In British Columbia, where the mild humid climate is particularly favourable to its growth, European Holly - Ilex aquifolium, L. - is rather extensively cultivated. Because it cannot be grown successfully in other parts of North America, an important industry has sprung up in this region, and the sales of cut holly from the Pacific North -'lest, which area includes West Washington and North-west Oregon in the United States, amount to several hundred thousand dollars annually. for planting as an ornamental tree in public parks and private estates it is also in good demand, and many fine specimens are to be found in this part of the world, especially in the city of Vancouver. The only 'fly in the ointment' so far as the growing of this tree is concerned, is literally a fly - Phytomyza ilicis, Curt., or the Holly Leaf -miner, an insect whose larvae burrow in the mesophyll and produce, in course of time, large unsightly blotches or mines on the surface of the leaves. As many as 75 to 80 per cent of the latter may be disfigured annually in this way, and as a consequence, the cut foliage, from a commercial point of view, is considerably reduced in value. The trees themselves are also rendered less ornamental and attractive, and although the effect is not very apparent, it is quite possible that they suffer somewhat in health by the wholesale removal of such a large mass of chemically active cells.
Like its host, the Holly Leaf -miner is a native of North -west Europe and the phenomenal success which it has attained in British Columbia is said to be due to the absence of its parasites from this area. Since these natural enemies apparently failed to accompany the fly when the latter was accidentally introduced into Canada, the Dominion authorities decided to make good this deficiency in the fauna and asked the Imperial Institute of Entomology to secure the necessary parasites. When the great difficulties involved in chemical control, - such as for example, the invulnerability of the larvae in their secure situation underneath the protecting cuticle, and the leaf- shedding reaction of the tree to strong sprays - are realised, the fact that, at the outset, natural control offered the best, if not the only prospects of success, will be better appreciated. As a preliminary to the work of collection and export, the present writer undertook a general study of the fly and its parasites in England, in order to obtain a good working knowledge of the latter, and to find out what part they play in the control of the pest in this country. The results of this investigation are set down in the following pages.
The paper opens with a general account of the systematics, morphology, biology and distribution of the fly itself. This introductory section is followed by the largest division of the work, which deals with the various parasites reared from English material. Keys to the adult and later developmental stages of these insects precede the comprehensive account of the systematics, distribution, host relationship, biology and general morphology of the six most important species. Other mortality factors, such as the predatory Blue Tit, bacterial disease, etc., are discussed in some detail, while the final section is concerned with the natural control of the fly in England, and touches briefly on such points as population density and oscillations therein, parasite efficiency, reaction of parasites on each other, the interrelationship of biological and chemical forms of control, and the use of immune varieties of holly.