Insecticides for Vegetables

THE perfect insecticide for vegetables must meet several definite specifications. It must leave no residue or deposit on the fruit or edible leaves of the plant to endanger the health of the consumer—a point that is especially important in controlling insects on leafy vegetables like cabbage, broccoli, kale, lettuce, celery, and asparagus, and almost as important in the case of vegetables that bear edible pods or fruits. The residues on leafy vegetables are not readily removed by washing. The insecticide for vegetables must be poisonous to a number of different kinds of insects and have no effect upon plant growth, whatever the climate. Temperature and humidity should not affect its toxicity to the insect or its physical qualities. It should be compatible with fungicides.

A material or combination that meets fully all these specifications has not yet been discovered, although many hundreds have been tested. But insecticides containing pyrethrins and rotenone as the principal toxic agents, developed during the past 15 years, have given satisfactory control of some important insect pests and have practically eliminated the hazard of residues.

The ground flowers of the plant Pyrethrum cinerariaefolium is the principal raw product from which pyrethrum insecticides are prepared.  Before the war, Japan was the principal source of the raw product, but during the war supplies were obtained largely from Kenya Colony, Africa. The two toxic ingredients of the flower buds are known chemically as pyrethrins I and II. The raw product from Kenya Colony contains, on an average, 1.3 percent of total pyrethrins.

Rotenone insecticides are prepared from the ground roots of two kinds of plants, Derris elliptica (derris) from the Malay States and Dutch East Indies, and Lonchocarpus species (cube, timbo, barbasco) from South America. The ground roots of these plants contain, besides rotenone, other ingredients toxic to insects, but rotenone is considered the most important, and is used as a basis for the preparation of rotenone insecticides.

In the late 1920’s and early 1930’s the condemnation by food officials of vegetable products, such as celery and cabbage, because of undesirable insecticide residues emphasized the necessity of developing insecticides less hazardous to man than the arsenical compounds and of providing schedules for applying insecticides that would reduce or eliminate the spray-residue problem for certain types of vegetables.

In general, the problem was approached from two angles: To study the growth of the plant to determine the latest period in its development that an arsenical insecticide could be applied without involving a risk of contaminating the part of the plant that is used as food; to develop the use of pyrethrum and rotenone products and extend the use of nicotine insecticides for leaf-feeding insects.

The problem of residues was particularly acute in areas where arsenicals were used for the control of cabbage caterpillars, especially where the cabbage is marketed with several loose leaves around the head. Therefore, the investigations initiated in 1932 were concentrated on the control of insects affecting cabbage and related cole crops. Plant-growth studies on the Wakefield variety of cabbage grown near Charleston, S. C., which is marketed with four outer loose leaves, revealed that objectionable residues will follow the use of arsenicals applied 30 days before harvesttime, or after the head begins to form. On cauliflower similar residues remained if the application was made after the curd or head had begun to form.

The investigations on rotenone and pyrethrum insecticides showed that either or both of the materials could be used to protect the crop from green caterpillar damage after it became unsafe to apply arsenicals or similar poisons. Since the insecticidal value of both pyrethrum and rotenone varies with the species of insect involved, the determination of which material to use depends up:n the species of worms that are predominant in the infestation. For example, pyrethrum insecticides are more effective than rotenone against the cabbage looper; rotenone is more effective for the control of the larva of the diamondback moth and the imported cabbageworm. Therefore, when the predominating caterpillars are the imported cabbageworm and the larvae of the diamondback moth, a dust containing 1 percent of rotenone is recommended. If the cabbage looper is the most abundant species, a dust containing 0.3 percent of pyrethrins will yield more satisfactory results.

