CYCLES OF PLANT VIRUSES IN INSECT VECTORS 159 



1952) in which, the virus is probably merely mechanically transferred by the 

 biting mouth parts. Recently an unusual kind of mechanical transmission 

 was indicated by the evidence that the ovipositors of certain leaf miner flies 

 may transfer tobacco mosaic and sowbane mosaic viruses (Costa et al., 1958). 



There are a number of soil-borne plant viruses whose means of survival in 

 the soil and transmission from plant to plant has been a tantalizing mystery 

 for many years. For this reason the recent discovery by Hewitt et al. (1958) 

 that the ectoparasitic nematode, Xiphinema index, transmits the soil-borne 

 grape fanleaf virus is of great interest. The potential significance of this 

 finding is diificult to assess at this time but it may well herald future 

 discoveries of many nematode vectors with new intriguing relationships to 

 the viruses they carry. It has definitely demonstrated how some soil-borne 

 viruses are spread. 



Before entering upon the discussion of the main theme of this paper, a few 

 additional introductory remarks are in order regarding two broad generaliza- 

 tions on arthropod transmission of plant viruses. One is that the vectors of a 

 single plant virus are almost always restricted to one of the major taxa, such 

 as that comprising the aphids, the leafhoppers, the white flies, the beetles, 

 the thrips, the mealy bugs, or the mites. Most records to the contrary ua the 

 literature are not well authenticated and others could well be reinvestigated 

 in the fight of the bulk of evidence supporting the rule. The second genera- 

 Hzation is that a virus is almost always transmitted by only one of the 

 principal types of transmission, that is, by propagative transmission, by 

 circulative transmission, or by one of the mechanical processes, simple or 

 not so simple. However, there are indications that some of the aphid-borne 

 and some of the beetle-borne viruses are transmitted in more than one way. 

 Even if this should prove to be true, this second rule has wide vafidity. 



II. The Nature of the Insects and Viruses Involved in 

 Biological Cycles 



The problem of ascertaining whether or not any plant viruses have a 

 biological or reproductive cycle in their insect vectors was worked out with 

 those plant viruses that are transmitted by leafhoppers and the reader needs 

 to be aware of certain characteristics of both the leafhoppers and the viruses 

 they carry in order to understand the necessity for certain experimental 

 procedures used in investigations. Leafhoppers are smaU insects, weighing 

 about 1 or 2 mg., which live by sucking nutriment, usually from the vascular 

 structures of plants. They insert their tubelike mouth parts and commonly 

 withdraw plant juices from the phloem, but some of them feed on the xylem. 

 Although some may be maintauied for a few days on artificial nutrient 

 solutions, no one has discovered how to rear them on such solutions. Thus 



