BACTERIA AS DESTROYERS OF INSECTS. 95 



and others we know of diseases due to those simplest of fungi, the bac- 

 teria, which produce the most deadly ravages among insects. Profes- 

 sor 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 similar 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 mus- 

 cardine " 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 in- 

 sect-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 mi- 

 nute fungi besides the Saccharomyces cerevisice, 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 deliber- 

 ate cultivation of an insect's disease-producing fungus, and the applica- 

 tion 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. 