Studies were also conducted to find means of increasing the efficacy of the simple dust mixtures containing these insecticides, especially rotenone.  For this purpose various wetting agents and oils were added, and different types of diluents or carriers were tried. Both mineral and vegetable oils were found to increase the effectiveness of rotenone dust mixtures against some insects, but were of little or no value against others. For example, 2 percent of mineral oil increased the toxicity of rotenone to the pea aphid and cabbageworms, but for Mexican bean beetle control rotenone dusts containing oil showed little superiority over simple dust mixtures. Experiments with the various carriers or diluents, such as clays, talc, sulfur, bentonite, and lime, for the rotenone-containing root powders showed talc and particularly pyrophyllite to be the most satisfactory.

Liquid extracts of pyrethrum flowers and of rotenone-containing roots, as well as powders impregnated with pyrethrum extracts, have also been prepared and used successfully.

The work on pyrethrum and rotenone conducted during the past 10 years by Federal, State, and commercial workers has shown that rotenone has a greater over-all usefulness on vegetables than pyrethrum and that the use of either product does not involve a residue hazard. The raw materials of both can be imported into this country and processed, and the products can be prepared and distributed at a reasonable cost.

As previously stated, these insecticides are selective in their action.  Rotenone insecticides are outstanding for the control of the Mexican bean beetle on the green bean crop, the pea weevil, and the asparagus beetle, and they are useful for the control of the green cabbage caterpillars on cabbage and other cole crops, loopers on lettuce, several kinds of flea beetles and aphids, the Colorado potato beetle, and the striped cucumber beetle. However, they are of little or no value against the tomato fruitworm, the tomato pinworm, webworms, the cabbage aphid, the pepper weevil, the celery leaf tier, leafhoppers, the garden flea hopper, or plant bugs. The term “plant bug” is applied to a group of insects that obtain their food by sucking the juices from the plants. Common among these are the squash bug, the harlequin bug, and the tarnished plant bug.

Pyrethrum insecticides, on the other hand, are outstanding for their usefulness against the celery leaf tier, webworms, the cabbage looper, and the bean or potato leafhopper. They are useful for the control of the imported cabbageworm, the garden flea hopper, and some species of plant bugs. Careful and frequent applications of pyrethrum dusts will control the Mexican bean beetle and the immature forms of the squash bug and the striped cucumber beetle. As is the case with rotenone, pyrethrum insecticides are of little or no value against the tomato fruitworm, the tomato pinworm, the pepper weevil, or the cabbage aphid.

Rotenone insecticides for vegetables are currently manufactured in three general types:
   1. Rotenone dust mixtures containing from 0.75 to 1 percent of rotenone, designed for applying in the dry form without any further dilution.  An examination of the label on a package of a dust containing 0.75 percent of rotenone should show the following:

   2. Undiluted ground-root powder containing 4 to 5 percent of rotenone, designed for mixing with water and applying as a spray.
The label on such a package should read about as follows:

   3. Extracts containing 1.5 to 2 percent of rotenone, designed for dilution with water and application as a spray.

Pyrethrum insecticides are also prepared in three general forms: Pyrethrum flowers mixed with a diluent such as talc or sulfur; dust mixtures prepared by incorporating a pyrethrum extract with a powder such as pyrophyllite; and pyrethrum extracts. All these preparations are made up to contain at least 0.3 percent of total pyrethrins. The first and second are applied in the dust form at the rate of 20 to 30 pounds to the acre, and the third is diluted with water and applied as a spray.

The toxic ingredients of extracts of both pyrethrum and rotenone when applied to plants in the liquid form dissipate more rapidly and therefore lose their effectiveness faster than they do in the dust mixture or powders mixed with water and applied as sprays. However, rotenone dust mixtures remain effective not longer than 4 days, and pyrethrum for even a shorter period.

The far-reaching results of this work with rotenone and pyrethrum were brought into focus during the war, when the War Food Administration set up controls as to their use on essential food crops.

