Insects, Viruses, and Seed Crops

ORIN A. HILLS, KENNETH E. GIBSON, AND W. F. ROCHOW.

MANY serious plant diseases are due to viruses that insects carry. Some of the viruses may be transmitted in other ways, but the diseases would have little importance without insect vectors.

Sometimes a single species of insect is the carrier. Sometimes many different insects, usually of a related group, are to blame.

The aphids are the worst. The green peach aphid exists everywhere, is not overly particular about the plants it feeds on, and is said to transmit more than 50 plant viruses.

One reason therefor is that it is active. One winged aphid can spread more disease than a whole colony of comparatively inactive wingless forms.

Leafhoppers, next to the aphids in destructiveness, also are active. Often they feed for a comparatively short time in one locality and then move on, spreading disease as they go.

A plant unsuitable as a breeding host or even for food of a particular insect is not safe from a virus the insect may be carrying. Plants susceptible to the disease but unfavorable to the insect may be damaged more than a plant the insect likes better. The green peach aphid, for example, carries several mosaic-type diseases of muskmelons. It does not like muskmelons and cannot live long on them. It moves quickly over a field, sampling the plants and spreading disease as it goes.

Several hundred virus diseases are spread by insects.

One group of them is nonpersistent, because the vectors can continue to transmit the virus only a short time, often only a few minutes.

A second group is called persistent, because the vector generally can transmit the virus for a long time often as long as the insect lives.

Nonpersistent viruses usually are transmitted by the aphids and can be acquired by the insects after a short feeding time on an infected plant.

Beet mosaic virus can be picked up by certain aphids after feeding only 10 seconds. Leafhoppers or aphids that transmit most persistent viruses may have to feed for several hours or more before they can acquire a virus of the persistent type.

At least part of the explanation for these differences in feeding time lies in the fact that nonpersistent viruses infect most of the tissues of a plant and that aphids can easily acquire virus by feeding in the outermost cells. Persistent viruses often seem to be restricted to the phloem, and a longer feeding time is required to penetrate into this internal tissue.

Aphids that have acquired a non-persistent virus usually can transmit it immediately. A leafhopper or aphid that has acquired a persistent virus usually is unable to transmit it for a time that varies with each specific case a few hours to many days. This period before the vector becomes infective may be the time needed for the persistent virus to penetrate the gut, enter the blood, and be injected into the plant along with saliva during feeding. The delay also may be due to the time needed for the virus to multiply in the insect vector.

Another difference between the non-persistent and persistent viruses is the effect of fasting on transmission. Aphids that have not been allowed to feed for some time before they reach a plant infected by a nonpersistent virus are more efficient vectors than are aphids that had been feeding continuously. Furthermore, fasting after feeding on the infected plant may prolong the time during which an aphid carrying a nonpersistent virus is infective. Fasting has little effect on the transmission of persistent viruses.

These effects have been noted mostly in experimental work, but the observations are of practical as well as fundamental importance because the winged insects undergo periods of fasting in nature.

Viruses of the nonpersistent group sometimes are transmitted by many different aphid species. Western celery mosaic, a nonpersistent virus, has been transmitted by 17 species of aphid.

Viruses of the persistent group usually have more specific vector relations. Sometimes only one insect species may be the vector.

Viruses of the nonpersistent group generally have been transmitted experimentally from plant to plant by mechanical sap inoculation, although it is quite clear that transmission of these viruses by aphids involves more than mere mechanical contamination of mouth parts.

Differences between the nonpersistent and persistent virus-vector relationships have practical applications.

Because aphids can transmit non-persistent viruses for such a short time, plants that are the source of these viruses obviously must be near the crop fields. Insects may acquire a persistent virus at great distances from the fields, however, and still cause infection when they feed.

Diseases caused by the nonpersistent viruses sometimes may be controlled by breaking the succession of crops necessary to maintain the virus. Such a succession will not affect the maintenance of persistent viruses, since they can be retained for long periods by the insect. Because a persistent virus may even overwinter in its vector, the insect does not have to feed on source plants before it can infect new crop plants at the start of the next growing season.

