The phenological window for western spruce budworm: seasonal decline in resource quality

1 Western spruce budworm Choristoneura occidentalis Free. (Lepidoptera: Tortricidae) emerge in the spring before budburst and then face a rapidly deteriorating host quality each season. 2 Measures of fitness, survival and fecundity, were made on cohorts of final-instar spruce budworms deployed on host trees at several times during the season in four field locations in coastal and interior British Columbia, Canada. 3 Survival and fecundity were strongly correlated throughout the season and varied as much as four-fold from maxima at mid-season to minima at the end of the season. 4 Fitness values overall were greatest in the coastal compared with interior locations. Among interior locations, fitness was greatest at the highest elevation and least at the lowest elevation. Both cohort and sample-based estimates of survival of wild, final-instar budworms were relatively high in these outbreak populations. 5 The influence of the phenological window and degree of synchrony with the host plant on herbivore abundance often depends on other processes affecting population rates of change.