Conifer-feeding budworms emerge from overwintering sites as small larvae in early spring,several days before budburst,and mine old needles.These early-emerging larvae suffer considerable mortality during this foraging period as they disperse in search of available,current-year buds.Once buds flush,surviving budworms construct feeding shelters and must complete maturation before fresh host foliage senesces and lignifies later in the summer.Late-developing larvae suffer greater mortality and survivors have lower fecundity when feeding on older foliage.Thus,there is a seasonal trade-off in fitness associated with host synchrony:early-emerging budworms have a greater risk of mortality during spring dispersal but gain better access to the most nutritious foliage,while,on the other hand,late-emerging larvae incur a lower risk during the initial foraging period but must contend with rapidly diminishing resource quality at the end of the feeding period.We investigate the balance that results from these early-season and late-season synchrony fitness trade-offs using the concept of the phenological window.Parameters associated with the variation in the phenological window are used to estimate generational fitness as a function of host-plant synchrony.Because defoliation modifies these relationships,it is also included in the analysis.We show that fitness trade-offs characterizing the phenological window result in a robust synchrony relationship between budworm and host plant over a wide geographic range in southe British Columbia,Canada.