness consequences of phenological mismatch at the individual level and how this mismatch affects population dynamics in the winter moth. To determine the fitness consequences of mistimed egg hatching relative to timing of oak budburst, we quantified survival and pupation weight in a feeding experiment. We found that mismatch greatly increased mortality rates of freshly hatched caterpillars, as well as affecting caterpillar growth and development time. We then investigated whether these individual fitness consequences have population-level impacts by estimating the effect of phenological mismatch on population dynamics, using our long-term data (1994–2021) on relative winter moth population densities at four locations in the Netherlands. We found a significant effect of mismatch on population density with higher population growth rates in years with a smaller phenological mismatch. Our results indicate that climate change-induced phenological mismatch can incur severe individual fitness consequences that can impact population density in the wild.