Further studies on the relationship between hyoscyamus virus 3 and the aphis Myzus persicae (Sulz.) with special reference to the effects of fasting

The means by which insects transmit plant viruses are largely unknown. It is improbable that the viruses are transmitted merely by contamination of the mouthparts of the insect, because there appears to be a close specific relationship between the viruses and their vectors. For this reason it has usually been assumed, on analogy with insect transmitted animal disease such as Malaria, that the virus is taken in by the vector through the mouth, passes through the wall of the alimentary canal into the haemocoele, and is introduced into a new host with the saliva. There is good evidence to suggest that at least one class of viruses is transmitted in this way, but only the viruses causing Streak disease of Maize, and Curly-top of Sugar beet, have been shown to be present within their insect vectors, and of these only the first has been obtained from the blood stream. So far no virus has been obtained either from the salivary glands, or directly from the saliva, of any infective insect. Viruses of the type causing Streak disease of Maize (Storey 1928) and Curly-top of Sugar beet (Bennett 1934, 1935) are not transmitted by insects immediately after feeding on the infected plants, bu t a so-called incubation period of the virus in the vector is necessary. Also these insects become infective for long periods, sometimes for the whole of their lives, after a single feeding on the source of infection, and the duration of their infectivity seems to depend, to some extent, on the length of this feeding. In these insects transmission is obviously not merely mechanical transfer, for it must be assumed that the virus is in the body of the vectors for some time before it is able to cause an infection, and possibly the insect acts as a kind of reservoir of virus from which successive infective doses can be withdrawn as long as the supply lasts.