Repository: UIHistories Project: Board of Trustees Minutes - 1886 [PAGE 301]

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295 drink with its baneful germs, we can see no prima Jacie reason why we may not pursue with success a similar policy in respect to the contagious diseases of our injurious insects. Acting on this idea, I have used such opportunities and time as I could find during the three last years for careful study of insect diseases, and for some preliminary experiments of a practical character with such as promised any useful results. My studies are still in progress, and I shall not attempt anything like an exhaustive account of them or description of their results, but will confine myself here to a single one of the diseases investigated—the most destructive, rapid and easily propagated with which I have experimented. But first it may be well to introduce this special subject by a brief account of the diseases of insects at large, such as will show you about how the particular malady I have selected for discussion is related to the others. Contagious diseases may be caused either by plant or animal parasites. If the former, they are called mycoses, being always due, as far as known, to parasitism by fungi. Hyphomycosis, schizomycosis, and the like, are names applicable to varieties of fungous will serve to indicate the group of fungi to which the disease parasite belongs. The only animal parasites, known to me, which cause insect diseases worthy of the name of plagues, belong to Protozoa; and to a class of that sub-kingdom now called the Sporozoa, all the members of which are parasitic on animals. The genus Gregarina is the disease, and best known example of the class. A disease caused by such parasites might, I suppose, be called a sporozosis. The insect maladies of this group are distinguished from all others by being, so far as known, the only ones that are directly hereditary, the thing passing from parent to offspring being not merely a constitutional peculiarity, a diathesis, a predisposition, a susceptibility, but the disease germ itself. Pebrine of the silkworm is the best known example. The position of what I here propose to call the white plague of the cabbage worm—the disease to which the greater part of this paper is given—will be sufficiently indicated in the classification which I have outlined, by saying that it is a schizomycosis, not certainly known to be hereditary, but apparently conveyed from year to year by the persistent vitality of the bacteria which characterize it. It is not impossible, however, that it may be directly inherited, since the bodies of insects affected by it are so generally penetrated by its characteristic bacteria that these may be reasonbly supposed in many cases to fill the ovaries and to enter the forming egg. In studying experimentally on insect disease, it is necessary, in the present state of our knowledge, (1) to determine precisely the symptoms and character of the disease itself, in order that it may be subsequently recognized with certainty; (2) to learn whether it is characterized by bacteria; and (3) whether it is practically contagious. Determining these questions affirmatively, (4) cultures of the bacteria must be made artificially; and (5) these