CYCLES OF PLANT VIRUSES IN INSECT VECTORS 171 



wound tumor virus particle is approximately 10"^^-' cubic millimeters. 



Merrill (1936) assumed a specific gravity of 1.0 for all antigens considered in 



deriving liis generalization relating number of virus particles per cubic 



centimeter at serological thresholds and the mass of each particle. On the 



same assumption, the mass of each wound tumor particle would be lO"^--'' mg., 



and it can be estimated from Merrill's experimental curve (1936, Fig. 1) that, 



at the end-point titer, a solution of wound tumor virus would contain lO^** 



virus particles per milliliter. However, Merrill was dealing with end points 



determined by mixing antigen and antiserum solutions, whereas Black and 



Brakke were using the ring test. This gives approximately tenfold higher 



titers. Therefore, the end-point solution containing 10"^-^ gm. leafhoppers per 



milliliter is estimated to contain 10^ wound tumor virus particles. Adult 



leafhoppers weigh about 2 mg., so that the average leaf hopper would contain 



2 X 10^ / 

 — — — -r — = 10^-^, or about 10^ virus particles. This figure is halfway 



between the maximum and minimum probable numbers estimated by Black 

 (1950, 1953b). 



It may well be, of course, that the average number of particles of rice 

 stunt virus in its vector is much less. Wolcyrz and Black (to be published) 

 were unable to detect potato yellow-dwarf virus in extracts of its vector, 

 Aceratagallia sanguinolenta, at dilutions of 1/16. When this virus is photo- 

 graphed under the electron microscope it rarely assumes a roughly spherical 

 shape. When it does, its diameter is about 130 m/x. (Brakke and Vatter in 

 Black (1955), Fig. 4). By calculations similar to those above, it may be 

 estimated that the number of particles of potato yellow dwarf virus per 

 leafhopper is less than 10^. 



Black (1953b) concluded that the multiplication of rice stunt virus, 

 clover club leaf virus, aster yellows virus, and wound tumor virus in their 

 vectors had been demonstrated. The evidence for the propagative relation- 

 ship between some plant viruses and their leafhopper vectors has continued 

 to accumulate. 



Kunkel (1954) studied groups of about 50 Macrosteles fascifrons fed for 2 

 hours, 1 day, 7 days, or 14 days on asters with yellows, and subsequently 

 tested individually on a fresh aster plant each day as long as they lived. 

 Kunkel considered that the insects that did not live for 26 days or more after 

 the start of virus acquisition had not been adequately tested. Among the 

 others, 6/47, 47/68, 42/50, 53/53, or 13 %, 69 %, 84 %, and 100 %, trans- 

 mitted in the 1/12-, 1-, 7-, and 14-day groups. If one considers only the plants 

 that were used in the tests after each indi\adual began to transmit, the 

 percentages of plants infected by infective insects in the four groups were 71, 

 72, 75, and 78. This is a remarkably uniform result, consistent -o-ith 

 the multipUcation of this virus in its vector. Kunkel also found that a 



