The virus diseases of agricultural crops are of such great economic importance and the part played by insects is so fundamental to the spread of many of these diseases that a book on agricultural entomology would be incomplete without some account of this partnership between insect and virus.

It may be well, in the first instance, to state briefly some of the outstanding characteristics of viruses. All types of organisms from man to bacteria are liable to attack by virus diseases and about two hundred have now been described affecting plants of different kinds. Most animal viruses and, so far as we know, all plant viruses are beyond the resolving power of the microscope using visible light and some are of molecular size. The virus of turnip yellow mosaic, for example, is about five times the diameter of a haemoglobin molecule. No virus has been cultivated on a cellfree medium such as agar, as have so many bacteria and fungi; a living susceptible cell is essential for virus multiplication. Most plant viruses are systemic in their hosts, all parts of the plant being invaded with the usual exception of the seed. It is a curious fact that very few plant viruses are seed-transmitted but any plant which is vegetatively propagated will, if virus-infected, give rise to virus-infected progeny. It is this fact which makes virus diseases of fundamental importance in such crops as potatoes, hops, strawberries, raspberries, dahlias, etc., all of which are vegetatively propagated.

Several plant viruses have been isolated in pure crystalline form and shown to be nucleo-proteins. One of these is the virus causing turnip yellow mosaic (i) which is described in a later paragraph.

Not all plant viruses are insect-transmitted but in this chapter attention is confined to those which are spread in this manner. The most important group of insects concerned with the spread of plant viruses is the Hemiptera, especially leaf-hoppers, aphides and whitefly, but in Great Britain the only insects of this order known to be vectors of viruses are the aphides. The mechanics of inoculation of a healthy plant by a Hemipterous insect are thought to be briefly as follows.