C. P. Clausen's and W. R. Thompson's concepts on the time factor in biological control are compared with the writer's experience. All three viewpoints emphasise the influence of environment correlated with time. As Thompson points out, we are almost always forced to ascribe the limitations of the numbers of pests to a combination of factors, biotic and physical, varying from time to time and from point to point. Clausen's views concerning the maximum time (three host generations or three years) necessary for an introduced parasite to demonstrate that it will be fully effective are supported by the observed facts. There is associated with the initial establishment of introduced parasites, however, a time lag that is not evident in the control of subsequent outbreaks. The principal factors are the attainment of an “equilibrium of distribution” and the rate of dispersal in building up a controlling density. When effective biological control can be anticipated, it is characteristic that marked outbreaks are brought rapidly under control, and the pest population then oscillates at a relatively low level. Several examples of fully effective parasites and their limitations are discussed which indicate that the value of a parasite or predator should be regarded not as a constant but rather as a variable.

An equilibrium of distribution is obtained when there is no decrease in the density of a parasite due to outward dispersal into the uncolonised area; the forward and backward surges of dispersal offset each other, producing a minimum uniformity of distribution independently of other influences. Parasites with slow dispersal rates produce an equilibrium of distribution and effective control more rapidly, but in a proportionately smaller area, than those with fast dispersal rates.

When these considerations are applied to the introduced, partially effective parasites of the gipsy moth, the time element is seen to be proportionately extended beyond the three-year period. After establishment, however, the effectiveness of these parasites is conditioned by the normal duration of the heavy outbreaks of the gipsy moth which rarely exceeds three years. Many of these enemies are as effective in the new environment as in the old. The degree of control of the gipsy moth achieved in the north-eastern United States does not differ greatly from that which occurs in central Europe. Examination of this relationship indicates that an enemy that is to become useful, but not completely effective, does not show its value within three years; consequently the adoption of Clausen's conclusions involves a danger of discontinuing programmes prematurely.