162 L. M. BLACK 



days. The virus did not pass through the insect egg. If the insect acquired 

 virus during the nymphal stage only, the virus persisted in the insect, even 

 when it underwent molts which must have removed virus from aU external 

 parts, including the mouth parts. By moving them to a fresh, healthy plant 

 each day or by maintaining them on rye, which he showed was not susceptible 

 to the virus, Kunkel made sure that the insects did not pick up fresh virus 

 after molting. These facts clearly indicated that the virus was carried inside 

 the insect, and Kunkel concluded "It seems probable that the incubation 

 period of aster yellows m Macrosteles fascifrmis^ is due to a development and 

 multiplication of the causative agent in some tissue of the leaf hopper." 



Kimkel looked for some difference between the viruliferous and non- 

 viruliferous leafhoppers but found none. The average length of Hfe of the 

 virus-bearing and virus-free vectors was the same. He also sought for 

 microscopic evidence of disease in the infective insects but was unable to 

 find any. 



Storey (1928), w^orking with corn streak virus in Africa, found that the 

 minimum incubation period in the vector Cicadulina mhila was 6 to 12 hours 

 and that once insects were infective they retained infectivity, when trans- 

 ferred daily, throughout their hves, although occasional exceptions occurred. 

 Infective leafhoppers captured as soon as a nymphal skin was shed were still 

 infective. Storey (1928, p. 22) beheved "the evidence clearly indicates a 

 multiplication of the virus in the insect." 



In 1933, Storey transmitted corn streak virus extracted from plants or from 

 vectors by introducing it into nonviruliferous Cicadulina mhila by means of a 

 needle or glass micropipette. Using this technique to test for virus he deter- 

 mined that the virus occurred in the blood before the end of the incubation 

 period. He also detected it in the general contents of the thorax and abdomen, 

 but in the contents of the rectum, only if the insect had recently fed on a 

 diseased plant. It did not occur in the naturally voided feces. 



In the same year, Fukushi (1933) reported the first authentic and well- 

 demonstrated instance of plant virus transmission through the egg of the 

 vector. According to Fukushi, the Japanese entomologists, Onuku and 

 Murata, had discovered, about 1902, that leafhoppers {NepJwtettix apicalis var. 

 cincticeps) which hatched from eggs collected in Shiga prefecture produced 

 stunt (dwarf) disease in rice plants, whereas leafhoppers of the same species 

 from the vicinity of Tokyo did not. They subsequently fomid that non- 

 infective leafhoppers from Tokyo became infective after feeding upon diseased 

 plants. According to Fukushi, Murata stated in 1915 that infectivity was 

 transmitted from parents to progeny for 3 or 4 generations. Fukushi stated 

 that no experimental procedures or results were reported in detaO to support 



^ Currently accepted binomials are used throughout and have been substituted for the 

 originals in quotations. 



