REPORT OF THE SOCIETY 59 



INSECT CARRIERS OF PLANT DISEASES 

 E. Melville DuPorte, Macdonald College 



There are many ways in which insects affect the health of plants. The work 

 of those insects which destroy the foliage, bark, wood or roots is well known, as is 

 also the work of those which suck the sap. Some insects produce galls, contor- 

 tions or discolourations; others interrupt the circulation by ringing the twig, or in 

 some other way disturb the normal metabolism of the plant. These all cause 

 direct injury; but there is a method more insidious and not so well known by 

 which insects may cause serious injury to crops, and that is by carrying or aiding 

 in the spread from plant to plant of the germs of bacterial or fungous diseases of 

 plants. This paper will deal exclusively with this last aspect of the question. 



How Insects Spread Disease 



If we examine the methods by which disease may be spread among plants, 

 through the agency of insects, we find a general similarity between these methods 

 and those by which disease is spread by insects among men or other animals. The 

 following summary brings out this similarity. 



1. The pathogenic organism may undergo a part of its development 

 within the body of the insect. This raises the question as to whether the same 

 disease organism can normally attack both an animal and a vegetable host. The 

 question, which is an important one from the standpoint of human health, has not 

 been extensively studied, but results thus far obtained by investigators have 

 been for the most part negative. A note-worthy exception is the fact that John- 

 ston^ has discovered that the causal organism of the bud rot disease of the 

 cocoanut is a bacillus, indistinguishable from j5. coli which lives in the intestines 

 of man and other animals. Ophionectria coccicola, a fungus parasite of scale 

 insects, has been observed by Noack to pass from the scale insects to orange 

 twigs causing gummosis. The only case known to the writer in which the 

 evidence indicates that an insect may serve as a host of an organism normally 

 parasitic on plants is that of the beet-leaf-hopper Eutettix tenella which spreads, 

 the curly-leaf disease of beets in the southwestern states. 



2. The insect may introduce the disease directly into the tissues of 

 the host. Insects feeding on diseased plants may carry the disease germ on 

 their mouth-parts or bodies and introduce these into the tissues of healthy plants, 

 through the punctures or burrows made in feeding, or oviposition. Many 

 diseases are thus spread. 



ijohnston, J.R. Is Bacillus coli ever a plant parasite? Phytopathology I, pages 97-99, 1911. 



