CYCLES OF PLANT VIRUSES IN INSECT VECTORS 165 



acquisition feeding on diseased plants. Kunkel also demonstrated that, if 

 insects were heat-treated while undergoing natural incubation periods, the 

 effect of the high temperature was greater than if the insects were already 

 infective. Kmikel believed that these results indicated that heat treatments 

 of 12 days or more completely destroyed the virus in the insects, that heat 

 treatments for shorter periods destroyed virus in proportion to the duration 

 of the heat treatment, and that the time required for regaining infectivity 

 represented an induced incubation period in which the virus must once more 

 multiply to an infective concentration. The greater effect of the treatment on 

 insects in which virus was still undergoing incubation, was attributed to a 

 lower virus content in these insects when the heat treatments were started, 

 than that in insects already infective. 



Kunkel observed that the heat-treated insects frequently transmitted 

 milder strains of the aster yellows virus and suggested that the heat treatment 

 exerted some kind of selective action for mild strains of the virus. The 

 observation indicated that the heat treatment was actually preferentially 

 inactivating the more virulent strains of the virus and was in itself evidence 

 for a direct effect on the aster yellows virus. 



Before Kunkel's paper appeared, Bennett and Wallace (1938) had sent to 

 press a report of their extensive investigation of curly top. Among other 

 things, they found that, although the curly top virus could be acquired in as 

 short a time as one minute of feeding, individual leafhoppers did not acquire 

 their maximum ability to transmit unless the acquisition feeding lasted for 

 two days. Acquisition feedings for longer than two days increased the virus 

 concentration in the leafhoppers but did not increase ability to transmit. They 

 were able to ascertain the latter point by preparing extracts of the leafhoppers, 

 by allowing other nonviruliferous leafhoppers to feed on the extracts through 

 a membrane, and by then testing the infective ability of these insects on beet 

 seedlings. The essentials of this artifical feeding technique had been described 

 by Carter (1927) and by Severin and Swezy (1928). Although infections can 

 be obtamed by multiple pin pricks through virus extracts into the crowns of 

 beet plants, the successful moculations are so rare and irregular that the 

 method has never been used to assay curly top virus. 



Subsequent to acquisition of virus and passage of the incubation period, 

 Bennett and Wallace fomid that the infectivity of the leafhoppers and their 

 virus content gradually decreased over periods of 8 to 10 weeks. However, 

 reduction of infectivity was slight if the original virus content was high. 

 Ability to transmit at a higher level of efficiency could be restored by renewed 

 access to a diseased plant. Bennett and Wallace also determined the minimum 

 incubation period in the leaf hopper to be four hours. By the feeding technique 

 described above they ascertained that the virus occurred in the blood, 

 alimentary tract, salivary glands, and the feces. The blood was the richest 



