142 



extreme, or rather that from an exaggerated idea of what may be ex- 

 pected from this uew means of contending with insects disappointment 

 must inevitably follow, and then abandonment of a method which, taken 

 in its proper limit, may prove of very great advantage. 



It is my object in this paper to briug together some facts to show 

 what may reasonably be expected from this source and to point out as 

 far as possible the limits, as they appear to me, of its applicability. 



Naturally the only diseases over which we can have any practical 

 control, and which can therefore be encouraged at our pleasure, are 

 those of a contagious nature, or we may probably say with safety those 

 due to the spread and multiplication of specific disease germs. 



Bearing this in mind three inquiries will naturally arise: First^ 

 what diseases have we as a basis upon which to work ? Second, to 

 what extent can we control, encourage, and disseminate such diseases^ 

 what limit will naturally surround their distribution, and what insects 

 can be reached ? Third, how will such methods compare in cost and 

 effectiveness with other methods of destroying injurious species? 



As regards the first point we are well aware of various diseases that 

 attack and destroy many species of insects, some of the most common of 

 which have been characterized under the names of Muscardine; Gras- 

 serie, Jaunes or Jaundice; Pebrine; Flaccidity, Flacherie or Schlaff- 

 sucht, and Foul Brood, as well as the various kinds of Entomophthora 

 affecting flies, locusts, cicadas, and the Chinch Bug. These diseases are 

 so well known and their characteristics have been stated so often that 

 a repetition here is unnecessary. It may be stated, however, that care- 

 ful descriptions of some of the most important are given by Prof. C. V. 

 Riley in the Eeport of the United States Entomologist for 1885, and by 

 Prof. S. A. Forbes in a pamphlet entitled "Studies on the Contagious 

 Diseases of Insects;" also a summary of them in my paper already 

 mentioned. Transactions Eastern Iowa Horticultural Society, in report 

 Iowa State Horticultural Society for 1886, pages 400-405. 



Summing up these diseases, I think no one will deny their great 

 economic importance, on the one hand, as destructive agents to very im- 

 portant industries, such as sericulture and apiculture, and on the other 

 hand as natural checks working more or less constantly as efficient 

 agents in destroying insects of an injurious nature. Nevertheless, in 

 order that our knowledge should give us a really practical advantage, 

 it is necessary that it should provide us with means for controlling the 

 multiplication and spread of the various forms so that we may prevent 

 the destruction of insects of domestic value and encourage and hasten 

 their action where used as agents in preventing or counteracting in- 

 juries of destructive species. It is exceedingly fortunate and profita- 

 ble when a sudden epidemic carries off hosts of chinch-bugs or cabbage- 

 worms, but until we can start the disease in localities where it is not 

 at work, and carry it over from year to year so as to set it to work at 

 our pleasure, we can not consider that our knowledge of the disease or 



