CHAPTER VI 



THE RELATION OF INSECTS TO PLANT DISEASES 



The injurious relationships of insects to plants may be roughly- 

 divided as follows : 



1. Direct injuries of the nature of wounds (traumatisms). 



2. Systemic or organic disturbances originated by sucking 



insects (stigmonoses). 



3. The exposure of internal tissues to infection. 



4. Transference of infection. 



Insects as the Cause of Injuries and Diseases. 



The principal of the relationships specified above is that which 

 in practice is the concern of the economic entomologist, and 

 consists in the production of injuries mainly direct and of the 

 nature of wounds. Thus the plant or any of its parts may be 

 destroyed by biting or boring insects or its juices be drawn upon 

 and its tissues perhaps poisoned by sucking species. 



Whatever the nature of the injury it must have a reaction on 

 the condition of the plant, and as regards this there is in many 

 cases no essential difference between the results produced by 

 insects and by fungoid parasites. The reduction of leaves, 

 flowers, or fruit, the girdling of a stem, the destruction of roots, 

 have much the same results when caused by one or the other. 

 From this point of view insects as well as mites and nematodes 

 are on the same footing as fungi and bacteria in the causation 

 of plant diseases. The main difference is that with insects the 

 injury and the manner of its production are usually direct and 

 obvious. 



Stigmonoses. 



There are exceptions, however, especially where sucking 

 insects are concerned. The attacks of plant-feeding Rhynchota 

 — hemipterous bugs, aphides, scale insects, leaf hoppers and the 

 like — to which may be added thrips and red spider, often result 

 in more or less general symptoms not obviously related to the 

 direct injury inflicted. To this type of affection the term 

 stigmonose has been applied. No better example could be quoted 

 than the froghopper blight of sugar-cane, or the effects of thrips 

 on cacao and sweet potato. The starved and chlorotic condition 

 induced in certain Solanaceous and other plants by lace-wing 

 bug infestations is another familiar example. 

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