IN the Lake District plantations, and elsewhere throughout the country, those interested in the welfare of the larch have viewed with no little apprehension, for some time past, the yearly ravages of the large larch saw-fly (Nematus erichsonii). As direct methods of control are out of the question over most of the affected areas, interest has been centred upon those natural agencies which in any way tend to limit the indefinite multiplication of this saw-fly, and it has been recognised that the atmospheric conditions, several of the common insectivorous birds, voles, certain insects, and parasitic, fungus, are all capable of exercising a considerable if variable influence upon the numbers of the pest. To a combination of forces such as these must be attributed such respite as the trees have gained in certain of the infested areas, and in those cases where accurate observation of the facts was possible it was found that cessation of the saw-fly attack coincided with an overwhelming increase in the numbers of one of its insect enemies, an ichneumon, hitherto unknown to science, Mesoleius tenthredinis, Mor. This parasite, by eventually accounting for more than 70 per cent, of the larv within the cocoons, undoubtedly in these instances played a very large part in the reduction of the pest.