More than 2,000 tons of Paris green are now used annually in America against insect pests.
It is the standard poison spray, and is used at the rate of 1 pound in 100 gallons on orchards, except plum and peach, where only about half this amount is safe ; on potatoes it is used at least twice as strong.
The arsenite of copper or green arsenite is simi lar to Paris green.
The arsenate of lead, which was first used against the gypsy-moth in 1892, is coming into general use. It adheres better to the foliage and can be used very strong with safety, thus making it especially useful against certain insects like the elm leaf-beetle, codling-moth, plum curculio, rose-chafer, and grape root-worm. It is sold in a paste form, one pound of which contains only about half as much arsenic as Paris green, thus necessitating using twice as much of the arsenate of lead, or 2 to 4 pounds per 100 gallons for apple orchards and 4 pounds per 50 gallons in vineyards for grape root-worms.
Hellebore is still much used for currant-worms, but has been largely replaced by the Paris green spray.
Sucking insects.—The insecticides used for kill ing sucking insects are largely powders, oils or soaps, which kill by contact or when they hit the body of the insect.
Pyrethrum powder is often used for house-flies, but it is too expensive for general use in spraying.
Tobacco in various forms is largely used for fighting plant-lice in greenhouses, and sometimes as a spray outdoors or in "washes" or "dips" for domestic animals. Tobacco stems may be burned slowly, creating a killing smoke, or tobacco dust may be freely scattered over the plant, or decoctions and extracts may be sprayed on the plants.
Whale-oil and fish-oil soaps and various common soaps are effective insecticides for plant-lice, scale insects and many other sucking insects. Two pounds of soap dissolved in one gallon of water is the neces sary strength for killing scale insects on dormant plants in winter, and one pound in four to six gal lons will kill plant-lice and recently hatched scale insects.
Kerosene and crude petroleum are among the most effective materials for killing sucking insects. Sometimes they can be applied in a fine spray on dormant trees with little or no injury, but usually it is necessary to combine them with soap in an emulsion, which can then be diluted with water. The emulsion is made by dissolving pound of soap in 1 gallon of hot water, then adding 2 gallons of the oil and thoroughly agitating the mixture into a stable emulsion. This should be diluted with 3 or 4 parts of water for scale insects and with about twice as much water for plant-lice and other suck ing insects. Pumps have been designed for combin ing the oils and water into a good mechanical emul sion, but usually the percentage of oil can not be satisfactorily controlled.
So-called soluble or miscible oils which quickly emulsify with water are now made and are very effective against scale insects.
A lime, salt and sulfur mixture (often without the salt) is a very effective and safe spray to use on dormant plants for the San Jose scale and the peach leaf-curl fungus. This " wash " is made by boiling for about an hour 15 pounds of flowers of sulfur and 20 pounds of stone lime in 50 gallons of water ; by using about 6 pounds of caustic soda this " wash " can be made without boiling and is nearly as effective, but costs more.
Fumigation.
Both sucking and biting insects succumb to the fumes of carbon bisulphid or to hydrocyanic acid gas.
Carbon bisulphid is largely used in killing insects infesting stored grains or seeds. It is poured into shallow dishes set on top of the grain in tight bins, or it may be sprinkled over the grain. The fumes are heavier than air and sink all through the grain ; as the fumes are explosive, no lights should be near. A little of the liquid poured on clothing stored during the summer will kill the destructive clothes moths. Cucurbitaceous vines have been covered with cloth and successfully treated for plant-lice with carbon bisulphid.