The relationship between larval growth, body size, and the number of larval stages and fecundity ofQuadricalcarifera punctatella(Motschulsky) was investigated by rearing larvae under constant conditions. Fecundity, adult weight, and forewing area were strongly correlated. Forewing area measurement method may be good for estimating fecundity of field-trapped adults in future studies. Larvae were not able to develop at constant temperatures below 12°C, but the developmental threshold was estimated as ≍8.5°C for first and second instars. High temperatures>25°C also seemed to be unfavorable to larvae, because body size decreased and larval duration was prolonged. Larval development was comprised of four or occasionally five instars. This extra molt lengthened the duration of the larval stage and enlarged body size. Because larger mature larvae became larger adults with greater fecundity, larval growth was important in determining fecundity. There was a tendency for larvae exhibiting five instars to be smaller during the third stadium. There was also an increased frequency of extra instars at high temperatures. This phenomenon can be explained by a cost-benefit trade-off; prolongation of the larval period caused by an extra molt may result in increased mortality (a cost), but i also causes increased body size and fecundity (a benefit). And, the larval period of individuals with four instars was longer at 25°C than at 20°C, but no such developmental delay was observed for larvae with five instars. An extra molt is, therefore, also an adaptation to high temperatures. These trade-offs could also explain the variation in numbers of instars among years in field populations; extra instars were less common in 1982 when larval duration was longer than in the other 2 yr, especially at low temperatures.