Potato Aphids

W. A. Shands, B. J. Landis.

Four species of aphids commonly attack potatoes : The buckthorn aphid, the green peach aphid, the potato aphid, and the foxglove aphid.

All attack potatoes in New England, but only the green peach aphid and the potato aphid are pests of potatoes in some areas of the Western States. In the wingless form the aphids usually are green or yellowish green. The winged form may vary from green tinged with brown to completely black. They reproduce rapidly under favorable conditions. A few aphids per plant may increase to such a degree that they cover the leaves of the plant in a few weeks.

All four species can transmit potato virus diseases when they feed. The winged aphids can infect more plants over a wider range than the wingless forms. The greatest spread of some of the virus diseases follows the periods when large numbers of aphids fly into, within, and between the potato fields in midsummer. Most important among the diseases are spindle tuber, mild mosaic, rugose mosaic, and leaf roll, the worst. Under natural conditions leaf roll is spread only by aphids.

All these diseases affect yields. If all plants in a field are diseased, the marketable yields may be reduced 20 to 100 percent. Spindle tuber causes the tubers to become elongate with growth malformations about the eyes. Leaf roll disease causes an internal dark netting net necrosis within the flesh of some varieties; the defect in even a small percentage of the tubers lowers the market grade of the entire crop.

Virus diseases of the potato first attracted attention in Europe about 1770. The progressive deterioration of potato crops was generally regarded then as a weakening or running out of the seed stocks. Before 1913 no distinctions were draw between leaf roll and other potato virus diseases; in 1920 scientists learned that aphids transmit leaf roll. In 1937 a serious outbreak of the green peach aphid in Maine was accompanied by an unusual spread of leaf roll. Yields were reduced and few seed stocks were sufficiently free of the disease for further propagation. Because nearly half of the certified seed stocks in the United States were grown in Maine at that time, many eastern gardeners, truck farmers, and potato growers could get no good planting stock.

In 1940, men in the Department of Agriculture began to cooperate with those of the Maine Agricultural Experiment Station, which had been engaged in aphid research for many years, in an investigation of the aphid problem on potatoes in northeastern Maine. Their studies of the biology and control of aphids in large measure have led to the production of better seed potatoes and increased yields of table-stock potatoes.

In 1938 leaf roll occurred in damaging amounts in Washington. Until that time the green peach aphid had not been recognized as an important pest of potatoes in that State. Widespread losses from 1941 to 1949 were accompanied by unusually large populations of the green peach aphid. In 1947 the Department, the Washington Agricultural Experiment Station, and the Washington Department of Agriculture began an investigation of aphids on potatoes in central Washington.

CLIMATE DETERMINES the period during which potatoes can be grown and also whether other host plants, which are essential to the survival of the aphid, may occur in the region. Aphids generally require more than one kind of host plant, cultivated or wild, for existence. In areas where climatic conditions are unsatisfactory for the growth of the host plants aphids are not usually a factor in potato production. In Maine the climate favors the growth of host plants of the green peach aphid, the potato aphid, the buckthorn aphid, and the foxglove aphid. In Washington the buckthorn aphid is not a pest of potatoes because of the lack of its primary host plant, and the foxglove aphid has shown little inclination to adopt the potatoes to gain a beachhead.

These species are not all equally important in spreading the virus diseases, but all have been reported to be carriers of leaf roll. The potato aphid is of little moment as a carrier of leaf roll, but it transmits mild mosaic and it can spread spindle tuber from plant to plant. The buckthorn aphid transmits leaf roll but is of minor importance in its spread because of its sedentary habits. The foxglove aphid is an important vector of leaf roll but has a relatively limited occurrence on potatoes, except in Maine. The green peach aphid is the most important vector of leaf roll and also transmits rugose mosaic, mild mosaic, and spindle tuber. In Maine and Washington the greatest spread of leaf roll has occurred in seasons when the green peach aphid was abundant. Considerable variation in the annual abundance and the relative numbers of the various kinds of aphids present on potatoes has been observed. Without regard to their ability to carry disease, the buckthorn aphid in Maine and the green peach aphid in Washington usually are the most abundant species on potatoes.

A reduction in yield from aphid feeding in Maine results chiefly from a reduction in the number and size of marketable potatoes. In a series of experiments, potato plants of three varieties were given varying degrees of protection from aphids. The Katandin variety proved to be the most susceptible to aphid damage. The Green Mountain was moderately affected. The Chippewa was affected least. From the standpoint of plant appearance, Kennebec seemed to be susceptible to aphid damage in 1950, but at harvest we found that the yield was reduced only to a degree comparable to that in Chippewas. Protection from aphids increased the yield of No. 1 grade potatoes considerably. Increased yields of the Green Mountain could be attributed about equally to an increase in the number of tubers reaching grade size and to an increase in the size of tubers within that grade. In Chippewa about 25 percent of the increase in yield was the result of the increase in numbers of tubers that reached the grade size and 75 percent to the increase in size.

Green peach aphid. Winged form at top; wingless form middle; last instar nymph at bottom. (All about 15 times natural size.)

Greater direct losses from aphid feeding occur in Washington in the White Rose variety than in the Russet Burbank. Both are susceptible to leaf-roll infection, but the greater losses occur in Russet Burbank. White Rose is grown mainly for an early crop, Russet Burbank for a late crop. The amount of damage in the late crop depends largely on the amount of leaf roll and the number of aphids that develop in the early crop.

STUDIES WITH the Green Mountain, Katandin, and Chippewa varieties in Maine showed that leading factors in the rate of leaf roll spread in plots of the three varieties were the numbers of aphids feeding on the plants and the amount of leaf roll present in the planting stock. In Green Mountains and Chippewas the amount of leaf roll spread was in proportion to the size of population of wingless and winged aphids allowed to develop within the plots, but in Katandins, leaf roll was spread principally by winged aphids that developed elsewhere and flew into the plots.

In the Southern States, the females of the green peach aphid propagate by depositing young aphids, most of which develop into wingless females during much of the year, on various host plants. Winged females, however, are produced at intervals; they serve primarily as a means of transfer to progressively younger crops. In Maine and Washington, where winter temperatures prevent aphid development, the aphids survive the winter in the egg stage. In Maine some of the winged females produced in August, September, and October fly to primary hosts, where wingless females capable of laying eggs are produced. Males appear from August to November and fertilize the females. The females then lay the overwintering eggs on the buds and bark of the trees.

The aphid eggs hatch in late February and March in Washington. In Maine they hatch in late April and early May. From then until the following fall, all the individuals produced are females.

Starting with the eggs that hatch in the spring, several generations of the green peach aphid are produced on the peach or wild plum trees. In Maine some of the aphids of the second generation and most of those of the third generation are winged; they leave the wild plum trees and relocate on summer host plants, such as weeds and potatoes, where successive generations, mainly wingless, are produced until fall. In Washington, however, the winged forms appear first in the third generation on peach trees, and the production of winged aphids continues through each successive generation until fall.

In Maine when the spring migrations start before potatoes are up, as frequently happens, the migrant aphids land chiefly on weeds, where they continue to breed. Winged aphids developing here soon infest potatoes. If diseased potato plants are present, the aphids become infected and carry the disease from plant to plant as they breed and feed through the season.

In Washington the winged spring migrant aphids fly to weeds and various spring crops, including potatoes, where wingless aphids are produced. In July and August large numbers of the winged aphids appear and fly to younger plants, including late-crop potatoes. Winged and wingless forms are produced on the potato crop until fall. As in Maine, most of the leaf roll spread occurs in summer during the period when winged aphids are dispersing.