INSECTS AND PLANT DISEASES — CARTER 335 



studies in Hawaii revealed a disease of pineapple which is thrips- 

 transmitted and which is called yellow spot. Another study in South 

 Africa described a disease of tobacco called kromnek (crooked neck) 

 disease. Workers now realize that they are all dealing with the same 

 disease, either on tomato or on other economic species of plants that 

 are attacked (pi. 4). 



Mealybug wilt of pineapples appears to be present wherever a sus- 

 ceptible variety of pineapple is grown and mealybugs are present in 

 any numbers. The mosquito bug has been recognized as a serious pest 

 of tea in India and Java for many years, but the same insect is now 

 known to affect not only tea but many other crops in inland Africa. 

 A virus disease of cassava, which is a staple starchy food of many 

 native peoples, is known throughout tropical Africa and in Java 

 (pi. 5). On the other hand, the virus diseases of Irish potatoes are 

 world-wide, and it is doubtful whether potatoes absolutely free of 

 virus diseases are to be found anywhere. 



HOW THE INSECT ACTS AS A CARRIER 



Reference has already been made to the spread of fungus and bac- 

 terial diseases. Usually the organisms of these diseases adhere to the 

 insect's body and are carried from blossom to blossom and from leaf 

 to leaf or tree in that way. In some cases the organisms are taken 

 into the intestinal tract of the insect and develop and reproduce there. 



The toxic effects of insect feeding depend on the sensitivity of par- 

 ticular species of plants to the injections of specific insects. Some in- 

 sects, such as the potato leafhopper and the mosquito bug, affect many 

 species of plants, but with each plant species reacting in a somewhat 

 different way. The pineapple mealybug, however, although it lives 

 and flourishes on many kinds of plants, from short field grasses to 

 bananas, produces wilt disease symptoms on only one, the pineapple. 



The actual toxic principal has not been isolated from any insect 

 associated with this type of disease for the very good reason that tha 

 insect's salivary glands, which are presumably where the toxic sub- 

 stances arise, are extremely small and their contents very unstable 

 chemically. There has been a measure of success with experiments 

 on the injection of whole salivary glands in some cases but the isolation 

 of the toxic principles themselves is probably beyond the capacity of 

 even modern analytical technique. 



The manner in which insects acquire and spread virus diseases of 

 of plants has been studied in great detail, subject to the important 

 limitation that, since the virus itself cannot be seen in the insect's tis- 

 sues, the appearance of disease symptoms in the plant fed upon is the 

 sole criterion which the experimenter can use. In simple outline, the 

 process whereby an insect acquires and transmits a virus is as follows : 



