The oyster-shell or mussel scale (Lepidosaphes ulmi, L.) (fig. 1) is too well known to need any particular introduction ; suffice it to say that this insect has followed its principal food-plant, the apple tree, over the world. Though usually considered a serious pest in places in which it has been long established, such as England and

Canada, it is not increasing. There must then be agencies at work destroying annually about 98 per cent. of the progeny of each pair of scales. In different countries these agencies may differ greatly. It is the purpose of the present paper to give a general outline of the principal factors operating in the control of this scale in Canada, and more especially in Eastern Canada. The study, which has been carried on under the direction of the Dominion Entomologist, Dr. C. Gordon Hewitt, is based on the literature of the subject and on the examination of about eighteen thousand egg-masses of scales collected, between September 1916 and April 1917, from representative places throughout Canada. Most of this material was gathered by the officers and friends of the Entomological Branch and mailed to the Dominion Entomological Laboratory at Fredericton, N.B., for examination. The names of these gentlemen appear in the analysis table (on page 191) opposite their respective collections and it is a pleasure to acknowledge their kindly assistance. Each 100 of these egg-masses was, so far as practicable, an average 100 selected from several twigs of one tree or bush. So far as possible, collections were made from five trees or bushes at each observation point.