Insect Parasites on Trees

THE note in NATURE of April 11 (p. 144) about the ravages of insect parasites upon the chir pine (Pinus longifolia) in the Himalayas suggests a consideration which, I think, is not enough present to foresters and planters in this country. I am too destitute of biological or physiological knowledge to venture an opinion upon the causes which lead to the excessive multiplication of parasites, whether animal or fungoid, upon animals and plants whereof the vitality has been impaired by some other agency; but the phenomenon must be familiar to most people, though it is generally wrongly interpreted. Normally vigorous organisms may, and do, entertain a reasonable number of parasitic guests without appreciable loss of vigour; but these guests seem to bide their time until the host is weakened by accident or disease, when they display a surprising amount of latent fecundity. In the case of the chir pine, the opportunity occurs when the vitality of the tree is lowered by tapping for resin; in other words, when it is depleted of its protective juices, the diminution of which gives easy access to the Platypus larvæ.