THE RELATION OF INSECTS TO PLANT DISEASES 67 



In some such affections there is reason to believe that besides 

 the drain on the juices of the plant there is a poisoning effect 

 from a toxin injected by the insect into the tissues, the immediate 

 action of which is to overcome the resistance of the protoplasm 

 lining the cell wall and thus allow abstraction of its liquid con- 

 tents. 



Insect Injuries as Openings for Infection. 



The protective function of surfaces of cuticle or cork and the 

 dependence of many fungus parasites on wounds to secure an 

 entrance have been described. Most commonly such injuries 

 originate from the attacks of insects. Not only do fungi gain 

 access by the grosser wounds, but it has been repeatedly shown 

 that the infection of leaves or fruits may depend on injuries of 

 this nature not appreciable to the naked eye, and the presence 

 or absence of an unconsidered insect may in this way determine 

 the prevalence of a fungoid or bacterial disease. 



Insects as Carriers of Infection. 



In addition to their function in opening up ways for infection 

 evidence is accumulating that in plant as in animal diseases 

 insects frequently transfer the actual means of infection from 

 plant to plant. The covering of the insect body is usually very 

 suitable for the carriage of fungus spores or bacteria, and next 

 to wind insects are probably the most effective agents in distri- 

 buting these, and are capable of extending the range of a disease 

 over the long distances sometimes covered in insect migration. 

 It is not necessary that the insects themselves should be of in- 

 jurious kinds ; they may visit the affected plant to feed from its 

 flowers or be attracted by and perhaps bred from the tissues 

 soured or decayed as a result of disease. 



The fungi causing the internal disease of cotton bolls are 

 transferred from the fruits or seeds of numerous wild and cul- 

 tivated plants to the bolls, and from one boll to another, by 

 plant-feeding bugs and by no other known means. The form 

 in which the fungus concerned is carried has not been detected, 

 but there is good reason to believe that the infected material 

 is introduced into the boll or other fruit at the time of feeding. 



In the mosaic diseases infection commonly occurs through the 

 agency of aphides, and in one such disease, spinach blight, the 

 virus is handed on to succeeding generations of the insects, as 

 shown by the power of these to produce infection. The virus 

 of curly-top of beets is transferred by a Jassid bug, and it has 

 been recently demonstrated that a period of incubation of about 

 24 hours in the body of the insect is necessary to the process. 

 The difference between these diseases and the stigmonoses should 

 be noted : in the latter the affection is caused by the insect 

 itself. 



