Popular Science Monthly/Volume 17/May 1880/Bacteria as Destroyers of Insects

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"WHAT is the good of a knowledge of microscopic creatures? What is the good of prying into the anatomy of insects? It is all very well as an amusement, but serious persons can not be expected to assent to the devotion of endowments or state funds to such trivial purposes. Chemistry, geology, electricity, if you please, have their solid commercial value, but biology is an amusement for children and old gentlemen." Such is the opinion of many a "practical man," ignorant and short-sighted as the genus invariably proves itself.

Already the practical man may be told, in reply, that surgery is entirely reformed by our knowledge of the minuter fungi; that, by avoiding the access of bacteria to wounds, we avoid a large destruction of human life. Already we see our way to avoiding some deadly diseases caused by these same bacteria now that we know them to be the active cause of such disease. Already silk is cheaper in consequence of our knowledge of the bacteria of the silk-worm disease; already better beer is brewed and better yeast supplied to the baker in consequence of Pasteur's discovery of the bacterian diseases of the yeast-plant; already vinegar-making, cheese-making, butter-making, winemaking, and other such manufacturing trades are on the way to benefit by like knowledge. Potato-disease and coffee-disease have been traced to their causes and means suggested by biologists for dealing with the parasitic plants causing those diseases, whereby not thousands but millions of pounds sterling a year may be saved to the community.

Insect-pests which have depopulated whole provinces, such pests as the phylloxera and the Colorado beetle, are about to receive a check at the hands of the same class of scientific students. The application of knowledge of natural facts is in this case a very remarkable one; for it is actually proposed to make use of our recently acquired knowledge of diseases due to bacteria—not that we may arrest such diseases, but that we may promote them. Insect-pests are to be destroyed by poisoning them not with acrid mineral poisons which damage plants as well as the insects, but by encouraging the spread of the disease-producing bacteria which are known to be fatal to such insects. Professor Hagen, of Cambridge, Massachusetts, has called attention to the old practice of destroying greenhouse pests by the application of yeast. He conceives that this method may be applied to other insect-pests, such as phylloxera, Colorado beetle, cotton-worm, etc. He imagines that the yeast-fungus enters the body of the insect on which it is sprinkled, and there produces a growth which is fatal to the insect's life. It is a well-known fact that insects are very subject to fungoid diseases, and it is also ascertained that the application of yeast to the plants frequented by such insects favors their acquisition of such disease. Professor Elias Metschnikoff, the celebrated embryologist has, however, made some investigations on this subject, and given an explanation of the possible value of yeast application ("Zool. Anzeiger," No. 47), different and more satisfactory than that which Professor Hagen appears to adopt.

The general result of the most accurate investigations of the beer-yeast fungus (Saccharomyces cerevisiæ) is entirely opposed to the notion that it can enter an insect's body and produce a disease. Beer-yeast is beer-yeast and appears always (or within experimental limits) to remain so. On the other hand, De Bary has made known the life-history of some simple fungi which destroy insects, and from Pasteur, Cohn, and others we know of diseases due to those simplest of fungi, the bacteria, which produce the most deadly ravages among insects. Professor Metschnikoff has examined some of these minute parasitic fungi and cultivated them by passing them from one insect to another, and has experimentally proved their very deadly character to the insects exposed to infection. The "green muscardine" (Isaria destructor) is the name given by Metschnikoff to one of the minute fungi the effects of which he most successfully traced. Now, it is perfectly evident that if green muscardine spores could be produced in large quantity, or spores of smilar disease-producing fungi, and applied to the ground and shrubs infested by insect-pests liable to harbor those fungi, we should have the best of all means for effecting the destruction of the insects, viz., a poison which once set at work would spontaneously multiply and spread its destroying agents around.

Accordingly, Professor Metschnikoff endeavored to cultivate the "green muscardine" apart from insects, so as to obtain its spores if possible in great quantity, in a liquid which might be applied to places attacked by injurious insects. He at last succeeded in effecting this cultivation by the use of beer-mash; in this decoction the "green muscardine" produced a rich mycelium and finally spores.

It is exceedingly probable that we have here the true explanation of the value of the application of yeast to plants, etc., affected by insect-pests. If there are a few spores only of such parasites as the "green muscardine" about, the fluids of the yeast will serve them for nourishment and so cause the muscardine to spread until it comes into contact with the insects. There is no reason to suppose that the beer-yeast plant itself is capable of generating a disease in any insects; at the same time we must remember that yeast as ordinarily used by the brewer is by no means pure: it contains in small quantities other minute fungi besides the Saccharomyces cerevisiæ, and it is quite possible that a given quantity of it, say a pint, may, if the brewery from which it came were not conducted on the most perfect system (such as that lately introduced by Pasteur), contain a few spores of such a disease-producing parasite as muscardine. A diseased insect once in a way falling into the mash-tub would sufficiently keep up the supply, and thus it is possible that yeast may carry infection to insect-pests and destroy them.

At the same time, Professor Metschnikoff's suggestion of a deliberate cultivation of an insect's disease-producing fungus, and the application of the cultivated fungus in quantity to places infested by these insects, is in the highest degree ingenious, and likely to give results the value of which will be estimated in thousands, of pounds, and so do something to persuade "practical" men that all science is deserving of their respect and encouragement.––Nature.