176 L. M. BLACK 



for curly top virus. By injecting extracts from aphids into virus-free aphids 

 which were later tested on plants, Harrison (1958a) demonstrated such 

 decrease for potato leaf roll virus. In the propagative viruses, aster yellows 

 and womid tumor, measurements indicate an increase in virus after acquisition 

 (Black, 1941; Maramorosch, 1956). The progressive decrease in virus content 

 following acquisition would seem to be the most significant datum that can be 

 obtained as evidence against multiplication. However, it serves to illustrate 

 the difiiculty of attempting to prove this negative proposition. Apart from 

 the purely theoretical impossibility of proving the complete absence of 

 multiplication, there is the practical obstacle of being imable, at present, to 

 quantify these viruses accurately or to know how many virus particles are 

 present in an infective dose. Thus, it is theoretically possible that, with the 

 techniques now available, a progressive and consistent decrease in the activity 

 of a massive dose of acquired virus could be measured while, at the same 

 time, the multiphcation of some virus might occur below the level of the 

 concentration that can be measured. 



(2) The demonstration that transmission by single vectors consistently 

 decreases following the termination of certain appropriate acquisition 

 feeding periods. This was proved for curly top virus by Freitag (1936) and by 

 Bennett and Wallace (1938). Bennett and Wallace found that this decrease 

 was not very noticeable when the acquisition feeding period was such that the 

 original virus content was high. A minimum latent period of 4 hours, during 

 which the insect is unable to transmit, is detectable in suitably designed 

 experiments with curly top. Therefore, a period of maximum transmission, 

 at some point between the latent period and the period of decreasing trans- 

 mission, is to be expected. The possibility that this decrease in transmissive 

 ability may be due to aging of the leaf hoppers was thoroughly investigated in 

 the case of curly top virus. Freitag (1936) showed that the percentage of 

 leafhoppers able to acquire and transmit virus decreased with age. Neverthe- 

 less, age was not the only factor involved, because transmissive ability could 

 be increased by a second acquisition feeding (Freitag, 1936; Bennett and 

 Wallace, 1938). Giddings (1950) showed that individual leafhoppers trans- 

 mitting curly top virus, strain 2, could transmit the strain 3 virus following 

 a 16-hour acquisition feeding period. The proportion of transmissions of strain 

 3 virus rose to 50 % and subsequently fell to 9 %. A second acquisition 

 feeding period on a source of strain 3 increased the proportion of strain 3 

 transmissions. This effectively ruled out age as a sufficient explanation for 

 decrease in transmissive ability with time. 



(3) The demonstration that transmissive ability of individual leafhoppers 

 is roughly proportional to the length of the acquisition feeding period. 

 Bennett and Wallace (1938) showed this to be true for the curly top virus 

 when they found that although virus could be acquired in as short a time as 



