In the first paper of this series (5) the ecologica1 approach to apple orchard insect problems was discussed and it was pointed out that the spray program must be considered as an ecological factor. It is obvious that because of the complex relationships of the plant and animal species in an orchard it is impossible to change the relationships of any single species without, in greatcr or lesser degree, altering the whole community. In some cases the disturbed relationship may show immediately in the repression of one pest and the rise of another, while in other cases the changes are much more obscure and the economic effects nlay not appear for a long time. The cause of these change is that a spray aimed at some particular species also, directly or indirectly, aifects many other species in the orchard environment and thus may be expected to alter the numerical relationship of many predacious and parasitic forms to certain pests or potential pests which are their prey. The oystershell scale, Lepidosaphes ulmi (L.), and the European red mite, Metatetranychus ulmi (Koch) = (Paratetranychus pilosus (C & F)) are examples of species which react quickly to changes in spray treatments owing to the large populations they may build up in a short time and to the rapid increase of their predators and parasites when not hindered by the spray treatments.