The development of colonies of the aphid Pterocomma populifoliae (Fitch) was studied in sucker stands of Populus grandidentata Michx. Estimates of the losses from the colonies were made by subtracting the actual numbers present at four day intervals from the potential number calculated from the reproductive rates of the individual females. Based on their development the colonies were divided into three groups. The disappearance of many colonies before the first nymphs had matured (Group 1) was attributed to predation by wandering predators such as spiders and adult coccinellids. In larger colonies which were destroyed before dispersal started (Group 2) Scymnus larvae accounted for up to 40% of the losses, and they and other predators resident in the aphid colonies such as syrphids and chamaemyids were largely responsible for suppressing these colonies. Losses from the remaining colonies (Group 3) were much higher than could be explained by predation or by the dispersal of alatae. Such losses were attributed to the dispersal of nymphs and apterae. This was verified by trapping the aphids leaving large, caged colonies. Parasitism was unimportant, never accounting for more than 8% of the losses. It was concluded that there were three levels in the development of a colony where numerical growth might be limited. At the lowest level suppression was by errant predators, at the next highest by resident predators and errant predators. When suppression by predators failed, numbers increased to where dispersal was triggered. The chance of success of a colony appeared to increase with an increasing number of founding females.