166 L. M. BLACK 



source and the salivary glands a relatively poor source. They considered that 

 the sahvary glands might be unfavorable to the virus. 



It is apparent that their findings confirmed and extended those of Freitag. 

 The authors considered it probable that there was no multiplication of curly 

 top virus in its leafhopper vector and that, if multiplication occurred, it was 

 insufficient to maintam the vims content of the vector. As a result of their 

 work, Bennett and Wallace suggested that if the same conditions held 

 generally for other insect vectors that retain viruses for long periods "much 

 of the evidence now accepted as supporting the idea that certain plant vii'uses 

 multiply in their insect vectors must be re-evaluated." 



In support of his earlier work, Kunkel (1938) reported that newly hatched 

 nymphs of the aster leafhopper can grow to the adult stage, produce fertile 

 eggs, and otherwise live what appear to be normal lives when held con- 

 tinuously at 95°F., a temperature 5° higher than that used to cause temporary 

 or permanent loss of infectivity. He demonstrated again that insects that 

 have been rendered permanently noninfective by heat treatment may 

 reacquire and transmit the virus by feeding on diseased plants. Because of 

 these facts, and the fact that heat treatment of plants indicated inactivation 

 of the virus in plants, Kunkel concluded that the effect of heat treatments on 

 insect infectivity must be attributed to action on the virus. He considered 

 that multiphcation in the insect was the basis for the specific relationship 

 existing between the aster leafhopper and the aster yeUows virus. 



In 1939, the first edition of Bawden's widely read and influential book, 

 "Plant Viruses and Virus Diseases," was published. He accepted and strongly 

 supported the conclusions of the workers on curly top virus and, although 

 he considered that multiplication of plant viruses in vectors could not 

 definitely be excluded, he advanced many arguments against the evidence 

 that such multiplication occurred. It would be tedious to recapitulate here 

 all the arguments that were advanced against multiplication of plant viruses 

 in their insect vectors. Instead, only those ideas that stiU have pertinence in 

 the light of our present knowledge will be dealt with, Bawden's opposition to 

 the evidence for multiplication was continued through the 1943 and 1950 

 editions of his book, except for an addendum in the latter. Some of his 

 criticisms were vahd and are still valid today. For example, Bawden's 

 important arguments regarding the possible number of virus particles 

 which a single leafhopper might contain were actually extremely con- 

 servative, as will be seen later. On the other hand, some of the arguments 

 advanced against multiplication were erroneous and some seem very 

 involved and improbable, though tenable until proof to the contrary was 

 forthcoming. 



In 1939, Fukushi briefly reported further studies on the transmission of the 

 rice stunt virus through the egg and, in 1940, he pubhshed complete details of 



