Insecticides may be divided into three classes, namely: Stomach poisons, contact poisons and repellents. Stomach poisons are used in destroying insects with biting mouth parts; for example, the potato beetle and the asparagus beetle. Contact poisons are used in killing sucking insects, as aphides and the stink squash bug. Repellents, as lime and tobacco, may not kill insect foes, but they may be effective as deterrents.
Arsenate Of Lead is the most valuable of the arsenical poisons. It is a stomach poison and has three distinct advantages over other arsenical mixtures, which are: (i) It is harmless to the foliage, (2) it adheres better to the foliage; (3) it remains longer in suspension. The usual strength is 3 pounds of lead arsenate to 50 gallons of water. Weaker mixtures are often effective, while 5 pounds to 50 gallons may be an advantage in killing insects difficult to poison. The commercial preparation which comes in the form of a paste should be mixed thoroughly with a small quantity of water before diluting in the sprayer. It may be used with bordeaux mixture without diminishing the value of either.
Paris Green, a stomach poison which has been used extensively for many years in combating chewing insects. In order to prevent injury to foliage, it is always desirable to add some lime in the preparation of the spray. One pound of lime and one pound of paris green are used with 75. to 200 gallons of water, depending upon the susceptibility of the foliage to burning. It is always safer to have a slight excess of lime in the mixture. The paris green becomes more thoroughly diffused in the water if it is first mixed to a paste. As it is simply held in suspension and as it sinks quickly, the spray pump should be provided with an agitator to keep the mixture constantly stirred while being applied.
White hellebore finds favor among some home gardeners mainly because it soon loses its poisonous principle when exposed to the air. As a stomach poison it is effective for cabbage worms and other pests, if the material is fresh when applied. It may be used as a powder, but the distribution is more thorough if applied as a spray, using 1/2 to 2 ounces of powder in 2 gallons of water.
Kerosene Emulsion is probably the most extensively used of contact poisons for sucking insects. It is prepared as follows: Dissolve 1/2 pound of hard, soft or whale-oil soap in a gallon of hot water; while hot, add 2 gallons of kerosene. Then use a force pump provided with a direct nozzle to churn or agitate violently for 5 or 10 minutes, or until the mixture is of the consistency of thick cream. If thoroughly emulsified the oil will not separate and the stock solution may be kept almost indefinitely. Various dilutions, ranging from 10 to 20 parts of water, with one of the stock solution, are used for various sucking insects. It is essential, of course, that the emulsion come in contact with the enemy. This is not always easy to accomplish, especially when the insects are on the undersides of the leaves.
As a contact poison and a repellent carbolic acid emulsion is valuable in combating root maggots of various crops, such as onion, radish and cabbage. It is made in the same manner as kerosene emulsion (132), by using 1 pound of soap, I gallon of water and 1 pint of crude carbolic acid.
Tobacco is used in various forms. The powder is often effective in destroying plant lice. Stems are sometimes strewn along the lines of peas to repel the pea louse. Tobacco decoction, made by steeping or soaking the stems in water, is an excellent insecticide for plant lice. Numerous nicotine extracts and powders have been placed on the market and are used for the various species of aphides.
Whale-oil soap, 1 pound dissolved in 5 to 7 gallons of water, makes a useful insecticide to control aphides and other minute insects. Hard and soft soaps may be substituted, but whale-oil soap makes a more effective spray.