Systemic insecticides may soon drastically change our present pest-control practices.
These chemicals are absorbed either through the roots or leaves of actively growing plants in sufficient amounts to kill insects and related pests feeding on or breeding in them.
They are effective even at considerable distances from the point where they are applied.
They also make possible the control of pests, such as root aphids and leaf nematodes, which spend some or all of their lives in areas not readily accessible to standard surface sprays.
The available materials are effective mainly against aphids, mealy bugs, and mites and not so effective against scales, thrips, leaf miners, and leaf hoppers.
Arsenic As A Promising Chemical
Leonardo da Vinci, most famous as an artist, is credited with being the first to consider pest control by treating plants internally.
More than four hundred years ago, he wrote that arsenic was a promising chemical for use in what we now call systemic applications.
Although arsenic never became popular as a systemic, it did become one of the most widely used insecticides for surface applications. Systemic insecticides as we know and use them today were developed only recently.
Among the earliest studies on the internal therapy of plants were those in the middle 1920s by a German, Adolf Muller, who found that pyridin was a plant therapeutic for insects.
About 10 years later, other Germans (Gerhard Schrader and his associates) noted that certain toxic compounds could be absorbed through plants’ roots and leaves, thus protecting them from insect attack.
Types Of Organic Phosphate
An organic phosphate, octamethylpyrophosphoramide, was one of the first found to have this unusual property.
This compound is also known by the names OMPA and Schradan, the latter in honor of its discoverer Gerhard Schrader.
At present, two organic phosphates are perhaps the most widely used systemic insecticides.
These are known by the common names Demeton (trade name Systox) and Schradan (Pestox 3 and OMPA).
These are used mostly as soil drenches to control aphids and mites on several important ornamental plants—roses and chrysanthemums grown outdoors and on many ornamental plants grown under glass.
On outdoor roses and chrysanthemums, Systox is used at the rate of one liquid ounce, well-diluted with water, for every 100 square feet of the flower bed. Following are the basic dosages of the systemic for use on greenhouse ornamentals:
For chrysanthemums and carnations, 1 pint of 21% to 23% percent Demeton (Systox), or 2 pints of 90% percent Schradan (OMPA) per 1,000 square feet of bed area.
Ornamental Plant Pest Control
For African violets, cyclamen, delphinium, and foliage plants, 2 pints 21% to 23% percent Demeton (Systox), or 2 pints 90 percent Schradan (OMPA) per 1,000 square feet.
Sufficient water must be used to ensure good distribution through the soil. The solution must be used as a soil drench, never as a spray.
Although Systox was first used primarily to control pests on ornamental plants, it became the first systemic insecticide to be approved for use on food crops.
At present, its use in this country is permissible early in the growing season on apples, potatoes, and English walnuts. In Europe, its use is permitted on an even wider range of edible crops.
Permitted Use Of Systox
Only recently has the state of California permitted Systox on cabbage, cauliflower, brussels sprouts, and broccoli to control cabbage, green peach, and turnip aphids.
Treated crops must be consumed within the state, and none can be shipped over the state line.
Only one pint of Systox per acre will control the aphids. Therefore, a second application is permitted, if necessary, but it must be made at least 21 days before harvest.
One reason for permitting the use of Systox on certain food crops is that it does not last long after it gets inside the plant.
In fact, it breaks down so rapidly inside the plant that the residues are essentially non-toxic.
It remains effective for a maximum of 3 weeks or long enough to destroy the insects against which it is applied.
Systox is harmless to bees unless they are hit directly with it. Therefore, no appreciable loss occurred when fields were sprayed with a relatively heavy dose directly over the bee colonies.
Some wildlife species were unharmed when directly exposed to Systox spray outdoors or fed on freshly treated plants in screened cages.
Some of the newer systemics can be applied as foliage sprays. For example, one investigator found that 9% percent of the Schradan applied to bean leaves entered within 14 hours.
Another found that 50% percent entered citrus leaves within 24 to 48 hours of the foliar application. The less toxic an insecticide is to humans and pets, the more extensive its application.
With this thought in mind, chemists have concentrated their efforts on developing less toxic organic phosphates.
The first such systemic to be developed is Meta-Systox, which is far less toxic to test animals (rats) than Systox, but it must be used at twice the strength to achieve the same degree of aphid and mite control.
Investigations On Organic Phosphate
About the time that Schrader began his investigations on organic phosphates, two United States Department of Agriculture research workers.
A. M. HurdKarrer and F. W. Poos discovered that selenium compounds could be introduced into plants via the roots in sufficient quantities to protect plants against certain insects.
They grew cereal crops in soils containing ten parts of selenium (as sodium selenate) in a million parts of soil and noted that aphids that fed on these plants soon died.
Other research workers then found that about 100 parts per million of sodium selenate in plants would control all red spiders and that less than half this amount could be used to control aphids.
Soon it was discovered that selenium absorbed by chrysanthemums would also control the widespread and highly destructive leaf nematode.
The use of selenium as a systemic insecticide for controlling aphids, mites, and leaf nematodes is absolutely restricted to ornamental plants.
It should never be used on plants that are to be consumed as food by man or animals. Moreover, the soils on which it is used cannot be used to grow food crops for many years.
Using Sodium Selenate
Sodium selenate, a readily soluble compound of selenium, is the form most commonly used to treat ornamental plants. Unfortunately, it is a deadly material and must be handled with the greatest care.
It is prepared and used as follows: First, dissolve one ounce of sodium selenate crystals in a gallon of water and label the jug “Sodium Selenate Concentrate, Poison!”
To use, dilute one-half pint of this concentrate in water to make one gallon of diluted mixture. Sprinkle one pint of this diluted mixture on each square foot of the flower bed.
Treatment Using The Diluted Solution
A gallon of this diluted solution will treat 8 square feet of bed or 8 plants. A gallon of the concentrate will make 16 gallons of the diluted mixture, which will treat 128 square feet of bed or 128 plants.
After treating with the diluted sodium selenate mixture, sprinkle the treated area with plain water and keep the area well watered during dry spells.
The treatment should be repeated on the same plants or in the same beds three to four weeks later.
Sodium selenate is also available in capsule form under the name Kapsulate, for the control of pests of African violets.
In a 2% percent concentration, absorbed on superphosphate, under the name P-40, for direct application to soils in which chrysanthemums are growing.
Effective Systemic Insecticides
Systemic insecticides, particularly sodium selenate, have proved most effective on vigorously growing herbaceous ornamentals.
They are less effective, however, on woody plants and slow-growing nursery plants and trees.
44659 by P. P. Pirone