164 L. M. BLACK 



e\ddence earlier (1934) that the insects did not acquire virus in less than 3 days 

 of feeding on diseased plants. Moreover, insects in general are unable to 

 acquire virus from plants until a day or two before plants develop symptoms; 

 Fukushi (1934, pp. 85, 86, 90) had determined that the incubation period in 

 plants varied from 6 to 37 days. In his experiment, one of the young nymphs, 

 weighing about 0.06 mg., was a female that infected 38 plants. It also ovi- 

 posited eggs from which the nymphs, handled in the same manner, included 

 15 infective progeny that infected 201 plants. Fukushi reasoned that the 

 amomit of virus in the original 0.06 mg. nymph must have been "extremely 

 small" and that the passage of the virus to this nymph and to the next 

 generation of progeny, and the infection of so many plants, meant that the 

 virus multiplied in the vector. 



One year later, Freitag (1936) pubhshed his studies with the curly top 

 virus in sugar beets and interpreted his data as evidence that the virus did 

 not multiply in the beet leafhopper, Circulifer tenellus. In this case the insects 

 never receive virus through the egg. After allowing the leafhoppers to acquire 

 virus by feeding on diseased plants for various periods, he transferred 

 individual insects to fresh healthy beet seedlings each day. This ensured that 

 the only virus acquired from plants was that obtained in the initial acquisition 

 feeding period. During successive equal intervals after the acquisition of virus, 

 the percentage of beets infected by single insects tended to decrease progress- 

 ively. In some cases infective ability was lost entirely. Moreover, the ability of 

 the leafhoppers to infect was roughly proportional to the length of the acquisi- 

 tion feeding period. Although the ability of adult insects to acquire and to 

 transmit virus decreased with age, this effect was less marked than the pro- 

 gressive decrease in transmission with time, which occurred when insects 

 were not allowed renewed access to virus. In other words, the age effect could 

 not entirely account for the decrease. Furthermore, it was demonstrated that a 

 second feeding on a source of virus in late adult life increased the infectivity 

 of insects that were transmitting poorly. Freitag reasoned that, if multiplica- 

 tion occurs in the vector, the insects should not only retain infective capacity 

 during their entire adult life, but that those fed for only a short period on 

 diseased beets should be able to cause as many infections as those fed for 

 longer periods. He considered that the virus did not multiply in the insects. 



Not long afterwards, Kunkel (1937) published his first paper on the effect 

 of heat on the ability of Macrostelesfascifrons to transmit aster yellows virus. 

 He showed that infective leafhoppers, after being kept at approximately 

 32°C. for varying numbers of days, were unable to transmit the virus when 

 returned to temperatures of about 24°C. The period of their inability to 

 transmit was roughly proportional to the length of time they were held at the 

 high temperatures. If they were kept at the high temperatures for 12 days or 

 longer they lost infective ability permanently, unless allowed another 



