Insects and Vegetables

The Potato Psyllid

R. L. Wallis.

Psyllid yellows, a disease of potatoes and tomatoes, is due to a substance injected into the plants by the nymphs of the potato psyllid (Paratrioza cockerelli) while feeding. The disease, if not controlled, will completely destroy the crops in years when psyllids are abundant. The psyllid is a native of the Western States. It originally subsisted on wild plants of the nightshade family. Soon after people began growing potatoes and tomatoes in the West the potato psyllid adapted itself to those crops and became a pest of economic importance.

The potato psyllid does not injure crops every year. Sometimes it causes total crop failures. In other years the psyllids are so few as to be difficult to find, except at the higher elevations where the cooler climate is more favorable for their development. In 1911 and 1912 a reduction in the potato crop in Colorado to only 35 to 95 bushels an acre was due to the potato psyllid. Earlier the injury to tomatoes was thought to be caused by the direct feeding of the psyllid rather than the disease it transmits. Other epidemics of the psyllid yellows have occurred in 1927-28, 1929-33, 1938-39, and 1949, with a varying degree of injury in the intervening years. Considerable injury is done to potatoes in years of light infestation even though the symptoms of the disease or the effect of direct feeding by the insects are not readily apparent.

The injury to potatoes by psyllid yellows results in an excessive root growth, the production of a large number of small tubers, and a reduction in quality. If infection is severe, all the tubers may be below marketable size. They cannot be stored successfully because their rest period is shortened and they will sprout early. They may even sprout in the ground before harvest.

The first visible symptoms of infection on the above-ground part are an upward curling of the base of the younger leaflets near the top of the plant. As the disease progresses, most of the leaflets turn slightly yellow and the curled parts have purple edges. The leaves finally become leathery, the plant is stunted, and it turns brown and dies. If infestation is light, the plants show no symptoms of disease but the tubers are noticeably smaller in size and yield and the storage qualities of the tubers are poorer.

The injury to tomatoes causes a reduction in the yield and quality of the fruit. Young plants that are attacked may fail to produce any fruits or set only a few, small, yellowish, coarse, and rubbery fruit with a low content of juice.

The symptoms on the tomato plant are a thickening and upward curling of the older leaves. Some purpling of the veins is evident. The leaves are lighter green than normal. The new leaves at the tips of the branches are much smaller in size and give the appearance of a sparsely foliated and stunted plant.

The potato psyllid occurs throughout the continental area west of the 100th meridian, except in Washington, Oregon, and most of Idaho. It has occurred east of that area in the lower Rio Grande River Valley in Texas in small numbers, and in eastern Nebraska and eastern North Dakota to the extent of causing slight injury in some outbreak years.

The greatest injury is caused in Colorado, southern Wyoming, western Nebraska, southeastern Montana, and Utah, particularly at the higher elevations where average summer temperatures range below 70 F. At the lower elevations the July temperatures are usually above 700 and psyllid infestations are retarded in their development. In Texas, New Mexico, and Arizona the insect may cause considerable damage to early spring crops.

THE ADULT or parent potato psyllid is a tiny, delicate insect. When full-grown it is about one-tenth of an inch long. The transparent wings are held at rest in a raised roof-like position over the body. The newly emerged adult is green but after 2 or 3 days becomes black. Its white markings give it a grayish appearance. It has two white marks a broad transverse one on the first abdominal segment and an inverted V-shaped one on the last abdominal segment. It has strong hind legs, which aid it in taking a quick flight or jump and give it the common name of jumping plant-lice. Mating may take place within a few hours after emergence, and egg laying begins in about 5 days. The females deposit the eggs mostly on the edges or under sides of the leaves in the shady parts of the plants. Egg deposition takes place most freely at temperatures of about 80 . Few eggs are laid at temperatures of 60 or 100 . Each female deposits about 300 eggs during a lifetime at the rate of about 15 a day.

