The Spread of Scale Insects and their Parasites

Abstract

MANY years ago I was an industrious collector of scale insects and mealy-bugs, especially in Jamaica. I found them in great abundance on cultivated plants, and obtained many species. When recently travelling in the Oriental tropics, I was struck by the relative scarcity of these insects, and the occurrence of various well-known injurious forms only in small patches or isolated individuals. Perhaps the difference was partly due to the relative poorness of my eyesight, but I could not help speculating on the causes which might lead to a diminution of scale insects on cultivated plants, aside from the operations of economic entomologists. World-wide commerce has spread the injurious Coccidæ over the earth, as they are so easily carried with plants. In their native countries they are efficiently controlled by parasitic and predatory enemies. In several well-known cases a plague has been abated by going to these countries and obtaining the natural enemies, which had failed to arrive with the first (accidental) importation of the coccids. Thus, following the modern expansion of trade and rapid transit, there has been in many regions a great increase in the damage done by scale insects, at times reaching the magnitude of a calamity. But by the same process, gradually but surely, the natural enemies will also spread. In the course of time, almost imperceptibly, they will gain the ascendancy, and the coccid plague will cease, never to return unless through the importation of a new sort of cocoid. Thus it may even happen in some cases that a rigid quarantine, after a pest has arrived, may be harmful, preventing natural enemies from following it. These latter may, however, be brought in by entomologists, through special permission, provided they have been found and recognised.