168 L. M. BLACK 



in vivo was very readily inactivated by high temperatures in plants as well as 

 in insects, and supported Kunkel's interpretation of his work. 



Black fouiid that, by using Storey's (1933) technique of insect injection, 

 he could transfer aster yellows virus from viruliferous to nonviruliferous 

 leaf hoppers. In 1941, he titrated the virus in samples of leaf hoppers from a 

 colony that had been allowed to feed on diseased plants for one day. After the 

 acquisition period of one day, the colony, minus the samples that were used 

 for titrations at different times, was subsequently maintained on healthy 

 aster plants and transferred at frequent intervals. Counting the day in which 

 the insect fed on diseased plants as 1, two samples of 50 insects each, were 

 withdrawn at random from the colony on the 2nd, 4th, 8th, 12th, and 16th 

 days and tested for virus at different dilutions. During this time the source 

 insects were maintained on susceptible aster plants, none of which became 

 diseased. It was impossible, therefore, for the insects to have introduced 

 virus into these plants and to have withdrawn virus from them after it had 

 multiplied in the plant host. From the 18th to 31st day the remainder of the 

 colony infected the two fresh aster plants to which it was transferred each day. 

 It was found that on the 2nd day no virus could be detected in insect juices 

 diluted to 1 : 10 or 1 : 100, but on the 12th day, virus could be detected at 

 1 : 100 and 1 : 1000; Black interpreted his results as showing at least a 

 100-fold increase of virus in the vectors during the incubation period. Similar 

 results were obtained by Maramorosch (1953a, 1956). Whitcomb and Black 

 (to be published) have followed the increase of viral soluble antigen in leaf- 

 hoppers of Agallia constricta after injection of wound- tumor virus. The 

 leaflioppers were maintained at 27.5°C on alfalfa which is immune to the 

 virus. By means of the precipitin ring test they demonstrated that no 

 antigen could be detected at a dilution of 1/10 tlirough the 4th day following 

 the injection. Between the 5th and 8th day the titer of viral soluble antigen 

 rose from 1/10 to 1/120 and after the 12th day it was maintained at a titer of 

 1/160 for at least 50 days. This curve resembles that for the increase of a 

 virus in Droscphila melanogaster (L'Heritier, 1958, p. 205), 



In 1948, Kunkel pointed out that in three different viruses affecting corn 

 there was a correlation between the length of the incubation period in the 

 specific vectors and in the corn plant. He drew attention to the fact that this 

 approximate correlation held for six other virus diseases. In regard to the 

 viruses affecting corn, Kunkel pointed out that the three different vectors 

 were about the same size and considered that there was no obvious reason 

 why one of the viruses should require only one-half day to pass from gut to 

 saliva, while another virus should require 14 days or 28 times as long to make 

 a similar trip in another leaf hopper. If physical passage of the virus from one 

 location to the other alone were involved, he found it difiicult to understand 

 why the incubation periods in the corn plant should be approximately the 



