INSECTS AND PLANT DISEASES — CARTER 333 



some cases the spot is visible before the uisect has finished feeding, 

 indicating that the feeding process is a very vigorous one. On tea, it 

 has been estimated that six mosquito bugs, during their development, 

 can destroy a pound of green leaves. 



The typical spot occurs when the insect reaches certain cells in 

 the plant. If other cells are reached, then different symptoms result. 

 For example, on mango fruits, if the mouth parts of the insect 

 reach the middle skin, scab symptoms occur, while if the inner sldn 

 is reached, rot results. On the stems of mango and tea, cankers 

 formed by split and swollen bark result if the mouth parts reach cer- 

 tain other cells. 



In most cases where sucking insects cause local disturbance in the 

 leaves of plants, it is possible to trace the route that the mouth parts 

 took, because in penetrating the leaf tissue the insect injects a kind 

 of saliva which hardens in the leaf to form a well-defined track. 

 It is thus possible to learn whether the insect pierces directly through 

 cells in a direct line to the particular cells which are its objective, 

 or whether the objective is reached by a tortuous course between 

 the plant cells. 



One case is known where one strain of an insect species produces 

 a local spotting while another strain does not. There are two 

 strains of the pineapple mealybug, and one of these produces a dark 

 green circular spot at the point where the insect feeds on the leaf. 



THE VIRUS DISEASES 



The virus diseases constitute the third great division of plant 

 diseases that are spread by insects. This is perhaps the most im- 

 portant group of diseases of plants carried by insects, although this 

 statement would be difficult to prove in the absence of actual com- 

 parative figures on the damage caused. The number of plant viruses 

 known is very large, at least 100 being recognized apart from numer- 

 ous variants which are known as virus strains, and new ones are 

 continually being recognized and described. 



There is considerable debate at the present time as to whether vir- 

 uses are living things or highly complex chemical structures with 

 some characteristics similar to those usually associated with living 

 matter. Since this paper is primarily concerned with the role of in- 

 sects in spreading these agents of disease we can proceed on the basis 

 of what is universally agreed upon, namely, that whatever their true 

 nature, viruses are spread principally through the agency of insects. 



The number of plant species affected is very large. The virus of 

 tomato spotted wilt is known to affect over 100 species of plants ; the 

 virus of curly top of sugar beet, a disease that has been studied in 

 great detail for many years, has over 200 known plant hosts; aster 



