To elucidate how galling adelgids (Hemiptera: Adelgidae) vary their larval (=galling) periods between species, we compared larval development of Adelges japonicus (Monzen) to that of Adelges pectinatae (Cholodkovsky) on Picea jezoensis (Siebold & Zucc.) Carr. (Pinaceae). Although eggs of the two species hatch when new buds begin to burst in spring, A. japonicus has a much longer larval period (≈3 mo) than A. pectinatae (≈1 mo). Larvae of A. pectinatae developed rapidly in every stadium and completed their growth by mid-June. In contrast, larvae of A. japonicus developed slowly through each stadium and became the last instar between early August and early September. There was no instar with particularly long developmental period. Despite the larval period difference, percentage of each instar period differed little between the two species, indicating that the longer larval period in A. japonicus is not caused by summer diapause in a specific instar. Nitrogen contents in galls of the two species were highest at budburst and declined rapidly until late June. A. japonicus larvae stayed as first instar when nitrogen contents in galls were high. In addition, nitrogen contents in galls were always higher for A. japonicus than for A. pectinatae, suggesting that delayed development in A. japonicus larvae is not a result of nutrient limitation, but is a naturally programmed response to pass the summer in larval stage and wait for autumn to become adults. This study is the first to clearly show that insects reduce their development in each stadium to prolong their larval period in summer.