The meaning of the incubation period of the viruses is moot. Some investigators believe it is a true incubation period, during which the virus goes through some kind of necessary developmental or reproductive stage. Others consider it merely the time necessary for the virus particles to make their way through the intestinal walls of the insect into the blood stream and thence into the salivary glands, where they can be introduced with saliva into healthy plants during feeding.

Both points of view can be justified, depending on the virus involved. Some leafhoppers transmit persistent viruses throughout their lives once they become infective. Others may lose the ability after a period. In some individuals the ability to transmit a virus may become much less pronounced as they near the end of their life span perhaps the original supplies of virus taken in have become exhausted during the intervening feeding periods on healthy plants and there has been no multiplication of the virus particles within the insect, or at least not sufficient reproduction to maintain an infective charge of the virus. There is no evidence that viruses undergo biological changes in insects, but one scientist has reported that clover club-leaf virus reproduces in its leafhopper carriers. The leafhoppers remain infective through successive generations long after there would be any chance for the original quantity of virus to be involved. There also seems to be convincing proof that the virus causing aster yellows and the one that causes stunt disease of rice in the Orient multiply in their insect carriers.

Some insects that transmit viruses can become infective after a feeding period of only 1 minute or after only a single feeding on a diseased plant. Different species vary with respect to their efficiency in transmitting virus diseases, and there are instances where the nymphs or immature stages seem to be less efficient than the adult insects. Some vectors can pick up viruses while they are in immature stages but cannot transmit them until the adult stage is reached. The suggested explanation, in the case of a leafhopper carrier of aster yellows, is that the incubation period is not completed before the nymphs reach the adult stage. But that is not the explanation in the case of thrips, which transmit spotted wilt virus, because adults become infective only after picking up the virus while in the larval stage.

Macrosiphum ambrosias, aphids.

A plant virus may have a single species of insect serving as its vector, or there may be several kinds able to transmit the same virus. Sometimes the latter are entirely unrelated species. Single insects can infect plants with virus diseases, but even an infective individual cannot cause an infection every time it feeds on a healthy plant. In some instances this seems to be because the virus must be introduced into certain types of plant tissue which the vector does not always reach with its mouth parts; in other cases the reasons are not apparent. Viruses do not seem to affect their insect carriers in any way, even though they cause serious diseases of plants.

THE APHIDS, or plant-lice, have developed the ability to serve as carriers of plant viruses to the greatest degree. These minute, soft-bodied insects feed by sucking sap through their beaks, which they insert into plant tissues. They attack practically all kinds of plants. Most species produce both winged and wingless individuals. The former are chiefly responsible for the spread of virus diseases in fields.

The green peach aphid is outstanding among aphid carriers of plant virus diseases. It is known to transmit more than 5o kinds, mostly of the nonpersistent type.

The green peach aphid occurs almost everywhere and feeds on many kinds of plants. It is a serious pest of Potatoes because it can transmit leaf roll and other viruses. In potato-growing areas where the winters are mild, the green peach aphid spends the winter on weeds and such vegetables as spinach and kale. Winged individuals produced on the winter host plants migrate into the potato fields when the plants are small. As they move from plant to plant, the winged migrants leave a few young aphids here and there and spread potato viruses from diseased plants to healthy plants. The young aphids left behind start new aphid colonies throughout the potato field. When the colonies become overcrowded, enormous numbers of winged aphids may be produced. They swarm over the field and cause another wave of infection. Individual potato farmers are helpless in their efforts to protect their crops when tremendous numbers of migrating aphids are present.

In northern Maine and other potato-growing areas where winters are cold, the green peach aphid over-winters in the egg stage. The eggs are laid on twigs of peach and plum trees by female aphids, which are produced in the late summer or early fall. Relatively few winged aphids are produced in colonies developing from these eggs, and consequently infestations in potato fields are extremely light early in the spring. Although large numbers of winged aphids may be present later in the summer, there is usually not so much spread of virus diseases in northern potato-growing areas as there is in areas with warmer winters. The amount of potato leaf roll in the following year's crop may be predicted rather accurately from the abundance of the winged forms of peach aphid during the summer.

The green peach aphid and other aphids that develop on potatoes and other plants may migrate across a gladiolus field and pick up yellow bean mosaic virus. The virus causes only mild symptoms in gladiolus, but when the aphids transmit it to beans, a destructive disease results. Celery in Florida is infected with. cucumber mosaic by aphids which pick it up as they feed on commelina, a weed that grows along ditchbanks.

Lilies in fields containing a few plants infected with the nonpersistent coarse mottle and cucumber mosaic viruses soon become almost completely diseased when, the fields are planted near potatoes or other plants where the aphid carriers of these diseases develop. Lily rosette, a persistent virus, is transmitted by the melon aphid after an incubation period of the virus in the aphid lasting 3 or 4 days. This aphid develops on young lily plants; both the wingless aphids (which crawl to adjacent plants) or winged migrants (which fly to plants farther away) may spread lily rosette.

The melon aphid also transmits a virus that causes a condition known as lily symptomless disease. The disease has spread slowly throughout most commercial stocks of lilies. In itself it is not serious, but when the same plants get cucumber mosaic the double infection termed necrotic fleck makes them worthless. Necrotic fleck was chiefly responsible for the failure of Easter lily bulb production in the United States. To meet our needs, as many as 25 million Easter lily bulbs have been imported in a year.

The strawberry aphid in England transmits three viruses of strawberries, which cause the "running out" of desirable varieties. This aphid and two related species occur in the United States and live throughout the year on strawberry plants. Similar diseases and possibly others are devastating strawberries in the United States. These three strawberry aphids have been shown to be vectors of strawberry viruses in America and are believed to be chiefly responsible for their dispersal under field conditions. The Department of Agriculture has helped the strawberry industry by locating virus-free strawberry plants of the more valuable varieties and furnishing foundation stocks to cooperating nurseries for mass propagation and replacement of infected plants.

Winged aphids from overwintering Pea aphid colonies on alfalfa transmit a serious virus disease of peas, which kills the tips and interferes with the Productivity of the plants.

Aphids may also spread viruses that affect trees. An example is the citrus quick decline disease, which in a few years has caused the loss of many thousands of orange trees in California. The vector of quick decline is the melon aphid. Another aphid, which does not occur in the United States, is the vector of a similar virus disease of citrus in South America.

THE LEAFHOPPERS are our second most important carriers of plant viruses. They are small, slender, variously colored insects, which have sucking beaks similar to those of the aphids. They are active jumpers. The adults fly freely and some of them can cover long distances in migratory flights. A characteristic habit of young and adults is that of walking sideways. All leafhoppers are plant feeders. Certain kinds are called sharpshooters, and other names such as whitefly and greenfly have been used for some of them.

Leafhoppers transmit at least three serious virus diseases to peach trees. The oldest is peach yellows. Its vector was a mystery until the 1930's, when it was discovered that the plum leafhopper is the carrier.

The plum leafhopper feeds on the twigs and is seldom seen on the leaves. Plum is its favored host. Rarely is it found on peach. The leafhopper may obtain the virus, which it transmits to peach trees, from peach and plum trees. The latter are symptomless carriers of the yellows virus. In orchards adjacent to woodlands, correlations between the numbers of the leafhoppers, the abundance of wild plum, and the amount of yellows disease in peach have been noted. No other vectors of peach yellows have been discovered.