THE CURLY TOP virus, transmitted by the beet leafhopper (Circulifer tenellus), is an example of a persistent virus that affects some important seed crops. This disease was a serious menace to sugarbeets in the Western States before 1900. Before long, the occurrence of curly top was associated with outbreaks of the beet leafhopper. Now we know that curly top is transmitted solely by this insect species. Many vegetable crops, ornamentals, and seed crops are affected.

The beet leafhopper breeds on weeds in deserts and semiarid wastelands and moves from them to cultivated crops. Some 4o thousand square miles in the drier sections of southern Arizona are breeding areas of the insect. When rainfall is sufficient to maintain the annual herbaceous plants on which it breeds, astronomical numbers are produced. When winds and weather conditions are right, the leafhoppers migrate some 500 to 600 miles from southern Arizona to the agricultural areas of eastern Utah and western Colorado.

Many other breeding areas exist in all Western States. In outbreak years, movements of beet leafhoppers cover most of the agricultural land west of the Rocky Mountains. Some of the weeds on which the beet leafhopper breeds serve as reservoirs of the curly top virus. The virus is of the persistent type. It remains in the body of the insect and during migration plants far from the breeding area are infected.

CURLY TOP limits the production of garden beans for seed in the arid or semiarid regions of the West. This is true in south-central Idaho, but, because other conditions are favorable to the crop, about 8o percent of the national requirement of seed beans is grown here.

The disease is most severe on beans when the plants are infected in the crookneck, or seedling, stage. The problem is therefore particularly serious in Idaho, since the emergence of the bean plants frequently coincides with spring migrations of beet leafhoppers from breeding areas in deserts and wastelands.

Oddly enough, the leafhopper does not like beans. It cannot live long on the plants and does not breed on them. During migration, however, the insects feed briefly on the plants and transmit curly top.

Large acreages of garden beans are grown in Idaho for seed and for commercial dry beans. Some varieties are highly resistant to curly top, but none of the varieties of snap beans grown for seed is resistant to curly top. Some are extremely susceptible to curly top. Others may be less so, but in years of heavy infestations, all the seed varieties suffer heavy losses.

Research and control efforts have been directed primarily into two channels chemical and cultural practices and development of resistant varieties through an extensive breeding program.

Chemical and cultural control practices involve the beet leafhopper in its desert and wasteland breeding areas as well as in cultivated crops.

Chemical seed treatments, leaf sprays, and dusts are applied directly to the susceptible crops. The aim of the chemical control in the breeding areas is to reduce the extent of the migration into cultivated lands. Applications of insecticides as seed treatments or as foliar dusts or sprays to crops susceptible to curly top are intended to kill the beet leafhoppers soon after they appear and so reduce the spread of curly top.

Chemical control of the beet leafhopper in its breeding area has been practiced in southern Idaho since 1949 in years when the numbers of the leafhoppers, as measured by a spring survey, threatened destruction of cultivated crops.

For control, an oil solution of DDT is applied at the rate of 1 pound per acre to desert and wasteland areas where there are heavy concentrations of the spring nymphal brood. The application is made before the insects have matured as winged adults.

Work was started in southern Idaho in 1958 to reseed some of the large desert breeding areas with suitable perennial range grass. Russian-thistle, the worst summer host plant, is eliminated by the competition of the perennial range grasses when the grass becomes well established. About 87 thousand acres were reseeded, primarily with crested wheatgrass by 1960.

Systemic insecticides also have been used. They are used as seed slurry treatments, seed soaks, foliage sprays, drenches, and dips. These toxicants move from the point of application through the growing plant, and may be effective against insects for several weeks after application.

In laboratory experiments at Twin Falls, Idaho, curly top in garden beans was reduced materially by spraying the plants with systemic insecticides combined with juice from sugarbeets highly resistant to curly top.

The climate of southern Arizona is ideal for the production of seed of sugarbeets, and about 60 percent of the Nation's crop is grown here. The seed-producing areas, however, are close to large desert breeding grounds of the beet leafhopper, and curly top has been a problem. Because of the extent of the breeding grounds surrounding the beet-seed-producing areas, desert control is impractical or impossible.

Some of the varieties grown are resistant to curly top, but as a large percentage of the seed is for use in areas where the beet leafhopper is not a problem, varieties susceptible to the disease also are grown. Even varieties resistant to curly top are damaged in the early stages of development.