Adults of Apanteles melanoscelus (Ratzeburg) (Hymenoptera: Braconidae) were most abundant in Connecticut in 1975 in mid to late June when most gypsy moth larvae (Lymantria dispar L.) (Lepidoptera: Lymantriidae) were 4th instars or larger. Fourth instars are much less acceptable than earlier instars as hosts because of the vigorous defensive movements and long body hairs. Also, developmental rates in the laboratory of A. melanoscelus from egg to adult was always slower than development of gypsy moths from 1st to 4th instars. Thus even if the parasite lays eggs in 1st stadial hosts, adults of the next generation cannot emerge before most gypsy moths have become 4th instars. The inability of A. melanoscelus to parasitize 4th instars together with the long developmental rates of the parasite explain the decreased field parasitism of larger gypsy moths observed in June.