Interpopulation differences in pupal size and fecundity are not associated with occurrence of outbreaks in Epirrita autumnata (Lepidoptera, Geometridae)

Among‐population differences in pupal mass were studied in a geometrid, Epirrita autumnata. Some Epirrita autumnata populations regularly reach outbreak densities while others are never known to do so. Because adults do not feed, pupal mass of females correlates strongly with fecundity. Larvae were collected from twelve field sites. Ten of our sample populations originated within the outbreak range of the species and represented different phases of outbreaks. Two populations originated outside the outbreak range. Pupal mass of field‐collected E. autumnata varied significantly among populations. The peak phase populations had the smallest pupae and the biggest were found in low density populations outside the outbreak range. Offspring of moths from each population were reared under identical conditions in two larval densities. Significant differences were not found in pupal mass among populations. That is, the inherent size, correlated with fecundity of moths, was not different between populations originating within and outside the outbreak range, nor among collections from different densities or phases of the outbreaks. Rearing density did not interact in a consistent way with population. As far as size and fecundity are concerned, the results do not support Chitty's hypothesis that differences in genetic composition of the population at low and high density phases generate cyclic fluctuations of population density. Because no hereditary or maternal differences were found in size and fecundity between E.autumnata originating within and outside the outbreak range, variation in reproductive capacity cannot explain why outbreaks occur only in some populations.