The ecology of black pineleaf scale which occurs on several species of pine and Douglas fir causing defoliation, discoloration of foliage, and reduced growth is reviewed. All known outbreaks of this scale have been caused by reduction in the population of Prosphaltella parasitoid that normally keeps the scale populations at very low densities. Abnormal mortality of Prosphaltella has been found to be caused either by the presence of sorptive dusts or by drift of insecticides. High population densities of scale have been found mainly in areas of high dust fall resulting from industrial sources and roadways. No known causal relationships have been found to exist between atmospheric fluorides and the black pineleaf scale population density. Environmental improvement to conserve the Prosphaltella parasitoids appears to be the most effective control measure.