Studies in a hemlock plantation and in four hemlock forests in Connecticut revealed that hemlock woolly adelgid,Adelges tsugaeAnnand, severely reduced the growth and survival ofTsuga canadensis(L.) Carriere, which had a major impact on the subsequent performance and population dynamics of this insect. The presence of adelgids at all but relatively low densities (93% of the adelgids produced in each of the subsequent 3 yr became sexuparae; in the fourth year only sexuparae were produced. Density-dependent production of the unsuccessful sexuparae played a major role in the decline ofA. tsugaepopulations on their deteriorating host.