CYCLES OF PLANT VIRUSES IN INSECT VECTORS 175 



Maramorosch (1950a,b, 1953b) has demonstrated that the greater the 

 inoculum the shorter the minimum incubation period following injection, and 

 has considered this as evidence for multiplication. However, even if there 

 were no multiplication, but mere internal passage, one would expect that the 

 greater the quantity of inoculum injected the shorter the minimum incubation 

 period that one could detect. Similarly, low temperature might be expected 

 to lengthen the incubation period or even arrest transmission whether or not 

 multiplication occurred (Maramorosch, 1950a, 1953b). 



Day (1955) and Heinze (1956) have succeeded in detecting potato leaf roll 

 virus in solutions. This was accomplished by the difficult operation of injecting 

 virus in solution into aphids. Aphids had not been successfully inoculated by 

 injection before. Both authors interpreted their results in obtaining successful 

 inoculations as evidence for multiplication in the vector. However, there 

 would seem to be no inherent reason why injections should not be successful 

 with a virus should it have the property of passing through the tissues of 

 the vector without multiplication. In prehminary experiments, Maramorosch 

 (personal communication) has been unable to transmit the beet curly top 

 virus serially in the vector, in spite of his successful transmission of the virus 

 to the vector by injection. 



The viruses of rice stunt, clover club leaf, wound tumor and rice stripe are 

 known to pass transovarially and to multiply in the vector, but there would 

 appear to be no a priori reason why a virus which does not multiply in its 

 vector but passes through vector tissues could not be transmitted transo- 

 varially. 



V. Evaluation of Kinds of Evidence for Absence of 

 Multiplication 



If any plant virus passes internally through the vector without multiplica- 

 tion, a demonstration of the fact is inherently much more difficult than a 

 demonstration of multiplication. In fact, with our present degree of refinement 

 in virological techniques, it does not seem possible to provide rigorous proof 

 of such a relationship. Nevertheless, a good deal of evidence has been obtained 

 indicating that the leafhopper-borne curly top virus does not multiply in its 

 vector although it is retained through the molt and is capable of being 

 transmitted for days after a single acquisition feeding. 



The data against multipUcation vary in significance. The most critical 

 would seem to be: 



(1) The demonstration that the virus content decreases progressively and 

 consistently following termination of the acquisition feeding period. By 

 feeding extracts of leafhoppers to non-viruliferous leafhoppers and testing 

 the latter on beets, Bennett and Wallace (1938) demonstrated such a decrease 



