VEGETABLE Insects Can be Controlled Without Arsenical-Residue Hazard
The control of insects that attack vegetables and small fruits by means that will not leave harmful residues on the marketed product has continued to receive the attention of the Department. The work of the previous year has been intensified and broadened in scope, and on the basis of this research a mimeographed circular has been issued containing revised recommendations for the control of a number of important pests of these crops. These recommendations emphatically provide that arsenicals or other poisons should not be used after the appearance on the plant of fruit or foliage that would be sent to market or consumed, except in cases in which washing or stripping would remove all harmful residues. [What this article fails to acknowledge is that arsenic cannot be destroyed by any biological or chemical means. Change of an atomic element requires nuclear intervention, something that itself has severe hazards associated with it. Thus, even if the leaves containing the arsenic aren't directly eaten by a consumer, the poison remains in the soil, prevents composting of the "waste" plant material and thus continues to pose a threat to life. The only effective way to use elemental poisons, like lead and arsenic, is to leave them deep in the Earth and NOT USE THEM. -ASC] In addition to stressing the importance of employing insecticides that do not incur the hazard of harmful residues, special emphasis is given to the time and method of applying insecticides, and supplementary control measures, such as field sanitation and cultural practices, particularly the thorough destruction or utilization of crop remnants after harvest, are recommended.
In this search for substitutes for arsenicals and other means of eliminating harmful residues, extensive experiments have been conducted in Ohio, Virginia, Florida, North Carolina, South Carolina, Louisiana, and California. These experiments have been concerned chiefly with cabbage, and in general, the results have substantiated those obtained during previous seasons, to the effect that arsenicals and similar inorganic insecticides may be applied to this crop up to within 40 days of harvest without danger of harmful residues remaining on the marketed product. This means that cabbage may be treated with arsenicals before the plant begins to form a head, since all leaves which develop prior to that time have dried or are discarded at the time of harvest.
These studies have indicated that derris-root powder containing from 0.5 to 1 percent of rotenone mixed with talc or tobacco dust as a diluent is effective in controlling the common species of cabbage worms. In general, the pyrethrum-dust mixtures and hellebore have been less effective than the derris combinations. The number of treatments and the cost involved in obtaining cabbage-worm control with derris combinations on a commercial basis have not yet been determined for application under the diverse conditions existing in the different parts of the country where cabbage is an important crop. A dust mixture composed of 1 part of paris green and 9 parts of hydrated lime is effective against the common species of cabbage worms, but its use is limited to the early stages of the plant growth, when there will be no danger of harmful residue.
The Department has not had an opportunity to conduct any extensive experiments on the control of cabbage pests on cauliflower, broccoli, kale, or collards in order to determine the possible utility of arsenical substitutes. It is believed, however, that the compounds containing rotenone and pyrethrum should give approximately the same results on these crops as when used on cabbage. There should be little or no danger in the treatment of these crops with arsenicals when they are in the seedling stage. The leaves surrounding the heads of cauliflower are often used for food, and the treatment of the crop should be so regulated that these leaves do not bear any harmful residue. Especial precautions should be exercised in the use of arsenicals or other poisonous materials on broccoli, since the nature of the edible portion of this plant is such that residues are retained for a considerable length of time and there is little likelihood that they will be removed by washing or stripping. Since fields of harvested cabbage and other cole crops serve as sources of infestation to new plantings, particularly in the South, crop remnants should be destroyed or utilized.
Extensive tests in Ohio and Virginia during 1934 have indicated that the Mexican bean beetle can be controlled effectively, without danger of arsenical residue, by the application of derris sprays or dusts. These sprays or dusts gave excellent foliage protection and increased the yield markedly over that of the untreated plots. In general, a better quality of control has been obtained with the derris-root sprays than with the derris dusts. The derris-root spray was prepared at the rate of 1½ to 2½ pounds of finely ground derris root, containing 4 percent rotenone per 50 gallons of water (equivalent to 0.015 to 0.025 percent rotenone in the spray mixture), with appropriate adjustments for varying rotenone content of the derris root. Cryolite at the rate of 3 pounds to 50 gallons of water has given results equal to those from magnesium arsenate at the rate of 2 pounds to 50 gallons of water, when applied properly. The derris-dust mixtures contained from 0.5 to 0.75 percent of rotenone with tale, tobacco dust, or ground marc as a diluent.
Tests with the celery leaf tier have shown that compounds containing rotenone are not effective against this insect and that pyrethrum is apparently a specific poison for the pest.
Damage by the pepper weevil has been materially reduced in some areas, especially in California, by the destruction of nightshade, the principal winter host plant of this insect. No insecticidal treatment for the control of this pest has yet been devised which does not involve an undue risk of harmful residue remaining on the market product.
Studies in the control of melon and pickle worms on fall-grown squash in South Carolina have indicated that a derris-root powder containing from 0.5 to 1.5 percent of rotenone is effective against these insects, and a profitable return from the crop has been attained even under conditions that render necessary several treatments at intervals of from 7 to 10 days. Sulphur appears to be an effective diluent for derris-root powder when employed against these pests, and the addition of from 10 to 25 percent of talc or clay, by weight, improves the dusting qualities of the mixture. Cryolite and paris green are effective, and there is no danger of harmful residues when they are applied prior to the formation of the fruits. Calcium arsenate has not proved satisfactory in the control of these insects.
In tests against the turnip aphid in the South, derris-root powder containing 1 percent of rotenone with equal parts of finely ground tobacco dust and sulphur as diluents gave good results, even under the relatively low temperatures prevailing when this pest is most numerous.
Preliminary tests indicated that compounds containing rotenone are effective against the harlequin bug a common pest of many of the important vegetable crops.
Unsatisfactory results were obtained with compounds containing derris or pyrethrum when directed against the tomato hornworm, which was unusually abundant in certain sections of the East during the past season. Fall plowing is an effective aid in the control of this pest.
As a result of extensive experiments in the State of Washington, it was shown that sprays containing approximately 0.01 percent of rotenone were effective against the raspberry fruit worm, particularly when they were applied after the blooms appeared on the plants, supplemented by a spray containing arsenicals prior to the development of the blossoms. With this procedure no harmful residues were left on the harvested berries.
Injury by the strawberry weevil can be lessened by burning over its hibernating areas. Since such burning is necessary only over areas within 100 feet of strawberry fields and can be conducted during the winter, this method has a very practical application.