During the course of investigations during the war period to develop insecticides from new materials, several hundred chemicals and combinations of chemicals were tested. This work was accentuated because of the shortage of pyrethrum, rotenone, and the arsenicals. Fluorine compounds were in good supply. One of these, cryolite or sodium fluoaluminate, recognized for many years as having possibilities as an insecticide, was experimented with extensively. Emphasis was placed on the improvement of its dusting qualities and on the determination of the most effective strength and dosage per acre to use against various vegetable pests. As a substitute for calcium arsenate it was found that, under conditions in southern California, a cryolite dust mixture containing 70 percent of sodium fluoaluminate with talc as the diluent gave satisfactory control of the tomato pinworm and the tomato fruitworm.  In the same area a mixture containing equal parts of sodium fluoaluminate and talc yielded good control of the pepper weevil. It was necessary, however, to establish a special washing device to remove the cryolite residues from the peppers.

In the Yakima Valley of Washington, where a cryolite dust mixture containing 55 percent of sodium fluoaluminate with pyrophyllite was used, 95 percent of the potato tubers were not damaged by the tuber flea beetle larva.

In eastern Virginia a satisfactory control of the corn earworm on late beans was obtained with two applications of a cryolite mixture containing 70 percent of sodium fluoaluminate with sulfur.

It was also found that sodium fluosilicate, a close relative of cryolite, in a wheat-bran bait was more effective than calcium arsenate against the Puerto Rican mole cricket and would also poison the southern mole cricket, a species not affected by a bait containing calcium arsenate. An effective bait against both species consisted of 8 pounds of sodium fluosilicate to 100 pounds of dry wheat bran, applied once at the rate of 20 pounds to the acre.

Investigations on the use of DDT as an insecticide for vegetables from the fall of 1942 to the spring of 1946 have led to the following conclusions: This chemical in its various formulations, including dust mixtures, emulsions, wettable powders, and aerosols, is toxic to a wide variety of insects. However, the range of usefulness of DDT is limited because of the residue factor. Its performance against several pests of potatoes, such as the potato leafhopper, the Colorado potato beetle, potato flea beetles, aphids, and psyllids, has been outstanding. It can be used with bordeaux mixture and the other so-called basic coppers. From the present knowledge it should not be used with copper-lime dusts, that is, mixtures of dehydrated copper sulfate and lime.

As an insecticide for tomatoes in southern California, DDT has proved to be more effective than either cryolite or calcium arsenate against the tomato fruitworm, and it is compatible with sulfur. Therefore, mixtures of DDT and sulfur can be used for the control of both the tomato fruitworm and the russet mite.

Its usefulness on beans is limited, as it will not control the Mexican bean beetle, although it is effective against the bean leafhopper and the corn earworm. The earworm attacks the tender shoots, flowers, and pods of snap and lima beans in some sections of the country.

On cabbage it has given a high degree of control of the various caterpillars, including the cabbage webworm and some kinds of cutworms, which attack cabbage in the South and oftentimes are destructive to the cabbage crop.  However, because of the residue factor, the precaution must be followed that is necessary with arsenicals; that is, the crop should not be treated when there is foliage on the plant that will remain on the product prepared for market.

On peas the control of the pea weevil with DDT has been equal, if not superior, to that obtained with rotenone. In the dust, emulsion, and aerosol forms it has yielded more satisfactory control of the pea aphid than any material tested heretofore. However, the use of DDT on the pea crop both for pea weevil and pea aphid control is recommended with the reservation that pea vines be withheld from livestock until more is known about the residue hazard.

On onions DDT has given a slightly higher degree of onion thrips control than either nicotine or tartar emetic-sugar mixture.

Against the pepper weevil dust mixtures containing from 2 to 5 percent of DDT have given excellent control and have also controlled the green peach aphid, which often causes damage in pepper fields following the use of either calcium arsenate or cryolite.

As an insecticide for cucurbits (squash, pumpkins, cantaloupes, watermelons, and cucumbers) the indications are that the usefulness of DDT will be limited, because it is injurious to some of these crops, particularly certain varieties of squash.

