Insect transmitted plant viruses may be divided into two groups according to their relationships with their insect vectors. The first, which may be called the “Presistent viruses”, survive in their vectors for long periods, sometimes for weeks or months; the second, or “Non-persistent viruses”, survive in their vectors for only a short period, always less that that during which they remain active in untreated infective plant sap. There are other differences between the two types of viruses besides those of persistance in the vectors, and an attempt has been made in Table I to indicated the properties and range of vectors of the viruses in each group. The table is necessarily incomplete, for the properties of many viruses which are known to be insect transmitted, particularly those of the non-persistent type, have not yet been fully investigated. The persistent viruses are rather variable in their general properties. Some can be mechanically transmitted, though usually with difficulty, while others can be transmitted only by grafting or by their insect vectors. The properties
in vitro
of the persistent viruses also vary. Curly top of sugar beet has been shown by Bennett (1935) to survive for several days in expressed sap, and to be resistant to chemical treatment, while spotted wilt virus of tomato (Bald and Samuel 1931, 1934) is very unstable, and survives in untreated sap for only a few hours. Within the persistent group there is also considerable variety of vectors, belonging to many families of the Hemiptera. These insects usually have a non-infective period after feeding on the infected plants which is known as the “latent” or “incubation” period. To account for the very high level of efficiency exhibited by these vectors some worker postulate that the viruses multiply in the bodies of the insects, thus maintaining the high virus content assumed necessary for the infection of successive healthy plants. The “latent” period is explained as being the time taken for the virus imbibed by the insects to reach an “infective” concentration. As, however, there is no information as to the size of an infective dose of virus, as delivered by an insect, or the number of infective doses which it could contain, this assumption is by no means essential. It is rendered less probable by the fact that the “latent period is not decreased by increasing the quantity of virus available to the insects; for instance by prolonging the period of feeding on the infected plants (Freitag 1936). Also the facts that the degree of vector efficiency, and the length of time for which the vectors remain infective, are increased by increasing the feeding time on the infected plants, in those insects for which this treatment has been tested. Their efficiency therefore seems to depend rather on the quantity of virus which they acquire on feeding than on any subsequent process.
Studies on the transmission of sugar-beet yellows virus by the aphis,
Myzus persicae
(Sulz.)
