The Pea Aphid

John E. Dudley, Jr. William C. Cook.

The pea aphid is a small, light-green insect less than one-quarter inch long. Its food is the sap it sucks from the plants on which it lives. Except for size, adults and young look alike. It was first recorded in the United States in 1879 on peas in Illinois. It is now found in every State and in several Provinces of Canada wherever peas or its other food plants are grown. In 13 States it is a serious pest nearly every year.

Peas are the most widely distributed and the most favored food plants of the pea aphid. It attacks all peas, whether grown as green peas for market, for canning, for seed, as field peas, as a cover crop, or sweet peas grown for their flowers. The country's commercial planting of peas for canning and freezing in 1951 amounted to 445,860 acres. Peas are grown for canning and freezing in at least 31 States. The Great Lakes States and Washington and Oregon are the centers of the industry. In them also are large acreages of alfalfa and clover, perennial crops that are a reservoir for the aphid during the months when peas are not growing.

Next to peas the aphid likes alfalfa best. Serious infestations may develop on alfalfa in some States, notably Kansas and California. About 18 million acres of alfalfa were grown in the United States in 1949. The pea aphid also feeds on vetch and red clover, alsike clover, crimson clover, and sweetclover, but they seldom are damaged seriously.

THE PEA APHID spends the winter in the egg stage in the Northern States. In the Pacific Northwest it may over winter in an active stage. In the South it remains active most of the winter. Tiny black eggs, glued to the stems and fallen leaves of alfalfa and clover and protected most of the time by a blanket of snow, remain throughout the winter in the North. In April or early May, depending on the latitude, the eggs hatch into young aphids, called nymphs, which feed upon the newly sprouted alfalfa and clover. They molt four times before becoming adults. After one or two generations of aphids have been reared on alfalfa and clover, a large proportion of the next generation will develop wings and fly to peas in late May and early June. They are weak fliers, but wind currents may carry them far, perhaps 50 miles. Arriving in pea fields, the winged aphids commence at once to produce young, most of them wingless. In a season 14 or 15 generations may be produced.

BECAUSE PEAS are more succulent and tender than the crops the aphids have left and the temperature is likely to be rising, reproduction on peas is rapid. Female aphids can produce young without fertilization; in spring and summer all aphids are females. In warm weather a female may produce 10 to 14 young a day on one acre of peas there might be millions of aphids which came from eggs laid the previous autumn on alfalfa and clover and which can quickly ruin the crop if insecticides are not applied promptly. As the peas approach maturity and become less favorable for feeding, winged forms again appear. Most of them die, but some find their way to later planted, more tender peas or go back to alfalfa and clover. Some aphids remain on alfalfa all summer.

In October the forms change. Some of the aphids then born become males. Others become egg-producing females. Some are winged. Others are wingless. After the males and females have mated, fertile eggs are laid, which overwinter on alfalfa and clover. Thus the yearly cycle is completed.

The aphid sucks the sap from the leaves, stems, blossoms, and pods of plants. The injury they do to alfalfa and clover is not conspicuous unless the plants are small and suffering from drought or the infestation is heavy. Occasionally a heavy infestation has destroyed the entire spring growth of alfalfa. A few aphids may kill small pea plants. A heavy infestation on more mature plants may reduce yield and quality or even destroy the crop. The plants become stunted and produce fewer and smaller pods than uninfested plants. Aphids frequently attack the pods and cause them to curl, shrink, and be only partly filled with peas. The deformed pods are low in market value and do not shell out in the viner.

The aphid also transmits several virus diseases of peas. One is the yellow bean mosaic. In the Pacific Northwest alfalfa is the reservoir for the virus; it is spread to peas by winged aphids. In years of heavy migrations from alfalfa severe epidemics often cause great damage to peas. Another virus carried by the aphid is enation mosaic, which roughens or toughens the pods. In late-planted peas it is more destructive to the crop than the yellow bean mosaic because infected pods become too. tough to be shelled in a viner, and the result is that half the crop often is lost.

Two other virus diseases sometimes are found in peas. One causes a wilting of the growing tips and the other a bronzing of leaves and stems.

Devastating infestations of the pea aphid do not appear without some warning. The number of eggs laid on winter hosts and the early spring population of nymphs in alfalfa and clover make possible an estimate of the infestation that may be expected in peas. Seasonal weather conditions are the main determining factors. Warm, sunny weather stimulates rapid reproduction. Cool, rainy weather slows it down or even stops it for a while.

