92 AN INTRODUCTION TO THE STUDY OF VIRUSES 
The biological control of insect vectors by the encouragement ot 
their natural enemies is a possibility and a start has been made on the 
collection of parasites of the mealy bugs which transmit the swollen- 
shoot disease of cocoa. 
Finally the method of trap crops may be mentioned; this is an 
attempt to draw off the insects from the main crop by early planting 
of a strip which is allowed to become infested and is then destroyed 
by chemicals. 
2. Elimination of Reservoirs of Infection 
This is a very important measure in the control of plant viruses 
because of the common occurrence of plant carriers and the wide host 
range of some viruses. 
Cutting-out measures similar to the slaughter policy adopted with 
some animal viruses may occasionally be necessary when it becomes 
advisable to sacrifice a heavily infected crop. : 
The importance of starting with a virus-free crop cannot be over- 
emphasized since the presence of virus-infected plants in a young, 
growing crop, at the start of the season, offers ideal conditions for the 
spread of an insect-borne virus. This is particularly true of potatoes, 
and is the basis of the important trade in seed potatoes between England, 
Scotland, and Ireland. In the two latter countries, there are large 
areas with a damp climate unsuitable for the aphis vectors where 
virus-free seed potatoes can be raised on a scale not possible in England. 
It must be understood, of course, in this connexion that if a potato 
plant is infected with a virus then all the tubers from that plant, 
including the “‘seed” potatoes, are also virus-infected. It is this fact 
which makes virus diseases of such fundamental importance in all 
crops which are vegetatively propagated, including, besides potatoes, 
strawberries, raspberries, hops, and all bulbous and tuberous plants. 
For a comprehensive survey of the factors underlying the spread in 
the field of potato viruses the reader is referred to a recent monograph 
(Doncaster and Gregory, 1948). 
The fact that so few viruses are transmitted through the seed makes 
it easy enough to start with a virus-free crop of sugar beet. But this 
crop is susceptible to two serious aphis-borne viruses, those of mosaic 
and yellows, and the question of sources of infection is a very important 
one to the grower. There seem to be three main sources, “volunteer” 
sugar beets left over from previous years, mangold clamps which may 
harbour both the aphis vector and the viruses, and thirdly the close 
