1919 ENTOMOLOGICAL SOCIETY. 63 



Insects as Disseminators of Bacterial Diseases. 



Compared with other agents insects play a much more important part in the 

 spread of bacterial than of fungus diseases. This is partly because bacteria do not 

 to any great extent lend themselves to dispersal by the wind, whereas wind is 

 the chief means of fungus spore dispersal. Another reason is that during the 

 growing season, — the time of greatest dissemination, — the bacteria in some plants 

 are wholly concealed within the plants and are only obtained for fresh inoculations 

 by penetration through the surface to them. This insects do. A third reason 

 is that about half of our worst bacterial diseases can enter plants only through 

 wounds and such wounds are made chiefly by insects. It is worth noting that 

 though insects like Aphids or Capsids with very slender, piercing mouth-parts 

 play but little part in the spread of fungus diseases they are often very important 

 in the spread of bacterial diseases. This is because they feed indisgriminately on 

 healthy and diseased portions of plants and thus by penetrating the diseased areas 

 get their mouth parts contaminated; for no set of mandibles and maxillae are too 

 small to carry numerous bacteria if once they reach them. Once the mouth-part 

 is contaminated inoculation of healthy parts is easy. It looks, however, as if White 

 Flies and Red Spiders were exceptions and did not play much part as spreaders. 

 There are also cases like Cucumber Wilt in which it is doubtful whether Aphids 

 can act as inoculators. Further study will doubtless explain such exceptions. 



The fact that out of the eight common and important bacterial diseases of 

 plants in Ontario three are disseminated almost exclusively by insects, shows the 

 importance of insects in relation to bacterial diseases. 



Examples of Bacterial Diseases Disseminated by Insects. 



Cucumber Wilt (Bacillus tracJieiphilus) . It has been definitely proven 

 that the Striped Cucumber Beetle (Diahrotica vittata), and to a less extent the 

 12-Spotted Cucumber Beetle (Diahrotica 12-punctata) are the chief and probably 

 almost the sole distributors of this destructive disease and that if they could be 

 exterminated the disease would almost disappear. It is very interesting to learn 

 too that the disease is not only disseminated by these insects but is supposed to 

 be carried over from one year to another by them and not through the soil. 



Pear Blight (Bacillus amylovorus) . This, as is well known, is a very 

 destructive disease of pears, apples and quince, causing an enormous amount of 

 damage some years and a considerable amount every year. It is nearly unanimous- 

 ly agreed that insects are the great factors in its dissemination both in the stage 

 known as " blossom blight " and in the later twig blight stage. If a list were to 

 be compiled of all the insects that had a part in the spread of thid disease it would 

 be a long one, for it includes many of the blossom frequenting insects, most sucking 

 insects with piercing mouth-parts found on the apple and pear, and at least one 

 bark beetle. In connection with this disease I may say that we have on several 

 occasions found the gummy exudate at blossom time and have several times found 

 ants feeding upon it. We also know that ants are common frequenters of the 

 nectaries of blossoms. We consider ants therefore as the probable cause of the 

 earliest cases of blossom infection. 



Soft Rot of Vegetables (Bacillus carotovorus) . This fairly common 

 disease of cabbage, turnips, carrots, tomatoes, potatoes and celery is believed to 

 enter solely through wounds, and insects and slugs are believed to be the main 

 carriers of the organism as well as the chief inoculators. There has been a lot of 



