Pyrethrum and Rotenone

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:

Table 3

Dust mixtures of this kind should be used at the rate of 20 to 30 pounds to the acre.

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:

Table 4

Root powders are mixed with water at the rate of 2 1/2 to 3 pounds to 100 gallons of water and applied at the rate of 120 to 150 gallons per acre for Mexican bean beetle control.

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.