CYCLES OF PLANT VIRUSES IN INSECT VECTORS 161 



inoculated directly into plants. Extracts from plants, known to contain virus, 

 cannot, in most cases, be successfully used to inoculate very susceptible 

 plants. The few exceptions have not helped in the solution of the question 

 about biological cycles, because in none of these cases nor in any others has a 

 leafhopper virus in an extract of the vector been successfully inoculated into 

 plants. Yet until very recently the only methods of detecting such a virus or 

 measuring changes in its concentration depended upon obtaining infections 

 in plants. No virus assay by serology, electron microscopy, or by disease 

 effects in the vector was available for any of the viruses under investigation. 



Ill, History of Research on the Problem 



Research on the problem of multiplication of plant viruses in their insect 

 vectors has been carried out in various parts of the world on viruses that 

 scientists were afraid to ship freely from one country to another because of 

 their possible devastating effects on crops. This meant that men in one part 

 of the world, or even in two different parts of a single country like the United 

 States, worked on one virus that they considered appropriate, and men 

 elsewhere chose another. The subject was healthily controversial; Bawden, in 

 England, played a stimulating role as a critic of some of the work. However, 

 neither he nor his colleagues in Western Europe were able to work on the 

 problem, because no leafhopper transmission of virus was discovered in 

 Western Europe mitil de Fluiter and his colleagues (1955) in Holland success- 

 fully obtained transmission of Rubus stmit virus with Macropsis fuscula in 

 June, 1953. 



The multiphcation of certain plant viruses in their insect vectors, as well as 

 in the plants they infect, is now generally accepted. In fact, it now seems that 

 the probability of virus multiplication in certain vectors of plant viruses is 

 sometimes too readily accepted and that ideas that were advanced during the 

 controversy over the problem are sometimes forgotten. Some of them may, 

 however, merely be in eclipse and will prove eventually to have a place in the 

 total virus picture. 



As early as 1915, Smith and Boncquet had found that the beet leafhopper, 

 after acquiring curly top virus from plants, required an mcubation period of 

 24 hours before the insects could uifect healthy plants. They thought the 

 causal agent underwent some development or change in the insect before it 

 could be transmitted. 



In 1926, Kunkel published a comprehensive accomit of his studies on aster 

 yellows. Kunkel was impressed by the fact that, after aster leafhoppers had 

 acquired the causal virus by feeding on plants with yellows, a period of at 

 least 10 days intervened before the leafhoppers were able to infect healthy 

 plants. However, once the insects became infective, they usually continued 

 to be so during the remainder of their lives, in some cases for as long as 100 

 VOL. II — 11 