The bright-yellow, oval eggs are about one thirty-second inch long. One end of the eggshell lengthens into a stalk about the length of the egg. It is attached to the leaf and supports the egg. The eggs hatch in 4 to 15 days, depending on the temperature. The flat, scale-like nymph has a row of short hairs around the entire margin of the body. It is roughly elliptical in shape, about one one-hundredth of an inch wide when first hatched, and about one-twentieth of an inch wide when fully grown. In their development the nymphs pass through five stages, becoming progressively larger each time. In the first, they are orange or pale yellow but later they become light green. They complete their nymphal development in 12 to 21 days.

DURING FEEDING the nymphs inject into the potato plant a substance that disturbs the proper relation between the foliage of the plant and the tubers. Tubers developing on the stolons of infected plants cease normal growth. From the eyes of the undersized and immature tubers, short sprouts soon appear. Other small tubers may form on the sprouts. If the psyllid yellows infection continues, the abnormal growth may proceed until a chain of several tubers occurs on one stolon, none of which will be of marketable size. Tubers that have been checked in their growth by drought or by other causes not due to disease and then start growing again ordinarily produce knobby potatoes totally unlike those formed by potato plants infected with the psyllid yellows.

The potato psyllid restricts its feeding almost entirely to the plants of the nightshade family, to which the potato and tomato belong. Its preferred host plants are the ornamental plant known as Chinese lantern (Physalis franchetz) and horsenettle (Solanum carolinense). They feed in considerable numbers also on buffalo-bur (Solanum rostratum), several species of ground-cherry (Physalis), and matrimony-vine (Lycium), as well as potatoes and tomatoes. Potato sprouts growing in cull piles bulk large in the spring buildup of psyllid populations in the potato-and tomato-growing areas. Adults have been collected from members of several other species of plants, but only on two species, field bindweed (Convolvolus arvensis) and sweetpotato, have occasional eggs and nymphs been found, indicating that the insect can adapt itself to those species when other food plants are not available.

BECAUSE of an incorrect identification of the insect, people long believed that the potato psyllid passed the winter in the northern part of its range on cedar trees or other evergreens. Further investigations disclosed, however, that the psyllid on evergreens in the northern areas in winter is a different species. Extensive surveys have revealed no positive evidence that the potato psyllid overwinters in areas where subzero temperatures occur. The surveys have shown, however, that the potato psyllid passes the winter in southern Texas, southern New Mexico, and southern Arizona in an active condition on wild host plants, principally species of wild matrimony-vine. The insect increases in numbers during the spring, but completely disappears therefrom in June, coincidentally with its movement northward, and returns to its southern range in October or November. In the more temperate areas farther north, wild plants also serve as a source of subsistence for psyllids that come from the south in early spring. Late in the spring they move into potato and tomato fields.

A study of the potato psyllid in its overwintering area in southern Texas and southern New Mexico showed that small numbers appeared in the area in November and persisted in the territory bounded, roughly, by Crystal City, Tex., on the south, San Angelo, Tex., on the east, Big Spring, Tex., on the north, and Las Cruces, N. Mex., on the west. It increased in numbers during the late fall and early winter but ceased breeding temporarily during January under the influence of the relatively low temperatures normally prevailing then. In winters when temperatures are above normal, however, such as occurred in the winter of 1949-50, breeding activities continued throughout the winter, as evidenced by the abundance of eggs, nymphs, and adults during this period. In winters of below-normal temperature such as the winter of 1950-51 the wild plants on which they feed are killed. So are the psyllids. As a result, none could be found on potatoes and tomatoes in northern sections in the summer of 1951.

THE MOVEMENT of the potato psyllid for long distances depends on wind currents. P. A. Glick, of the Bureau of Entomology and Plant Quarantine, has captured this psyllid at elevations of 4,000 feet and in considerable numbers up to 2,000 feet. Prevailing winds during most of the year in the plains east of the mountains from southern Texas and southern New Mexico to northern Colorado are from the south. In Wyoming and Nebraska prevailing winds are from the northwest. That may account for the generally lower populations in the North Platte Valley in Wyoming and Nebraska. When wind currents blow from the south for several days in the spring the movements of the potato psyllid into the North Platte Valley are much faster. Conversely, when the psyllid is moving in Colorado and wind directions are northwest in Wyoming and Nebraska, it reaches those areas in small numbers. The northern Colorado potato-growing area is the center of the summer population an indication that unfavorable wind currents north of that point greatly retard movements beyond that area.