To know when to begin control operations, growers have to keep a close watch of the pea fields. They can determine how many aphids are present by several methods. A common way is to sweep the plants with an insect net that has an opening about 15 inches in diameter. The number of aphids captured in one sweep of the net is counted. After many such counts from sweeps in different parts of the field, the average number per sweep is determined. When an average of 35 aphids per sweep has been found, it is time to apply insecticides.

MORE THAN 70 species of predatory and parasitic insects, which vary greatly in abundance from season to season, attack the aphid. Now and then in sections s with warm, humid weather, a fungus disease may attack the aphids and kill a high percentage of them in a short time. The disease is of small account in dry climates. All these natural checks on aphids usually come too late to prevent damage to the pea crop.

The first attempts at commercial control of the pea aphid in the United States were made in Maryland about 1900 by W. G. Johnson, of the Department of Agriculture. He experimented with sprays of kerosene and fish oil soap and with tobacco dust. He concluded that sprays were not practical for use on large acreages. Then he developed brushes of pine boughs, which, attached to a cultivator, knocked the aphids off the plants.

Research on methods of control began in 1922 by the Maryland Agricultural Experiment Station, the Department of Agriculture, and the Wisconsin Agricultural Experiment Station. Gradually, as the aphid became a widespread menace, 12 State experiment stations initiated experiments. The work has involved tests with mechanical devices, insecticides, parasites, predators, and cultural methods.

Cultural methods have included heavy pasturing of alfalfa fields with sheep or cattle in fall or early spring, cutting as soon in the spring as a hay crop could be obtained, clipping the tender alfalfa tops, and regulating the time of irrigation in reference to the infestation. All gave varying degrees of control, but all had disadvantages and generally fell short of preventing aphid damage to the hay crop. Efforts to breed alfalfa plants resistant to aphids have been carried on in Kansas, California, and Wisconsin. Research in Wisconsin has shown that some varieties of peas Onward, Pride, Yellow Admiral are partly resistant.

About a million coccinellids, or lady beetles, collected in great masses from hibernation in the mountains of California, were liberated in the middle of a large alfalfa field in California. Several hundred thousand were shipped to Wisconsin and liberated in a field of peas. In both instances the beetles soon took wing to scatter far and wide without appreciably reducing the aphid infestation.

Burning alfalfa fields in late fall or early spring was tried by igniting the stubble and weeds and later by spraying the fields with fuel oil before firing them. Neither way achieved complete combustion, and enough unburned areas were left to allow the surviving aphids to build up again. Machines called stubble burners were developed and tested. They burned oil and produced a hot flame, which consumed all the green foliage both alfalfa and weeds as they were driven through a field. They destroyed practically all living aphids and their eggs but also killed the aphid's natural enemies. New growth appeared soon after the burning, winged aphids flew in from nearby unburned fields, and the aphids were able to reproduce so rapidly that in a short time the field again became critically infested.

Dragging alfalfa with brush, platform, or chain drags, cultivating it with a spring-tooth harrow, and rolling with a field roller have been tried in several States. But the consensus is that even when the operation is repeated several times during the season too many aphids are left unharmed and reproduction goes on. An aphid-collecting machine, called an aphidozer and tested in several States, consisted of a hopper and revolving brushes mounted on wheels and pulled by a horse or mule. It collected bushels of aphids from an acre of peas, but it had the same fault as most other mechanical devices it had no residual or lasting effect and left too many aphids on the plants to continue reproduction.

INSECTICIDES are the answer to the control problem on peas grown for processing. Many types of insecticides have been tested, but for one reason or another all but three or four have been abandoned for commercial or experimental use. Insecticides may be applied with ground equipment or by aircraft.

The most common methods with ground equipment are the power duster; the high-gallonage power sprayer, which applies 100 gallons or more of dilute spray per acre; the low-gallonage weed sprayer, which applies about 25 gallons per acre; and the mist blower, which applies about 10 gallons per acre. The power duster is the most common way because it is rapid and causes a minimum of damage to the peas. Few canners use the high-gallonage sprayer; it is slow and costly and is apt to damage the plants. The low-gallonage sprayers are used experimentally and commercially but need further improvements.

Aircraft apply insecticides as dusts or concentrated sprays. Both methods are fast and do no mechanical damage to the peas. Dusting by airplane is the most common method, although the application of concentrated sprays is increasing. Canners have been quick to adopt the use of aircraft because of the rapidity with which large acreages with threatening aphid infestations can be treated.

Each type of control has its advantages and disadvantages but, everything considered, more effective aphid control may be expected when ground rather than airborne equipment is employed. In either case the degree of control depends on the use of efficient equipment and thoroughness and timeliness of application.