VECTORS AND VARIATIONS
Many plant virus diseases involve not only the interaction between virus and plant, but also interaction with a vector. Such diseases are usually complex because this interaction among three biological systems is influenced and often controlled by the environment, which not only influences each biological system individually, but also affects interactions among the separate systems. The importance of this 3-way interaction is generally recognized both in experimental work and in natural outbreaks of disease; it provides the focus for much research. A more complete view, however, includes other interactions that are often given little emphasis. In many virus diseases in the field, as well as in most experimental work, the vector interacts not only with the plant (suscept) that becomes diseased, but also with two additional plants, or groups of plants. One is the plant on which the vector is reared for experimental purposes or on which it develops in the field. The second is the plant from which the vector acquires the virus which may eventually be transmitted to the suscept. In many cases the plant on which the vector develops and the plant from which it acquires virus are the same. An over-all view of the basis for this discussion, therefore, includes the possible interactions between a vector and three separate plants, the interactions between a virus and two of the plants, and the interaction between virus and vector, the component of the picture probably studied most (Fig. 1).
Figure 1 illustrates the individual sources of variation that affect virus transmission by vectors. Each of the five separate biological systems has its own range and kinds of variations. Other types of variations occur for the six interactions (shown by arrows) between any two of the systems. Moreover, the investigator himself introduces another biological system that frequently adds still more complications. The environment plays a key role in all of this. Eventual understanding of the total picture must be based first on understanding the individual variations. Many writers have emphasized the need for greater understanding of the ecology of viruses and vectors, especially when broadly based control procedures