Considerable experimental work has been done on various aspects of the transmission process by the leafhopper. The mechanics of the mouth parts, the tissues of the plants reached by the mouth parts, and the location of the virus within the insect vector have all been subjects of investigation. Most leafhoppers apparently acquire virus from and introduce virus into the phloem. However, some vectors feed on the xylem and acquire virus from and inoculate it into this tissue. Some viruses show a corresponding specialization in regard to the tissues they attack.

WHITE FLIES transmit a number of plant viruses. Nymphs of the white fly are attached to the plant and therefore cannot themselves spread virus, but virus can be acquired by the nymphs, can pass through the pupal stage, and can be transmitted by the adults. The adults also can acquire the virus directly from plants.

When white flies were shown in 1946 to be vectors of a virus of abutilon, a puzzle of long standing was partly solved. Variegated abutilon had been used as an ornamental for many years in Europe and other parts of the world where the variegation never spread naturally to nonvariegated plants.

Since there was no natural spread of the condition, the disease was sometimes set somewhat apart from other virus diseases even though the variegation could be transmitted by grafting. It is now evident that in some countries a virus causing a similar variegation is disseminated in the field by a white fly.

MEALYBUGS, tended and transported by ants, are the vectors of destructive viruses that attack cocoa trees. Some of the viruses are closely related; in other cases relationships are uncertain. The viruses, which are not transmissible to plants by juice inoculations, do not remain long in the mealybugs unless they fast before the acquisition feeding; then virus may be retained about 36 hours. In spite of the nonpersistence of the virus, there is some specificity of transmission certain mealybug species transmit certain of the virus strains not transmitted by others, and vice versa. Moreover, some strains of one of the mealybug species failed to transmit a virus that other strains of the same species transmitted.

THE ONLY VIRUS definitely known to be transmitted by thrips is the one that causes tomato spotted wilt. Forty-one different plant viruses have been tested for transmission by thrips with negative results. Although three species of thrips are vectors of tomato spotted wilt, several other thrips species are not. Tomato spotted wilt virus can be acquired only by larvae, but it passes through the pupal stage, so that both adults and larvae can inoculate plants. After an incubation period of 5 days or more the insects remain infective for life.

We have relatively few authenticated accounts of transmission by insects with biting mouth parts. In some of these isolated cases, there is better transmission of the virus by biting insects than by those with sucking mouth parts. The virus of turnip yellow mosaic is an example of this kind of virus. It apparently is not transmissible by insects with sucking mouth parts but is transmitted by a number of insects with biting mouth parts. The most important of these are flea beetles, the larvae of which may retain virus for as long as 4 days. It is believed that the insects regurgitate the virus.

All viruses of this sort are readily transmitted by rubbing juice from diseased plants on the leaves of healthy ones, and some are readily isolated.

UNTIL RECENTLY the only viruses photographed under the electron microscope were the more stable ones, which are readily transmitted mechanically and occur in relatively high concentration in plants. Some are insect-transmitted. Among them are the tobacco mosaic and squash mosaic viruses. Less stable and less concentrated viruses that have a more intimate relationship with their vectors have been identified recently under the electron microscope. Considerable interest attaches to the nature of those viruses that are known to reproduce in both plants and animals.

When one considers that the occurrence of a virus disease of plants usually involves three entities the virus, the plant, and the vector and may involve more than one of each, it should at once be apparent that the interactions between them and their environment may be exceedingly complex. In the laboratory, certain single factors may be demonstrated to be decisively important. Thus the virus of aster yellows is inactivated at 89 F. But when one tries to explain the vagaries of the spread of virus diseases in the field, relationships are not always readily discerned. For example: Northern regions and high altitudes with climates inimicable to aphids generally favor the production of potatoes with a low virus content, but hot and dry climates are also unfavorable for aphids and under certain conditions can be used to produce seedpotatoes with a low incidence of virus.

The flight habits of aphids in the field in relation to the spread of potato virus diseases have been extensively studied at Rothamsted in England and the Maine Agricultural Experiment Station. It was determined in Maine that early and sustained flights containing a high proportion of Myzus persicae, an aphid vector of leaf roll disease of potatoes, were associated with considerable spread of the disease. But flights late in August or in September usually resulted in little or no spread.

Detailed work has been done on the ecology of the vector of the curly top virus. This leafhopper is an active flier, and large numbers can easily be borne long distances by the wind. It often multiplies in the spring on a variety of succulent weeds on uncultivated or abandoned lands. If those plants dry up after the insects have reached the winged adult stage, the insects take flight. One such migratory flight was estimated at 60 miles. Forecasts of leafhopper invasions based on studies of the breeding areas have been used to reduce losses.

In general the insect vectors of a virus tend to be confined to one of the major phylogenetic divisions, such as the families of the Hemiptera or the order Thysanoptera. Some of the viruses multiply in their insect carriers and more may be expected to be placed in this category. The vector relationships of such viruses are obviously quite different from those of the viruses that do not multiply in the vector. Although more extensive research on certain viruses has greatly increased the number of known vectors for each, other research has at the same time indicated more highly specific virus-vector relationships than were indicated in early work.

L. M. BLACK is a professor in the department of botany in the University of Illinois. Before joining the University in 1952, he was curator of plant pathology in the Brooklyn Botanic Garden.