In 1910 in a paper on the parasites of the Gypsy and Brown-tail moths, W. F. Fiske calculated the mortality necessary to stabilize the gypsy moth population and stressed the importance of a sequence of parasites and predators attacking successive stages of the insect host, in the establishment and maintenance of satisfactory control. In 1912, in the bulletin entitled “The Importation into the United States of the Parasites of the Gypsy Moth and Brown-tail Moth”, L. O. Howard and W. F. Fiske studied the problem of a sequence of parasites much more fully. They then made the very important point that aggregate percentage of kill necessary for the control of an insect pest could not be secured by simply adding together the figures representing the kills resulting through attack by each of two or more species. It was going to be necessary, they said, to combine these several aggregates in a different manner. Thus a 50% parasitism of the eggs if it could possibly be secured, followed by another 50% parasitism of the caterpillars, could not by any possibility be considered as resulting in 100% parasitism or complete extinction but only in 50% parasitism added to 50% of what remained which amounted in effectto 25% of the whole. In this manner an aggregate of 75% only is secured. They said further that any specific amount of parasitism as 20% of the eggs, was neither more nor less, but exactly as effective as 20% parasitism of the caterpillars and pupae, in so far as its value in constructing the final aggregate was concerned. The principle they thus established, though it may seem obvious enough, had not apparently been previously recognized by workers on biological control and it is of the very greatest importance.