INTERPOPULATION DIFFERENCES IN PUPAL SIZE AND FECUNDITY ARE NOT ASSOCIATED WITH OCCURRENCE OF OUTBREAKS IN EPIRRITA-AUTUMNATA (LEPIDOPTERA,GEOMETRIDAE)

1. Among-population differences in pupal mass were studied in a geomet rid, Epirrita autumnata. Some Epirrita autumnata populations regularly reach outbreak densities while others are never known to do so. Becau se adults do not feed, pupal mass of females correlates strongly with fecundity. 2. Larvae were collected from twelve field sites. Ten of ou r sample populations originated within the outbreak range of the speci es and represented different phases of outbreaks. Two populations orig inated outside the outbreak range. 3. Pupal mass of field-collected E. autumnata varied significantly among populations. The peak phase popu lations had the smallest pupae and the biggest were found in low densi ty populations outside the outbreak range. 4. Offspring of moths from each population were reared under identical conditions in two larval d ensities. Significant differences were not found in pupal mass among p opulations. That is, the inherent size, correlated with fecundity of m oths, was not different between populations originating within and out side the outbreak range, nor among collections from different densitie s or phases of the outbreaks. 5. Rearing density did not interact in a consistent way with population. 6. As far as size and fecundity are c oncerned, the results do not support Chitty's hypothesis that differen ces in genetic composition of the population at low and high density p hases generate cyclic fluctuations of population density. 7. Because n o 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 occu r only in some populations.