DDT has been tested on a wide variety of vegetable plants for plant tolerance and, with the exception of the cucurbits, it appears that in most sections of the United States the insecticide can be used without injury to the crop. However, injury to peas and tomatoes has been reported from New Jersey.

DDT in aerosol form has been tested extensively in Maryland against the pea aphid, and this method of applying DDT to the pea crop appears to have a decided advantage over other methods because of the lightness of the load. However, analyses of residues on pea vines indicate that there is a greater residue from the aerosol treatment than from the use of dusts or emulsions, which may prohibit the use of DDT in the aerosol form on peas where the vines are to be used as cattle feed.

DDT as a soil insecticide has been tested against wireworms, and while it is slow acting against these pests, the indications are that it may be useful as a means of keeping wireworm infestations to a minimum.

Sabadilla was introduced into the vegetable-insecticide field during the war by T. C. Allen, of the University of Wisconsin. Sabadilla insecticides are prepared from the ground seed of a tropical lily or lilies. The active principle of the seeds is a complex mixture of alkaloids called veratrine. This material appears to have its greatest usefulness in the control of the squash bug, harlequin bug, and the potato or bean leafhopper.

Following the successful use of a mixture of dichloropropane and dichloropropene, known commercially as D-D, for nematode control in pineapple plantings in Hawaii, by Walter Carter, of the Pineapple Growers Association, this material was tested in California and Washington against wireworms. The results were promising. Another product having for its toxic agent ethylene dibromide gave equally good, if not superior, results against the sugar-beet wireworm. Success in the use of a soil fumigant is dependent on the method of application, and within the last 2 years machines for effectively applying soil fumigants have been developed. This, together with more effective soil fumigants, should aid materially in reducing wireworm infestations in lands devoted to vegetables where the returns per acre will justify a comparatively large output of funds.

THE AUTHOR
W. H. White is in charge of the Division of Truck Crop and Garden Insect Investigations in the Bureau of Entomology and Plant Quarantine.

FOR FURTHER READING
Reid, W. J., Jr., Smith, C. E., Reed, L. B., and Bare, C. O.: Studies on the Control of Cabbage Caterpillars with Derris in the South, U. S. D. A. Circular 615, 1942.
Reid, W. J., Jr., Smith, C. E., Reed, L. B., and Thomas, W. A.: Field Studies of Insecticides Used to Control Cabbage Caterpillars in the South, U. S. D. A. Technical Bulletin 782, 1941.
Smith, C. E., Reid, W. J., Jr., Harrison, P. K., and Bare, C. O.: A Study of Arsenical Dusting of Cabbage in Relation to Poison Residues, U. S. D. A. Circular 411, 1937.

Entomologists at the Toledo, Ohio, station of the Bureau have tested hundreds of preparations offered as effective weapons against the European corn borer. In preliminary laboratory tests, corn leaves treated with the experimental insecticide are infested with young borers raised for the purpose. Here David Questel prepares to treat a corn leaf with a test preparation.


Again, preparations that pass laboratory tests are tried out in a field as sprays or dusts under natural conditions. To be considered good enough for commercial use, an insecticide must protect crops and not cost too much.  Here a dust preparation is being applied to corn with a self-propelled power duster developed by engineers of the Department of Agriculture.

Tests on DDT and other insecticides for use in controlling insect pests on sugar beets grown for seed are made at the Bureau’s station at Phoenix, Ariz. Here the late K. B. McKinney adjusts around a sugar beet seed plant a cage into which he has put 100 tarnished plant bugs.  All plants in this test plot are of the same age and have been treated with different insecticides.


At its Whittier, Calif., field station the Bureau of Entomology and Plant Quarantine seeks a new way to control the California red scale, a costly citrus pest that resists fumigation with hydrocyanic acid gas, once used to good effect against it. Here two members of the laboratory staff, H. D. Nelson and Ruth L. Busbey, determine the required concentration of gas.