36 



INSECTS AND DISEASES 



gists. These persons have become as much a part of our modern needs as, in a related realm, have the 

 physicians and sanitarians. 



For the most part, the work of insects is at once recognizable ; but plant diseases are obscure as to 



cause, and it is only within the past fifty 

 years that very careful study has been 

 made of them. The special study of para- 

 sitic fungi, which cause many of the dis- 

 eases of plants, is cemmonly dated from the 

 work of M. J. Berkeley (1803-1889) in Eng- 

 land about the middle of the century just 

 passed. It is also astonishing that the life- 

 histories of most of the common insects were 

 not understood a century ago ; and there 

 are numerous insects all about us whose 

 life-cycles have never been worked out. A 

 good part of our current, economic ento- 

 mological study is devoted to discovering 

 the main phases of the insects rather than 

 to the adding of new facts and incidents. 

 The subject of the intimate relationship of 

 insects to each other, to weather, to food 

 supplies, and to other factors 

 of their environment, where- 

 by their relative prevalence 

 is largely determined, is yet 

 practically an unexplored 

 field ; yet it is in this eco- 

 logical domain, rather than 

 in merely destroying in- 

 sects by what may be called 

 mechanical means, that the 

 greatest permanent pro- 

 gress in contention with in- 

 sects is to be looked for. 



The gradual growth of 

 the idea that one plant may 

 be parasitic on another and 

 cause what may be called a 

 disease, would be a subject 

 of great attractiveness to one who is interested in human history. The idea is so recent 

 that it should not be diificult to trace. A recent development of it is the discovery 

 that there are germ diseases of plants as well as of animals, a history that is recorded 

 by its literature in E. F. Smith's "Bacteria in Relation to Plant Diseases" (Carnegie 

 Institution, 1905). Other classes of diseases are yet known only by their external 

 manifestations. Of these, peach-yellows and other peach diseases are examples. 

 What causes the mal-nutrition and what carries the disease are undetermined. No 

 doubt -many ailments of plants are physiological and organic, — using these words in 

 their human-medicine sense — rather than due to germs or filamentous fungi. Plants 

 have scarcely begun to be studied in respect to their intimate pathological processes 

 and their response to sanitary or unsanitary environment. Very likely we await a new 

 era in plant cultivation. 



The plant diseases that are likely to be most clearly recognized by the general 

 observer are those occasioned by the filamentous fungi. These low spore-bearing 

 plants are related to the molds that appear on bread and decaying substances. It is 

 impossible for one who has not studied these forms patiently under a microscope really 

 to understand what they are. The ragged and spidery pictures of them that appear in 



Fig. 53. Example of a bitmg or chewing Insect. Army-Trorms 

 eating up a stalk of corn. 



Fig. 54. Example 

 of a suckiDg in- 

 sect. San Jos6 

 scales (enlarged) 

 attached to a 

 twig and ex- 

 tracting the 

 juices from it. 

 Plant-lice are 

 sucking insects; 

 also the stink- 

 bu^ and their 

 kin. 



