Nuculaspis californica(Coleman) occurs on several species of pine and on Douglasfir with dense populations of outbreaks causing defoliation, discoloration of foliage, and reduced growth. All known outbreaks of this scale are caused by reducing the population ofProspaltellaparasitoid that normally keeps the scale populations at very low densities. Abnormal mortality ofProspaltellais caused either by presence of sorptive dusts or by drift of insecticides. The scale is excluded from many areas by rapid onset of freezing and in other areas it is regularly reduced in numbers by such freezing. In areas where the scale is cold-conditioned annually before freezing occurs it persists without significant winter reduction.Prospaltellaand winter freezing are the only factors that reduce the populations of scales to acceptable levels on forest or ornamental trees. Some reduction of numbers occurs through predation by the coccinellids of the generaChilocorusandMicroweisiaand by snakeflies. At extreme population densities scales die either because of crowding or because they kill host tree twigs or whole trees. Scale populations apparently become adapted to specific host individuals, and population densities can become high only with genetic fitness of the population to the host species and individual. Nymphs have the greatest chance of survival on the parent tree, less chance on other trees of the same species, and little chance when transfer is made to another host species. Hence scale outbreaks are largely confined to one host species even when several known host species occur in the area. No known causal relationships exist between atmospheric fluorides and black pineleaf scale population density. Infestations may be reduced with insecticides, but environmental improvement to conserve theProspaltellaparasitoids is the most effective control.